Measurement of growth rate, non-linear saturation coefficients, and mode-mode coupling coefficients of a `Van der Pol' plasma instability

Results are presented which show that a self-excited ion-sound instability in a plasma behaves in a similar manner to a classical Van der Pol oscillator, both ( a ) in a single mode régime, and ( b ) in a multi-mode régime. In the single mode régime various methods are described which enable the linear growth rate α 0 to be measured. These are checked by direct measurement and a consistent value is obtained. In the multi-mode régime, values are obtained for mode-mode coupling coefficients, and non-linear saturation coefficients for each mode present in the system, as well as for the parameters associated with the growth rates of these modes. Comparison with a theory developed to describe these two systems, shows good agreement with the experimental measurements.