Asymmetric linear efficiency and bunching mechanisms of TM modes for electron cyclotron maser

Abstract

This study examines the transverse magnetic (TM) waveguide modes, which have long been considered as the unsuitable ones for the operation of the electron cyclotron maser. The beam-wave coupling strength of the TM modes, as expected, is found to be relatively weak as compared with that of the transverse electric (TE) waveguide modes. Unlike TE modes, surprisingly, the linear behavior of the TM modes depends on the sign of the wave number kz. The negative kz has a much stronger linear efficiency than that of the positive kz. The bunching mechanism analysis further exhibits that the azimuthal bunching and axial bunching do not compete but cooperate with each other for the backward-wave operation (negative kz). The current findings are encouraging and imply that TM modes might be advantageous to the gyrotron backward-wave oscillators.

Linear studies of spurious backward-wave instabilities in a dielectric-loaded smooth-wall gyrotron beam ducts

“…In our cases, the interaction is always occurring in the backward wave regime, but the assumption of considering only TE modes was found to be valid. Evaluating the TE and TM mode coupling coefficients 32 for a typical case treated in this work, the TM mode coupling coefficient was found to be three orders of magnitude lower than the TE mode coupling coefficient.…”

“…When the resonance condition takes place with an important Doppler shift (k z >> 0), the beam-wave coupling with a TM mode could be important. It has been shown 32 that the beam coupling to a TM mode is much stronger for the backward wave interaction, due to the fact that the azimuthal and axial bunching processes cooperate with each other, while for a forward wave interaction, the two bunching processes compete against each other. In our cases, the interaction is always occurring in the backward wave regime, but the assumption of considering only TE modes was found to be valid.…”

Intrinsic resonance in gyrotron using non-resonant RF structure

“…Previous research has investigated the bunching mechanism of TM-mode gyrotrons under the assumption of weakly relativistic electron beam. 46 An important finding has been obtained for gyro-BWO, indicating that the azimuthal bunching (through Dc) and the axial bunching (through Dv z ) induced by E z always cooperate. The cooperation becomes prominent during the far from cutoff operation.…”

“…Without loss of generality, the behaviors of electron bunching in the phase space can be examined by applying the temporal variation on the effective electron cyclotron frequency 1,46 We then obtain…”

“…Based on the analyses presented in the previous work 46 The beam-wave energy transfer can be further examined by the change in Lorentz factors Dc of all electron ensembles. Figure 6(a) demonstrates Dc of 21 representative electrons calculated in particletracing simulation at near-cutoff operation (B 0 ¼ 36 kG), while the similar plot at a relative far from cutoff region (B 0 ¼ 38 kG) is shown in Fig.…”