Conceptual study of a purely electrostatic energy recovery system for negative-ion-based neutral beam injectors

Abstract

We have conducted a conceptual study of a new energy recovery system for negative-ion-based neutral beam injectors. It could be used on deuterium beams with energies around 1 MeV. It is a purely electrostatic system based on the following principles: (i) the residual positive and negative ions are electrostatically separated; (ii) the electrostatic deflector is made of baffles in order to reduce the gas pressure and trap the secondary electrons by an electric field; (iii) the negative ions are decelerated and the positive ions are dumped at full energy or eventually decelerated; (iv) the decelerated negative ions are collected on a composite electrode acting as an electrostatic trap for secondary electrons. This system might be rather simple to operate and cheap, since no additional power supplies would be required. Narrow beams, with a width around 0.1 m, are more favourable for electrostatic deflection, for differential pumping and also with respect to current density limitations. Calculations show that current density limits should appear only at the level of 150 Am−2 of decelerated current in the case of 1 MeV D− beams with 0.1 m width. The unneutralised D− ion current would be recovered with an efficiency of about 90%, provided the problem of recycled gas evacuation is very carefully treated. In these conditions, and if a low neutraliser gas target of 6 × 1015 molcm−2 is used, energy recovery might yield a reduction of 15% on the overall cost of the injector lines.