ION-CYCLOTRON EMISSION MEASUREMENTS DURING JET DEUTERIUM-TRITIUM EXPERIMENTS

In the course of the Preliminary Tritium Experiment in JET, where comb ined deuterium and tritium neutral beam injection generated a DT fusio n power of 1.7 MW, ion cyclotron emission (ICE) was measured in the fr equency range nu less-than-or-equal-to 180 MHz. The ICE spectra contai n superthermal, narrow, equally spaced emission lines, which correspon d to successive cyclotron harmonics of deuterons or alpha particles at the outer midplane, close to the last closed flux surface at major ra dius R approximately 4.0 m. Above about 100 MHz the lines merge into a relatively intense continuum. The ICE signal fluctuates rapidly in ti me, and is extinguished whenever a large amplitude edge localized mode (ELM) occurs. In pure deuterium and mixed DT discharges ICE spectra a re similar in form, but on changing from pure D to mixed D + T neutral beam injection at constant power, the intensity of the ICE rises in p roportion to the increased neutron flux: this indicates that fusion al pha particles - and not beam ions - provide the free energy to generat e ICE. The JET ICE database, which now extends over a range of six dec ades in signal intensity, shows that the time averaged ICE power incre ases almost linearly with total neutron flux. The rise and fall of the neutron flux during a single discharge is closely followed by that of the ICE signal, which is delayed by a time of the order of the fusion product slowing down time. This feature is well modelled by a TRANSP code simulation of the density of deeply trapped fusion products reach ing the plasma edge. Calculations reveal a class of fusion products, b orn in the core, which make orbital excursions of sufficient size to r each the outer midplane edge. There, the velocity distribution has a r ing structure, which is found to be linearly unstable to relaxation to obliquely propagating waves on the fast Alfven-ion Bernstein branch a t all ion cyclotron harmonics. The paper shows how ICE provides a uniq ue diagnostic for fusion alpha particles.