Multistage acceleration of light ions for fusion

Summary form only given, as follows. Theory and experiments indicate that post acceleration can reduce ion divergence. In addition, overall system efficiency should be better in multistage accelerators. Yet even the small step from single-stage to the use of two acceleration stages introduces a significant increase in the system complexity. As an example, the ion beam must remain space-charge-neutral between acceleration stages and the use of a conventional gas cell results in beam stripping. Diode configurations to circumvent this problem are presented. Another issue is the effect of diamagnetic pressure on the virtual-cathode of the second stage, which causes a bending of the magnetic flux surface that defines the virtual cathode. This results in a focusing of the ions beam. Analytic calculations of this process are presented. Divergence reduction during post acceleration depends of the level of electromagnetic fluctuations generated by instabilities in both stages. Three dimensional particle-in-cell simulations are presented to study this process.