High current $\mathrm{H}_2^+$ beams from a filament-driven multicusp ion source

Abstract

Recently, the use of $\mathrm{H}_2^+$ ions instead of protons to overcome space charge challenges in compact cyclotrons has received much attention. This technique has the potential to increase the available beam current from compact cyclotrons by an order of magnitude, paving the way for applications in energy research, medical isotope production, and particle physics, e.g. a decisive search for sterile neutrinos through the IsoDAR experiment. For IsoDAR we go a step beyond just using $\mathrm{H}_2^+$ and add pre-bunching through a Radio-Frequency Quadrupole (RFQ) embedded in the cyclotron yoke. This puts beam purity and beam quality constraints on the ion source that no published ion source has simultaneously demonstrated so far. Here, we report results from a new multicusp ion source (MIST-1) that produces the world's highest steady-state current of $\mathrm{H}_2^+$ from this type of ion source (1 mA), with exceptionally low emittance (0.05 $\pi$-mm-mrad, RMS, normalized) and high purity (80% $\mathrm{H}_2^+$). This result shows the feasibility of using a multicusp ion source for IsoDAR and the RFQ direct injection prototype, and paves the way to record breaking continuous wave (cw) beam currents of 5 mA $\mathrm{H}_2^+$ (equivalent to 10 mA protons) from compact cyclotrons, ideal for underground installation. This represents a significant advance, with impact on neutrino physics specifically and high power cyclotron design in general.