Experimental determination of the electric field and charge distribution in magnetically insulated ion diodes

Abstract

We have measured the electric field distribution across the high-voltage gap of a magnetically insulated intense ion beam diode by observing the Stark shift of line emission from ions accelerating in the gap. The measured distribution yielded the time dependence of the actual diode gap. We observed rapid gap closure early in time resulting from expansion of the electric-field-excluding electrode plasmas in the diode. This can contribute substantially to the previously observed ion-current-density enhancements over the values calculated based upon the mechanical diode gap. Assuming planar geometry the electric field distribution was used to determine the electron density across the diode gap. This was compared with one-dimensional Brillouin-flow model calculations using the actual acceleration gap. The electrons were observed to spread towards the anode beyond the region of the theoretical electron sheath. The electron presence near the anode was further enhanced when a cathode vane was used. In addition, an increase in the total electron number in the gap relative to analytic theory was inferred. This serves to explain the enhancement of the measured ion current density over the one calculated with use of the actual diode gap that we observed for the configurations tested.