Parametric ion-acoustic turbulence in a helicon discharge

The spectra of oscillations are examined in a short helicon source excited at a frequency of 13.56 MHz. The parametric interaction of an rf field with plasma was found to give rise to turbulence in the low-frequency (LF) range about 1 MHz. The excitation of LF oscillations is shown to have thresholds on the rf antenna current (input power) and the do magnetic field. The direction of propagation, phase velocities and correlation characteristics are measured for the components of the LF spectrum. Turbulent pulsations are attributed to the ion-acoustic waves; their amplitudes and spatial profiles are determined. Special measurements are performed and theoretical estimations are made to ascertain the physical origin for turbulence. Computations show that under experimental conditions both strong parametric instability and supersonic do electron flow induced by the ponderomotive force can give rise to the excitation of ion-acoustic waves.