This paper presents the results of measurements of space-charge effects on the efficiency of a direct converter. The device consists of a 22-stage, electrostatic periodic-focusing, direct energy converter, a magnetic expander, and a hydrogen plasma source. At low beam density, the measured average efficiency is 86.5 ± 1.5% as compared with the predicted one of 88.6 ± 1.5%. At higher beam density, the measured efficiency decreases with increasing space charge in agreement with predictions. The effect of space charge is increased if the ion energy is decreased or if the energy distribution is made narrower. Design criteria for scaling the recovery system for a reactor show that this direct converter is probably economical only if the mean ion energy is greater than 500 keV. – In addition to presenting the space-charge measurements, this paper is intended to serve as a final summary of the authors' work on this type of direct converter.
MHD gas turbine energy conversion for mirror fusion reactors