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Abstract
Ion cyclotron emission (ICE), characterized by a spectrum of discrete harmonics of the ion cyclotron frequency evaluated at a particular magnetic field strength, is commonly observed in fusion plasma devices. In the Joint European Torus (JET), ICE power varies linearly with the fusion alpha particle population in the plasma edge, suggesting that ICE may serve as a useful alpha particle diagnostic. Here, we use a system of wire loop probes to monitor the behavior of ICE in the Tokamak Fusion Test Reactor (TFTR). ICE in TFTR exhibits more complicated behavior than in JET. The harmonic structure varies with plasma species, and typical discharges exhibit two distinct classes of emission--one occurring transiently after the onset of neutral beam injection (NBI), and the other rising more slowly and persisting throughout NBI. Correlations with the fusion rate are observed, but the dependence is less straightforward than in JET.
To understand the differences between TFTR and JET observations, we review the theory of the magnetoacoustic cyclotron instability (MCI). We then consider the theory of compressional Alfven eigenmodes (CAEs), which describes the same physics using a global approach. A numerical investigation of CAE behavior demonstrates that in TFTR, ICE behavior should depend sensitively on many quantities, including the Alfven speed near the plasma edge and the shape of the fusion product velocity distribution, while in JET, ICE behavior should depend primarily on the size of the alpha particle population. Dissimilar observations of ICE in TFTR and JET are thus consistent with a single theory, which predicts that the behavior of ICE will depend strongly on the ratio of the alpha particle birth velocity to the local Alfven speed. When alpha particles are super-Alfvenic near the plasma edge, ICE power will vary primarily with the alpha particle density, and will thus provide a simple method of monitoring this population. However, if the alpha particles are sub-Alfvenic, then ICE growth rates will be a sensitive function of the shape of the alpha particle distribution function. In this case, it is difficult to extract useful diagnostic information from the observed emission.
