Low-emittance 08 AK⁺ ion source for the LBL induction linac system experiment (ILSE)

Summary

A high-current injector for heavy-ion fusion presently under construction at Lawrence Berkeley Laboratory requires large sources of up to 17 cm in diameter and total potassium ion current of 790 mA with a low normalized transverse emittance of less than 0.5 mm mrad. A 1″ diameter hot alumino silicate (zeolite) source was fabricated and tested in the injector set-up of SBTE, an existing facility at LBL. Initial measurements showed a maximum space-charge-limited ion current of 95 mA. The corresponding density of 19 mA/cm² was limited by the source diode optics and not by the source emission. The density required for the high-current injector is 4 mA/cm². The normalized emittance was measured to be 0.06 mm mrad, corresponding to a transverse temperature of 0.2eV. Nondestructive life tests showed that the source can be operated under continuous operating conditions of the high-current injector for more than a month (twenty eight-hour days, with 1 μs long pulses at 1 Hz). d.c. destructive life tests showed that ∼30% of the total stored K can be ionized and extracted, allowing, in principle, for years of operation for the high-current injector. Furthermore, we shall describe the extension of the fabricating technique to large-diameter sources (up to 17 cm) and report on measured ion emission performance, measured surface temperature uniformity and heating power considerations.