Effects of Different Magnetic Field Profiles on Output Power and Efficiency of a Second-Harmonic Gyrotron

As the magnetic field used in nuclear magnetic resonance (NMR) research increases (up to 23 T), the irradiation frequency of the electromagnetic field used for dynamic nuclear polarization (DNP) needs to be in the terahertz band (140–600 GHz). This article analyzed and compared the influence of different external magnetic field profiles on start current and beam–wave interaction of a 394-GHz gyrotron. As the simulation results show, the minimum start current corresponding to the linear and parabolic external magnetic field profile is lower than that corresponding to the uniform magnetic field. The minimum start current in the case of parabolic magnetic field profile is a little lower than that in the case of a linear magnetic field profile. Based on the nonlinear self-consistent calculation, it is observed that the optimum output powers in both linear and parabolic magnetic field profile cases are nearly equal when the beam current is 0.25 A and both of them are larger than the optimum output power when the magnetic field is uniform. In the calculation of orbital efficiency
$\eta _{\bot } $
, the maximum orbital efficiency in the soft-excitation region in the inhomogeneous magnetic field profile cases is larger than that in the case of uniform magnetic field profile. After comprehensive consideration of beam voltage, beam current, and efficiency, the final operation point is located in the place where the beam current is 0.25 A and the beam voltage is 15 kV. The output power is more than 300 W and meets the requirements of DNP-NMR experiment.