The electron distribution over residual energy after electron interaction w
ith an rf field was determined using the method of decelerating electric fi
eld. It is demonstrated that, under the maximum efficiency conditions, elec
trons with an energy lower than 30% of their initial energy are absent in t
he spectrum. Hence, the efficiency of energy-recuperation gyrotrons can sub
stantially be increased. (C) 2000 MAIK "Nauka/Interperiodica".