Experimental results of energy recovery on a neutral beam injector

Abstract

A neutral beam injector, based on the concept of energy recovery by electrostatic deceleration of the un-neutralized beam fraction, has been designed and operated. A new type of electrode, the STEM, has been developed for the energy recovery system in view of electron suppression and trapping. The results of a series of experiments with H, D and He beams, up to 70 keV, 16 A of extracted beam, 0.5 s duration, are presented. The recovery of the un-neutralized part of the beam is clearly demonstrated. At least 90% of the full energy residual ions are decelerated and collected at a potential as low as 5% of the accelerating beam potential. Seventy per cent of the secondary electrons emitted by ionic impact are trapped in the STEM electrode. An extrapolation of the energy recovery system to higher energies shows that this new scheme should make the operation of a positive ion based 200 keV deuterium injector with an acceptable efficiency possible.