3-D design of inverse magnetron injection gun for 0.24 THz, 1 MW coaxial cavity gyrotron

Inverse magnetron injection gun (IMIG) provides several advantages over simple MIG especially in high-power, high-frequency gyrotron. Sub-millimeter (> 230 GHz), high power (≥ 1 MW) gyrotrons would be required in futuristic plasma fusion machines to fulfill the need of tens of megawatt RF power for electron cyclotron resonance heating (ECRH). Here, in this manuscript, an inverse MIG is designed for 0.24 THz, 1 MW coaxial cavity gyrotron. Coaxial cavity configuration provides better control on the mode competition and space charge compared to the simple cavity configuration. First, the trade-off design equations and technical constraints are used to synthesize the electrodes geometry and electrical parameters of IMIG. This synthesized IMIG model is further simulated to finalize the design using 3-D particle trajectory simulator CST. Simultaneously, the magnet system including the auxiliary gun coils and superconducting magnets (SCM) is also designed and the obtained magnetic field profile is used in the IMIG simulations. This IMIG is designed for a very high order mode (TE_46,17) based coaxial cavity. A detail parametric and misalignment analyses are also performed with respect to the electron beam parameters, such as pitch factor, velocity spread and guiding center radius spread. These parametric and misalignment analyses would be utter helpful in the fabrication, assembling and testing of the IMIG as well as gyrotron. (author)