Evaluation of a simple method for chopping Penning surface‐plasma source H− beams

Abstract

Accumulator rings proposed for use in high‐intensity spallation‐neutron sources require a chopped beam with ∼100‐ns‐wide particle‐free gaps at 1–2 MHz rates, with fall and rise times ≤20 ns. Chopping the beam directly in the ion source may be an attractive way to provide the desired beam structure. Previous measurements showed that placing a grounded collar in the drift region just before the emission aperture lowers the e−/H− ratio in the Penning surface‐plasma source H− beam. We electrically isolated the collar and biased it to modulate the extracted H− current. Positive collar bias decreases the H− beam by up to 90%. The fastest H− current fall and rise times achieved to date are 400 ns and 2 μs, respectively. The current fall time is close to the 300‐ns pulser rise time. The current rise time is considerably longer than the 500‐ns pulser fall time. Negative collar bias lowers the H− beam by up to 50%. Simulations indicate that the beam time structure will be preserved in transport from the ion source to the radio‐frequency quadrupole entrance.