Wave-wave resonance instabilities and electron velocity distribution tail structure

The existence and instability of extended suprathermal tails in the electron velocity distribution are fundamental to r.f. heating and current drive schemes. The linear stability against emission of longitudinal plasma waves of flat tails with that of tails which have a bump is contrasted. The wave-wave resonance instabilities are found to have higher growth rates than the wave-particle resonance instabilities associated with a flat tail. They occur for waves of different frequency, and in different regions of wavenumber space. The role of wave-wave resonance instabilities in the evolution of tail distributions with a bump may therefore be significant, and they offer in addition an experimental indicator of tail shape. (author)