Plasma wall interaction in long-pulse helium discharge in LHD – Microscopic modification of the wall surface and its impact on particle balance and impurity generation

Abstract

Ultra-long-pulse helium discharge with ion and electron cyclotron heating (ICH+ECH) in the Large Helical Device (LHD) was achieved in a 48min plasma (ne∼1.2×1019 m−3, Ti,e∼2keV) with an average heating power of 1.2MW. The temperature of the first-wall surface during discharges remained at nearly room temperature. However, even in ultra-long-pulse helium discharge, the discharge conditions cannot be said to be in a steady-state, because of two major issues interrupting the steady-state condition. One is the “dynamic change of the wall pumping rate” and the other is the “termination of the discharge with the exfoliation of the mixed-material deposition layers.” Microscopic modifications, such as helium radiation damage and the formation of the mixed-material deposition layers composed of C (∼98%) and Fe (∼2%), on the plasma facing components (PFMs) were clarified to possibly influence the major issues.