ICRF heating for the non-activated phase of ITER: From inverted minority to mode conversion regime

In the initial phase of ITER H plasmas will be used in order to avoid activating the machine. The reference ICRF healing scenarios rely on minority species such as Helium (³He) or deuterium (D). These schemes' distinctive feature comes from the presence of the fast magnetosonic wave ion-ion hybrid resonance/cut-off pair, between the antennas and the minority cyclotron layer. In order to document these unusual heating schemes, ICRF experiments were carried out recently on JET. First, the use of ³He ions in H plasmas was investigated with a sequence of discharges in which 5 MW of ICRF power was coupled to the plasma and the ³He concentration was varied from below 1% up to 10%. The inverted minority heating regime was observed at low concentrations (up to ∼2%). Energetic tails in the ³He distribution were observed with effective temperatures up to 300 keV and central electron temperatures up to 6 keV. At around 2%, a sudden transition was reproducibly observed to the mode conversion regime, in which the ICRF fast wave couples to short wavelength modes, leading to efficient direct electron heating and central electron temperature up to 8 keV. All these experiments systematically used power modulation techniques to assess the radial profiles of the wave absorption by the electrons. Secondly, experiments to study the ICRF heating of D minority ions in H were performed. This heating scheme proved much more difficult since modest quantities of C⁶⁺ impurity, which has the same Z/A ratio than the D minority ions, led us directly into the mode conversion regime. This effect preventing any absorption by D ions at minority cyclotron layer, could make the (D)H scenario not suitable for the non-active phase of ITER.