73592760
High Resolution HST-STIS Spectra of CI and CO in the Beta Pictoris Circumstellar Disk
High resolution FUV echelle spectra showing absorption features arising from CI and CO gas in the Beta Pictoris circumstellar (CS) disk were obtained on 1997 December 6 and 19 using the Space Telescope Imaging Spectrograph (STIS). An unsaturated spin-forbidden line of CI at 1613.376 A not previously seen in spectra of Beta Pictoris was detected, allowing for an improved determination of the column density of CI at zero velocity relative to the star (the stable component), N = (2-4) x 10^{16} cm^{-2}. Variable components with multiple velocities, which are the signatures of infalling bodies in the Beta Pictoris CS disk, are observed in the CI 1561 A and 1657 A multiplets. Also seen for the first time were two lines arising from the metastable singlet D level of carbon, at 1931 A and 1463 A The results of analysis of the CO A-X (0-0), (1-0), and (2-0) bands are presented, including the bands arising from {13}^CO, with much better precision than has previously been possible, due to the very high resolution provided by the STIS echelle gratings. Only stable CO gas is observed, with a column density N(CO) = (6.3 +/- 0.3) x 10^{14} cm{-2}. An unusual ratio of the column densities of {12}^CO to {13}^CO is found (R = 15 +/- 2). The large difference between the column densities of CI and CO indicates that photodissociation of CO is not the primary source of CI gas in the disk, contrary to previous suggestion.
36086311
Space charge neutralization of high brightness ion beams
Experiments are under way to investigate the extent to which various space charge neutralization methods will allow one to exceed the space charge limited transport condition. Imperfect spatial neutralization will result in nonlinear fields which have the effect of diluting the phase space density of the beam, and thereby increasing the emittance. These experiments give quantitative results on the problem of vacuum neutralization in the final focus/transport for a heavy ion fusion reactor.
93743668
Neutralization of positive particle beams by electron trapping
7155669
EXPERIMENTAL EVIDENCE FOR WATER FORMATION VIA OZONE HYDROGENATION ON DUST GRAINS AT 10 K
The formation of water molecules from the reaction between ozone (O3) and D-atoms is studied experimentally for the first time. Ozone is deposited on non-porous amorphous solid water ice (H2O), and D-atoms are then sent onto the sample held at 10 K. HDO molecules are detected during the desorption of the whole substrate where isotope mixing takes place, indicating that water synthesis has occurred. The efficiency of water formation via hydrogenation of ozone is of the same order of magnitude as that found for reactions involving O-atoms or O2 molecules and exhibits no apparent activation barrier. These experiments validate the assumption made by models using ozone as one of the precursors of water formation via solid-state chemistry on interstellar dust grains.
119206019
Sticking coefficient of hydrogen and deuterium on silicates under interstellar conditions
Context. Sticking of H and D atoms on interstellar dust grains is the first step in molecular hydrogen formation, which is a key reaction in the interstellar medium. Isotopic properties of the sticking can have an incidence on the observed HD molecule. Aims. After studying the sticking coefficients of H2 and D2 molecules on amorphous silicate surfaces experimentally and theoretically, we extrapolate the results to the sticking coefficient of atoms and propose a formulae that gives the sticking coefficients of H and D on both silicates and icy dust grains. Methods. In our experiments, we used the King and Wells method for measuring the sticking coefficients of H2 and D2 molecules on a silicate surface held at 10 K. It consists of measuring with a QMS (quadrupole mass spectrometer) the signals of H2 and D2 molecules reflected by the surface during the exposure of the sample to the molecular beam at a temperature ranging from 20 K to 340 K. We tested the efficiency of a physical model, developed previously for sticking on water-ice surfaces. We applied this model to our experimental results for the sticking coefficients of H2 and D2 molecules on a silicate surface and estimated the sticking coefficient of atoms by a single measurement of atomic recombination and propose an extrapolation. Results. Sticking of H, D, HD, H2 ,a nd D 2 on silicates grains behaves the same as on icy dust grains. The sticking decreases with the gas temperature, and is dependent on the mass of the impactor. The sticking coefficient for both surfaces and impactors can be modeled by an analytical formulae S (T ) = S 0(1 + βT/T0)/(1 + T/T0) β , which describes both the experiments and the thermal distribution expected in an astrophysical context. The parameters S 0 and T0 are summarized in a table. Conclusions. Previous estimates for the sticking coefficient of H atoms are close to the new estimation; however, we find that, when isotopic effects are taken into account, the sticking coefficient variations can be as much as a factor of 2 at T = 100 K.
56163569
IPHI, the Saclay high-intensity proton injector project
High-power accelerators are being studied for several projects based on high-flux neutron sources driven by proton or deuteron beams. Since the front end is the most critical part of such accelerators, it has been decided to build a high-intensity proton injector (IPHI) designed to accelerate a CW 100 mA beam up to 11 MeV. The aim is to validate design codes, test technical choices, demonstrate operational reliability and measure the beam parameters in order to optimize the high-energy part of the accelerator. The design chosen for IPHI (ECR source, RFQ, DTL and diagnostic line) and the construction schedule are presented.
118184102
Determination of coronal temperatures from electron density profiles
The most popular method for determining coronal temperatures is the scale-height-method (shm). It is based on electron density profiles inferred from White Light (WL) brightness measurements of the corona during solar eclipses. This method has been applied to several published coronal electron density models. The calculated temperature distributions reach a maximum at r > 1.3 RS, and therefore do not satisfy one of the conditions for applying the shm method. Another method is the hydrostatic equilibrium method (hst), which enables coronal temperature distributions to be determined, providing solutions to the hydrostatic equilibrium equation. The temperature maximas using the hst method are almost equal to those obtained using the shm method, but the temperature peak is always at significantly lower altitude when the hst-method is used than when the shm-method is used. A third and more recently developed method, dyn, can be used for the same published electron density profiles. The temperature distributions obtained using the dyn method are regular solutions of the hydrodynamic equations. They depend on the expansion velocity of the coronal plasma considered as a free input parameter in the calculations. At the base of the solar corona, where the coronal bulk velocity is small (subsonic), the dyn and hst methods give similar temperature values. Larger differences are found at higher altitudes where the expansion velocity approaches and exceeds the velocity of sound. This paper shows also the effects of the flow tube geometry and ion temperatures on the electron temperature distributions. The dyn method is a generalization of the hst method, where the electron temperature distribution tends to zero at infinite radial distances. It is a new diagnostic tool for determining from WL coronal observations the range of radial distances where the coronal heating rate is at its maximum.
44015141
The effect of a southward interplanetary magnetic field on Stormer's allowed regions
The motion of a charged particle in a magnetic dipole has first been studied by Stormer. The different applications of Stormer's theory to aurorae, cosmic rays and Van Allen radiation belt particles are recalled in an historical perspective. In this paper, we expand the Stormer theory in order to take into account the effects produced by an additional uniform and stationary interplanetary magnetic field (IMF) whose orientation is parallel or antiparallel to the magnetic moment of the dipole. A new expression is derived for the Stormer potential taking into account the additional IMF component. It is shown how Stormer's allowed and forbidden zones are influenced by the implementation of a northward or a southward IMF, and how a southward turning of the IMF orientation makes it easier for Solar Energetic Particle and Galactic Cosmic Rays to enter into the inner part of the geomagnetic field along interconnected magnetic field lines. (C) 2003 COSPAR. Published by Elsevier Science Ltd. All rights reserved.
119222326
Coronal Temperature Profiles Obtained from Kinetic Models and from Coronal Brightness Measurements Obtained During Solar Eclipses
Coronal density, temperature, and heat-flux distributions for the equatorial and polar corona have been deduced from Saito’s model of averaged coronal white-light (WL) brightness and polarization observations. These distributions are compared with those determined from a kinetic collisionless/exospheric model of the solar corona. This comparison indicates similar distributions at large radial distances (> 7 R⊙) in the collisionless region. However, rather important differences are found close to the Sun in the acceleration region of the solar wind. The exospheric heat flux is directed away from the Sun, while that inferred from all WL coronal observations is in the opposite direction, i.e. conducting heat from the inner corona toward the chromosphere. This could indicate that the source of coronal heating extends up into the inner corona, where it maximizes at r>1.5 R⊙, well above the transition region.
116897797
What Happens When the Geomagnetic Field Reverses
During geomagnetic field reversals the radiation belt high-energy proton populations become depleted. Their energy spectra become softer, with the trapped particles of highest energies being lost first, and eventually recovering after a field reversal. The radiation belts rebuild in a dynamical way with the energy spectra flattening on the average during the course of many millennia, but without ever reaching complete steady state equilibrium between successive geomagnetic storm events determined by southward turnings of the IMF orientation. Considering that the entry of galactic cosmic rays and the solar energetic particles with energies above a given threshold are strongly controlled by the intensity of the northward component of the interplanetary magnetic field, we speculate that at earlier epochs when the geomagnetic dipole was reversed, the entry of these energetic particles into the geomagnetic field was facilitated when the interplanetary magnetic field was directed northward. Unlike in other complementary work where intensive numerical simulations have been used, our demonstration is based on a simple analytical extension of Stormer's theory. The access of GCR and SEP beyond geomagnetic cut-off latitudes is enhanced during epochs when the Earth's magnetic dipole is reduced, as already demonstrated earlier.
119298597
Convective Instability Of The Solar Corona: Why The Solar Wind Blows
Chapman’s conductive model of the solar corona is characterized by a temperature varying as r−2/7 with heliocentric distance r. The density distribution in this non‐isothermal hydrostatic model has a minimum value at 123 RS, and increases with r above that altitude. It is shown that this hydrostatic model becomes convectively unstable above r = 35 RS, where the temperature lapse rate becomes superadiabatic. Beyond this radial distance heat conduction fails to be efficient enough to keep the temperature gradient smaller than the adiabatic lapse rate. We report the results obtained by Lemaire who showed that an additional mechanism is then required to transport the energy flux away from the Sun into interplanetary space. He pointed out that this additional mechanism is advection: i.e. the stationary hydrodynamic expansion of the corona. In other words the corona is unable to stay in hydrostatic equilibrium. The hydrodynamic solar wind expansion is thus a physical consequence of the too steep (superadiabatic...