Study of the multi-modes behaviour in a high harmonic large orbit gyrotron

The gyrotron oscillator has been demonstrated as one of the leading candidates for powerful sub-millimetre and millimetre wave generation¹. However, this device is not yet widespread in the THz frequency range due to the requirement of intense external magnetic field. To solve this problem, a high harmonic operation of the gyrotron should be considered. A large orbit gyrotron² (LOG), by restricting the equality between the azimuthal index m of the operation TE_mn modes and the electron cyclotron harmonic number s, can enable the high harmonic oscillation. For high harmonic operation of a LOG, the multi-modes behaviour is important in two distinguished ways. On the one hand, the designed operation mode still faces considerable competition from the axial modes within the same harmonic as well as modes from the neighbouring harmonics. On the other hand, by effective control of the multi-modes behaviour, broadband frequency step tunable radiation can be generated. Thus, this paper is dedicated to characterise the multi-modes behaviour of a LOG operating in the THz range.