In the course of the Preliminary Tritium Experiment in JET, where combined deuterium and tritium neutral beam injection generated a DT fusion power of 1.7 MW, ion cyclotron emission (ICE) was measured in the frequency range Y I 180 MHz. The ICE spectra contain superthermal, narrow, equally spaced emission lines, which correspond to successive cyclotron harmonics of deuterons or alpha particles at the outer midplane, close to the last closed flux surface at major radius R -4.0 m. Above about 100 MHz the lines merge into a relatively intense continuum. The ICE signal fluctuates rapidly in time, and is extinguished whenever a large amplitude edge localized mode (ELM) occurs.In pure deuterium and mixed DT discharges ICE spectra are 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 proportion to the increased neutron flux: this indicates that fusion alpha particlesand not beam ionsprovide the free energy to generate ICE. The JET ICE database, which now extends over a range of six decades in signal intensity, shows that the time averaged ICE power increases 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 reaching the plasma edge. Calculations reveal a class of fusion products, born in the core, which make orbital excursions of sufficient size to reach the outer midplane edge. There, the velocity distribution has a ring structure, which is found to be linearly unstable to relaxation to obliquely propagating waves on the fast Alfv6n-ion Bernstein branch at all ion cyclotron harmonics. The paper shows how ICE provides a unique diagnostic for fusion alpha particles.

The theory of the magnetoacoustic cyclotron instability, which has been proposed as a mechanism for suprathermal ion cyclotron harmonic emission observed in large tokamaks, is generalized to include finite parallel wave number k∥. This extension introduces significant new physics: the obliquely propagating fast Alfvén wave can undergo cyclotron resonant interactions with thermal and fusion ions, which affects the instability driving and damping mechanisms. The velocity–space distribution of the fusion ions is modeled by a drifting ring, which approximates the distribution calculated for the emitting region in tritium experiments on the Joint European Torus (JET) [Cottrell et al., Nucl. Fusion 33, 1365 (1993)]. Linear instability can occur simultaneously at the fusion ion cyclotron frequency and all its harmonics when the fusion ion concentration is extremely low, because the finite k∥ gives rise to a Doppler shift, which decouples cyclotron damping due to thermal ions from wave growth associated with fusion ions. Doppler shifts associated with finite k∥ may also be related to the observed splitting of harmonic emission lines.

Ion cyclotron emission ͑ICE͒ has been observed during neutral beam-heated supershots in the Tokamak Fusion Test Reactor ͑TFTR͒ ͓Phys. Rev. Lett. 72, 3526 ͑1994͔͒ deuterium-tritium campaign at fusion product cyclotron harmonics. The emission originates from the outer midplane edge plasma, where fusion products initially have an anisotropic velocity distribution, sharply peaked at a sub-Alfvénic speed. It is shown that the magnetoacoustic cyclotron instability, resulting in the generation of obliquely propagating fast Alfvén waves at fusion product cyclotron harmonics, can occur under such conditions. The time evolution of the growth rate closely follows that of the observed ICE amplitude. Instability is suppressed if the fusion products undergo a moderate degree of thermalization, or are isotropic. In contrast, the super-Alfvénic fusion products present in the outer midplane of the Joint European Torus ͑JET͒ ͓Nucl. Fusion 33, 1365 ͑1993͔͒ can drive the instability if they are isotropic or have a broad speed distribution. This may help to account for the observation that fusion product-driven ICE in JET persists for longer than fusion product-driven ICE in TFTR supershots. ͓S1070-664X͑96͒01002-1͔

Ion cyclotron emission (ICE) has been observed during D-T discharges in the Tokamak Fusion Test Reactor (TFTR), using RF probes located near the top and the bottom of the vacuum vessel. Harmonics of the alpha cyclotron frequency ( Omega a,) evaluated at the outer midplane plasma edge are observed at the onset of the beam injection phase of TFTR supershots and persist for approximately 100-250 ms. These results are in contrast with observations of ICE in JET, in which harmonics of Omega a evolve with the alpha population in the plasma edge. Such differences are believed to be due to the fact that newly born fusion alpha particles are super-Alfvenic near the edge of JET plasmas, while they are sub-Alfvenic near the edge of TFTR supershot plasmas. In TFTR discharges with edge densities such that newly born alpha particles are super-Alfvenic, alpha cyclotron harmonics are observed to persist. These results are in qualitative agreement with numerical calculations of growth rates due to the magnetoacoustic cyclotron instability