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Progress in Space Charge Lens Development

A number of space charge lens electrode geometries have been studied for use at very high beam currents at low energy, and for precision optics at MeV energies. One such lens mounted very near a Duo-plasmatron extractor delivered a 30-keV beam of H+ ions through an analyzing magnet at a current of 175 mA for a period of six hours. Another produced a focal spot of diameter 0.2 mm when a 1-cm diameter beam of 1.2-MeV protons was focussed 90 cm from the lens. This paper also describes observations on self-sustaining discharge in the lenses. A very interesting rotation of the lens optic axis about the geometric axis at a frequency near the drift frequency occurs when the vacuum is spoiled by electrode outgassing.