The existence and instability of extended suprathermal tails in the electron velocity distribution are fundamental to RF heating and current drive schemes. The authors contrast the linear stability against emission of longitudinal plasma waves of flat tails with that of tails which have a bump. Tails with a bump support additional wave modes, whose wave-wave resonance instabilities have already been pointed out. Previous authors have ignored the role of these instabilities in the evolution of the tail distribution. The authors examine this topic, and relate it to negative-energy wave considerations. 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.
Mach cone structure generation by the interaction of a short microwave pulse with a plasma