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

Abstract

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 m 3 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.

Advanced Thomson scattering system for high-flux linear plasma generator

“…Although only one point at z = 165 mm was recorded, the measured T e is in agreement with the results obtained by Ref. 18 from double Langmuir probe measurements.…”

“…Figures 11 and 12 show that on the axis of the jet, the measured n e and n 0 decay approximately as z −2 for large z. The shock is expected at about 270 mm distance from the nozzle, 18 which implies that the measurement region corresponds to the supersonic expansion zone.…”

“…Due to the high pressure difference between the source and the Magnum-PSI vessel, the plasma expands into the source chamber. The ionization degree of the expanding plasma is in the range of 5%-20% 18 depending on the arc current, which allows for treating the plasma dynamics inside the jet as that of a hot gas of heavy particles (neutrals and ions) since the coupling between ions and neutrals is strong. Starting from the nozzle, the macroscopic velocity of the plasma accelerates to supersonic velocity and the diameter of the plasma jet rises linearly (rarefaction effect), and results in a z −2 decay of the neutral density along the z-axis.…”

“…with c 0 the local sound speed (∼1700 m/s at 0.7 eV 18 ). This results in a value of n res,0 ∼ 1.2 × 10 22 m −3 .…”

High heat flux capabilities of the Magnum-PSI linear plasma device

“…This requires the use of differential pumping which has to be optimized for the high gas flows required for the plasma generation. More details about the design of the differential pumping can be found in [16].…”