Nonlinear plasma instability effects for subharmonic and harmonic forcing oscillations

Results are presented which show that a marginal ion sound instability in a plasma behaves in a manner similar to that predicted by a Van der Pol type of nonlinear equation including anharmonic terms. Experiments have been performed in which the system is subjected to an external driving force at the fundamental frequency omega 0 , and the subharmonic frequencies, which shows that the instability exhibits characteristics similar to the classical nonlinear phenomena of 'jumps' including the associated hysteresis effect. When the system is driven by a forcing oscillation at the harmonic frequencies 2 omega o or 3 omega o , the instability behaves differently from linear forced resonance, and exhibits a behaviour similar to a parametric oscillator. A theory developed to describe this ion sound instability under these various conditions, is compared with the experimental results, and good agreement is obtained.