Expanding the operating space of ICRF on JET with a view to ITER

This paper reports on ITER-relevant ICRF physics investigated on JET in 2003 and early 2004. Minority heating of helium three in hydrogen plasmas (He)H was systematically explored by varying the He concentration and the toroidal phasing of the antenna arrays. The best heating performance (a maximum electron temperature of 6.2keV with 5MW of ICRF power) was obtained with a preferential wave launch in the direction of the plasma current. A clear experimental demonstration was made of the sharp and reproducible transition to the mode conversion heating regime when the He concentration increases above ~2%. In the latter regime the best heating performance (a maximum electron temperature of 8keV with 5MW of ICRF power) was achieved with dipole array phasing, i.e. a symmetric antenna power spectrum. Minority heating of deuterium in hydrogen plasmas (D)H was also investigated but was found inaccessible, because this scenario is too sensitive to impurity ions with Z/A=1/2 such as C, small amounts of which directly lead into the mode conversion regime. Minority heating of up to 3% of tritium in deuterium plasmas was systematically investigated during the JET Trace Tritium experimental campaign (TTE). This required operating JET at its highest possible magnetic field (3.9 to 4T) and the ICRF system at its lowest frequency (23MHz). The interest of this scenario for ICRF heating at these low concentrations and its efficiency at boosting the suprathermal neutron yield were confirmed, and the measured neutron and gammay ray spectra permit interesting comparisons with advanced ICRF code simulations. Investigations of finite Larmor radius effects on the RF-induced high-energy tails during second harmonic (ω=2 ωc) heating of a hydrogen minority in D plasmas clearly demonstrated a strong decrease of the RF diffusion coefficient at proton energies ~1MeV, in agreement with theoretical