Abstract
Abstract
Ion cyclotron range of frequencies (ICRF) antenna operation and plasma-wall interaction were investigated on the WEST tokamak in the regime where the density at the antenna limiters was sufficiently low for the slow wave (SW) to propagate in front of the antenna. Using a reciprocating emissive probe magnetically connected to the antenna, we measured for the first time the DC plasma potential, VDC, during a radial scan of the LH resonance layer across the antenna limiter. VDC peaks when the density at the antenna limiter edge approaches the LH resonance density, but never exceeds typical values of a few hundred volts. From the plasma-wall interaction standpoint, this regime is highly favorable: because particle fluxes are reduced while sheath potentials remain comparable to standard operating conditions at the same antenna voltage, local tungsten sources at the ICRF antenna and other outer-wall components become nearly undetectable. In general, tungsten sputtering from active WEST ICRF antennas is dominated by the particle flux rather than the sputtering yield; that is, it follows variations in the local density rather than the plasma potential. By contrast, sputtering in the divertor is primarily governed by the sputtering yield. Core impurity contamination is likewise significantly reduced when the antennas are positioned far from the separatrix, both with and without ICRF power, and the radiated power fraction decreases accordingly. Despite the modest coupled powers obtained at large antenna-plasma clearance, satisfactory ICRF heating is maintained as the density at the antenna limiter edge falls below the LH resonance, and no deleterious effects are observed in any key core plasma metrics when operating the antenna from this low-density region. Overall, the experimental results point toward minimal coupling to the SW by the fast wave antenna despite it being located in a region where the SW can propagate.