Time-resolved emittance of a bismuth ion beam from a pulsed vacuum arc ion source

Abstract

The emittance of a pulsed bismuth vacuum arc ion source has been measured using a time-resolving double-slit emittance scanner. Each arc pulse was 250 μs long. Ion extraction was performed with 20 μs pulses 200 μs after arc triggering, when Bi ions are almost exclusively singly charged. This was motivated by a possible use of the ion source for heavy ion fusion. Pulsed extraction implies perveance mismatch during extractor voltage rise and fall time. The emittance increased with arc current. The beam current is very constant when operating in the slightly overdense plasma regime because fluctuations of the ion supply current are partially offset by extraction optics. For an extraction voltage of 35 kV, a bismuth current density of up to 18 mA/cm2 with a normalized emittance of 0.006 π mm mrad was obtained for a beamlet of 2 mm nominal diameter.