Higher order resonances in a plasma

Resonances of a plasma driven into a weakly non-linear régime are investigated by two different models.

(1) A cold plasma driven by an extended wave source is considered. It is found that resonances can occur at fractions as well as multiples of the normal mode resonances. For the fractional resonances the response amplitude grows linearly with time, whereas for the harmonic resonances it grows exponentially with time.

(2) An unmagnetized Vlasov plasma is excited by a two-grid source below the plasma frequency. A resonance is shown to occur when the driving frequency approaches one half the plasma frequency. It is argued that such non-linear resonances may be responsible for the Alouette II observations of resonances at fractions of the plasma frequency.