Two-frequency plasma heating in a high charge state electron cyclotron resonance ion source

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Abstract
The performance of the Lawrence Berkeley Laboratory (LBL) advanced electron cyclotron resonance ion source, which is a single stage source designed to operate at 14 GHz alone (single‐frequency heating), is enhanced by heating the plasma simultaneously with microwaves of 10 and 14 GHz (two‐frequency heating). Production of high charge state ions was increased a factor of 2–5 or higher for the very heavy ions such as bismuth and uranium, as compared to single‐ frequency heating. Plasma stability was improved and the ion charge state distribution shifted to higher charge state. With two‐frequency heating, the source can produce more than 1×109 pps of fully stripped argon. High charge state ion beams of bismuth and uranium produced by the source were injected into the 88‐Inch Cyclotron at LBL. After acceleration to energies greater than 6 MeV/nucleon, the extracted beam intensities were 1×106 pps or higher for Bi50+,51+ and 238U52+,53+.