A high-resolution, variable-energy electron beam from a Penning–Malmberg (Surko) buffer-gas trap J. R. Machacek, T. J. Gay, Stephen J. Buckman and Sean S. Hodgman Eur. Phys. J. D, 76 2 (2022) 33 Published online: 15 February 2022 DOI: 10.1140/epjd/s10053-022-00349-y