Superthermal Radiation from Fusion Products in JET

Abstract

Observations of superthermal ion-cyclotron emission from JET Ohmic- and neutral-beam-heated discharges are presented. Previously unobserved narrow-band spectral features correspond in both cases to multiple harmonics and half-harmonics of the proton gyrofrequency at the outer edge of the plasma. In this region, fusion products with large radial excursions and injected fast ions produce anisotropic velocity distributions with positive perpendicular gradient. It is discussed how these may relax by maser action, giving rise to the observed localized radiation.