ICRF Performance with Metallic Plasma Facing Components in Alcator C-Mod

Abstract

To utilize ion cyclotron range of frequency (ICRF) heating for ITER and future fusion reactors where high Z metallic plasma facing components (PFCs) will be employed, impurity production needs to be minimized and controlled. With high Z PFCs, the acceptable fractional concentration of high Z material in the plasma, tungsten {approx}10{sup -5} and molybdenum {approx}10{sup -4}, is significantly more restrictive compared to low Z material, carbon {approx}0.02. Furthermore, high power ICRF heating needs to be compatible with the use of low Z coatings, e.g. boronization, which in current tokamaks is used to control plasma radiation that has been shown to be very important for high performance H-modes, particularly in devices with high Z PFCs. In Alcator C-Mod, we have investigated the compatibility of high power ICRF heating with high performance plasmas and high-Z PFCs with, and without, boronization. Without boronization, excess radiation particularly from Mo, a strong edge radiator, resulted in lower H-factors. Upon boronization, record C-Mod stored energy and world record plasma pressures were achieved but the beneficial effect of boronization degrades after {approx}50 MJ of injected power. The erosion rate is estimated to be quite significant at {approx}10-15 nm/s. Areas outside the divertor were identified as the importantmore » Mo source and B erosion locations and found to be isolated to the active antenna. Furthermore, we observed that erosion rate associated with ICRF heating was unaffected by the heating scenario's single pass absorption.« less