Density depletion and double layer formation in a current-carrying plasma jet

The Caltech plasma jet experiment produces an MHD-driven, current-carrying laboratory plasma jet with an initial radius of a few centimeters, a length increasing to roughly 40 cm, and a nominal electron temperature of 2eV. >6 keV X-ray and 20-60 eV extreme ultraviolet radiation have been observed simultaneously when the cross-section of the plasma jet is choked by Rayleigh-Taylor instability. Visible light images of the plasma jet show that the choked region is likely of low density. Based on these observations, an evacuation instability that can lead to density depletion and the formation of a double layer is proposed. This instability becomes active when the electron drift velocity surpasses the electron thermal velocity. A double layer is formed when a current-aligned, resistive electric field is enhanced due to a reduction in the plasma jet’s cross-sectional area. Charged particles can be accelerated by the enhanced electric field and produce the observed high-energy radiation. This evacuation instability will be compared to the two-stream and ion-acoustic instabilities. Potential applications of this model to particle acceleration in some space plasmas will be discussed.