Observation of nonthermal turbulent electric fields in a nanosecond plasma opening switch experiment

Abstract

Nonthermal turbulent electric fields due to plasma instabilities were studied in a 100-ns duration plasma opening switch using observations of hydrogen line spectral profiles. The ${\mathrm{H}}_{\ensuremath{\alpha}}$ and ${\mathrm{H}}_{\ensuremath{\beta}}$ widths were seen to rise by 2--3 times during the current pulse, shown to result from the presence of nonthermal electric fields in the plasma. The spectral profiles are analyzed using two recently developed methods based on short and intermediate time behaviors of the line profile Fourier transforms. One method gives the mean amplitude of the nonthermal fields with no dependence on their frequencies. The second method uses calculations of the autocorrelation functions for various field amplitudes and frequencies to yield bounds on these two parameters. The field amplitude is determined to be 14.5\ifmmode\pm\else\textpm\fi{}2.5 kV/cm, and the fluctuation frequency is found to be of the order of the electron plasma frequency. Based on their high frequency, the oscillations probably result from Langmuir waves, driven by the voltage drop on the plasma opening switch (POS). The waves have no significant effect on the POS operation, since they do not give rise to anomalous resistivity, and therefore have no effect on the magnetic-field evolution. We obtain an upper limit for the amplitude of possible low-frequency fields (ion-acoustic waves), that may give rise to anomalous resistivity, and estimate the resulting diffusion velocity and current channel width. Both quantities are found to be much lower than the values observed in the experiment, and the low-frequency field amplitude is much lower than the saturation limit predicted by previous theoretical treatments. This implies that in our experiment possible low-frequency waves have little influence on the magnetic-field distribution.