M Spolaore
email: monica.spolaore@igi.pd.cnr.it

G Serianni

A Leorato

F Degli Agostini

Design of a system of electrostatic probes for the RF negative ion source of the SPIDER experiment

Keywords: 52.25.-b, 52.50.Dg, 52.50.Gj, 52.55.Fa

come

Design of a system of electrostatic probes for the RF negative ion source of the SPIDER experiment

M Spolaore, G Serianni, A Leorato, F Degli

Introduction

The realization of the Heating Neutral Beam (HNB) for ITER represents a technology challenge. ITER will be equipped with at least two HNB systems, each one delivering an effective power to the plasma of about 16 MW of the particles will be accelerated at 1MV [START_REF]Neutral beam heating and current drive (NB H and CD) system[END_REF]. The required initial current of 40A of negatively charged deuterium for each system will be provided by an RF ion source. In order to test, commission and optimize the ion source and all the beam line components of the HNB system for ITER a test bed is foreseen to be built in Padova (Italy). Within this project a specific facility, SPIDER (Source for the Production of Ions of Deuterium Extracted from Rf plasma), will be devoted to test and optimization of the ion source [START_REF] Sonato | [END_REF]. The negative ions extracted in this case will be accelerated up to 100 kV. In SPIDER, a system of electrostatic probes will measure the plasma parameters and monitor their homogeneity in the region facing the plasma grid, where most of the negative ions are expected to be produced and extracted. In the next sections after briefly describing the working principle of the SPIDER source, the details of the design of the electrostatic probe system will be provided including the design guidelines, the sensor description as well as integration within the SPIDER facility.

The RF negative ion source facility SPIDER

A detailed description of the working principle of a RF source for negative ion production in existing systems can be found in [3], see also [START_REF] Fantz | Diagnostics of Negative Hydrogen Ion Sources for ITER: A Comparative Study[END_REF]. For the present purpose it is worth mentioning that in RF negative ion sources for ITER neutral beams the plasma is produced inside RF drivers (see fig1.a), then it expands in a so called expansion region. Negative ions, for the ITER purpose H -or D -, are produced both in the plasma volume and due to surface processes, i.e. on the grids facing the source plasma. Negative ions are extracted through grid holes and then accelerated so as to build the negative ion beam. It is worth mentioning that the surface production processes, which in the existing systems are believed to provide the majority of negative ions, are favored by the presence of a thin layer of Cs, deposited during the source operation on the plasma grid surface through which the negative ions are extracted. For this reason in SPIDER such surfaces have to be maintained at 150°C, by an active heating system. A further element to be considered is constituted by the magnetic field, which is essentially parallel to the grids and plays the role of filtering out the most energetic electrons in order to fulfill the requirement of ITER HNB concerning the reduction of the co-extracted electrons. The SPIDER source will be equipped with 2x4 RF drivers. In figure 1a the vertical section of the SPIDER source is shown and some details of the plasma facing components upstream of the acceleration step are also given in fig. 1b and1c: the plasma grid is characterized by the presence of 4x4 modules of 16x5 apertures through which the ion beam is extracted. The extraction region in the SPIDER experiment is constituted by the plasma grid (PG) and the bias plate (BP), which is a further grid with wide openings corresponding to the modules in the PG [3]. The system is completed by the extraction grid and the acceleration grid, not shown in fig. 1. Given the key role of the extraction region in the negative ion production, the electrostatic probe system exploits PG and BP as supporting structures. It is worth mentioning that also other diagnostics will be installed in this region, in particular spectroscopic ones, so that horizontal cuts of the BP are foreseen. In the SPIDER facility the whole RF source is in vacuum.

The design of the electrostatic probe system

Design guidelines

The main aim of several diagnostics in the SPIDER system is troubleshooting in case the ion beam data is not as expected and providing information for fine tuning and optimization of ion source operation. Specifically the main aim of the electrostatic probe system is to provide information about uniformity of plasma parameters in the extraction volume where most of negative ions are produced, as well as the investigation of the local plasma parameters in different plasma conditions. The sources of non homogeneity that can be expected [START_REF] Fantz | Diagnostics of Negative Hydrogen Ion Sources for ITER: A Comparative Study[END_REF][START_REF] Fantz | AIP Conf. Proc[END_REF] are mainly due to differences between the edge and the central part of the extraction region, and to the possible influence of the discrete number of RF drivers located in the ion source. Furthermore it is particularly interesting to investigate the plasma features as close as possible to the grid apertures through which the negative ions are extracted. Finally the electrostatic probe system would provide input data for other diagnostics such as spectroscopy. The features of the electrostatic probe system, given its exploitation of PB and PG as supports, have to guarantee high vacuum compatibility and operation at 150°C in a hydrogen or deuterium plasma environment, as described in section 2. In order to meet all these needs, an accurate choice of materials has been performed; considering the long duration of operation, 1 hour pulses with maintenance interventions foreseen about twice a year, the design requires robustness as well as easy assembly and maintenance operations. The use of caesium vapor during the operations requires accurate short circuit prevention, despite the continuous metallic deposition that characterizes the source operations. This is accomplished by a specific design of the insulating part of the sensors accompanied by a suitable procedure for periodical cleaning and maintenance. The system design has to account for the presence of a RF plasma produced by the drivers and for a magnetic field of the order of some mT mainly parallel to the grid plane; such magnetic field is originated by a current flowing in the PG itself, aiming at filtering out the most energetic electrons, and by the electron suppression magnets in the extraction grid. These source features constitute a challenge in the electrostatic probe data interpretation [START_REF] Boschi | [END_REF][START_REF] Hippler | Low Temperature Plasma Physics[END_REF]. The solution adopted in the design consists in the RF compensation through a reference electrode, with an area relatively larger than that of the electrode itself [START_REF] Chatterton | [END_REF]. The presence of a magnetic field is known to affect the Langmuir probe characteristic and this issue has been accounted for through a design solution with suitable orientation between the sensor collecting surface and the magnetic field. This is the reason of protruding electrodes or sensors placed in the lateral side of BP as described in the next section. In order to make the data interpretation easier a planar probe concept has been adopted for the electrodes and the design accounts for this guideline in the sensor integration on different components of the ion source. Concerning the operation of Langmuir probes, all sensors will be continuously used to measure the ion saturation current, providing a relative plasma density homogeneity map on three different planes parallel to the PG. It has to be noted that such operating way provides data not affected by RF nor by the presence of magnetic field. The presence of various positive ion species [9] will be accounted for in the ion sound speed [START_REF] Stangeby | The interpretation of plasma probes for fusion experiments[END_REF] and by comparison with spectroscopy measurements. However also information on local plasma parameters, like electron temperature or Electron Energy Distribution Function, can be deduced upon biasing the sensors and collecting the I-V characteristic. It is worth noting that the collection of the current-voltage characteristic for planar probes, limited to very few electron temperatures above the floating potential allows to obtain reliable plasma parameters [START_REF] Stangeby | The interpretation of plasma probes for fusion experiments[END_REF][START_REF] Spolaore | [END_REF][START_REF] Serianni | Proc. of XXIV International Conference on Phenomena in Ionised Gases[END_REF]; this avoids the need of bringing the electrodes to the electron saturation current branch, which can provide the electron density but is potentially perturbing for the system and where the data interpretation becomes more difficult due to RF and magnetic field. Around the floating potential the contribution of negative ions to the slope of the I-V curve is expected to be negligible, due to the different mass and temperature compared to electrons. In addition the design has been carried out with the aim of providing the system array with large versatility: an easy design concept simplifies the assembly and maintenance operations. The system will then follow the experimental needs of SPIDER operation: the change of configuration is allowed by the same assembly concept adopted both for the sensing electrodes and for the reference electrodes, which are required for local RF compensation.

Sensor description

The design concept of the sensors to be installed on the PG and of their constituent parts is described in the following. The sensors will be installed from the front side of the PG. As shown in fig. 2 the main components are the electrode and the cable, which is soldered to a conducting disk. The electrical continuity is guaranteed by a contact spring, whereas an insulating disk and an insulating ring provide the electrical insulation from the surrounding metallic component (the PG in this case) respectively on the back side and laterally. The shape of the insulating ring involves the presence of surfaces not directly facing the plasma, due to the need of preventing the risk of short circuits during the operations. All sensor components are held together and fixed to the PG by a locking ring, which is screwed to the PG itself, allowing the sensor installation and compressing the spring so as to provide the electrical contact.

Given the dimensions reported in fig. 2 the size of the collecting area for ions is 75 mm 2 .

According to the basic plasma parameters measured in such kind of RF negative ion sources [3], a density of 0.5•10 18 m -3 and an electron temperature of 1 eV, the expected ion saturation current is 30 mA; with a density of 10 18 m -3 and an electron temperature of 2 eV, the ion saturation current results 85 mA. These values of ion saturation current are not difficult to be measured and allow the estimation of few percent variations to satisfy the main goal of the electrostatic sensor system, namely the measurement of source uniformity.

In order to fulfill an easy change of configuration of the system array, the design of the reference electrode for the PG is based on the same concept as for the sensing electrode, shown in fig. 2.

The main difference consists in a conducting plate directly screwed on top of the electrode. The reference plate should provide a relatively larger area exposed to plasma with respect to the collecting area of the sensing electrodes. This is achieved in the PG while considering the constraint due to the area of the grid itself not covered by the BP. On the other hand the reference electrode placed on the BP does not suffer from this constraint, so that a ratio of about 10 can be easily achieved between the plate and the respective sensor.

The integration of the system of electrostatic sensors in the ion source components has been obtained by conceiving the design of essentially three different kinds of sensors. The first one is the sensor to be installed on the plasma grid and has already been described; concerning the BP two different kinds of sensors have been designed: the sensor on the front side of the bias plate and the sensor on the lateral side of the bias plate. Concerning the sensors on the front side of the BP the concept described in fig. 1 has been used as a guideline; however in this case the sensor can be installed from the rear side of the BP, so that the role of the locking ring is played in this case by a metallic plate screwed on the back side. The reference electrode for such sensors exploits the same methods as already seen for the sensors on the PG. Also the sensors to be installed on the lateral sides of the bias plate adopt the concept of the contact spring, however they have the peculiarity of exploiting the BP thickness itself (10 mm). This solution provides a clear advantage from the data interpretation point of view, since the electrode collecting surface is perpendicular to the magnetic field. In this case however the tight available space does not allow the installation of reference electrodes so that they will be used essentially as ion saturation current measurements, with the possibility of using opposite facing electrodes as a double probe, given that they are placed in the same flux tube. In figure 3 the details of the components of the three different types of sensors are shown and how they are installed respectively on PG or BP. The sensor orientation with respect to the magnetic field direction is also shown, furthermore for the electrodes to be installed on the PG and on the front side of the BP the respective reference electrodes are also shown.

Probe system integration in the SPIDER facility

The design of the electrostatic sensor arrays exploits PG and BP as supporting structures. The main technical constraints then concern the cooling channels that allow an active heating of the PG and BP up to 150°C during the operations, and the supports of these components. The available thickness of PG and BP (9mm and 11 mm respectively) has to be accounted for. Furthermore all the sensors have to be exposed to the ion source plasma and due to the high voltage differences in the accelerator, the sensors on the PG have to be installed on the ion source side of the grid. In order to minimize the number of sensors, while pursuing the scientific objectives mentioned in section 3.1, the design involves basically two main complete arrays, to investigate vertical and horizontal non-uniformities of plasma parameters, supplemented by other partial arrays so that the effect of a discrete number of drivers is diagnosed. Specifically the measurements are performed in several vertical positions, corresponding to the center of the RF drivers and in between them. The three different types of sensors are then arranged on 2D arrays placed on planes perpendicular to the beam direction and respectively at 0, 15 mm and 20 mm from the PG surface, to study the axial gradients of plasma parameters. The resulting arrangement of sensors is shown in fig. 4. The total number of each kind of sensors is shown in Table I. The integration of the electrostatic sensor system in the extraction region requires taking care of cable path and their safety during the source operations. For this scope miniaturized coaxial insulated cables, UHV compatible, have been chosen, with diameter of 1.5 mm. Cables are protected from plasma exposure through their entire length: at first the cable paths exploit the 1 cm gap between PG and BP, so that cables are not exposed to driver plasma; subsequently the cables are installed in trenches and as a further protection a metallic cover will be applied over cable paths. It is worth mentioning that cable path and covers do not reduce the gap space that has to be maintained free for spectroscopic diagnostics purposes.

Conclusions

The design of an electrostatic probe system for the SPIDER facility has been presented, including the purpose of the measurements, the design criteria and the adopted solutions. The presence of RF and magnetic fields, deposition of conducting materials during the operations, robustness and easy maintenance are some of the aspects guiding the design work, as well as the constraints related to the integration of the probes in the source design.

Figure 1 Figure 2

 12 Figure 1 Section of the RF ion source of SPIDER and of the extraction region(a); the bias plate and the plasma grid are evidenced (b, c)

Figure 3

 3 Figure 3 Overview of details of different sensors installed respectively on the plasma grid (right panel), on front side of Bias plate (left panel top) on lateral side of bias plate (right panel bottom). For each case details of the sensor assembly are shown and how it is installed on the respective component of the ion source. The central figure shows the expansion of a region where different sensors are installed on SPIDER ion source.

Figure 4

 4 Figure 4 Layout of sensor assembly in the extraction region; the yellow disks on the background indicate the RF driver position. Sensors on PG (blue), sensors on BP front side (red), sensors on BP lateral side (green) and reference electrode on BP (yellow). The reference electrodes on PG are not indicated in this schematic

Acknowledgments

This work was set up in collaboration and financial support of Fusion for Energy


H Gebavi
email: gebavi@yahoo.com

D Milanese

R Balda

S Chaussedent

M Ferrari

J Fernandez

M Ferraris

Spectroscopy and optical characterization of thulium doped TZN glasses

Keywords: tellurite glass, thulium, Judd-Ofelt, refractive index, energy transfer PACS: 42.70.Hj, 78.40.-q, 78.55.-m, 78.55.Qr

This paper reports spectroscopic properties of the 3 H 4 and 3 F 4 Tm 3+ levels in TZN glass host with a concentration range from 0.82 to 22 10 20 cm -3 . Precise refractive index measurements are performed on five different wavelengths by the prism coupling method. Judd -Ofelt intensity parameters have been determined in order to obtain transition rate, branching ratio, and radiative lifetime. Spectroscopy measurements showed the most promising concentration for 1.8 µm short cavity laser emission at 6.84 10 20 cm -3 Tm 3+ with a 24% quantum efficiency Confidential: not for distribution.

Introduction

Huge scientific effort was devoted in investigation materials and devices for emission in the infrared wavelength region where optical telecommunications play their main role, in the so called S-band from 1460 to 1530 nm, C-band from 1530 to 1565 nm and L-band from 1565 to 1625 nm. On the other hand, infrared (IR) domain at wavelengths around 2 µm includes different properties and offers a number of applications. First of all, there is an atmospheric transparency window that allows remote sensing in free space. Detection of water vapor is also possible in such range of wavelengths, thanks to strong vibrational absorption lines [START_REF] Yamane | Glasses for Photonics[END_REF].

Thulium (Tm 3+ ) is a good choice for IR domain applications because of its absorption at 790 nm which is available using commercial diodes, and its interesting emission at ~1.8 µm for a number of sensing and medical applications [START_REF] Gibbs | Population dynamics of the 3 F 4 and 3 H 4 levels in highly-doped Tm 3+ :ZB(L)AN glasses[END_REF]. In addition, the phonon assisted energy transfer process called "cross relaxation"(CR: 3 H 4 , 3 H 6 3 F 4 , 3 F 4 ) allows obtaining two ions in upper laser level for each pumping photon, which enhances the 1.8 µm emission and is one of the main reasons for utilizing thulium as an active ion for IR applications. An important feature of Tm 3+ as emitter in a host glass is the possibility of continuous tuning in the 1.8 -2 µm region depending on the host matrix, thanks to its wide emission peak. Tunability of Tm is also demonstrated in the 2.2 -2.46 µm region in Tm:YLF host (crystal host) for small dopant concentrations [START_REF] Godard | Infrared (2-12 µm) solid-state laser sources: a review C[END_REF].

Another interesting emission in the eye-safe region and telecommunication S-band is at 1.47 µm which occurs for 3 H 4 3 F 4 transition. However, inverted population for such lasing transition is more difficult to obtain without depopulating the lower energy level since the lifetime of 3 H 4 level is shorter than the one of 3 F 4 which creates the so called 'bottleneck' effect.

The aim of this study is a detailed glass examination in order to improve fiber lasing emission and develop short cavity fiber laser with lasing emission in the infrared region.

Advantages of a short laser cavity lies in a background loss reduction which occurs due to scattering or contaminants absorption and so can yield the same gain in a shorter device [START_REF] Digonnet | Rear -Earth doped fiber laser and amplifiers[END_REF] and narrow emission linewidth. Such objective requires highly doped active ion concentration which carries a potential risk of clustering and increasing of excited state absorption effect (ESA) or reverse cross-relaxation. Fiber lasers have potential for narrow linewidth (few kHz) applications compared with for example semiconductor lasers (~MHz). [START_REF] Digonnet | Rear -Earth doped fiber laser and amplifiers[END_REF]. Narrow linewidth is specially related to single frequency laser which operates in a single resonator mode emitting quasi monochromatic radiation with very small linewidth and phase noise which guarantees spectral purity. The origin of the phase noise is spontaneous emission of the gain medium related with optical and technical noise influence. The short cavity should give contribution to stabile long-term, single mode operation avoiding hopping mode [START_REF]Encyclopedia of Laser Physics and Technology[END_REF]. On the other hand, too narrow linewidth can create problems due to Brillouin scattering which can be partially reduced by resonator length decrease [START_REF]Encyclopedia of Laser Physics and Technology[END_REF].

As the host matrix tellurite glass is used because of its unique properties whilst the IR emission is ensured with thulium ions. In a previous work [START_REF] Gebavi | Spectroscopic investigation and optical characterization of novel highly thulium doped tellurite glasses[END_REF], some of the authors reported a characterization of some structural and optical properties of Tm 3+ ions in 75TeO in the framework of diffusion -limited regime and dipole -dipole interaction.

Experimental techniques

Glass fabrication

Glasses were prepared by melt quenching from mix powder batches, inside a glove box in a dry atmosphere with water content of about 7 ppm. The chemicals employed (together with their purity) were the following: TeO 2 (99+%), ZnO (99.99%), Na were performed by collecting the luminescence at the emission wavelength of 1680 nm. Data were processed by a Tektronix oscilloscope. All measurements were performed at room temperature.

Experimental results

Glass table and refractive index measurements

Table 1 shows the samples examined in this study together with their dopant concentration and refractive index values at 633, 825, 1061, 1312 and 1533 nm respectively.

Table 1

The refractive index decreases with increasing Tm 3+ concentration and increasing wavelength [START_REF] Gebavi | Spectroscopic investigation and optical characterization of novel highly thulium doped tellurite glasses[END_REF]. Values of the refractive index of the sample T5 are used for obtaining Cauchy fitting parameters to be used in the Judd -Ofelt theory (3.2).

Judd-Ofelt theory results

The VIS-NIR absorption measurement was obtained in the 300-2100 nm range on the sample doped with 11.3 10 20 cm -3 Tm 3+ , T5 (58151 ppm of Tm 3+ ). Absorption transitions originating in the ground state are assigned as 1 G 4 , 3 F 2,3 , 3 H 4 , 3 H 5 and 3 F 4 (Fig. 1.).

Fig. 1

Judd-Ofelt (J-O) theory is used to calculate transition probabilities between different manifolds, radiative lifetimes and emission branching ratios as found in the literature [START_REF] Rolli | Erbium-doped tellurite glasses with high quantum efficiency and broadband stimulated emission cross section at 1.5 µm[END_REF][START_REF] Walsh | Optical properties of Tm 3+ ions in alkali germanate glass[END_REF].

Obtained values of J -O parameters by using five main absorption bands are summarized in Table 2 together with the literature overview.

Table 2.

The absorption bands of Tm 3+ are all dominated by electric dipole transitions except the transition 3 H 6 3 H 5 , which contains electric-dipole and magnetic-dipole contributions which can be calculated as shown in the literature [START_REF] Walsh | Optical properties of Tm 3+ ions in alkali germanate glass[END_REF]. The largest error in a J-O calculation is

the assumption that all Stark levels of a given multiplet are equally populated. An overview of radiative rate, lifetime, and branching ratio of the main transitions is given in Table 3.

Table 3

In section 3.4 is introduced the experimental value obtained on a low doped sample and considered as radiative lifetime. Comparison of lifetime results given by the J-O theory with experimental values and the results from other authors is presented in Table 4.

Table 4

All reported data indicate the radiative value of 3 H 4 level as ~0.35 ms whilst a significant discrepancy for 3 F 4 level between J-O theory and the experimental values is observed.

Emission spectra

Emission spectra of 3 transitions. Saturation occurs for Tm 3+ concentrations higher than 6 mol% of Tm 3+ . In that case 3 H 4 level is totally depleted by CR to 3 F 4 level and no further increase in the 1847 nm emission is possible.

Fig. 3b presents the ratio between the same two emission peaks normalized to the number of ions. This ratio could give an insight about the concentration dependence of the efficiency of the cross-relaxation process. Two regions can be observed: the first one, when the emission at 1847 nm increases, up to 3 mol%, and the second one for higher concentrations when it decreases. The peak value is for the sample doped with 3 mol% of Tm 3+ . CR process is beneficial up to 3 mol% of Tm 3+ considering the amount of dopant Tm 3+ ions. Above this concentration, quenching of 3 F 4 level begins.

Fig. 3

Emission cross section can be calculated by using the expression [START_REF] Taylor | Spectroscopy of Tm 3+ -doped tellurite glasses for 1470 nm fiber amplifier[END_REF]:

eff p em c n λ τ π β λ σ ∆ = 0 2 4 .

8

, where p is the peak fluorescence wavelength, is branching ratio, n is refractive index, c is light velocity, 0 is radiative lifetime and eff. is the effective linewidth.

The effective linewidth of the transition has been calculated by using the relation

( ) d I I max eff =
. The emission cross section obtained values for level 3 H 4 are in the range 2.6-2.8 10 -25 m 2 whilst for level 3 F 4 emission values of the T0.36 sample is 5.4 10 -25 m 2 . This value is in agreement with those reported in the literature for tellurite glasses [START_REF] Walsh | Optical properties of Tm 3+ ions in alkali germanate glass[END_REF][START_REF] Taylor | Spectroscopy of Tm 3+ -doped tellurite glasses for 1470 nm fiber amplifier[END_REF] and higher than those reported in fluoride and oxyfluoride glasses [10].

Fluorescence waveforms were measured at 1475 nm ( 3 H 4 level) and 1680 nm ( 3 F 4 level) by pumping with 793 nm excitation wavelength at room temperature. Further experimental results and discussion will be separately presented for both emissions.

Concentration quenching of the 3 H 4 emission

The decay curves for the 3 H 4 level at 1475 nm were found to be non-exponential for all dopant concentrations except for the 0.36 mol% Tm 3+ doped sample (T0.36) which shows an exponential decay i.e. linear curve in semilogarithmic scale, as reported in Fig. 4.

Fig. 4

Average lifetime values for the non-exponential decays are calculated by utilizing the expression [START_REF] De Sousa | Evidence of higherorder mechanisms than dipole-dipole interaction in Tm 3+ -Tm 3+ energy transfer in fluoroindogallate glasses[END_REF]:

⋅ = = dt t I t I ) ( ) 0 ( 1 τ
. Calculated lifetime values from this expression are given in Table 5. The lifetime value of the lowest doped sample T0.36 is taken as the radiative lifetime in further assumptions.

Table 5

Photoluminescence quantum efficiency (η ) is expressed as a ratio between number of emitted and incident excitation photons. By the formula, η can be described either as a ratio between radiative and total decay or as the ratio between measured and radiative lifetime η = m / 0 [START_REF] Ajroud | Study of the spectroscopic properties and infrared-to-visible up-conversion fluorescence of Er 3+ -doped germanate glasses[END_REF]. Quantum efficiencies of all glass samples are reported in Table 5.

The reduction of the lifetime and quantum efficiency of the 1475 nm emission as concentration increases, together with the change from exponential to nonexponential decays have been previously observed in Tm-doped glasses and attributed to cross-relaxation processes such as 3 H 4 , 3 H 6 → 3 F 4 , 3 F 4 . In this process part of the energy of an ion in level 3 H 4 is transferred to another ion in the ground state with both ions ending up in level 3 F 4 .

Energy transfer (ET) occurs when more than one optically active center are present inside the critical domain. Its value depends on sensitizer emission and activator absorption spectra overlapping, mutual ion distance, quantum transition efficiency, relative orientation of sensitizer and acceptor transition dipoles [START_REF] Lakowicz | Principles of fluorescence spectroscopy[END_REF]. ET processes can be considered as radiative or non-radiative. In the case of radiative processes characterized by photon transfer, there is no coupling between active ions, and their emission probability will remain the same. However, non-radiative energy migration can be described by Coulomb interaction via electromagnetic field or by quantum mechanic and virtual photon exchange. The latter can be transferred to a classical mechanic frame by a dipole oscillator interaction assumption. Dipole -dipole (dd)

and multipole interaction theory is described in the work by Dexter and Förster [START_REF] Walsh | [END_REF].

Non-radiative energy transfer processes such as cross-relaxation are generally described in terms of three limiting cases: a) one step process involving direct energy transfer, b) diffusion -limited relaxation, and c) fast diffusion [START_REF] Weber | Luminescence decay by energy migration and transfer; Observation of diffusion -limited relaxation[END_REF]. In the case of very fast diffusion, the decay of the donor fluorescence is purely exponential.

Decay curve of sample T0.36 (Fig. 4) shows a clear exponential decay and can be described by direct energy transfer model. However, for the higher dopant concentration it was not the case. Difference between diffusion limited process and fast diffusion can be determined by the concentration dependence of the quenching rate R Q given by:

0 1 1 τ τ - = m Q R (1)
-where m is the measured value and 0 the radiative lifetime.

A quadratic dependence R Q ~N2 corresponds to diffusion -limited processes whilst the linear R Q ~N infers fast diffusion processes [START_REF] Auzel | The interplay of self-trapping and selfquenching for resonant, transitions in solids; role of a cavity[END_REF]. Hence a logarithmic plot of the quenching rate versus concentration should show a slope equal to two if the decay is diffusion -limited and equal to one in the case of fast diffusion.

Fig.5

Figure 5 shows a logarithmic plot of the quenching rate of the 3 H 4 level as a function of the Tm 3+ concentration. As can be observed in this concentration range the slope is 2.2 which indicate the presence of a dipole-dipole mechanism in the framework of a diffusionlimited regime. As we have mentioned before, for Tm 3+ concentrations higher than 6 mol%, the emission from 3 H 4 level is nearly totally quenched due to CR to 3 F 4 level.

Non-exponential decay curves of 3 H 4 emissions which are attributed to diffusionlimited regime when energy transfer between sensitizers and sensitizer to activator via dipole -dipole interaction occurs can be fitted by the Yokota -Tanimoto (Y-T) equation [START_REF] Gibbs | Population dynamics of the 3 F 4 and 3 H 4 levels in highly-doped Tm 3+ :ZB(L)AN glasses[END_REF]:

+ + + ⋅ ⋅ ⋅ ⋅ ⋅ - ⋅ ⋅ = - 4 / 3 2 2 / 1 2 / 3 / ) 743 . 8 1 5 . 15 87 . 10 1 ( ) ( 3 4 exp ) 0 ( ) ( 0 x x x t C N e t SA t π φ φ τ (2)
where 0 is radiative lifetime, N is dopant concentration,

3 / 2 3 / 4 3 / 1 3 / 4 ) 3 4 ( 5 . 0 t N C C x SA SS ⋅ ⋅ ⋅ ⋅ ⋅ = - π
, C SA and C SS are ET microparameters for SA and SS interaction respectively. The following parameterization can be used:

2 / 1 2 / 3 3 4 SA C N A ⋅ ⋅ = π , 3 / 4 3 / 1 3 / 4 ) 3 4 ( 5 . 0 N C C B SA SS ⋅ ⋅ ⋅ ⋅ = - π
. Value of parameter B can be expressed also as B = DC SA -1/3 with D as a diffusion coefficient [START_REF] Balda | Spectroscopic properties of the 1.4µm emission of Tm 3+ ions in TeO 2 -WO 3 -PbO glasses[END_REF]. Example of the fit curve for the sample T1 in a semilogarithmic plot is shown in fig. 6. For Tm 3+ doping levels higher than 6.84 . 10 20 cm -3 , the emission from 3 H 4 level is practically quenched.

Fig. 6

Obtained values of Y-T parameters are shown in Table 6.

Table 6

This table also shows the values for the critical radius R 0 . The critical distance R 0 taken as 6 0 R = 0 C SA indicates that the energy transfer can occur among ions which are located within a given domain. A longer critical distance value means faster energy transfer.

Parameters A and B can be extended for the various concentration values by plotting them versus dopant concentration (Fig 7a,b).

Fig 7

From the slope of the parameter A, C SA = 2.15 10 -50 m 6 /s can be obtained which in turn is used together with the slope of the parameter B to give C SS = 1.16 10 -50 m 6 /s. These values are of the same order as those obtained from fits of the experimental decays from level 3 H 4 to the Yokota -Tanimoto model. In this case C SS < C SA so the probability of energy transfer to acceptor is higher than to sensitizer.

Another possibility to evaluate the effect of the energy transfer on the lifetimes of the 3 H 4 level is to use rate equation formalism. By using this formalism and considering a low excitation density, Camargo et al. [START_REF] De Camargo | Spectroscopic properties and energy transfer parameters of Tm 3+ ions in gallium lanthanum sulfide glass[END_REF] obtained the following expression for the lifetimes of the 3 H 4 level as a function of concentration:

0 0 . 1 τ τ τ ⋅ ⋅ + = N W ET calc ( 3
)
where 0 is the radiative lifetime of 3 H 4 level, W ET the total energy transfer for dd interaction and N sensitizer ion concentration. In this study ET microparameters of dd interaction are obtained trough Yokota -Tanimoto fitting and not by Kushida model. Therefore is taken: [START_REF] De Sousa | Microscopic and macroscopic parameters of energy transfer between Tm 3+ ions in fluoroindogallate glasses[END_REF]. W ET should include contribution from dq and qq interaction which is in our case excluded. Values of calculated and measured lifetimes are shown on Fig.

N C C W dd SS dd SA dd ET 4 / 3 4 / 1 ) ( ) ( 28 =

8.

Fig. 8 ET microparameters of Tm 3+ in ZB(L)AN glasses for the3 H 4 level reported in literature [START_REF] Gibbs | Population dynamics of the 3 F 4 and 3 H 4 levels in highly-doped Tm 3+ :ZB(L)AN glasses[END_REF] are: C SS = 2.8 10 -51 m 6 /s, C SA = 3 10 -51 m 6 /s. Values reported by Balda [START_REF] Balda | Spectroscopic properties of the 1.4µm emission of Tm 3+ ions in TeO 2 -WO 3 -PbO glasses[END_REF] where the host material was TeO 2 -WO 3 -PbO glass and Burshtein model applied are ~10 -51 m 6 /s. Values of 5.15 10 -51 m 6 /s obtained by Yokota-Tanimoto fit on the 3 H 4 decay curve of Tm 3+ in TeO 2 -CdCl 2 glass is reported by [START_REF] Sennaroglu | Effect of cross relaxation on the 1470 and 1800 nm emissions in Tm 3+ :TeO 2 -CdCl 2 glass[END_REF]. C SA parameter from this study obtained by Yakota-Tanimoto fitting for the sample T1 is of the same order of magnitude as in the literature whilst by parameter extension by fitting gives higher values. Both parameters, C SA and C SS showed good range of order by inserting them to ET constant and comparing obtained lifetime values with the experimental results.

Concentration quenching of the 3 F 4 emission

Concerning the 3 F 4 emission, the relative luminescence intensity increases while its lifetime decreases with concentration. Figure 9 shows the logarithmic plot of the experimental decays of the 3 F 4 level for all samples. As can be seen the decays can be described by an exponential function to a good approximation for all concentrations and the lifetime decreases as concentration increases. Lifetime values and the corresponding quantum efficiency for the Fig. 9 In this case, where the 3 F 4 level is the first excited state, the quenching of luminescence when active ion concentration increases can not be due to cross-relaxation between various excited states and it has been mainly considered as due to diffusion towards unidentified impurities (such as OH or other impurities present in the starting materials). The problem can be separated into two cases: diffusion limited regime and fast diffusion. The first case is considered when the order of magnitude for transfer probability between sensitizers and sensitizer to activator are the same. As we have seen, in the case of the diffusion limited situation the quenching rate is proportional to the square of concentration. As it is shown in Fig. 5 a logarithmic plot of the quenching rate of level 3 F 4 as a function of Tm 3+ concentration gives a slope close to 2, which indicates that we are dealing with a diffusion limited regime.

In such a case, and assuming a dipole-dipole interaction, the self quenching behavior can be described by the formula proposed by Auzel [START_REF] Auzel | The interplay of self-trapping and selfquenching for resonant, transitions in solids; role of a cavity[END_REF]:

2 0 0 ) ( 2 9 1 N N π τ τ ⋅ + = (4)
where is measured lifetime at given concentration N, 0 is lifetime for low concentrations i.e.

radiative lifetime, N 0 as the critical sensitizer concentrations is linked with critical distance between sensitizer and trap as Experimental values are in very good agreement with the fit curve for the whole range of concentrations. ET parameter is lower than in the case of 3 H 4 level by three orders of magnitude and the critical radius, shorter.

Conclusions

The present study concerns the investigation of various highly doped Tm 3+ tellurite glasses in order to provide optimum dopant concentration for short cavity laser. Lifetime values of low doped sample have been measured and the obtained values of 3.09 and 0.347 ms for 3 F 4 and 3 H 4 level, respectively. The optimum emission intensity at 1.8 µm considering single ion was obtained for the concentration of 6.84 10 20 cm -3 Tm 3+ with 24% quantum efficiency. For such a high dopant concentration CR process strongly quenches emission from 3 H 4 level. An analysis of the fluorescence decays of the 3 H 4 → 3 F 4 emission as a function of concentration reveals that the electronic mechanism responsible for the ion-ion interaction is a dipole-dipole quenching process in the framework of a diffusion-limited regime. The self-quenching of the 1.8 µm emission can be attributed to limited diffusion within the active centers. This means that the probability for the diffusive steps between active centers is of the same order of magnitude than the one for quenching between impurities and centers. A critical sensitizer concentration for self quenching of 4.2 10 20 cm -

Figure captions

  correspond to 1475 and 1847 nm emissions, respectively. Fig2shows the spectra of all prepared samples: they are normalized on the area under the 3 H 4 level emission band. Some of the emissions (sample T7 and T4) from 3 F 4 level are shifted towards longer wavelengths which is due to measurements on thicker samples and therefore affected by reabsorption effects.

Fig. 2

 2 Fig.2

Fig. 3a presents 3 F

 3 Fig.3apresents the ratio between the areas under 3 H 4 and 3 F 4 emission bands. Increase of 3 F 4 emission with dopant increase is likely due to CR between3 H 4

.

  By fitting of experimentally obtained lifetime values on the above formula (Fig.10) the following characteristic parameters can be obtained: 0 = (3.29±0.07) ms, N 0 = (4.2±0.1) 10 20 cm -3 , R 2 = 0.997, R 0 ( 3 F 4 ) = 8.3 Å C SA = 9.8 10 -53 m 6 /s.

Fig. 10

 10 Fig.10

Fig. 1 .Fig. 2 .

 12 Fig.1. Absorption cross section of Tm 3+ doped TZN glass

Fig. 3a )

 3a Fig.3a) Emission ratio presents the ratio between area under 3 F 4 3 H 6 and 3 H 4 3 F 4

Fig. 3b ) 3 F 4

 3b34 Fig.3b) Ratio between the areas under the 3 F 4 3 H 6 and 3 H 4

Fig. 4 . 36 Fig. 5 . 3 H 6

 436536 Fig. 4. Lifetime decay curves of 3 H 4 level. Exponential decay is only in the case of sample

Fig. 6 .

 6 Fig. 6. Yokota-Tanimoto fit for 3 H 4 level of the sample T1.

Fig. 7a ) 3 Fig. 8 .

 7a38 Fig. 7a) Yokota -Tanimoto fit parameter A as a function of concentration: A = (1.089±0.003) 10 -24 N

Fig. 9 .

 9 Fig. 9. Decay curves of 3 F 4 emissions. Sample T1 shows non-linear behavior.

Fig. 10 .FiguresFig. 1

 101 Fig. 10. Experimental data of 3 F 4 level fitted on theoretical curve. Values of 8.3 Å for critical distance and corresponding N 0 = (4.2±0.2) 10 20 cm -3 and ET microparameter 9.8 10 -53 m 6 /s

  Fig. 1.

  Fig. 2.

  Fig.3a) Fig.3b)

  Fig.7a) Fig.7b)

Fig. 10

 10 Fig. 9.

  2 -20ZnO-5Na 2 O tellurite glass. In this work, we report together with Judd-Ofelt analysis to calculate transition rates, radiative lifetimes, and branching ratios the study of the effect of concentration on the 3 H 4 → 3 F 4 and 3 F 4 → 3 H 6 emissions of Tm 3+ . Wide concentration range strongly changes the nature of the ion-ion interaction which is characterized by quenching parameters. The analysis of the fluorescence decays from the 3 H 4 level indicates the presence of a dipole-dipole quenching process assisted by energy migration. Energy transfer microparameters and critical ion distance are determined for both emission levels 3 H 4 and 3 F 4 ,

Table 1 .

 1 Concentration and refractive indexes for Tm-doped 75T-20Z-5N glasses

	Tables
	Sample name	Tm 3+ x10 20 (cm -3 )	n(633nm) n(825nm) n(1061nm) n(1312nm) n(1533nm)
	T0	0	2.0514	2.0243	2.0091	2.0012	1.9962
	T0.36	0.82	2.0488	2.0224	2.0077	1.9998	1.9950
	T1	2.28	2.0454	2.0189	2.0046	1.9965	1.9918
	T2.1	4.87	2.0412	2.0152	2.0007	1.9928	1.9880
	T3	6.84	2.0396	2.0136	1.9994	1.9917	1.9867
	T4	9.06	2.0356	2.0097	1.9954	1.9983	1.9833
	T5	11.3	2.0306	2.0053	1.9913	1.9839	1.9792
	T6	13.5	2.0268	2.0021	1.9879	1.9805	1.9761
	T7	15.7	2.0239	1.9993	1.9853	1.9780	1.9733
	T10	22.1	2.0116	1.9878	1.9746	1.9672	1.9629
	* T0 signs host composition

Table 2 .

 2 J-O parameters comparison with literature.

	Reference	2	4	6
	[8]*	3.38	2.17	1.17
	[10]*	4.64	1.61	1.26
	[11]**	5.106	1.174	1.082
	This study *	4.4	1.97	1.22
	* host composition: 75TeO 2 -20ZnO-5Na 2 O
	** host composition: 80TeO 2 -10ZnO-10Na 2 O
	*** [10 -20 cm 2 ]

Table 3 .

 3 Transition rates, radiative lifetimes, and branching ratios

	Transition λ (nm)	A J'J (s -1 )	0 (ms)
	1 G 4	3 H 6	470	2803.82	0.19 0.5445

F

emission are reported in Table

was obtained. This dopant concentration could be extended to the 6.84 10 20 Tm 3+ with a satisfactory QE in order to obtain short cavity fiber laser devices.

Acknowledgments

The authors wish to thank the Regione Piemonte Converging Technologies "Hipernano" research project. R. Balda and J. Fernández acknowledge financial support from the Spanish Ministerio de Ciencia e Innovacion under project MAT2009-14282-C02-02 and from the Basque Country Government (IT-331-07).


E Robert
email: eric.robert@univ-orleans.fr

S Point

S Dozias

R Viladrosa

J M Pouvesle

Study of pulsed neon xenon VUV radiating low pressure plasmas for mercury free fluorescent sign optimization

Keywords:

This work deals with the study and optimization of mercury free fluorescent discharge tubes for publicity lightning applications. The experimental set up allows for the time resolved spectroscopy from 110 nm up to 900 nm, the photometric characterization in a large volume integrating sphere, and the current and voltage measurement of microsecond duration signals delivered by lab-developed pulsed drivers. The glow and afterglow radiative process analysis indicates that the best performance measured with the pulsed excitation of rare gas plasma, in comparison with the conventional AC excitation, essentially originates from the efficient plasma relaxation during the afterglow at the benefit of the VUV resonance line radiated at 146.9 nm for xenon. The fit of the VUV time resolved experimental measurements with the results issued from a simplified kinetic model of neon xenon plasmas, evidence the crucial role of molecular ions production during the glow phase and of their radiative recombination during the afterglow. The pulse duration and the gas mixture pressure appear as two experimental parameters which influence, studied over an extended range, has been demonstrated to allow a significant sign performance enhancement. There exists an optimum pulse duration range, which results in the appearance of limited stepwise excitation and ionization processes, favourable for an intense afterglow VUV production. The pressure dependence study shows that the best performance for pulsed excitation are obtained in Ne/Xe (100/1) mixtures around 50 mbar, at the difference of AC driven Ne/Xe plasma for which the best conditions were reported to be of a few mbar. This pressure increase results both in the VUV and sign light output enhancement and the successful continuous operation of pulsed mercury free signs for time as long as 4000 hours with neither electrode erosion, nor glass or phosphor degradation nor chromatic coordinate variation. For the green phosphor covered, 65 cm long and 13 mm inside diameter signs, the efficiency of pulsed neon xenon discharge likely to be operated for a few thousand hours reaches 50 % of that of the same tube filled with mercury based mixtures.

Introduction

As enlighten through the last issues of the international lighting conference [START_REF] Zissis | Proc. of the 10 th Int. Symp. on the Science and Technology of Light Sources, IOP Conf. Series Number[END_REF][START_REF] Liu | Proc. of the 11 th Int. Symp. on the Science and Technology of Light Sources[END_REF][START_REF][END_REF], there exists a growing interest and research effort devoted to the development of new environmentally friendly substitute to the mercury based fluorescent technology. Dealing with gas discharge technology, various Confidential: not for distribution. Submitted to IOP Publishing for peer review 20 January 2010 excitation techniques of gas mixture plasmas are studied or already used in different fields of applications. DC and AC discharges using hot filaments or cold cathodes, pulsed excitation of such discharges, but also RF excitation of electrode-less lamps have for example been studied for a long time for lightning applications. The main issue is to achieve an efficient excitation of the phosphor luminescence by an intense UV-VUV emitter. Rare gas mixtures, including xenon, close to the atmospheric pressure have already been applied for display applications. Major progress have been reported in the optimization [4][5][6][7], and modelling [7][8][9], of low pressure rare gas mixtures based on Xe resonance line. A large effort is still devoted to the study of the three following and sometime antagonist objectives: light output flux, lamp efficiency and the lamp lifetime. At GREMI, such a research was partly supported by the AUPEM SEFLI company for years, to find out Hg free alternatives to the luminous signs used for publicity lightning. Signs represent a specific case for the search of the efficient and long living operation of a plasma produced by a high voltage power supply connected across cold hollowed electrodes. While important, the luminous efficiency of signs is not such a crucial parameter in comparison with that of general lighting lamps, but very stringent conditions need to be fulfilled for publicity lighting devices such as a few tens of thousand hour lifetime and a very limited chromatic coordinate variation during the whole sign operation. Preliminary works [10,11] have shown evidence for the potentialities of pulsed excited low pressure rare gas mixtures which were shown to allow for a three time enhancement of the luminous output and energy efficiency in comparison with conventional AC excitation.

In this work, the time resolved spectroscopic study of neon-xenon plasma excited by microsecond and sub microsecond duration pulses delivered at repetition rate ranging from a few hundreds of Hz up to a few tens of kHz, the photometric and ageing characterization of sealed signs is proposed. The section 2 describes the experimental set up used for fluorescent tube conditioning, the pulsed driver technology, the electrical measurements, the VUV to near infrared spectroscopic study and the photometric measurements. The detailed analysis of the plasma emission during the glow phase and the afterglow documented in the section 3, allow for an approach of the main kinetics pathways through a simplified model of the molecular ion formation and of their successive recombination leading to a very significant population of the xenon resonance level also described in this section. The experimental perspectives inferred from the modelling have been investigated both for the pulse duration and gas mixture pressure as depicted in section 4. The section 5 deals with the lifetime and efficiency measurements and the last section draws some conclusion and suggests a few perspectives to this work.

Pulsed fluorescent sign experimental setup

The experimental set-up used for the fluorescence tube conditioning, ignition and diagnostics is presented in figure 1. The borosilicate discharge reactor is manufactured by the AUPEM SEFLI partner and consists in a T-shaped fluorescent tube, equipped with two conventional hollow electrodes. The electrodes are iron/nickel cylinders 40 mm in length and 10 mm in diameter, the inside of these electrodes is covered with an electron emitter barium fluoride layer. The 3 mm thick ceramic ring at their extremity is used to prevent from strong sputtering at the plasma facing electrode edge. Tubes with three internal diameters (10, 13 and 18 mm) and with length, defined as the distance between the two electrode ceramic rings, ranging from 10 cm up to 1 m, have been used in this work. When no specific indication is specified, the results presented through this manuscript refer to 13 mm inside diameter and 50 cm inter electrodes long tubes. The inner surface of the tube is covered with a zinc silicate manganese activated phosphor prepared and deposited by the AUPEM SEFLI company following the same procedure as that used for mercury based conventional signs. This green phosphor presents a peak emission located around 525 nm. The T branch of the fluorescent tube is first connected to the gas handling system, including gas flow controllers, pressure gauges, a needle valve and a vacuum pump to monitor the gas mixture composition. Gas flow rates, ranging from 0,1 to 50 cm 3 s -1 , are used during experiments to allow for a parametric study of the gas mixture composition influence on the tube performance without requiring the realization of a considerable number of sealed signs. Previous measurements [6,10] have shown that this fluorescent tube experimental procedure is valid for the prediction of the behaviour of sealed signs. Before the filling with high purity gases, N 45 or higher grade supplied by Air Liquide company, the tube is heated up to about 350 °C, via the neon tube bombarder, this temperature being measured on the outer surface of the glass at the centre of the tube. This procedure allows the removal of the impurities trapped in the glass, the phosphor and the metallic electrodes of the sign through the vacuum assembly of the bombarder. Finally, the T branch is optically connected to a VUV-UV-visible (110-900 nm) 0.4 focal length Acton Research spectrometer equipped with two gratings (300 and 1200 grooves/mm respectively).

Tube bombarder

Photomultiplier tubes (Hamamatsu R1220 and R955) are connected to the spectrometer for the measurement of the time resolved emission spectra of the discharge. A PImax ICCD camera, not shown in figure 1, was also used for imaging experiments discussed in section 4. The VUV measurement of the xenon 146.9 nm resonance line radiation has been performed through a vacuum evacuated glass capillary with an inside diameter of 3 mm, equipped with a MgF 2 window. This VUV window is set at the same position than the phosphor layer plane, through the T branch. In this way, the radiation trapping in the xenon gas is included in the measurement and corresponds to the phosphor excitation condition. The inset b) in figure 1 gives some schematic description of the pulsed power supply developed in this work. The pulsed power supply articulates around three main subassemblies. A DC power supply is used as a voltage source which amplitude ranges from about 100 V up to 1250 V and is typically set between 300 and 400 V. An IGBT based Hbridge together with its driving logic is used to produce voltage pulses with adjustable duration, in the microsecond range, and adjustable repetition rate, in the kilohertz range. The full H-bridge set up produces voltage cycles having one positive and one negative polarity pulse, which is required for fluorescent tube operation to avoid cataphoresis effect and to allow a similar degradation of both tube electrodes. These voltage pulses are finally applied to the primary coil of a high voltage transformer having a magnification factor of about 5, the secondary coil is connected across the fluorescent tube which requires voltage amplitude around one kilovolt to ignite. As discussed in section 4, some experiments have been performed with charging the H-bridge up to 1250 V and without using the voltage transformer to suppress power dissipation through this element and to obtain faster voltage rising edges and consequently shorter duration pulses. Voltage and respectively current measurements were performed using Elditest Electronic GE 3421 (100 MHz bandwidth) and Tektronix TCP202 (50 MHz bandwidth) probes respectively connected to a Tektronix 510 oscilloscope. Finally, photometric characterization consisted in the measurement of illuminance through a Minolta chroma meter, model xy-DF, and the measurement of the output visible flux through a 1 m diameter SLM-40 SphereOptics integrating sphere.

Pulsed neon xenon plasma kinetics

This section describes the essential features of the glow phase of neon xenon plasma produced with the use of the pulsed driver described in the previous section. The figure 2 presents the waveforms of the voltage and current signals together with those of the xenon resonant line radiated at 146.9 nm, the xenon infrared line at 823.1 nm, one of the most intense visible neon lines emitted at 640.2 nm, and the strongest ionic xenon line measured at 529.2 nm. This recording was obtained for a 45 mbar mixture containing 1% of xenon and excited by five microsecond duration pulses delivered at a repetition rate of two kilohertz. The glow is described by splitting this phase in two different stages. In the experimental conditions of figure 2, the transition between the two glow phase stages is measured two microsecond after the onset of the current pulse occurring at the time origin (t=0). At the instant of 2 µs, the current waveform turns from a low amplitude and slowly increasing signal to a quite linear ramp. The peak amplitude at the end of the current ramp, i.e. at the end of the current pulse, is of a few A. This transition corresponds also with the simultaneous decay of the resonance line emission, the strong enhancement of the neon line intensity and the rise of ionic xenon line radiation. The transition instant is essentially dependent on the pulse repetition rate, it occurs earlier for higher repetition rate and later for lower repetition rate.

The consideration of electron-impact excitation cross sections for neon and xenon both from the ground state and their metastable levels [12] together with the evolution of the amplitude of the current allow for a schematic description of the main kinetic reactions involved in the plasma species temporal evolution and the evolution in the electronic temperature during the pulse excitation. At the pulse onset, the plasma from the previous pulse is largely recombined, the tube impedance is high, in the kOhm range, the electronic density is low and the electronic temperature is high which is favourable for the direct excitation of the xenon and neon excited levels from their respective ground states. This allows for the population of the two rare gas metastable levels, the efficient excitation of the xenon 146.9 nm resonance line, and neon 73.6 nm transition, not measured in this work, but at the origin of the phosphor excitation in pure neon plasma [10]. These excitation processes require the participation of electron having energies above the threshold of 8.3 eV and respectively 16.62 eV for xenon and respectively neon. Then as the current density increases, the positive column plasma is established, the typical electronic temperature is around 1 eV, and the main electron impact reactions involve electrons having energies of a few eV. These plasma parameters are very favourable for stepwise excitation and ionization in the case of xenon and stepwise excitation for neon [12]. The spectroscopic analysis reveals indeed the decrease of all the observed xenon transitions and the large increase of all the neon transitions during the second stage of the glow phase. Stepwise excitation processes from xenon lower excited levels towards higher energy levels is depicted in figure 2 through the much more pronounced decrease of the resonant transition in comparison with that of the infrared line during the second phase of the glow signal. The production of xenon ions from xenon excited levels, the population of neon excited levels and the consecutive increasing importance of penning reaction lead to a strong rise of the electron density. This also leads, with a small time delay of less than 0.5 microsecond, to the production of high energy lying singly charged excited levels of xenon as depicted in figure 2 through the 529.2 nm line waveform which upper level have an energy of about 26 eV. The excitation of neutral neon and singly charged xenon transitions lasts up to the end of the current pulse.

The schematic description of the plasma kinetics during the glow phase indicates than the main portion of the power input lead to the production of visible radiation which are of no use for the phosphor excitation. When the pulse repetition rate is increased while the pulse duration is shortened to keep a constant electrical input power, the plasma recombination and relaxation is interrupted resulting in higher metastable state and electronic densities at the time of application of the next voltage pulse. This induces a faster transition between the two glow phases and this effect is even much more important if the pulse duration is not reduced, i.e. if the electrical power is increased. The neon xenon pulsed plasma kinetics analysis also confirms the unlikely stepwise excitation processes occurring with larger and larger amplitude when using AC excitation. With AC excitation, the increase of the power input across the fluorescent tube above about 5 W unfortunately led to the saturation at a constant level of the phosphor fluorescence which does not allow for the production of illuminance levels of interest for signs and lightning applications [10]. In the case of pulsed excitation, the next section evidences how the plasma recombination during the afterglow phase can be optimized to achieve a significant recovery of the power spent for the production of non UV radiating species. The figure 3 presents the temporal waveforms, of the resonant xenon transition at 146.9 nm, the 823.1 nm transition and of the most intense xenon visible radiation at 467.1 nm. The three signals are normalized to their respective maximum during the glow phase which lasts during the first ten microseconds. The fluorescence during the afterglow is measured up to the application of the next voltage pulse and is presented in figure 3 over 150 microseconds. The afterglow signal for visible neon transitions was measured to be negligible in our experimental conditions. Two essential features appear from figure 3. First the integration of the signal over the whole afterglow duration leads to the conclusion that the larger part of VUV radiation is emitted during the afterglow in comparison with the emission measured during the glow phase. Second, the radiative cascades and quenching reactions induce that the larger afterglow contribution is obtained for the lower lying energy level, as revealed by the comparison of the 146.9 nm signal with other xenon transitions, which is a very favourable situation for the production of VUV radiation. The development of a simplified kinetic model likely to allow the interpretation of the fluorescence signals measured during the afterglow phase for a radialy uniform positive column section was performed including the reactions summarized in table 1 together with the initial conditions for the different species densities. This initial densities are representative of a typical low pressure glow discharge and the different values were fitted to obtain the best matching between the calculated Xe * density and the 146.9 nm waveform. Besides xenon and neon ground state atoms, six species have been included in the modelling: electrons, xenon ions, neon and xenon excited atoms which are both considered in a rough approach as two unique states, the xenon dimer ion and the heteronuclear neon xenon ions. As described in section 4.2, the termolecular reactions leading to the production of xenon dimer, Xe 2 * , were not considered in the modelling as no evidence for the population of excited dimer was spectroscopically observed. On the contrary, the corresponding termolecular reactions involving the xenon ion and leading to the production of Xe 2 + or NeXe + molecular ions present reaction rate one order of magnitude higher than those for Xe * atoms and the xenon ion density is also much than one order of magnitude higher than the excited xenon state density. Our main goal was to describe the main kinetic paths leading to the strong afterglow signal depicted in figure 3. The initial value of the Xe * species, set to 1.6 x 10 10 cm -3 in the modelling, appears in relatively good agreement with that of [7] in which respective axis densities of 8 x 10 11 , 2 x 10 11 , 2 x 10 10 , and 5 x 10 9 were determined for Xe * (1s 5 , 1s 4 , 1s 3 and 1s 2 ) levels. In the present work, no radial analysis of the plasma column was considered and only one neutral excited xenon state was introduced. It was shown in [7], that at the wall species densities could be at least ten times smaller that the corresponding on axis values. Once again, the detailed modelling on the plasma was not the goal of the present paper, which would require a deeper analysis of the radial density profiles, the electron temperature calculation, the insertion of various xenon and neon states and corresponding reaction path and rates. The initial electron density, set to 6 x 10 11 cm -3 in the modelling, is larger than that of 4 x 10 11 cm -3 obtained in [7], while being of the same order of magnitude. This higher electron density value has to be moderated by the consideration of the pulsed, low duty cycle excitation mode of the plasma in our study. Typical duty cycle of 1%, corresponding to microsecond pulses delivered at kHz repetition rates, result in a mean electron density of a few 10 10 cm -3 while in [7] the excitation consisted in a AC 60 mA waveform. The neon excited level density of 10 8 cm -3 is consistent with the consideration that even with small xenon admixtures, the density of excited neon atoms remain far below those of xenon excited levels. Finally, the initial electron temperature for the afterglow phase description, set to 0.7 eV in the modelling, is consistent with the observed xenon and neon line intensities behaviour previously described in figure 3. The temporal evolution of the electron temperature, that is the fast decreasing during the first fifty microseconds during the afterglow, was shown to be crucial in the fit of the experimental records. It could be pointed out that such rapid thermalization of electron population was previously documented in [17] where the large xenon density growth at the onset of the afterglow was connected with a two times reduction of the electron temperature within a 20 µs duration, this evolution being very close to that inferred in our study to achieve the best fit between calculation and experimental data.

The figure 4 presents the comparison of the 147 nm fluorescence signal, measured for a Ne/Xe (100/1) mixture at a pressure of 45 mbar excited by two microsecond duration pulses, with the results of the modelling including the full set of reactions and initial conditions presented in table 1 (black trace) and without the consideration of R9 and R10 (thin grey trace). The calculations indicate that the main process responsible for the plasma recombination is the dissociative recombination of molecular ions (Xe 2 + and NeXe + ) while in our experimental conditions the collisionnal radiative reaction R11 plays no significant role in the plasma relaxation. This latter process would lead to an afterglow signal which would peak with a delay larger than 400 microseconds which is not measured in the experiments. It appears that the recombination of our pulsed neon xenon plasma, produced with xenon partial pressure of about 0.5 mbar, occurs at electron temperature lower than 0.7 eV, electron density of a few 10 11 cm -3 , and essentially involves dissociative recombination reactions. This regime appears in between the previously reported and modelled xenon low pressure (0.07 mbar), high density (a few 10 14 cm -3 ), low electron temperature (below 1 eV) and the constricted high xenon pressure (13 mbar), low electron density (10 11 cm -3 ) and high electron temperature (3.5 eV) modes of operation of pulsed low voltage discharges [17]. The model also confirms the fast decrease of the electronic temperature after the current pulse end, as the best fit were obtained if the temperature falls from 0,7 eV at the end of the pulse down to a few tenths of eV in the first tens of microseconds during the afterglow. Finally, the rough modelling of the afterglow phase has been used to determine the most promising pathways for the optimization of VUV production in neon xenon pulsed driven discharges. The table 1 indeed indicates that R9, R10 and the formation reactions R4, R5 have a strong influence on the production of VUV radiation so that it appears worthwhile to study higher pressure and higher density plasma. NeXe + 0,7 eV 6 x 10 11 cm -3 6 x 10 11 cm -3 1 x 10 8 cm -3 1.6 x 10 10 cm -3 10 10 cm -3 10 10 cm -3

Parametric optimization of mercury free signs

Pulse duration analysis

As discussed above from the analysis of figure 2, the second phase of the glow is dominated by the production of xenon ion, neon excited states and the successive increase of current amplitude and production of xenon ion excited states. The optimization of the post discharge fluorescence resulting from an increase of the xenon ion density during the glow phase could thus be experienced by the matching of the pulse duration. The figure 5 presents the temporal evolution of the current waveform, the resonant transition at 146.9 nm and of the xenon ion transition at 529.2 nm obtained in a 45 mbar Ne/Xe (100/1) discharge excited respectively by two and five microsecond duration pulses. A linear growth of the plasma charge, obtained by the time integration of the current pulse over the glow period, was measured as a function of the pulse duration on the whole 1 to 20 µs domain. The figure 5 indicates that while no strong increase of the resonant radiation is observed for longer duration pulses, the enhancement of the population of the xenon ions is very significant, in agreement with the two phase analysis of the glow signal in figure 2. A consecutive increase of the VUV afterglow was obtained resulting from the recombination of a larger amount of molecular ions. The figure 6 presents the experimental 146.9 nm afterglow signal together with the results from the model for 2, 5 and 10 µs pulses. The modelling was performed using the reaction and rate sets of table 1 and the initial densities reported in table 2. The initial electron temperature was kept to a constant value of 0.7 eV with the same temporal evolution as depicted in figure 4. The initial excited neon atom density was also kept constant, while it was experimentally observed that the neon transition intensity is enhanced through pulse duration increasing but this value has no critical impact on the simulated temporal profiles. It appears from figure 6 that the rough modelling of the afterglow kinetics is consistent with the experimental data if modulation of the initial density values is performed in agreement with the behaviour measured for resonant and xenon ion transitions such as illustrated in figure 5. While confirming the simplified description of the plasma kinetics in neon xenon mixtures, the increase of the pulse duration must be counterbalanced by an appropriate lowering of the pulse repetition rate to obtain a constant electrical input power which is of crucial importance for the efficiency issue of any light source. The figure 7 presents the evolution of the illuminance as a function of the pulse duration in a 45 mbar neon-xenon (100-1) mixture for two lab developed pulsed drivers. The data measured with the pulsed driver 1 including a voltage transformer to deliver high voltage pulse with an amplitude in the kilovolt range as described in the experimental set up section, have been obtained by keeping a constant electrical input across the sign regardless of the wall plug power which increases together with the pulse duration due to the power dissipation in the leak inductance of the transformer. A new pulsed driver 2 was designed with a H bridge inverter operating at voltage as high as 1500 V which allow both to suppress the power dissipation in the output transformer and to apply pulse with duration as low as 0,35 microseconds. In this latter case, the full line data in figure 7 corresponds to the evolution of the illuminance with the pulse duration for a constant electrical input of 30 W across the sign and also a constant wall plug power of about 35 W. It appears from figure 7 that there exists an optimum for the phosphor excitation efficiency for pulse duration ranging from two to five microseconds. For larger pulse duration, the high peak current amplitude leads to a small contraction of the positive column plasma along its longitudinal axis which results in a smaller VUV radiating plasma surface and thus a decrease of the tube illuminance. On the other hand, the reduction of the pulse duration below two microseconds correspond to the production of lower xenon ion and neon excited states densities which results in a very reduced afterglow fluorescence contribution and a smaller sign efficiency even at high pulse repetition rate. This last result indicates that in our experimental conditions, the best performances are achieved with an efficient recovery of the energy spent during the glow phase involving stepwise ionisation and ionization processes.

For shortest pulse duration operation, the limitation of these a priori undesirable processes is nevertheless less favourable for the sign efficiency.

Role of the pressure

The other parameter very likely to have a significant effect on the afterglow optimization is the mixture gas pressure. Both the xenon and neon pressure increase could play a positive role in the production of higher molecular ions (Xe 2 + or NeXe + ) densities. The increase of the xenon partial pressure was shown to be possible over a very limited range below a few mbars. As the xenon pressure is increased the discharge first contracts along the longitudinal axis and rapidly turns from a diffuse glow mode to a filamentary or arc mode which is non stationary, inducing premature damage to the electrodes and not allowing an efficient excitation of the phosphor layer. These limitations are probably strongly connected with the about three orders of magnitude lower secondary emission coefficient of xenon ions in comparison with that of neon ions [START_REF] Capdeville | Proc. XXIV ICPIG[END_REF]. As argued in section 3, in our experimental conditions, the pulsed excitation of neon xenon with small admixtures of xenon of a few per cent, does not lead to the efficient population of the excited dimer levels radiating the first and second continua on the red wing of the resonant transition. The figure 8 presents the VUV spectrum of a 100 mbar Ne/Xe (100/1) discharge powered by the pulse driver for an electrical input of 30 W, which shows no indication for the xenon dimer radiation. The main emission occurs at 146.9 nm, and the 129.6 nm and 119.2 nm resonant radiations are also detectable but with much more lower intensity. On the contrary, previous higher resolution VUV measurements [6] have shown that the (NeXe) * exciplex transitions are observed on the blue wing of the xenon 146.9 resonant line. In this later case, termolecular reactions inducing one xenon atom with two neon atoms probably account for the population of the upper state of the (NeXe) * exciplex which in our experimental conditions, i.e. high neon density, is a much more probable reaction that those required for the xenon dimer formation for comparable reaction rate values. The influence of such termolecular reaction for (NeXe) * population was nevertheless not included in the modelling calculation.

Figure 8. VUV spectrum of a 100 mbar Ne/Xe (100/1) discharge using pulsed excitation and an electrical input power of 30 W. A vertical offset was applied for clarity of the figure. The spectrum is not corrected for the spectrometer sensitivity over the VUV spectral range.

The illuminance evolution with the pressure for mixtures containing 1% of xenon is presented in figure 9. The data in figure 9 were obtained by using 2 microsecond duration pulses and a constant input power across the sign of 20 W, this latter condition was achieved by a slight increase of the pulse repetition rate over the pressure range. The best sign efficiency is obtained for mixture pressures between 60 to 80 mbar. The progressive reduction of the increase of illuminance level for each 10 mbar increment up to its saturation around 70 mbar and the slight decrease of the illuminance for the higher pressures are observed. This phosphor behaviour is quite different of that of the VUV intensity evolution measured through the vacuum evacuated glass capillary in the phosphor plane also presented in figure 9. The deviation between the two measurements originates from the progressive contraction of the positive column as the mixture pressure is increased. The glass capillary visualized a constant surface of the plasma column which is optically thick for the resonance line radiation, the surface of the magnesium fluoride window being always smaller in our experimental conditions than the plasma column surface even for the largest pressures. The plasma column diameter was measured through a visible ICCD imaging experiment and it evolves from 13 mm at 10 mbar down to 9 mm at 100 mbar for a 20 W tube input power. With the assumption that the VUV radiation radial distribution is not considerably different to that of the visible part of the spectrum, it appears that the VUV capillary measurement is not influenced by the contraction of the plasma column while the phosphor is excited by a smaller and smaller VUV emitting surface as the pressure is raised.

In comparison with the case of pure xenon plasma, the contraction is observed while the plasma column is homogeneous and not filamentary up to 100 mbar if the power applied across the sign is higher than about 10 W. It was also measured that the contraction appears for higher pressures as the tube diameter is reduced in agreement with some previous analysis [START_REF] Baranov | [END_REF]21] and that the contraction could be counterbalanced by an increase of the power injected across the sign. It was finally measured [6] that besides the 146.9 nm resonant radiation, part of the phosphor excitation originates from the (NeXe) * excimer, radiated in the blue wing of the xenon resonant line, as the mixture pressure is increased. This excimer contribution was estimated to be of about 10% at 50 mbar and up to 15% at 90 mbar of the whole VUV radiated intensity, and is not trapped during its propagation from the plasma to the phosphor layer, at the difference of the resonance line. These observations and the consideration that the VUV intensity is increasing in a small volume around the tube axis, as depicted by VUV data in figure 9, may be of interest for the development of efficient higher pressure and small diameter mercury free signs.

Lifetime and efficiency studies

The results of this section were obtained in sealed signs and thus include the potential cataphoresis, and any other long term plasma phosphor, glass and electrode interaction effects. With AC excitation, the continuous operation of neon xenon signs results in a selective trapping of xenon in the glass tube and tube electrodes which induces strong limitation on the lifetime of the signs [10,22]. For AC excitation, there exists a strong trade off between the lifetime and the visible output of the signs. On one hand, AC driven neon xenon signs offering the greatest illuminance level, which was measured to be lower than 1000 Lux for the green phosphor used in this work, unfortunately presents lifetime of a few tens of hours. On the other hand, successful continuous operation during time periods of 10 to 12 thousands of hours were measured but with illuminance output of a few hundreds of Lux. In both cases, AC driven mercury free systems exhibit performances which do not allow their development for publicity lightning applications. In the case of pulsed excitation, the optimization of neon xenon plasmas was obtained for mixture pressures of a few tens of The measurements were stopped after 2800 hours. No sputtering of the electrodes was observed as revealed both by the electrode outer surface aspect which show no evidence of demetallization together with the absence of any deposition layer on the inside surface of the sign in the vicinity of the electrodes which was the first indication of the electrode degradation in AC driven low pressure tubes. The figure 11 compares the evolution of the illuminance for 13 mm inside diameter, 50 cm long fluorescent tube as a function of the electrical power input for a Ar/Ne/Hg 10 mbar and a Ne/Xe (100/1) 60 mbar sealed signs. In both cases, the inside surface of the sign is covered by a green emitting phosphor layer processed by the same industrial procedure. While the mercury based sign exhibits a constant slope of the illuminance increasing versus the electrical power, the Ne/Xe sign illuminance tends to gradually saturate to a value of about 6000 Lux for an electrical input around 40 W. It must be reminded that a significant improvement of the mercury free signs performance was obtained first through the use of pulsed excitation in comparison with the AC excitation, in this latter the illuminance value never exceeds 1000 Lux whatever the electrical input is, and also thanks to the optimization of the filling pressure and pulse duration. These features also hold true for longer signs experienced up to 2 m in length and smaller (11 mm) or larger (18 mm) inside diameter tubes.

Complementary experiments were performed with the use of the integrating sphere. These measurements indicate first that the efficiency of the mercury based green signs is almost constant over the power range studied in this work up to 50 W, in agreement with the illuminance behaviour depicted in figure 10, and has a value of 30 Lm/W for 13 mm inside diameter tubes. The figure 12 presents the efficiency of four different mercury free 50 cm long signs, with one per cent of xenon content, as a function of the electrical power. The decrease of the efficiency is almost identical for all the signs for injected power higher than 15 W, whatever the pressure (45, 60 or 80 mbar) and the tube diameter (13 or 18 mm) for the 80 mbar filling. For electrical power lower than 15 W, the slightly higher efficiency of 13 mm diameter in comparison with the 18 mm illustrated for the pressure of 80 mbar is representative of the behaviour of Ne/Xe signs all over the pressure range studied in this work from 10 mbar up to 100 mbar. As depicted for the 13 mm diameter tubes, there exists an optimum for the sign efficiency which appears with larger and larger amplitude and for lower and lower electrical power when the tube pressure is decreased. For the 45 mbar pressure this efficiency maximum is not measured and is probably achieved for very low electrical power, below 2 W. For power input smaller than 15 W, the reduction of the efficiency with the pressure increase is attributed to the energy consumption in the electrode region of the discharge. Thus for a given electrical input power, a larger proportion of it is dissipated for the discharge maintenance considering that the pressure increase has a quite insignificant effect on the secondary emission coefficient [START_REF] Capdeville | Proc. XXIV ICPIG[END_REF] while it has a strong effect on the breakdown voltage for pure neon and even more drastically for neon -xenon mixtures.

Conclusion

It follows from the analysis of the pressure, the efficiency and the lifetime performance studies that medium pressure, i.e. around 50 mbar, neon xenon mixtures containing a few amount, around 1%, of xenon provide green light output of a few hundreds of lumen with an efficiency of about 50 % of that of mercury based signs and lifetime of a few thousands of hours. These features appear of interest for the development of mercury free interior signs and may also be valuable for rather low visible output applications requiring low injected electric power which correspond to higher efficiency operation of neon xenon plasma. It must be noticed that the mercury free sign performances are obtained without any change neither in the electrode arrangement nor in the green phosphor chemical composition, which allow for a very easy manufacturing considering that only the mercury and gas mixture blend as to be changed for a neon xenon filling. On the other hand, the design of specific phosphor likely to be much more efficient under 147 nm excitation is a very motivating issue to achieve substantial enhancement of the sign efficiency. The use of external electrodes eventually in combination with the conventional cold hollowed electrodes may also be of interest for the production of homogeneous higher pressure plasma [23].

Figure 1 .

 1 Photomultiplier tube

Figure 2 .

 2 Figure 2. Temporal evolution during the glow phase of: voltage (dark trace) and current (grey trace) waveforms upper graph, and on lower graph of : the xenon resonant level at 146.9 nm, the xenon infrared line at 823.1 nm, the xenon ion radiation at 529.2 nm and the neon fluorescence at 640.2 nm.

Figure 3 .

 3 Figure 3. Temporal evolutions of the xenon resonant radiation at 146.9 nm, the xenon infrared line at 823.1 nm, and of the xenon visible emission at 467.1 nm (dark trace). The three waveforms are normalized to their respective maximum during the glow phase. Ne/Xe (100/1) mixture at a pressure of 45 mbar was used.

Figure 4 .

 4 Figure 4. Comparison of the fluorescence signal at 146.9 nm with the result of the modelling including the full reaction set of table 1 (black smooth trace) and without R9 and R10 (thin grey trace). The thick grey trace presents the temporal evolution of the electronic temperature selected in the modelling.

Figure 5 .

 5 Figure5. Current across a 45 mbar Ne/Xe (100/1) tube, XeI 146.9 nm and XeII 529.2 nm waveforms for 2 microsecond (black traces) and 5 microsecond duration (grey traces) excitation pulses. In both cases, the electrical input power is set to 30 W.

Figure 7 .

 7 Figure 7. Evolution of the illuminance versus the pulse duration for a constant power across the sign with driver 1 (dots) and for a constant wall plug power input with driver 2 (square fitted trace). Ne/Xe (100/1) mixture at a pressure of 45 mbar.

Figure 9 .

 9 Figure 9. Evolution of the illuminance versus the pressure (lozenge dark fitted trace) and evolution of the 146.9 nm intensity with pressure (square grey fitted trace)

Figure 11 .

 11 Figure 11. Evolution of the illuminance with the input power for mercury based sign (lozenge fitted trace) and for neon xenon sign (triangle dashed fitted trace).

Figure 12 .

 12 Figure12. Evolution of the efficiency with the input power for different mercury free signs which pressure and inner/outer diameter are labelled in the inset. The xenon content is always of one per cent. The efficiency of mercury based signs of the same geometry and same phosphor coating is measured to be of about 30 Lm/W.

Figure 1 Figure 12 (

 112 Figure 1 (fig1_ROBERT.tif)

Table 1 .

 1 Selected reactions, rate coefficients and species initial densities at the beginning of the afterglow.

	__________________________________________________________________________________
	Reaction			Label	Rate coefficient	Reference
	Xe + Ne *		Xe + + Ne + e -	R1	7,5 x 10 -11 cm 3 s -1	[13]
	Xe + Ne *		NeXe + + e -	R2	2,3 x 10 -11 cm 3 s -1	[13]
	Xe + + 2 Xe Xe + + Xe + Ne Xe + + 2 Ne		+ + Xe Xe 2 + + Ne Xe 2 NeXe + + Ne	R3 R4 R5	2,0 x 10 -31 cm 6 s -1 1,5 x 10 -31 cm 6 s -1 1,5 x 10 -31 cm 6 s -1	[14] [15] [15]
	NeXe + + Xe		Xe + + Ne + Xe	R6	5,0 x 10 -10 cm 3 s -1	[16]
	NeXe + + Xe + + e -Xe 2 Xe 2 + + e -NeXe + + e -Xe + + 2 e -		+ + Ne Xe 2 Xe + + Xe + e -Xe * + Xe Xe * + Ne Xe * + e -	R7 R8 R9 R10 R11	5,0 x 10 -12 cm 3 s -1 1,0 x 10 -7 cm 3 s -1 -0,7 cm 6 s -1 2,3 x 10 -6 x T e -0,5 cm 6 s -1 2,0 x 10 -7 x T e 2,0 x 10 -27 x T e -4,5 cm 6 s -1	[16] [14] [17] [18] [17]
	Xe *		Xe + hν	R12	10 -5 s -1		[17]
	T e	e -	Xe +	Ne *	Xe *	Xe 2 +

Table 2 .

 2 Species initial densities for figure6calculations.

		1,2
	Normalized resonant level density	0,4 0,8
		0
		0	50	100	150	200
				Time (µs)
	Figure 6. Experimental and simulated afterglows
	for 2 (black trace), 5 (thick grey trace) and 10 µs
	(thin grey trace) pulses. The calculations use
	reactions and rates from table 1 and initial densities
	from table 2.
		Pulse	e -/Xe +	Xe *	Xe 2 + /NeXe +
	duration
		(µs)
		2	6 10 11	1.6 10 10	10 10
		5	1.1 10 12	2 10 10	5 10 10

Acknowledgments

This work is supported by AUPEM SEFLI company, the French Agency for Environment and Energy Management (ADEME) and la Région Centre. The authors are grateful to J. Mathias for VUV measurements and M. Pretre, J. Duchene and G. Cailleaux for their contribution to the experimental parametric study during their training periods.