Manipulating large-area, heavy metal ion beams with a high-current electrostatic plasma lens

We describe some experimental investigations of the manipulation of high-current, large-area beams of heavy metal ions using a high-current electrostatic plasma lens. Beams of carbon, copper, zinc, and tantalum ions (separately) were formed by a repetitively pulsed vacuum arc ion source, with energy in the range about 10-140 keV, beam current up to 0.5 A, initial beam diameter 10 cm, and pulse length 250 ms. The plasma lens was of 10-cm internal diameter and 20-cm length and had nine electrostatic ring electrodes with potential applied to the central electrode of up to 7 kV, in the presence of a pulsed magnetic field of up to 800 Gauss. The current density profile of the focused beam was measured with a radially moveable, magnetically suppressed Faraday cup. The results show that the focal length of the lens and the profile of the transported beam can be controlled by variation of the lens magnetic field and electrode potential distribution. Under optimum operating conditions the ion beam current density at the focus was compressed by a factor of up to 30. We also carried out some preliminary observations of the effect of the plasma lens on the ion charge state distribution of the beam. Here we outline the principles underlying plasma lens operation and summarize the experimental observations we have made of the lens performance in this large-area. high-current regime.