Parametric excitation of a plasma instability by modulation of its damping coefficient

Results are presented which show that a previously damped ion-sound instability ( omega 0 ) can be parametrically excited in a plasma by modulating the damping coefficient at a frequency omega 1 approximately=2 omega 0 . In this case, the damping coefficient is varied between a positive value and a negative value, and, in fact, this coefficient must be negative for some fraction of a cycle, for parametric excitation to occur. It is found that a certain threshold modulation depth, h, is required before parametric excitation occurs. At larger values of h, a classical symmetrical parametric resonance curve (instability amplitude versus frequency) is observed. However, at very large values of h, non-linear behaviour is apparent. In this case, the parametric resonance curve becomes asymmetric and the non-linear phenomena of 'jumps' and its associated hysteresis effect is observed. A theory developed to describe this ion-sound instability under various conditions, is compared with the experimental results, and good qualitative agreement is obtained.