The beam space-charge associated with a gyrating intense relativistic electron beam limits the current that can be propagated, and thus the ultimate gyrotron power. An experiment at NRL has investigate the use a background plasma to increase the gyrotron beam current beyond the vacuum space charge limit, and thus to permit higher power gyrotron operation.
S. H. Gold, S. H. Gold,
"The NRL plasma-filled gyrotron experiment", Proc. SPIE 1514, 15th International Conference on Infrared and Millimeter Waves, 151466 (1 December 1990); doi: 10.1117/12.2301632; https://doi.org/10.1117/12.2301632