Efficient energy transfer between fast and slow electron plasma oscillations

Finite amplitude ion fluctuations are shown to efficiently and rapidly couple energy from very high phase velocity electron plasma oscillations into much slower ones and then into the particles, even when the mode coupling is off-resonant. The magnitude of the ion density fluctuation δn/n0 which gives efficient off-resonant coupling is typically the frequency mismatch between the plasma oscillations normalized to the electron plasma frequency. Such modest level ion fluctuations are often present in plasmas or can be selectively excited. This coupling mechanism is particularly important for allowing energetic beams to rapidly transfer energy to a dense background plasma. A simple calculation shows the principal features of the off-resonant mode coupling. The predictions are then confirmed by computer simulations.