Asymmetry in particle load on divertor tiles in different magnetic field configurations of LHD

Abstract

The asymmetry of plasma-particle load on divertor tiles at helically symmetrical positions has been investigated in the Large Helical Device (LHD). The asymmetry reverses when the toroidal magnetic field direction changes, suggesting that particle drifts cause the asymmetry. This study conducted proton orbit tracing calculations under a vacuum condition to investigate the effects of grad-B drift and curvature drift on the asymmetry. The calculations were performed for the major radius of the magnetic axis, R_ax, at 3.75 m and 3.6 m configurations. The results showed that due to the effects of grad-B drift and curvature drift, a similar asymmetry to the experimentally observed one appeared in the number of protons reaching the divertor tiles. The degree of asymmetry (DOA), representing the ratio of protons reaching the symmetrical divertor tiles, increased with higher proton energy and was smaller for the R_ax = 3.75 m configuration than the R_ax = 3.6 m one. An analysis of the experimental data for these magnetic field configurations revealed a consistent asymmetry between the R_ax = 3.75 m and the R_ax = 3.6 m configuration. It was also found that the asymmetry increased with higher electron temperature and was smaller in the R_ax = 3.75 m configuration than in the R_ax = 3.6 m one, which was consistent with the calculations. On the other hand, the experiments showed that the DOA saturated, which was not reproduced in the calculations.