Continuous-wave operation of a 460-GHz second harmonic gyrotron oscillator

We report the regulated continuous-wave (CW) operation of a second harmonic gyrotron oscillator at output power levels of over 8 W (12.4 kV and 135 mA beam voltage and current) in the TE
_0,6,1
mode near 460 GHz. The gyrotron also operates in the second harmonic TE
_2,6,1
mode at 456 GHz and in the TE
_2,3,1
fundamental mode at 233 GHz. CW operation was demonstrated for a one-hour period in the TE
_0,6,1
mode with better than 1% power stability, where the power was regulated using feedback control. Nonlinear simulations of the gyrotron operation agree with the experimentally measured output power and radio-frequency (RF) efficiency when cavity ohmic losses are included in the analysis. The output radiation pattern was measured using a pyroelectric camera and is highly Gaussian, with an ellipticity of 4%. The 460-GHz gyrotron will serve as a millimeter-wave source for sensitivity-enhanced nuclear magnetic resonance (dynamic nuclear polarization) experiments at a magnetic field of 16.4 T