Developments in Gridded Ion Lenses

For most applications, the use of grids in focusing elements can improve beam current densities per unit solid angle by a factor of 10 or more. The coupled problems of high grid transparency and long grid life have limited their usefulness in accelerator systems. The development of machined grids with a 10 fold increase in life expectance will be described. These grids are for the Einzel lens of the ORNL Nanosecond Pulser in which wire mesh grids had an average life to failure of 75 hrs. The grids were machined by electrical discharge (Elox) techniques in a sheet of tungsten or molybdenum. Transparencies as high as 84% have been obtained. The sheets can be pre-shaped to compensate for residual spherical aberrations in the lens. Shaped grids have shown further increases of beam brilliance of 30-40%. For fast pulsing operating in which there is no space charge neutralization, beam brilliance figures of 1 amp per sq cm per steradian have been obtained with beam currents up to 3 mA.