A high-current density contact ionization source for heavy-ion fusion

Heavy ion fusion (HIF) sources will be low-emittance, high current (0.5-1 A), and suitable for injection into a multiple beam induction linac, where the diameter of each beam aperture at the beginning of the accelerator is 3-6 cm. Practical transport limits involved in matching the beam to the accelerator aperture in a reasonably-sized structure correspond to several mA/cm² of space charge limited flow with an injector extraction voltage of 1-2.5 MV. We have built a J≈8 mA/cm² Cs⁺ contact ionization source for a 160 kV diode; this represents a factor of four increase in the Cs⁺ current density over previous sources for HIF. The emitter is sintered iridium, operating at 950<T<1150°C. We are experimenting with several methods of alkali feeding capable of maintaining a fraction (<5%) of a monolayer coverage on the iridium surface