Accurate Modeling of Monotron Oscillations in Small- and Large-Signal Regimes

Abstract

In Klystron amplifiers, monotron oscillations may cause unacceptable beam instabilities. To facilitate fast and accurate analysis of such processes, rather than performing timeconsuming Particle-in-Cell simulations, the theory of the monotron oscillations has been further developed for the small and large signal regimes and implemented into the klystron computer code KlyC. This development includes full considerations of the space charge effects, relativistic effects and can operate with arbitrary field distributions of the resonant mode. The effectiveness of these methods, have been demonstrated through benchmarking against PIC codes and has shown good (at 1% level) agreement, whilst KlyC computation time is significantly (at least 100 times) faster than in the PIC simulations.

Numerical Analysis of Resonant Multipolar Instabilities in High Power Klystrons

“…Also, it is impossible to disentangle any other modes where Q beam is larger. A more efficient approach to calculate Q beam was extrapolated from the method used in [12]. It is related to the monotron small signal theory, when beam dynamic calculations (BDC) in PIC can be performed with known (imported from CST eigen solver) eigen field map of the mode.…”

“…A more rewarding strategy is to review the geometry of the triplets itself. In our previous studies, monopole monotron oscillations in the triplet were removed by adjusting the cells period length to 3.8mm [12]. We have tried the same technique in attempt to mitigate the multipolar oscillations, but this attempt was not successful due to the number of modes involved, see Fig.…”

“…However, with large beam power, such a topology appeared to be sensitive to spurious mode oscillations. The possibility of monotron oscillations [9][10][11] in this tube were investigated in detail and mitigated by adjusting the length of the triplet period [12]. The threshold current of the monopolar monotron oscillation only weakly depends on the magnetic field since longitudinal beam-wave interaction dominates the process.…”

Design Study of a High-Power Ka-Band High-Order-Mode Multibeam Klystron

“…Considering the higher frequency modes with indices m × N (m=2,3), their frequencies are not harmonics of 36 GHz (57 GHz; n=40). There are still more modes that can be considered dangerous, as far as they could satisfy both the spatial and the frequency harmonics synchronism [18]. Though, the possible effect of these modes will be studied by analyzing the results of the 3D PIC simulation in the next section.…”