Abstract
The successful operation of a third-harmonic gyrotron whose electron beam was azimuthally prebunched in an RF accelerator cavity is reported. In the unbunched nth-harmonic gyrotrons, axis-encircling electrons interact with a high-order aximuthal TE n1 mode. The rotating electron helical shell is produced in a gyroresonant TE 11 RF accelerator. In the new interaction the gyrotron's TE n1 frequency is also equal to the nth harmonic of the accelerator's frequency. The electrons are now injected into the gyrotron at the optimum RF phase. Higher power can be expected because all electrons lose energy to the wave, unlike in the standard negative-mass instability where energy loss only dominates energy gain. RF output of 60 W has been measured at 27.6 GHz, using a window with very poor transmission. An increase of power by an order of magnitude is expected after the window is replaced. The dependence of the output power on the square of the beam current has been verified. The oscillation can be started at any current as in an amplifier.