ALFVENIC BEHAVIOR OF ALPHA-PARTICLE DRIVEN ION-CYCLOTRON EMISSION IN TFTR

Ion cyclotron emission (ICE) has been observed during D-T discharges i n the Tokamak Fusion Test Reactor (TFTR), using RF probes located near the top and the bottom of the vacuum vessel. Harmonics of the alpha c yclotron frequency (Omega(alpha)) evaluated at the outer midplane plas ma 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 Om ega(alpha) evolve with the alpha population in the plasma edge. Such d ifferences are believed to be due to the fact that newly born fusion a lpha 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 a re 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.