First Results with a Surface Conversion H[sup −] Ion Source Based on Helicon Wave Mode-Driven Plasma Discharge

Abstract

The currently employed converter‐type negative ion source at Los Alamos Neutron Science Center (LANSCE) is based on cesium enhanced surface production of H− ion beams in a filament‐driven discharge. The extracted H− beam current is limited by the achievable plasma density, which depends primarily on the electron emission current from the filaments. The emission current can be increased by increasing the filament temperature but, unfortunately, this leads not only to shorter filament lifetime but also to an increase in metal evaporation from the filament, which degrades the performance of the H− conversion surface. In order to overcome these limitations we have designed and tested a prototype of a surface conversion H− ion source, based on excitation of helicon plasma wave mode with an external antenna. The source has been operated with and without cesium injection. An H− beam current of over 12 mA has been transported through the low energy beam transport of the LANSCE ion source test stand. The results of these experiments and the effects of different source parameters on the extracted beam current are presented. The limitations of the source prototype are discussed and future improvements are proposed based on the experimental observations.