Hydrogen Plasmas with ICRF Inverted Minority Heating and Mode Conversion Regimes in the JET Tokamak

During the initial operation of the International Thermonuclear Experimental Reactor (ITER), it is envisaged that activation will be minimised by using hydrogen (H) plasmas where the reference Iion Cyclotron Resonance Frequency (ICRF) heating scenarios rely on minority species such as Helium (3He) or Deuterium (D). This paper firstly describes experiments dedicated to the study of 3He heating in H plasmas with a sequence of discharges in which 5MW of ICRF power was reliably coupled and the 3He concentration, controlled in real-time, was varied from below 1% up to 10%. The minority heating regime was observed at low concentrations (up to 2%). Energetic tails in the 3He distribution were observed with effective temperatures up to 300keV and bulk electron temperatures up to 6keV. 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 bulk electron temperatures up to 8keV. Secondly, the use of D minority ion heating in H plasmas was also investigated.This proved much more difficult since modest quantities of carbon C impurity ions, which have the same charge to mass ratio than the D ions, led directly to the mode conversion regime. Finally, numerical simulations with the 2-D full-wave code CYRANO, were used to further interpret some of the results obtained in these two sets of experiments.
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