Energy recovery from mixed H- /H0 /H+ beams and collector simulations

An axisymmetric system to recover beam energy from partially neutralized H- beams was recently proposed, for a given beam acceleration voltage Vs [1]. In the case of ion source NIO1 Vs may range from 20 to 60 kV. A first nominal H- beam for simulation was assumed having 60 keV and 20 mA with a 4 mm radius and 0 mrad divergence. A second more realistic beam with 3 mrad divergence, and a composition of 25 : 50 : 25 of H- , H0 and H+ has also been considered. The collector works by decelerating the H- ions (into a system similar to a Faraday cup provided with an exit hole electrode), so that they are radially deflected by space charge and anode lens effects, and collected to a low kinetic energy Kc (less than 1 keV), while neutral and H+ ions can pass through the exit hole electrode. A following collector can recover H+ energy. Several tools were compared for simulations, from commercial programs to user prepared codes, which are challenging for highly nonlinear problem and for a possible (numerically unstable) virtual cathode phenomena; stabilization techniques are compared. Limits for local perveance are discussed. Also mesh asymmetry effects and the related transverse oscillations of H + beam may be observed. Efficiency over 90 % can be reached in typical conditions. The secondary yield (which is low thanks to low impact energy Kc and Faraday cup concept) is estimated.