Deuterium retention in tungsten and tungsten: tantalum alloys exposed to high-flux deuterium plasmas

A direct comparison of deuterium retention in samples of tungsten and two grades of tungsten-tantalum alloys-W-1% Ta and W-5% Ta, exposed to deuterium plasmas (ion flux similar to 10(24) m(-2) s(-1), ion energy at the biased target similar to 50 eV) at the plasma generator Pilot-PSI was performed using thermal desorption spectroscopy (TDS). No systematic difference in terms of total retention in tungsten and tungsten-tantalum was identified. The measured retention value for each grade did not deviate by more than 24% from the value averaged over the three grades exposed to the same conditions. No additional desorption peaks appeared in the TDS spectra of the W-Ta samples as compared with the W target, indicating that no additional kinds of traps are introduced by the alloying of W with Ta. In the course of the experiment the same samples were exposed to the same plasma conditions several times, and it is demonstrated that samples with the history of prior exposures yield an increase in deuterium retention of up to 130% under the investigated conditions compared with the samples that were not exposed before. We consider this as evidence that exposure of the considered materials to ions with energy below the displacement threshold generates additional traps for deuterium. The positions of the release peaks caused by these traps are similar for W and W-Ta, which indicates that the corresponding traps are of the same kind

Angle resolved electron spectra from grazing incidence He2+/Cu(110) collisions

Electron spectra from keV collisions of He2+ with a single crystal Cu(110) surface are reported. The spectra are obtained for well defined angles of incidence and detection. From the spectra and from their variation with detection angle the following conclusions are drawn. At vertical velocities of the order of 0.01 atomic units the He2+ neutralizes in two successive steps while it approaches the surface. In the first step He2+ is converted to He+ in the ground state by two competing processes, namely, double capture followed by autoionization or Penning ionization, and direct Auger capture. In the second step He+ is neutralized by Auger capture into ground state He. A model is developed that allows one to calculate electron spectra due to those processes using model functions for distance-dependent transition rates and transition energies. It is shown that the main features of the observed spectra can be well reproduced by the model calculations if it is assumed that the metal does not respond adiabatically to the sudden changes induced upon the emission of fast electrons in the various spontaneous ionization processes

A differentially pumped argon plasma in the linear plasma generator Magnum-PSI: gas flow and dynamics of the ionized fraction

Magnum-PSI is a linear plasma generator designed to reach the plasma–surface interaction (PSI) regime of ITER and nuclear fusion reactors beyond ITER. To reach this regime, the influx of cold neutrals from the source must be significantly lower than the plasma flux reaching the target. This is achieved by a differential pumping scheme, where the vacuum vessel is divided by skimmers into separate chambers which are individually pumped. The non-magnetized expansion of 5 Pa m3 s-1 (3 slm) argon in a low background pressure was studied in the differentially pumped vacuum vessel fitted with non-cooled flat skimmers. The behavior of the neutral component was studied with direct simulation Monte Carlo simulations and Rayleigh scattering measurements. Thomson scattering and double Langmuir probe measurements were performed on the ionized fraction. It was found that the electrons and neutral particles are not completely coupled in the shock front. The neutral fraction shows clear signs of invasion from hotter background gas, causing the average temperature and density to increase before the shock. This is also shown in the ionization ratio, which has been determined in front of and behind the first skimmer. This study helps us to understand the behavior of the gas flow in the machine and validates our modeling

Operational characteristics of the high flux plasma generator magnum-PSI

\u3cp\u3eIn Magnum-PSI (MAgnetized plasma Generator and NUMerical modeling for Plasma Surface Interactions), the high density, low temperature plasma of a wall stabilized dc cascaded arc is confined to a magnetized plasma beam by a quasi-steady state axial magnetic field up to 1.3 T. It aims at conditions that enable fundamental studies of plasma-surface interactions in the regime relevant for fusion reactors such as ITER: 10\u3csup\u3e23\u3c/sup\u3e-10\u3csup\u3e25\u3c/sup\u3e m \u3csup\u3e-2\u3c/sup\u3e s\u3csup\u3e-1\u3c/sup\u3e hydrogen plasma flux densities at 1-5 eV. To study the effects of transient heat loads on a plasma-facing surface, a high power pulsed magnetized arc discharge has been developed. Additionally, the target surface can be transiently heated with a pulsed laser system during plasma exposure. In this contribution, the current status, capabilities and performance of Magnum-PSI are presented.\u3c/p\u3