Ion cyclotron emission due to collective instability of fusion products and beam ions in TFTR and JET

Abstract

Ion cyclotron emission (ICE) has been observed from neutral beam heated TFTR, and JET tritium experiments at sequential cyclotron harmonics of both fusion products and beam ions. The emission originates from the outer midplane plasma, where fusion products and beam ions are likely to have a drifting ring-type velocity-space distribution that is anisotropic and sharply peaked. Fusion product driven ICE in both TFTR and JET can be attributed to the magnetoacoustic cyclotron instability, which involves the excitation of obliquely propagating waves on the fast Alfven/ion Bernstein branch at cyclotron harmonics of the fusion products. Differences between ICE observations in JET and TFTR appear to reflect the sensitivity of the instability growth rate to the ratio vbirth/cA where vbirth is the fusion product birth speed and cA is the local Alfven speed for fusion products in the outer midplane edge of TFTR supershots, vbirth < cA for alpha particles in the outer midplane edge of JET, the opposite inequality applies. If sub-Alfvenic fusion products are isotropic or have undergone even a moderate degree of thermalization, the magnetoacoustic instability cannot occur. In contrast, the super-Alfvenic alpha particles that are present in the outer midplane of JET can drive the magnetoacoustic cyclotron instability even if they are isotropic or have a relatively broad distribution of speeds. These conclusions may account for the observation that fusion product driven ICE in JET persists for longer than fusion product driven ICE in TFTR. Moreover, the time evolution of the maximum growth rate, obtained using the Sigmar model for the alpha particle distribution and TFTR data for the fusion product source rate, closely follows the observed time evolution of the ICE amplitude in TFTR supershot discharges. Other observed features of fusion product driven ICE that match the linear instability include the scaling with fusion product density, doublet splitting of spectral peaks, the relative strength of certain harmonics and source localization. A separate mechanism is proposed forthe excitation of beam driven ICE in TFTR: electrostatic ion cyclotron harmonic waves, supported by stronglysub-Alfvénic beam ions, can be destabilized by a low concentration of such ions with a very narrow spread ofvelocities in the parallel direction. Sufficiently narrow distributions are likely to exist in the edge plasma, close tothe point of beam injection.