Beam properties of a high-perveance single-aperture dc extraction system

Abstract

In continuous operation a positive hydrogen ion beam up to 240 mA has been extracted at 50 kV out of a single aperture, corresponding to a perveance of 2.1×10−8 A/V3/2. The effective mass of the extracted ion beam was measured to be about 1.8 amu. The current density distribution of the extracted ion beam was measured with a small calorimetric probe. It was found that the beam distribution strongly depends on the matching between the plasma current density and the extraction voltage. At overmatched conditions the ion beam may even become hollow. The distribution of the extracted ion beam depends on the pressure along the beam path. At pressures above 3×10−4 Torr H2, the distribution was observed to be Gaussian. At a pressure 2×10−5 Torr H2, the intensity in the wings of the beam is relatively high with respect to the intensity in the beam center. The beam divergence decreases with the square root of the extraction voltage. The space-charge potential of the neutralized beam is estimated to be about 5.6 V at 2×10−5 Torr decreasing to 1.6 V at 10−3 Torr independent of the beam energy.