Influence of magnetic field strength on the focusing properties of a high-current plasma lens

摘要

We present results of experimental studies of the operation of the high-current wide-aperture plasma lens in the range of low magnetic fields. Investigations of focusing of copper and carbon ion beams with current up to 0,5 A and energy up to 20 keV by a plasma lens with aperture ~7 cm were conducted in Kiev; studies of focusing of tantalum, copper, zinc and carbon ion beams with current up to 0,5 A and energy up to 50 keV were studied in Berkeley. In both cases ion beams were produced by a vacuum-arc (MEVVA-type) ion source. Substantial increase of the beam current density at the focus of the lens was found for low magnitudes of the magnetic fields. A maximum in beam current density is observed for magnetic fields 5-16 kA/m, in a notably narrow range. The optimal magnetic field increases with increasing voltage applied to the lens. For a copper ion beam the optimal current density reaches ~250 mA/cm2, then drops by a factor 3-4 with increasing magnetic field, after which it grows again and reaches a saturation value ~120 mA/cm2 for magnetic fields exceeding 40 kA/m. The effect is observed for different distributions of the external potential of the lens electrodes. Measurement of the radial distribution of potential in the mid-plane of the lens reveals a self-consistent optimal electric field topography with minimal spherical aberrations. It is observed also in the optimum case, a drastic decrease (by a factor of more than an order of magnitude) in the amplitude of oscillations in the lens and focused ion beam. A decrease of the halfwidth of the ion beam at the lens focus is also observed.