Recent advances in high current vacuum arc ion sources for heavy ion fusion

For a heavy ion fusion induction linac driver, a source of heavy ions with charge states 1+–3+, ≈0.5 A current beams, ≈20 μs pulse widths and ∼10 Hz repetition rates is required. Thermionic sources have been the workhorse for the Heavy Ion Fusion (HIF) program to date, but suffer from heating problems for large areas and contamination. They are limited to low (contact) ionization potential elements and offer relatively low ion fluxes with a charge state limited to 1+. Gas injection sources suffer from partial ionization and deleterious neutral gas effects. The above shortcomings of the thermionic ion sources can be overcome by a vacuum arc ion source. The vacuum arc ion source is a good candidate for HIF applications. It is capable of providing ions of various elements and different charge states in short and long pulse bursts and high beam current density. Under a Phase-I STTR from DOE, the feasibility of the vacuum arc ion source for the HIF applications was investigated. We have modified an existing vacuum arc ion source at LBNL to produce a gadolinium (A≈158) ion beam with >0.5 A beam current, 120 keV beam energy, ≈6 cm diameter extraction aperture and ≈20 μs pulse width. The average beam current density at the extraction grids was ≈17 mA/cm2. We have measured that >85% Gd ions were in the 3+ charge state, the beam current fluctuation level (rms) was ≈3%, pulse-to-pulse variation of the beam (rms) was about 3%, the uniformity of the beam density over its 6 cm diameter was ⩾98% and the ion longitudinal energy spread was ⩽1%. Additional measurements were made to improve charge state purity by using other materials and employing an axial magnetic field close to the cathode. Yttrium (A≈89), lead (A≈207), and Ba (A≈137) were tested at similar current parameters with Ba delivering nearly a pure charge state with >95% being in 2+ state. The results of the experiments indicate that the vacuum arc ion source is a good candidate for HIF applications.