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Linear plasma devices have widely been utilized to examine plasma-facing materials under fusion reactor relevant conditions. In addition to DC arc sources, helicon plasma-based devices have also been developed. However, impurity generation from dielectric vacuum windows surrounded by an RF antenna remains a critical issue, as observed in Proto-MPEX [1], since unwanted deposited impurities can have an adverse impact on plasma material interaction (PMI) studies.
To address this issue, we have explored plasma flow in PISCES-RF, where no source impurities deposition on a PMI target has been detected [2]. Radial profiles of the parallel plasma flow were measured using two radially reciprocating Mach probes at two axial locations z = 421 mm ($\alpha$) and 675 mm ($\beta$) near the RF source exit, defined as z = 0 mm. Doppler shift spectroscopy of a He II line (468.6 nm) was also carried out at two axial locations z ~ 573 mm ($\delta$) and ~ 1047 mm ($\epsilon$) near the PMI target at 1202 mm.
In this experiment, measurements were first performed in helium plasmas, and then we confirmed that Mach probe-measured Mach numbers agree reasonably with those from Doppler shift measurements. In helicon discharges, it is found that there is a stagnation point around the $\delta$ and $\beta$ locations; the plasma flow is directed toward the RF source and PMI target in the upstream and downstream of the stagnation point, respectively. With increasing gas pressure, the stagnation point tends to move closer to the RF source. At the $\epsilon$ location, the Mach number reaches ~ 0.23, insensitive to the RF input power, and decreases to ~ 0.1 with increasing gas pressure. In deuterium plasmas, the existence of a stagnation point around the $\beta$ location was also confirmed. These experimental findings can explain impurity migration observed in [2,3], and are consistent with SOLPS-ITER simulations [4]. Thus, the plasma flow toward the RF source is thought to play an important role to prevent the transport of impurities from the RF source to the PMI target.
References:
[1] C.J. Beers et al., Phys. Plasmas 28 (2021) 103508.
[2] M.J. Baldwin et al., Nucl. Mater. Energy 36 (2023) 101477.
[3] G. Dhammale et al., Plasma Phys. Control. Fusion 66 (2024) 095015.
[4] M.S. Islam et al., Plasma Phys. Control. Fusion 67 (2025) 025002.
Acknowledgement:
This work was supported by the Japan-US Cooperation in Fusion Research and Development and the US DOE Cooperative Agreement No. DE-SC0022528.