The rf acceleration of moderate current (≲1 A) electron beams to 500 keV in a TE111cylindrical cavity resonator is described. Experimental results are compared with theory. rf to beam energy conversion efficiencies in excess of 50% have been observed. The resultant axis‐encircling beam with a large ratio of perpendicular velocity to longitudinal velocity is ideal as a driver for a high‐harmonic gyrotron.