An investigation of the statistical properties of ion acoustic turbulence

The statistical properties of ion acoustic turbulence in a helium DC discharge are investigated by means of the first order probability density function of the fluctuating potential as detected by a Langmuir probe. Measurements are reported for several pressures in the range of 0.050 to 0.200 torr. At all of the pressures investigated significant deviations from normal statistics are found for a range of discharge currents near the threshold of the ion acoustic instability. As the current is increased, the fluctuations eventually become gaussian. Higher pressures result in less pronounced deviations from gaussian behavior, as well as a reduced range of currents over which the turbulence is non-gaussian. The non-normal behavior seems to be connected with changes in the spectral density of the turbulent waves, a fact which suggest energy transfer between modes associated with wave-wave coupling. A strong dependence of the measured probability densities on the DC bias of the Langmuir probe used to detect the fluctuations 1S found to be caused by the generation of harmonics of the ion wave signal by the probe itself. The nonlinearity of the probe response which gives rise to the harmonic generation is due to a modulation of the probe-to-plasma potential caused by the turbulent wave fields.U of I OnlyThesi