Preliminary study on the characteristics of a rf-driven H− source

The use of a rf-induction discharge as a H− source is attractive for long life, pulsed, or cw operation. Recently, a new rf-driven, magnetically filtered multicusp source has been developed at LBL to generate volume-produced H− beams with extracted current densities higher than 200 mA/cm2. A total current of ∼40 mA can be obtained with a 5.6 mm diam aperture and a rf input power of 50 kW. The operating frequency is ∼2 MHz. As part of a study of the H− production mechanism in rf plasmas, measurements of the electron energy distribution have been undertaken. Initial measurements were made with a modification of the standard Langmuir probe method. The probe characteristics were sampled at different phases of the rf cycle. Results show that the electron density does not vary significantly during the rf cycle. The dependence of electron density and temperature on source pressure and axial position has been investigated and the results are summarized.