A scheme for the energy recovery of the charged beam fraction in neutral injectors

For neutral injectors, working at higher energies, the equilibrium fraction of fast neutrals, after the passage of the ion beam through the neutralizer, falls to a low value and the recovery of the energy contained in the charged beam fraction is important, both for efficiency and costs reasons. A particular scheme, based on electrostatic deceleration of the residual ions is described. This scheme is particularly attractive for its compactness and simplicity and can be extrapolated to megawatts proton beams at 80 kV energy. The ion source is operated at ground potential, the neutralizer is biased at the negative high voltage, corresponding to the neutral beam energy. The expulsion of electrons from the neutralizer plasma is suppressed by means of a cylindrical grid around the beam, behind the neutralizer. This grid is biased negative compared to the neutralizer in order to realize a potential barrier for the electrons. The decelerated ions can be captured on an electrode around the beam. Experimental results on the operation of the suppressor grid, with a 17 A beam at 30 kV are reported