Experimental investigation of ICRF heating scenarios for ITER's halffield Hydrogen phase performed in JET

Two ICRF heating schemes proposed for the half-field operation phase of ITER in Hydrogen plasmas - fundamental H majority and 2nd harmonic 3He ICRF heating - were recently investigated in the JET tokamak. Although the same magnetic field and RF frequencies (B0=2.65T, f=42MHz and f=52MHz, respectively) were used, the density and particularly the plasma temperature foreseen for the initial phase of ITER could not be matched in JET. Modest heating efficiencies (h<0.4) with typically dominant electron heating were found for both scenarios and strong plasma-wall interaction manifested by large impurity content and high radiation losses was observed. This effect is stronger for the 3He ICRF heating case than for the H majority heating case. It was verified that concentrations as high as X[3He]≈20% are necessary to observe significant ion heating in the 2nd harmonic 3He ICRF heating scheme. ICRF accelerated ions up to 50keV in the fundamental H heating experiments and up to 200keV in the 2nd harmonic 3He ICRF heating experiments were detected with NPA diagnostics. While hints of an increase in the heating efficiency as function of the plasma temperature were observed in the H majority case, the He3 concentration was the main ‘handle’ for enhancing the heating efficiency in the 2nd harmonic 3He heating scenario.