Enhancement of high-charge-state ions in vacuum arc ion sources

Abstract

Summary form given only, as follows. Vacuum arc ion sources are used to generate high current broad beams of metal ions. The attraction of increasing the mean ion charge state in the beam is because of the possibility of thereby increasing the ion beam energy without applying higher extraction voltage. This is important both for heavy ion accelerator injectors and for ion implantation technologies. We have explored three different methods to enhance the degree of multiple ionization in vacuum arc plasmas. These methods are: application of a strong axial magnetic field in the arc region, generation of current spikes, and injected electron beam. A strong magnetic field increases the arc burning voltage because of improved confinement of the plasma column and leads to higher plasma electron temperature, which is primarily responsible for ionization. The B-field also increases the length of the ionization zone, beyond which there is no significant change in the ion charge state distribution (the distribution is frozen). Another approach to increase the arc voltage and electron temperature is to use an arc current spike of pulse duration comparable to the arc current relaxation time or less. This leads to a somewhat elevated arc voltage during the spike. Finally, additional ionization in the vacuum arc plasma can be brought about by injection into the plasma of an externally-produced electron beam. In this case the most important problem is to obtain optimum electron beam parameters and e-beam transport through the entire arc region. Here we describe these approaches and the experimental results of our investigations of ion charge state distribution. Mean ion charge states are presented, compared and discussed.