Interpretation of ion cyclotron emission from sub‐Alfvénic fusion products in the Tokamak Fusion Test Reactor

Ion cyclotron emission (ICE) has been observed during neutral beam‐heated supershots in the Tokamak Fusion Test Reactor (TFTR) [Phys. Rev. Lett. 72, 3526 (1994)] deuterium–tritium campaign at fusion product cyclotron harmonics. The emission originates from the outer midplane edge plasma, where fusion products initially have an anisotropic velocity distribution, sharply peaked at a sub‐Alfvénic speed. It is shown that the magnetoacoustic cyclotron instability, resulting in the generation of obliquely propagating fast Alfvén waves at fusion product cyclotron harmonics, can occur under such conditions. The time evolution of the growth rate closely follows that of the observed ICE amplitude. Instability is suppressed if the fusion products undergo a moderate degree of thermalization, or are isotropic. In contrast, the super‐Alfvénic fusion products present in the outer midplane of the Joint European Torus (JET) [Nucl. Fusion 33, 1365 (1993)] can drive the instability if they are isotropic or have a broad speed distribution. This may help to account for the observation that fusion product‐driven ICE in JET persists for longer than fusion product‐driven ICE in TFTR supershots.