Zumin Qi

A non-uniform three-gap buncher cavity with suppression of transverse-electromagnetic mode leakage in the triaxial klystron amplifier

The triaxial klystron amplifier is an efficient high power relativistic klystron amplifier operating at high frequencies due to its coaxial structure with large radius. However, the coaxial structures result in coupling problems among the cavities as the TEM mode is not cut-off in the coaxial tube. Therefore, the suppression of the TEM mode leakage, especially the leakage from the buncher cavity to the input cavity, is crucial in the design of a triaxial klystron amplifier. In this paper, a non-uniform three-gap buncher cavity is proposed to suppress the TEM mode leakage. The cold cavity analysis shows that the non-uniform three-gap buncher cavity can significantly suppress the TEM mode generation compared to a uniform three-gap buncher cavity. Particle-in-cell simulation shows that the power leakage to the input cavity is less than 1.5‰ of the negative power in the buncher cavity and the buncher cavity can efficiently modulate an intense relativistic electron beam free of self-oscillations. A fundamental current modulation depth of 117% is achieved by employing the proposed non-uniform buncher cavity into an X-band triaxial amplifier, which results in the high efficiency generation of high power microwave.

An improved suppression method of the transverse-electromagnetic mode leakage with two reflectors in the triaxial klystron amplifier

Suppression of the transverse-electromagnetic (TEM) mode leakage is crucial in the design of a triaxial klystron amplifier with high gain, because a small microwave leakage from the buncher or the output cavity could overwhelm the input signal with low power. In this paper, a specially designed reflector is proposed to suppress the TEM mode leakage, whose axial electric field is approximately zero at the beam radial position. Theoretical analysis indicates that the reflector introduces little influence on the normal modulation of the beam while keeping a high reflection coefficient. By using two such reflectors with different eigen frequencies located in front of the buncher cavity and the output cavity, respectively, an improved triaxial klystron amplifier is presented. The simulation results show that the reflectors substantially decrease the TEM mode leakage power and achieve very good isolation among the cavities. The improved triaxial klystron amplifier can operate normally with 10s kW microwave injection without self-oscillations.

An Improved Small-Signal Theory of the Triaxial Klystron Amplifier in the Dispersion Relation and Beam Coupling Coefficient

An improved small-signal theory of triaxial klystron amplifiers is proposed with a reduction factor and the beam coupling coefficient being taken into account. It may be concluded that the beam coupling coefficient amends the amplitude of the modulated current and the reduction factor mainly modifies the optimum drift distance, at which the modulated current is maximum. The modified theory is validated by means of PIC codes, which indicates that the improved theory predicts the first-order modulation current more accurately than the previous theory.

Design and analysis of a radio frequency extractor in an S-band relativistic klystron amplifier

A radio frequency (RF) extractor converts the energy of a strongly modulated intense relativistic electron beam (IREB) into the energy of high power microwave in relativistic klystron amplifier (RKA). In the aim of efficiently extracting the energy of the modulated IREB, a RF extractor with all round coupling structure is proposed. Due to the all round structure, the operating transverse magnetic mode can be established easily and its resonant property can be investigated with an approach of group delay time. Furthermore, the external quality factor can be low enough. The design and analysis of the extractor applied in an S-band RKA are carried out, and the performance of the extractor is validated with three-dimensional (3D) particle-in-cell simulations. The extraction efficiency reaches 27% in the simulation with a totally 3D model of the whole RKA. The primary experiments are also carried out and the results show that the RF extractor with the external quality factor of 7.9 extracted 22% of the beam power and transformed it into the high power microwave. Better results are expected after the parasitic mode between the input and middle cavities is suppressed.

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

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.

Design and Experimental Demonstration of a Long-Pulse, X-Band Triaxial Klystron Amplifier With an Asymmetric Input Cavity

A triaxial klystron amplifier (TKA) with an asymmetric input cavity is designed to generate a long-pulse high power microwave at X-band. The input microwave is equally divided into two parts and then injected into an asymmetric input cavity to keep the axial electric field azimuthally symmetric in the input cavity. An asymmetric mode competition resulting in a pulse shortening on the output power is observed in a TKA with an asymmetric input cavity. The analysis indicates that the asymmetric mode is excited and amplified in the buncher cavity for the coaxial waveguide cannot cut off the corresponding asymmetric TE mode. By a reflector with high reflection coefficients both to the asymmetric mode and the TEM mode, the asymmetric mode competition is effectively suppressed in 3-D particle-in-cell simulation. The designed TKA is demonstrated by the experiment, in which a microwave with pulse duration of 100 ns, a power of 240 MW, and a gain of

An X-band high-impedance relativistic klystron amplifier with an annular explosive cathode

\sim 34

A large-signal theory of bunching in the triaxial klystron amplifier

dB is generated.

Analysis on the mechanism of pulse-shortening in an X-band triaxial klystron amplifier due to the asymmetric mode competition

The feasibility of employing an annular beam instead of a solid one in the X-band high-impedance relativistic klystron amplifier (RKA) is investigated in theory and simulation. Small-signal theory analysis indicates that the optimum bunching distance, fundamental current modulation depth, beam-coupling coefficient, and beam-loaded quality factor of annular beams are all larger than the corresponding parameters of solid beams at the same beam voltage and current. An annular beam RKA and a solid beam RKA with almost the same geometric parameters are compared in particle-in-cell simulation. Output microwave power of 100 MW, gain of 50 dB, and power conversion efficiency of 42% are obtained in an annular beam RKA. The annular beam needs a 15% lower uniform guiding magnetic field than the solid beam. Our investigations demonstrate that we are able to use a simple annular explosive cathode immersed in a lower uniform magnetic field instead of a solid thermionic cathode in a complicated partially shielding magnetic field for designing high-impedance RKA, which avoids high temperature requirement, complicated electron-optical system, large area convergence, high current density, and emission uniformity for the solid beam. An equivalent method for the annular beam and the solid beam on bunching features is proposed and agrees with the simulation. The annular beam has the primary advantages over the solid beam that it can employ the immersing uniform magnetic field avoiding the complicated shielding magnetic field system and needs a lower optimum guiding field due to the smaller space charge effect.

Matching Conditions of the RKA Input Cavity Based on the Cavity Absorbing Property Under Intense Beam Loading

A large-signal theory of bunching of an intense relativistic electron beam with both velocity and density modulation in the triaxial klystron has been derived in this paper. The theory presents both the harmonic current and the transient velocity in the bunching process, and expresses the velocity modulation directly by the modulation voltage that is easier to be attained and controlled in practice. A few checks are performed on the large-signal theory by particle-in-cell simulation that suggested the large-signal theory of bunching can successfully predict the first-order harmonic current and the velocity on a beam, with both velocity and density modulation, after interacting with a modulation voltage on the order of magnitude with the beam voltage. The theory described in this paper can accommodate general situations such as the bunching of a modulated beam after interacting with a buncher cavity of the Triaxial klystron amplifier.