Production of Low Divergence Positive Ion Beams of High Intensity

Abstract

In this paper ion sources are described which take advantage of the focusing properties of a plasma boundary, thereby producing positive ion beams of very high directivity or brightness. It was found that by employing a considerable degree of space‐charge neutralization beyond the accelerating‐field region of the ion source, the beam formed by plasma‐boundary focusing can be transported away from the source with low and constant divergence. It is also shown that the divergence of the beam could further be decreased by reducing the space‐charge expansion of the ion beam in the acceleration region. The positive ions were removed from the plasma boundary in a region of high field gradient before being retarded to the required final energy as they left the ion source. By use of these three developments, low divergence, high intensity dc ion beams were obtained, and a comparison between these and earlier results is provided. Under optimum conditions, 80 mA of positive hydrogen ions at 30 keV, extracted from a 5.0‐cm‐diam plasma boundary, were transmitted through a 5.0‐cm aperture placed 75 cm from the ion source. The beam emittance was measured to be 11×10⁻³ rad·cm·MeV^½ with a maximum half‐angle divergence of 45×10⁻³ rad.