ICRF heating of TFTR plasmas fuelled by deuterium - tritium neutral beam injection

Abstract

Experiments to heat D - T plasmas with ion cyclotron range of frequency (ICRF) waves have been conducted for the first time on the tokamak fusion test reactor (TFTR) (Wilson et al 1995 Phys. Rev. Lett. 75 842). Experiments were performed with full-bore discharges in the low recycling, neutral-beam-heated supershot (Strachan et al 1987 Phys. Rev. Lett. 58 1004) regime with global energy confinement times exceeding twice the empirical L-mode value. Up to 5.9 MW of 43 MHz ICRF power was coupled into plasmas fuelled and heated by 18 - 24 MW of 100 keV D - T neutral beam injection. The fraction of neutral beam power in tritium was varied from 14% to 100% and the toroidal magnetic field was scanned to move the second harmonic tritium layer across the plasma magnetic axis. With the layer on axis, the central ion temperature was increased from approximately 25 to 33 keV when 5.5 MW of ICRF power was added to a plasma fuelled and heated by 13.5 MW of T and 10 MW of D neutral beam injection. Up to 60% of the ICRF power was absorbed via ion heating within the core of a plasma with reactor-relevant parameters. Amplitude-modulated ICRF power was used to measure RF power absorption directly. The results were consistent with models used to predict the performance of ICRF heating scenarios in future machines, such as the international thermonuclear experimental reactor (ITER). Despite extensive plasma conditioning, assisted by neutral beam heating and lithium pellet injection, many discharges were characterized by a degradation in performance and reactivity early in the neutral beam pulse. The degradation resulted from enhanced recycling of impurities and deuterium from the carbon tile limiters. The proximity of the outboard limiter exacerbated attempts to limit this enhanced influx.