Improved field geometries for SABRE extraction ion diode operation with passive ion sources

The SABRE Facility at Sandia National Laboratories is an integrated testbed for the study of high voltage ion beam production and transport in extraction geometry for inertial confinement fusion. Our major emphasis is on the development of active ion sources, but several techniques are under investigation to improve diode performance with passive hydrocarbon and LiF ion sources. The operation of passive sources is particularly sensitive to details of the magnetic and electric field geometries and can provide insight into divergence and parasitic load mechanisms which also affect high voltage diode performance with active sources. We demonstrate that an outer cathode tip extension can eliminate cathode feed electron participation in source turn-on and virtual cathode evolution. Experimental data and TWOQUICK PIC code simulations indicate that instability-induced cross-:field diffusion of cathode tip electrons at small radius plays a dominant role in diode operation with a passive source. An improved diode geometry is presented with reduced feed electrode participation and better electron confinement at the anode emission area for lithium sources.