Reflection influence on the operation of a 0.42Â THz second harmonic gyrotron with complex cavity

Since the average power of a 0.42 THz s harmonic gyrotron with complex cavity is small, the laser calorimeter AC2500H is adopted to measure the pulsed output power. However, the optimal spectral response of AC2500H is far from the operating frequency of the designed gyrotron. Thus the reflection on the laser calorimeter would be large, which would affect the mode competition in the designed gyrotron. In this paper, the influence of reflections from the laser calorimeter and output window is investigated by a multi-mode, time-dependent nonlinear code. From numerical simulation results, it is shown that the output power of the designed gyrotron is greatly decreased by the reflection from AC2500H although it doesn't cause excitations of competing modes. And it is found that when the reflection phase on the output window θr,n,1 is 5π/11, reflection magnitude on the laser calorimeter Γr,n,2 is 0.44 and the corresponding phase θr,n,2 is 3π/7, the variation of the calculated output power is very similar with the experimental result. Moreover, if the beam quality and Ohmic loss in the cavity wall are both considered, the calculated power can fit perfectly with the measured one.