Towards coherent combining of X-band high power microwaves: phase-locked long pulse radiations by a relativistic triaxial klystron amplifier

Scientific Reports, Aug 2016

The radio-frequency breakdown due to ultrahigh electric field strength essentially limits power handling capability of an individual high power microwave (HPM) generator, and this issue becomes more challenging for high frequency bands. Coherent power combining therefore provides an alternative approach to achieve an equivalent peak power of the order of ∼100 GW, which consequently provides opportunities to explore microwave related physics at extremes. The triaxial klystron amplifier (TKA) is a promising candidate for coherent power combing in high frequency bands owing to its intrinsic merit of high power capacity, nevertheless phase-locked long pulse radiation from TKA has not yet been obtained experimentally as the coaxial structure of TKA can easily lead to self-excitation of parasitic modes. In this paper, we present investigations into an X-band TKA capable of producing 1.1 GW HPMs with pulse duration of about 103 ns at the frequency of 9.375 GHz in experiment. Furthermore, the shot-to-shot fluctuation standard deviation of the phase shifts between the input and output microwaves is demonstrated to be less than 10°. The reported achievements open up prospects for accomplishing coherent power combining of X-band HPMs in the near future, and might also excite new development interests concerning high frequency TKAs.

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Towards coherent combining of X-band high power microwaves: phase-locked long pulse radiations by a relativistic triaxial klystron amplifier

Abstract The radio-frequency breakdown due to ultrahigh electric field strength essentially limits power handling capability of an individual high power microwave (HPM) generator, and this issue becomes more challenging for high frequency bands. Coherent power combining therefore provides an alternative approach to achieve an equivalent peak power of the order of ∼100 GW, which consequently provides opportunities to explore microwave related physics at extremes. The triaxial klystron amplifier (TKA) is a promising candidate for coherent power combing in high frequency bands owing to its intrinsic merit of high power capacity, nevertheless phase-locked long pulse radiation from TKA has not yet been obtained experimentally as the coaxial structure of TKA can easily lead to self-excitation of parasitic modes. In this paper, we present investigations into an X-band TKA capable of producing 1.1 GW HPMs with pulse duration of about 103 ns at the frequency of 9.375 GHz in experiment. Furthermore, the shot-to-shot fluctuation standard deviation of the phase shifts between the input and output microwaves is demonstrated to be less than 10°. The reported achievements open up prospects for accomplishing coherent power combining of X-band HPMs in the near future, and might also excite new development interests concerning high frequency TKAs. Introduction Stimulated by the significant progress of modern pulsed power technology pioneered by J.C. Martin and coworkers in 1960s1, intense relativistic charged particle beams in nanosecond-scale became available in experiments, which have provided opportunities for many multidisciplinary researches such as, nuclear physics, X-ray flash radiography, Z-pinch, relativistic vacuum electronics, as well as the newly emerged field of bioelectrics2. Particularly, as the research focus of relativistic vacuum electronic devices3, high power microwave (HPM) generator has gained tremendous progress over the past decades and is still experiencing unprecedented development. The produced HPMs are of great interest for both scientific and civil applications, e.g. high power radar, power beaming, particle accelerators, plasma heating, and space propulsion, to name but a few4. The state-of-the-art HPM generators are able to produce microwaves with peak power in excess of 1 GW over frequencies ranging from P-band to Ku-band, and they are developing towards the directions of high output power, high efficiency, long pulse duration, and repetitive operation5. Remarkably, X-band HPMs in 30 Hz output with peak power of 2 GW and pulse duration of more than 100 ns have been demonstrated recently in experiment using an overmoded relativistic Cerenkov generator6. Notwithstanding the inspiring achievements, the output microwave power of a single HPM generator can hardly be further enhanced owing to an intrinsic physical limit: surface plasma formation due to breakdown on metal surface induced by ultrahigh radio-frequency electric field strength, usually termed as radio-frequency breakdown3,4,5,6,7. Like in fiber optics8, a promising development trend of future HPM research is to combine coherently multi HPM generators in order to pursue an equivalent peak power of the order of hundreds of GWs4. It is known that coherent power combing requires exactly the same operation frequency and stable phase differences between the individual HPM sources. In terms of HPM oscillators, there have been some experimental demonstrations of coherent combing of Cerenkov superradiances to achieve an equivalent peak power up to ∼10 GW9,10. Nevertheless, the superradiance sources have to be driven by a common accelerator to realize channel-to-channel synchronization in time, and moreover the radiation pulse duration and energy are far below the requirements of most practical applications. Compared with oscillator, the relativistic klystron amplifier (RKA)11,12,13,14, owing to its specific capability to lock the output HPM frequency and phase by input signal, is naturally suitable for coherent power combing. Previous investigations regarding RKAs have demonstrated GW-class long pulse HPM radiations in L-band and S-band13,14,15,16,17. In order to achieve a higher Pf2 factor4,7, scaling RKA to higher frequency bands (such as X-band) leads to the birth of triaxial klystron amplifier (TKA). It is expected that the TKA possesses higher power handling capacity by using coaxial interaction cavities with large radius18,19,20,21, where P and f denote HPM power and frequency, respectively. Nevertheless, the introduction of coaxial structure can easily induce self-excitation of parasitic modes (TEM mode and high order asymmetric TE modes), which could influence beam wave interaction, and result in pulse shortening as well as phase-unlocking. No significant progress therefore has been made in experiment since the proposal of TKA20,21. Furthermore, by far demonstration of phase-locking of the generated HPMs, the most important charac (...truncated)


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Jinchuan Ju, Jun Zhang, Zumin Qi, Jianhua Yang, Ting Shu, Jiande Zhang, Huihuang Zhong. Towards coherent combining of X-band high power microwaves: phase-locked long pulse radiations by a relativistic triaxial klystron amplifier, Scientific Reports, 2016, Issue: 6, DOI: 10.1038/srep30657