Design and performance of an UHV beamline to produce low and hyperthermal energy ion beams

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Abstract
We have constructed and tested an UHV beamline to produce beams of alkali metal and noble gas ions over the energy range E/E of 1% or better. At the sample position, we have attained tens of nanoamps of current for keV energy beams in a 1–2‐mm beam spot with about ±1° angular divergence, and nanoamps of current at 25 eV in a 4‐mm beam spot with ±2° angular divergence. By applying Liouville’s theorem to the beam’s emittance and using waist‐to‐waist transport through the beam optics, the current on the sample is maximized while limiting the spot size and angular divergence. To achieve useful current at the lowest energies, special attention was paid to minimizing space‐charge effects. Beam emittances measured at the sample position are consistent with Liouville’s theorem. Equations for waist‐to‐waist transport are derived in the Appendix.