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The increasing demand for powerful, reliable, and efficient gyrotron oscillators as millimetre-wave sources for Electron Cyclotron Resonance Heating (ECRH) in fusion plasma experiments requires a close look at the various factors in gyrotrons that determine gyrotron performance. In this frame, the influence of emitter surface roughness, emission inhomogeneity, and secondary electron generation on gyrotron operation is presented, with focus on Low Frequency Oscillations (LFOs) and Electron Beam Halo (EBH) generation, both being unwanted. A new surface roughness model is developed and implemented into the KIT Magnetron Injection Gun (MIG) codes ESRAY and ESPIC. Using this new model, it is possible to determine the influence of surface roughness on the gyrotron’s electron beam quality and efficiency. To investigate the influence of electrons reflected back to the emitter, a modified secondary electron emission model with high accuracy on the angular distribution of high energy secondary electrons (elastic and rediffused electrons) is developed and implemented in the codes ESRAY and ESPIC. In addition, a new evaluation method for the emission inhomogeneity is developed. With these new tools, the influence of emitter surface roughness and of emission inhomogeneity on electron beam quality and gyrotron efficiency is investigated numerically. Simulations show that both factors affect the electron beam quality and gyrotron efficiency significantly. It was found that back-reflected electrons can be generated due to these two factors. Two phenomena caused by back-reflected and secondary electrons in the gyrotron have been found: space charge LFOs and EBHs. Numerical

Influence of Emitter Surface Roughness and Emission Inhomogeneity on Efficiency and Stability of High Power Fusion Gyrotrons