Operation of a dc large aperture volume-production H− source

In testing a multicusp volume-production H− ion source (20 cm diameter, 23 cm long), we optimized the gas pressure, the plasma electrode bias potential, and the magnetic filter. At the optimum pressure of 9 mTorr, the H− beam output increased linearly with discharge power. The maximum H− beam, measured with a current transformer downstream of the accelerator, was 100 mA while using a 6.67-cm2 aperture. Presently we are limited by overheating of the cathodes by the plasma ions. Under similar discharge conditions the maximum H− current density was found to vary as a−0.7, where a is the aperture radius. Results from emittance measurements showed that the effective H− ion temperature increased with a for a>0.8 cm. Thus, the brightness of the beam decreased with increasing aperture radius. Operating the source with cesium would increase the H− output; however, our accelerator must be improved to avoid breakdowns caused by the cesium contamination.