Low-divergence, high-brightness lithium ion source for plasma diagnostics

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Abstract
Plasma diagnostics employing energetic lithium beams are in routine use on several present‐day tokamak experiments. These measurements are constrained by the availability of a suitable lithium ion source. Necessary and/or desirable parameters include narrow energy spread, high‐emission current density, simplicity, and long lifetime. The existing ion gun on the TEXT experiment uses the thermionic emitter β‐eucryptite and meets the above requirements, except for being limited to a useful emission of about 4 mA. We will present design criteria and accelerator results for a new ion gun capable of 15‐mA emission based on an increased emitter area. Important design considerations are the thermal and mechanical properties of the large (∼20 cm2) emitter area and the ion optics required to focus the emitted flux into a beam of ∼1‐cm2 cross section. The potential improvement in the performance of both the TEXT and ASDEX diagnostic systems will be discussed.