140119573
ELECTROMAGNETIC PARAMETERS STUDY OF MICROWAVE- ABSORBING MATERIAL FeSiAl FOR COLLINEAR LOAD OF LINAC*
Microwave-absorbing material is an essential part of LINAC collinear load. It is coated on the inner walls of several trailing accelerating cavities to transform the remnant microwave power into heat. Fe-85%Si-9.6%Al5.4% alloy, which reveals low outgassing rate and high attenuation, is selected for collinear load R&D. To measure the permittivity and permeability of FeSiAl at 2856 MHz, the coaxial transmission-reflection method is adopted. The system is firstly examined by testing the hollow coaxial fixture and comparing the results with the electromagnetic parameters of the air. Measurements of two PTFE rings show that the air gaps between the fixture and samples influence the test results seriously. CST is utilized to simulate the effects on the FeSiAl measurements. Eventually a scheme of molding the samples of FeSiAl powder mixed with paraffin to form a wax mold is proposed and the permittivity and permeability of FeSiAl are derived from the electromagnetic parameters equivalent formulas of mixed medium. INTRODUCTION In order to ensure LINAC running safely, the remnant microwave power has to be absorbed at the end of the accelerator tube. Waveguide load is regularly adopted, but the structures would make it difficult to miniaturize the LINAC. Collinear load is a new structure of LINAC load, which consists of several accelerating cavities coaxially extending the normal accelerating tube with efficient microwave-absorbing material coated on the inner walls. The remnant power is dissipated and transformed into heat, which is carried out through cavity walls by external cooling water. Without output coupler, the structure not only brings about simplification on focusing coils installation, but also makes the construction compact and the quality of beam current improved [1]. Absorbing material is an essential part of the collinear load and its absorbing capability depends on its electromagnetic parameters [2], which would be significant to the performance of the collinear load. There are two types of electromagnetic measurement methods for microwaveabsorbing materials: the network parameters method and the resonance cavity method. The resonance cavity method is fit for low loss materials, while the network parameter method is more suitable for higher loss materials. The latter consists of coaxial transmissionreflection method [3], free space method and so on. In this paper, coaxial transmission-reflection method is used to test the FeSiAl alloy which is an efficient absorbing material and prepared by Institute of Electronics, Chinese Academy of Sciences. THEORY OF TRANSMISSIONREFLECTION METHOD The transmission line is a reciprocal two-port network when the sample is a homogeneous, linear and isotropic medium. As shown in Fig. 1, the electromagnetic wave transmits and reflects back and forth in the medium. 1 R 2 (1 ) R T R 2 2 (1 ) R R T 3 2 4 (1 ) R R T 5 2 6 (1 ) R R T 7 2 8 (1 ) R R T 2 2 3 (1 ) R R T 8 2 9 (1 ) R R T 6 2 7 (1 ) R R T 4 2 5 (1 ) R R T 1 .... ..... .... .... 11 S 21 S Figure 1: The electromagnetic wave reflection and transmission in medium. Supposing that the reflection coefficient of the airmedium interface (A) is R, reflection coefficient of the medium-air interface (B) is -R and the transmission coefficient of the medium is T with the length of L, the complex number S11 of reflection sum in the A interface and S21 of transmission sum in the B interface will be
55951342
STUDY ON FREQUENCY CHANGE BY 3D RECONSTRUCTION OF DEFORMED CAVITIES OF LINAC COLLINEAR LOAD*
Collinear load, consisted of several coaxial cavities, is a substitute for traditional waveguide-type load to absorb the remnant power of the LINAC and makes the accelerating structure compact and small-size. The power loss on the cavities of collinear load brings thermal deformation which affects their resonant frequency deeply. In this paper, a new approach of 3D reconstruction of the thermal deformed cavities is utilized to evaluate the accurate influence on frequency change caused by non-uniform deformation and water cooling strategies of collinear absorbing load are studied. Then the thermal behavior of a six-cavity collinear load, which is coated with Kanthal alloy and FeSiAl alloy and used on a 2856MHz, 2π/3 mode respectively, is researched. The results show that the collinear load with Kanthal alloy can only absorb up to 10kW, while with FeSiAl alloy it can dissipate 15kW when the water flow controlled within 3.0kg/s for energy saving.
59430123
Drive Laser and Optical Transport Line for Photoinjector
A Photo-Cathode RF Gun is under development at NSRL. In this paper, the drive laser system is introduced and performance parameters are presented. We adopt a BNL type gun with laser illuminating the cathode at oblique incidence. To correct “time slew” and “elliptical spot” problems arisen on the cathode, an adjustable optical transport line is designed. INTRODUCTION Drive laser system is the key component of photocathode high-brightness injector. To ensure the high quality of electron beams offered for FEL, the injector request a reliable laser with prominent performance parameters, such as: pulse duration, beam profile, energy stability, pulse to pulse stability, pointing stability and phase jitter. We adopted a BNL type gun with p-polarized laser at oblique incidence, as to acquire high quantum efficiency and low thermal emittance [1]. But, the problems of “time slew” and “elliptical spot” will arise on the cathode. To correct them, an adjustable optical transport line was designed. DRIVE LASER SYSTEM We employed a drive laser system developed by High Q Laser Production GmbH [2]. It consisted of three main parts: oscillator with a timing stabilizer, a regenerative amplifier, and the frequency conversion part. Fig.1 is the layout of the drive laser system. Figure 1: Layout of the drive laser system Oscillator The seeding laser is a passive mode-locked laser with a semiconductor absorber mirror (SESAM). Its repetition rate is 102MHz, equal to 1/28 of 2856MHz S-band radio frequency, which is used for RF-gun. The timing stabilizer measure the phase offset between laser pulses and reference RF signal,then adjust the cavity length by putting one laser resonator mirror on a voltage controlled piezo to synchronize the two signals. Regenerative Amplifier The Pockels Cell captures pulses from the continuous seeding laser and determines the pass times of pulse in the regenerative amplifier cavity. The repetition rate of amplified pulses is optional from 10 to 100Hz. Frequency Conversion Part The SHG convert the IR light (1047nm) to green light (523nm), then the FHG convert the green light to UV light (262nm). The conversion efficiency is about 20% from the fundamental to the 4-th harmonics. LASER PERFORMANCE Pulse Duration The autocorrelation measurement result of IR pulse width was 8.3ps FWHW (deconvolved value taken from the Fig.2). The UV pulse width could be close to this value. Figure 2: Pulse width @ 1047 nm measured after Regenerative Amplifier @ 10Hz Beam Profile and Diameter The beam profile and diameter of UV laser pulse was measured by a CCD camera. Beam profile was shown as Fig.3. The beam diameter in X and Y direction were 2.051mm and 1.927mm respectively, which were statistical results of 300 shots. ___________________________________________ *Work supported by the second phase of “985” project hezhg@mail.ustc.edu.cn Proceedings of IPAC’10, Kyoto, Japan TUPEA035 07 Accelerator Technology T24 Lasers 1405 Pulse to Pulse Stability The UV pulse to pulse stability was measured by a photodiode connected with an oscilloscope. The area integration was measured. The fluctuation was calculated as: σ/μ∗100%. The result was about 0.8% over two hours, as shown in Fig.4. Figure 3: Beam profile of UV laser Figure 4: Pulse stability of UV over 2h: measured with 50 Ohm feed-through-terminated input; 500 MHz Bandwidth Tektronix TDS 3052, DC-coupled) Figure 5: Pointing stability measurement Figure 6:Long term energy stability measurement results Pointing Stability Beam pointing stability was measured with a lens of 600mm focal length. The CCD camera was placed at a distance of 700mm from the lens. Pointing stability of the beam centroid were 6.13μrad rms in X direction and 19.31μrad rms in Y direction. Fig.5 showed us the measured pointing stability of centroid point and also peak point. Energy Stability The long term (over 17 hours) energy fluctuation of UV measured by an energy meter was about 0.6% rms, as shown in Fig.6. The measurement precision is dependent on resolution of energy sensor. We think the actual value is higher than the measured one. Phase Jitter The phase jitter between oscillator and reference signal (signal generator: E8257D) was measured. The output signal was presented in Fig.7. Jitter calculation as: 6 0 1 1 14.2 360 102 10 360 rms rms OSC Jitter U mV f Hz = = × ∗ × o 387 rms Jitter fs = Scaling: 1000 1 mV = o To gain a low jitter, a signal generator with low phase noise is required. Signal generators E8257D and E8663B from Agilent are suitable. All of the laser performance parameters were listed on Table 1. OPTICAL TRANSPORT LINE Because of oblique incidence design of BNL type gun, two problems of “time slew” and “elliptical spot” will arise on photo-cathode. To correct them, two prisms were used to add appropriate negative time slew and TUPEA035 Proceedings of IPAC’10, Kyoto, Japan 1406 07 Accelerator Technology T24 Lasers ellipticity on laser pulse wave front. The added time slew is: τ=f(r,α), α is the angle of prism, r is the laser diameter on the prism. A variable beam expander was used to adjust the r . Therefore, the added time slew can be adjustable. The beam diameter on cathode can be controlled by a movable lens at the end of optical line. The Y direction beam diameter was also fore-adjusted by prisms. The sketch of optical transport line was shown as Fig.8. The beam profile after the two prisms was measured by a CCD camera, which was showed in Fig.9. The X direction diameter was compressed to 1.0mm, The Y direction diameter was still about 1.9mm. To compress the Y direction diameter, one just needs to rotate the prisms. The transmission efficiency of the two prisms is about 92%. Figure 7: SYNC-phase output signal, RMS=14.2mV Figure 8: Sketch of optical transport line Figure 9: Beam profile after two prisms Table 1: The performance of drive laser system 1 Wave length/nm 262 2 Pulse duration (FWHM) /ps 8.3 3 Oscillator frequency/MHz 102 4 UV repeat rate/Hz 10-100 5 UV energy/mJ 0-0.65 6 Energy fluctuation/rms 0.6% 7 Pulse to pulse stability/rms 0.8% 8 X direction Point stability/μrad 6.13 9 Y direction Point stability/μrad 19.31 10 Phase jitter/fs 387 11 X direction beam diameter/(1/emm) 2.051 12 Y direction beam diameter/(1/emm) 1.927 SUMMARY We described the drive laser system and presented the measured laser performance parameters. We also reported a preparatory design of optical transport line used to correct “time slew” and “elliptical spot” problems. We are going to reshape the laser pulse. In the aspect of spatial shaping, a commercial production from pi-shaper [3] and GBS-UVH [4] may be considered. Both of them showed favourable shaping effects [5] [6]. In the aspect of temporal shaping, we are going to try the pulse stacking method [6] [7]. REFERENCES [1] Xiang D et al. Nuclear Instruments and Methods in Physics Research A, 2006, (562): 48-52. [2] http://www.highqlaser.at/ [3] http://www.pishaper.com/ [4] http://search.newport.com/?sku=GBS-UV-H [5] D.H. Dowell et al., Proceedings of PAC07, Albuquerque, New Mexico, USA TUPMS058. [6] A.K. Sharma, T, Tsang, and T. Rao, PHYSICAL REVIEW SPECIAL TOPICS ACCELERATORS AND BEAMS 12, 033501 (2009). [7] H. Tomizawa et al., Proceedings of FEL 2007, Novosibirsk, Russia WEBAU01. Proceedings of IPAC’10, Kyoto, Japan TUPEA035 07 Accelerator Technology T24 Lasers 1407
119481484
Open spin chain and open spinning string
In this Letter, we study the open spinning strings and their SYM duals. A new class of folded open spinning strings is found. At planar one-loop level in SYM, by solving the thermodynamic limit of the Bethe ansatz equations for an integrable open spin chain, we find good agreement with string theory predictions for energies of both circular and folded two-spin solutions. A universal relation between the open and closed spinning strings is verified in the spin chain approach.
122234832
EXTRA NEUTRAL GAUGE BOSON FROM THE 3-3-1 MODEL AND THE SPIN CORRECTION OF TOP QUARK PAIR PRODUCTION AT THE ILC
In the off-diagonal basis, we explore the effects of extra neutral gauge boson Z' predicted in two versions of SU(3)C × SU(3)L × U(1)X model on the spin configuration of the top quark pair production in the high energy linear e+e- collider (ILC). Our numerical results show that, the cross sections for the suppressed spin configurations can be enhanced with the effects of the Z' boson through the modification of the spin configuration by producing enough top quark pairs to be measured in the future ILC experiments, which provides the way to observe the effects of Z' predicted in the 3-3-1 model and discriminate the various versions of 3-3-1 model.
54894329
The ESS Cryogenic System
The European Spallation Source (ESS) is a neutron science facility funded by a collaboration of 17 European countries currently under design and construction in Lund, Sweden. Cryogenic cooling is vital for large sections at ESS. Mainly there is a 2.0 GeV proton linac using superconducting RF cavities operating at 2 K. In addition to cooling the SRF cavities, cryogenics is also used for the cold hydrogen moderator surrounding the target. ESS furthermore uses both liquid helium and liquid nitrogen in a number of the neutron instruments. There is also a cryogenic installation associated with the site acceptance testing of the ESS cryomodules [1]. This paper describes the conceptual design of the ESS cryogenic system including the expected heat loads and cryoplant features. Challenges associated with the required high reliability and turn-down capability will also be discussed.
119298211
Relics of Minijets amid Anisotropic Flows in High-energy Heavy-ion Collisions
Two dimensional low-$p_T$ dihadron correlations in azimuthal angle $\phi$ and pseudo-rapidity $\eta$ in high-energy heavy-ion collisions are investigated within both the HIJING Monte Carlo model and an event-by-event (3+1)D ideal hydrodynamic model. Without final-state interaction and collective expansion, dihadron correlations from HIJING simulations have a typical structure from minijets that contains a near-side two-dimensional peak and an away-side ridge along the $\eta$-direction. In contrast, event-by-event (3+1)D ideal hydrodynamic simulations with fluctuating initial conditions from the HIJING+AMPT model produce a strong dihadron correlation that has an away-side as well as a near-side ridge. Relics of intrinsic dihadron correlation from minijets in the initial conditions still remain as superimposed on the two ridges. By varying initial conditions from HIJING+AMPT, we study effects of minijets, non-vanishing initial flow and longitudinal fluctuation on the final state dihadron correlations. With a large rapidity gap, one can exclude near-side correlations from minijet relics and dihadron correlations can be described by the superposition of harmonic flows up to the 6th order. When long-range correlations with a large rapidity gap are subtracted from short-range correlations with a small rapidity gap, the remaining near-side dihadron correlations result solely from relics of minijets. Low transverse momentum hadron yields per trigger ($p_T^{\rm trig} <4$ GeV/$c$, $p_T^{\rm asso}<2$ GeV/$c$) in central heavy-ion collisions are significantly enhanced over that in p+p collisions while widths in azimuthal angle remain the same, in qualitative agreement with experimental data.