Plasma and Antenna Coupling Characterization in ICRF-Wall Conditioning Experiments
Ion Cyclotron Wall Conditioning (ICWC) discharges, in pulsed-mode operation, were carried out in the limiter tokamak TEXTOR and the divertor tokamaks ASDEX Upgrade (AUG) and JET to simulate the scenario of ITER wall conditioning at half-field (TEXTOR, AUG) and full-field (JET). ICWC-plasma and antenna coupling characterization results obtained during the ICRF-Wall Conditioning experiments performed in helium-hydrogen mixture in TEXTOR, AUG and helium-deuterium mixtures in JET are presented here. Safe operational regimes for the experimental parameters could be explored for possible ICWC in ITER at different magnetic fields. Satisfactory antenna coupling in the Mode Conversion scenario along with reproducible generation of ICRF plasmas and reliable wall conditioning were achieved by coupling RF power from one or two ICRF antennas at single (TEXTOR) and at two (AUG, JET) different ICR frequencies, in agreement with the predictions of 1-D TOMCAT code. The plasma breakdown results obtained in the TEXTOR tokamak have been compared with the predictions of a 0-D RF plasma production model. The present study of ICWC emphasizes on the beneficial effect of application of an additional (along with toroidal magnetic field) stationary vertical (BV<<BT) or oscillating poloidal magnetic field (BP<<BT) on antenna coupling and plasma parameters.