Impurity transport in helicon plasma source linear devices*

Linear plasma devices (LPD) designed to test plasma-facing components in fusion reactor-relevant plasma conditions often face impurity transport challenges originating from the plasma source. Proto-MPEX^[1] and PISCES-RF^[2] LPDs utilizing a similar helicon plasma source have observed a progressive erosion of the plasma source window caused by a rectified sheath voltage formed at the plasma-facing window surface. Generated impurities may be transported in the plasma, affecting the plasma-material interaction at target surfaces. As a result, a source-born impurity deposition was observed on Proto-MPEX’s target surface. However, it was not observed on PISCES-RF’s target surface. This work investigates distinguishing factors for impurity transport in both devices. Titanium impurity seeding from different axial locations and magnetic field configurations applied near the plasma source are performed to experimentally observe the impurity deposition at various locations in PISCES-RF^[3]. COMSOL sheath and GITR simulation results of PISCES-RF experimental conditions are compared with that of Proto-MPEX^[1] and have suggested that in PISCES-RF, generated impurities stay neutral and unmagnetized and are likely to be lost to the vessel wall near the plasma source. The findings are important and will be used to optimize the operation plan for MPEX at ORNL.

References:

[1] Beers C, Lau C, Rapp J, et al. Physics of Plasmas 2021 28

[2] Baldwin M, Nishijima D, Patino M, et al. Nuclear Materials and Energy 2023 36 101477

[3] G. Dhamale, M J Baldwin, M. S. Islam, et al. Plasma Physics and Controlled Fusion (submitted)