Gamma ray emission profile measurements from JET ICRF-heated discharges

Abstract

Helium 3 minority fuel ions have been observed to be accelerated to megaelectron-volt (MeV) energies by ion cyclotron radiofrequency (ICRF) heating. These energetic ions undergo nuclear reactions with impurity 9Be and 12C ions in the plasma, and characteristic gamma radiation is emitted. For special circumstances, this gamma ray emission has been detected with a 19 channel profile monitor constructed primarily for neutron measurements. Two-dimensional (2-D) profiles of the gamma radiation emission have been derived by tomographic reconstruction; these profiles correspond to a cross-section weighted density of 3He ions having energies of 2 or more MeV. As expected for ICRF heating, the observed spatial profile of the gamma radiation shows that access by the accelerated ions to the plasma volume on the high field side of the RF resonance layer is effectively inhibited. In addition, the profile appears to demand the presence of a group of passing particles that are localized in poloidal extent and are circulating some 15 cm to the low field side of the resonance layer. Apart from the 15 cm displacement, these features are well reproduced by a model that combines a 2-D bounce averaged Fokker-Planck calculation with an orbit-following code. A unique example of a gamma ray spatial profile captured at the time of a sawtooth crash shows that the profile is relatively unaffected apart from the sudden obliteration of the passing group, for which the ions are found to possess relatively high energy, presumably gained from the RF waves through the Doppler shift effect. On the basis of these observations, an explanation is proposed for the selective expulsion of RF heated fast ions from the central region of the plasma to the outer regions