Observations of low emittance‐neutralized alkali ion beams from a large‐area multistage injector

Experiments are reported on the operation of a three‐stage accel‐decel ion injector that is driven by a 500‐kV–200‐ns pulse line at a power density of ≳100 kW/cm2—previously unobserved for injectors with no simultaneous electron breakdown. Stable thermionic emission of sodium ions from an array of sixteen 3‐mm×10‐cm scintered zeolite strips is demonstrated. Observations of over 500 mA/cm2 beams are noted and attributed to uniform space‐charge–limited extraction in the first stage and 3:1 electrostatic focusing in the second stage of the injector. After passing through the third stage, 500‐V decel potential (to retard electron flow), 70% charge neutralization is achieved with a reflex triode electron source. A mean angular divergence of less than 5 mrad is deduced from profile data of the sodium ion beam and reported.