PIG Sources Utilizing Sputtering for Negative Ion Formation

Abstract

We describe recent measurements which give information about the formation of negative ion beams from the cathode material in SPIGS. We also report emittance measurements for SPIGS and the ANIS source of Andersen and Tykesson. In SPIGS the mechanism of negative ion formation from the cathode material is direct Cs-assisted sputtering of negative ions into the discharge followed by thermalization in the plasma before extraction. We find a correlation between the negative ion yield of SPIGS and the electron affinity (e.a) of the negative ion (the larger the e.a., the larger the yield). Correlation of sputtered negative ion output with electron affinity is also observed for ANIS and the Middleton and Adams surface ionization source. In SPIGS the low electron affinity negative ions are attenuated in the plasma more than the high electron affinity ions. Thus, SPIGS has lower output for the low e.a. ions than ANIS and the surface ionization source. Emittance measurements of the Cu- beam from SPIGS and ANIS indicate that both sources have two-dimensional emittance ~5 mm mrad MeV1/2 for 90% of the total beam. Thus ANIS has higher brightness than SPIGS for the low electron affinity negative ions.