Investigation of the stability of a tapered gyro-TWT amplifier

The stability of a wideband single-stage tapered gyrotron traveling wave tube amplifier (gyro-TWT) operating in Ka-band (27-38 GHz) has been investigated. The amplifier operates at 33 kV, 1.6 A in the TE/sub 10/ rectangular waveguide mode at the fundamental cyclotron frequency. Small signal gain in excess of 20 dB has been demonstrated over a wide (33%) frequency range with this device. Recent narrowband large signal measurements have demonstrated a saturated efficiency of around 10%, corresponding to a peak power of 5 kW at 35 GHz. Attempts to increase the amplifier gain and efficiency further have been limited, however, by the onset of oscillations due to insufficient output circuit match. A stability study has been performed to assist in the optimization of the present device, and to aid in the design of a follow-on two-stage experiment. The study has focused on the parametric dependence of the start oscillation current threshold on the velocity ratio, the magnetic field detuning, and the output port reflection coefficient.<>