Analysis of grid size and ion temperature effects in retarding field energy analyzers (RFEA)

The self-potential of a high current ion beam may be fairly balanced by a secondary plasma with plasma potential Vp; the retarding field energy analyzer (RFEA) measuring the secondary ion outflow is a promising diagnostic of these plasmas. A detailed analysis of a planar RFEA is here discussed, with a focus on the response (in stationary condition) of the detector to the secondary plasma characteristics, and on the determination of design rules for the parameters of a compact RFEA. First, energy distributions of the secondary ion plasma outflow are discussed, as a function of ion temperature Ti and electron temperature Te. Second, the modulation of grid potential (depending on grid pitch b and wire radius a) is calculated both for 2D and 3D models, reaching a good agreement with accompanying electrostatic simulations. Beam emittance (or temperature) growth and beam diffusion are then discussed, also when input ion energy matches the discrimination voltage Vd; corrections to the usual paraxial dynamics res...