Converting low-order circular electric modes to Gaussian beams in high-frequency gyrotrons

Gyrotrons have been found to hold great promise in the enhancement of signal-to-noise ratios in Nuclear Magnetic Resonance (NMR) spectroscopy experiments, through the use of Dynamic Nuclear Polarization (DNP). Such applications typically require low CW output power levels (10-100 W) at frequencies ranging from 250 GHz to 1 THz. A key challenge in the design of such gyrotrons is the conversion of the excited oscillation mode into a usable Gaussian output beam. In this regime, applicable mode converter design techniques differ somewhat from those employed in the higher-power, lower-frequency fusion gyrotrons that have been a primary driver of mode converter technology in recent years. Here, several mode conversion techniques developed over the past several decades are employed to design a compact internal mode converter for generating a Gaussian beam from a TE₀₃ interaction mode in a 263 GHz DNP gyrotron.