Universal CUSP-Type Electron Gun for Helical Gyro-TWTs for DNP-NMR Applications

Gyro-TWTs with helically corrugated interaction region require a high-quality axis-encircling electron beam. In this work, a theoretical design of a CUSP-type electron gun for a 263 GHz helical gyro-TWT is presented. The theoretical design is optimized for an operation with an acceleration voltage of 50 kV, a beam current of 0.25 A and a pitch factor of $\alpha$=1.0. At this operating point, the spread of the pitch factor is as low as 3.5 % (RMS). Furthermore, the gun is capable to create an electron beam over a wide variety of pitch factors with different beam currents and acceleration voltages. In addition to the operation at 263 GHz, the gun can be operated at lower frequencies, e.g. at 140 GHz and at 94 GHz.