Negative Ion Beam Emittance Calculations

Abstract

Computer simulations are commonly used to support design and optimisation of powerful negative ion sources for the needs of future thermonuclear reactors like e. g. ITER. The aim of the paper was to study changes of produced beam quality (described by its emittance and brightness) with the geometry of the extraction system as well as extraction voltage. A two-dimensional Particle-in-Cell (PIC) method based code was applied to model H- ions and electrons extraction from the ion source plasma chamber through the opening with bevelled surface. The root-meansquare emittance of the extracted beam was calculated according to Chasman and Lapostolle approach. Ion beam phase space portraits were also presented to enrich the discussion. Growth of ion (electron) beam emittance was observed both with the increasing radius of the extraction opening and the inclination of its bevelled surfaces. This degradation of beam quality is partially balanced by increasing extracted H- ion current. On the other hand, increasing length of the extraction channel improves the beam quality. It was demonstrated that for wider extraction opening the H- ion beam consist of two parts coming form the two different regions of the chamber. According to calculated beam brightness the optimal wall inclination was found to be near 26o in the studied case. The decrease of the beam emittance saturates for larger channel length values. In the considered case the optimal channel length was h = 1.7 mm. The evolution of ion beam emittance and brightness shows that the best beam quality is achieved for extraction voltages between 0.5 kV and 2 kV.