Particle trapping in carbon walls during ICRH heating in Tore Supra

The role of high-energy charge exchange (CX) particles during radio-frequency (RF) heating is examined in an attempt to develop a predictive model for wall saturation and desaturation processes. Tore Supra discharges in which an apparent saturated wall, previously obtained through a succession of ohmic discharges, was transformed into a pumping wall with the application of RF heating are analyzed. Direct measurements show the increased mean energy of the CX spectrum upon application of RF heating. This leads to deeper CX neutrals penetration reaching the remaining a:C–D unsaturated layer and restoring the wall pumping efficiency. The conversion from a saturated to a pumping state with the application of RF heating, and the time scales for saturation during the ohmic and the heating phases, are reasonably well explained by several existing models. The deeper particle wall penetration observed during RF plasma experiments points to a potential mechanism which can increase tritium trapping in a fusion reactor.