Experiments on ICRF Coupling with Different Phasings

High power density ICRF launchers will be needed for ITER and, after the end of the 2004 and 2005 campaigns on JET, an ITER-like ICRF launcher will be installed and tested in JET. The reference design coupling performance (dipole phasing) may be improved if the loss of heating efficiency for the launcher in toroidal monopole phasing is reduced. In older measurements of plasma energy content vs. input power at JET, practically no hydrogen minority heating was observed for monopole with the present A2 antennae [1], although the coupling resistance was better in monopole than in dipole. The unaccounted coupled power in monopole is likely deposited through parasitic absorption in the enhanced sheaths. To further investigate the phasing dependence of heating ef ficiency with the JET A2 antennae, L-mode coupling experiments were conducted varying input power and its modulation, antenna-plasma distance, plasma configuration, number of active antennas, and phasing. Comparative monopole/dipole 42 MHz ICRF power ramp-up (< 8 MW) discharges at B=2.7T with a hydrogen minority scenario for two plasma currents I=2.8 MA (so called standard flux expansion SFE configuration) and I=2 MA (diagnostics optimized DOC configuration) are shown in Fig.1. a) b)