Observation of nonthermal turbulent electric fields in a nanosecond plasmaopening switch experiment

Nonthermal turbulent electric fields due to plasma instabilities were studi ed in a 100-ns duration plasma opening switch using observations of hydroge n line spectral profiles. The H-alpha and H-beta widths were seen to rise b y 2-3 times during the current pulse, shown to result from the presence of nonthermal electric fields in the plasma. The spectral profiles are analyze d using two recently developed methods based on short and intermediate time behaviors of the line profile Fourier transforms. One method gives the mea n amplitude of the nonthermal fields with no dependence on their frequencie s. The second method uses calculations of the autocorrelation functions for various field amplitudes and frequencies to yield bounds on these two para meters. The field amplitude is determined to be 14.5+/-2.5 kV/cm, and the f luctuation frequency is found to be of the order of the electron plasma fre quency. Based on their high frequency, the oscillations probably result fro m 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 o f possible low-frequency fields (ion-acoustic waves), that may give rise to anomalous resistivity, and estimate the resulting diffusion velocity and c urrent channel width. Both quantities are found to be much lower than the v alues observed in the experiment, and the low-frequency field amplitude is much lower than the saturation limit predicted by previous theoretical trea tments. This implies that in our experiment possible low-frequency waves ha ve little influence on the magnetic-field distribution. [S1063-651X(99)0890 1-1].