Analysis of ICRF heated discharges wi th boron coated molybdenum tiles

Abstract

Minimizing impurity production associated with ion cyclotron range of frequency (ICRF) operation to an acceptable level in H-mode, particularly with metallic plasma facing components (PFC), is challenging. To identify important erosion and impurity source locations, we have vacuum plasma sprayed ~100 μm of boron (B) onto molybdenum (Mo) tiles. For ICRF heated H-modes, the core molybdenum levels have been significantly reduced and remained at low levels for increased injected RF energy. The core Mo level also no longer scales with RF power in L-mode. With boronization and impurity seeding (typically nitrogen or neon), the plasma and ICRF antenna performance were improved. Surprisingly, impurity seeding did not result in increased core Mo levels and also suppressed antenna faults. Spectroscopic monitoring of the plasma limiter found that the impurity profile at the limiter was centered near the plasma mid-plane and the profile did not change shape with plasma current. From post campaign inspection, the B coating was not significantly eroded except in locations where melting occurred or where it peeled. Trace material analysis also found that the B surface was contaminated with Mo and tungsten (W). Improved performance with impurity seeding, the lack of erosion, and metallic contamination of the B coating suggest ICRF impurity generation is related localized heat loads (that can lead to melting) rather than entirely a result of sputtering.