Optimization of surface plasma sources for efficient production of negative ions with high emission current density

Abstract

The main mechanism of negative ion formation in a Surface Plasma Source (SPS) is secondary emission of sputtered and scattered particles accompanied by capture of electrons from the electrodes. In the first, pulsed, versions of the SPS, adding a small amount of cesium increased the emission current density for light ions up to 3.7 A/cm 2 with a flat emitter and up to 8 A/cm 2 after optimization of geometrical focusing. Since this power density was too high for DC operation, LBL developed a large volume SPS with a hot cathode discharge, a large emitter-emission aperture gap and low emission current density. The LBL type of SPS was used for some accelerators and for heavy negative ion production with emission current density of 10 mA/cm 2. Researchers at Budker Institute of Nuclear Physics (BINP) developed a small SPS optimized for long time DC operation. In the BINP source, DC Hcurrent up to 2.5 mA and heavy ion current up to 1 mA have been extracted from a 1 mm diameter aperture using an improved SPS that employs a hollow cathode discharge and spherical focusing of negative ions toward the emission aperture. This paper describes further optimization of this type source for production of negative ion beam with a high emission current density.