Hydrogen plasmas with ICRF inverted minority and mode conversion heating regimes in the JET tokamak

During the initial operation of the International Thermonuclear Experimental Reactor (ITER), it is envisaged that activation will be minimized by using hydrogen (H) plasmas where the reference ion 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 5 MW 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 (MH) regime was observed at low concentrations (up to 2%).

Energetic tails in the 3He ion distributions were observed with effective temperatures up to 300 keV and bulk 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 bulk electron temperatures up to 8 keV.

Secondly, experiments performed to study D minority ion heating in H plasmas are presented. This MH scheme proved much more difficult since modest quantities of carbon (C) impurity ions, which have the same charge to mass ratio as the D ions, led directly to the mode conversion regime. Finally, numerical simulations to interpret these two sets of experiments are under way and preliminary results are shown.