EFFICIENT ION-BEAM EXTRACTION FROM A FLOWING PLASMA

A moving plasma with directed flow velocities v larger than the ion acoustic speed cs is used as a source of high-current ion beams in the 10-20-keV range of energies. Current densities up to 3 A/cm2 are obtained at the plasma boundary which is an order of magnitude larger than the limiting value of Bohm current in a stationary plasma. The observed current densities were proportional to v, unlike the Bohm current densities which are proportional to ion acoustic speed. Total ion currents up to ∼100 A were extracted from the plasma through a two electrode extraction system. Simple geometric shaping of the electrodes enabled an 8-cm-diam beam to be focused to ∼7 mm.