Alumino-Silicate Ion Sources for Accelerator Applications

As part of the program of Heavy Ion Fusion Accelerator Research at the Lawrence Berkeley Laboratory, ion sources have been developed using thermionic emitters of singly charged alkali metal ions. These emitters are flat surfaces of alumino-silicate, loaded with the apropriate ion. They have become convenient and reliable sources producing pulsed beams of very low emittance. Thermionic emission of ions from alumino-silicates has been known for a very long time [1]. Here we focus on the practical application as accelerator ion sources. We discuss the fabrication and heating of large area emitters, uniformity of emission and the maximum ion current density which can be extracted under space charge limited conditions, with zero electric field on the emitter surface. Results are presented for Na, K and Cs ions showing maximum space charge limited current densities of 25, 40 and 120 mAcm-2 respectively. In the case of cesium we have produced a 5 mA beam at a kinetic energy of 200 keV with normalized emittance 1.2×l0-7 ¿ m rad.