ION-CYCLOTRON EMISSION - A NATURAL DIAGNOSTIC FOR FUSION ALPHA-PARTICLES

There exist strong observational links between ion cyclotron emission (ICE) and fusion reactivity in tokamak plasmas. These links originally emerged from deuterium discharges in the Joint European Torus (JET) a nd were demonstrated most recently in the Preliminary Tritium Experime nt. They include the proportionality of ICE intensity to measured fusi on reactivity over six decades in signal intensity correlations in the time evolution of the ICE signal and neutron flux during discharges; the matching of the spectral peak frequencies to successive local ion cyclotron harmonics at the outer midplane edge; and correlations betwe en ICE and the observed impact of magnetohydrodynamic activity, such a s sawteeth and edge-localized modes, on energetic ions. The observatio ns are broadly consistent with the excitation of the fast Alfven wave through cyclotron resonance with the local non-Maxwellian fusion produ ct population -the so-called magnetoacoustic cyclotron instability. Th e theory of this instability is extended to the regime of arbitrary k( parallel-to), in which it is necessary to include both wave-particle c yclotron damping and the positive-energy loading due to resonant cyclo tron harmonic waves supported by the thermal ions. The consequences of arbitrary k(parallel-to) for the instability thresholds are described . An outline is given of the close similarities between ICE from tokam aks and signals at multiple ion cyclotron harmonics observed in the Ea rth's magnetosphere, which apparently originate from regions where the re is a ring-type population of energetic protons. This emission also appears to be explicable in terms of the magnetoacoustic cyclotron ins tability, and comparison with tokamak observations yields information on the distinction between features generic to the emission mechanism and those specific to particular magnetic geometries.