Alfvenic behaviour of alpha particle driven ion cyclotron emission in TFTR

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 (Ωα) 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 Ωα 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. (author). 11 refs, 9 figs