Measurements of the Cesium Flow from a Surface-Plasma H- Ion Source

We have constructed a surface ionization gauge (SIG) and have used it to measure the Cs0 flow rate through the emission slit of a surface-plasma source (SPS) of H- ions with Penning geometry. The equivalent cesium density in the SPS discharge is deduced from these flow measurements. For dc operation the optimum H- current occurs at an equivalent cesium density of ~7 × 1012 cm-3 (corresponding to an average cesium consumption rate of 0.5 mg/h). For pulsed operation the optimum H- current occurs at an equivalent cesium density of ~2 × 1013 cm-3 (1-mg/h average cesium consumption rate). We observe cesium trapping by the SPS discharge for both dc and pulsed operation. A cesium energy of ~0.1 eV is deduced from the observed time of flight to the SIG. In addition to providing information on the physics of the source, the SIG is a useful diagnostic tool for source startup and operation.