ICRF HEATING PLASMA EDGE INTERACTION IN JET WITH BERYLLIUM GETTERING

During Ion Cyclotron Range of Frequencies (ICRF) heating in JET (Joint European Torus) with beryllium gettered walls and RF antenna screens both the global and local impurity influxes are significantly reduced as compared to the previous operation. In particular oxygen is reduced by an order of magnitude and the deuterium is efficiently pumped. The deuterium recycling RD < 1 except at the onset of RF pulse (1 < RD < 1.1) for t = 0.75 s, indicating modest desorption. This provides the density control. Except for the lowest densities, Zeff = 2 at PRF = 10 MW is routinely achieved. At At comparable densities, PRAD/PTOT is 30-50% lower. If we assume that the long term evolution of the beryllium signal at the antenna screen is due to erosion, rather than coverage by carbon, we estimate the corresponding influx in monopole configuration (k11 = 0 m-1) to be ΦBeSCREEN = 1.16 × 1019 atoms MW-1 s-1 from an effective area AEFF = 0.5 m2. At the same coupling resistance the beryllium influx scales with the RF power of the antenna and with the plasma density. With dipoles (k11 = 7 m-1), the influx is reduced by more than a factor 3. The contribution of antenna influxes to Zeff is small, ΔZeffBe = 0.08 and ΔZeffNi = 0.05 at PRF = 5 MW. © 1990.