A low-perveance electron gun for a high-efficiency Ka-band klystron B. Spataro, M. Behtouei, F. Di Paolo and A. Leggieri Eur. Phys. J. Plus, 137 7 (2022) 769 Published online: 05 July 2022 DOI: 10.1140/epjp/s13360-022-02987-y