Low particle confinement H-mode observed during ICRF heating on JET

Abstract

During H-mode experiments with ion cyclotron resonance frequency heating on JET it was observed that the influxes of deuterium and of impurities can substantially modify the quality of both energy and particle confinement. In particular, a low particle confinement H-mode (LPCH-mode) was triggered either by deuterium input into the X-point region or as a result of a moderate impurity influx into the plasma boundary. In the LPCH-mode the particle confinement is at least three times less than in a normal H-mode. The electron density, the deuterium density and the radiated power are also less than in a normal H-mode. However, the deuterium dilution reaches a steady state. The approximately 20% decrease in the plasma energy content is due to the drop in density. The H-mode duration is extended to 2.8 s by the LPCH-phase and is limited only by the length of the heating pulse. The LPCH-mode particle confinement is linked to L-mode-like particle diffusion within the radius 0.5 < ρ < 0.8 and to the H-mode-like barrier due to the large ratio of the inward convective velocity to diffusion coefficient in the plasma boundary. This mode has the potential for substantially extending the H-mode phase to make it more reactor relevant