Investigation Of An X-band Triaxial Klystron Amplifier

The spatial coherent power combining is the significant development trend of the high power power(HPM) technology. As the core device, the relativistic klystron amplifier(RKA) is widely investigated. However, due to the restriction of the power capability, the investigations focus on the low band, such as S band at present. To pursue a high pf~2 factor, a RKA with high frequency, high power and long pulse should be developed. Therefore, this dissertation proposes a technical route of an X-band triaxial klystron amplifier(TKA). The dissertation focuses on the significant technology problems, such as the design of the coaxial cavities, the TEM mode isolation, the suppression of the asymmetric mode competition, etc. The X-band TKA is demonstrated by the experiments. The investigations of this dissertation can provide significant guides for the design of the HPM amplifiers at higher frequency band.1. The beam coupling coefficient and the beam loading conductance of the coaxial cavity are analyzed based on the single particle theory, by which the unique characteristics of the coaxial cavity compared to the hollow cavity are obtained. Considering the beam coupling coefficient and a reduction factor, a small-signal theory is modified to describe the interaction between an intense relativistic electron beam(IREB) and the input cavity of the TKA. The maximum of the first-order harmonic current and the optimized drift distance are attained by the small-signal theory. A large-signal theory is proposed to describe the bunching process of a strongly modulated IREB in the coaxial waveguide, by which the modulation characteristics of the velocity and density of the beam are obtained.2. An asymmetric input cavity with two injection ports, a three-gap buncher cavity and two output cavities with different gaps are designed and analyzed. An azimuthal uniformity of 92% is achieved in an asymmetric input cavity. A non-uniform three-gap buncher cavity is proposed, which not only can modulate the beam efficiently, but also can achieve low TEM mode leakage to the input cavity. When the output power of the output cavity is ~1GW, the maximum axial electric in the output cavity with double gaps is 560 k V/cm, which is about 37% lower than that of the output cavity with single gap.3. The TEM mode leakage in the TKA is systematicly investigated by theoretical and simulation methods. For the limitation of the TEM mode isolation method by spreading lossy material on the surfaces of the coaxial waveguide, a new method is proposed by locating two TEM mode reflectors with different eigen frequencies in the TKA. When the input power is 100 k W and the output power is 1.04 GW, the TEM leakage power from the buncher cavity to the input cavity reduces to about 50 k W, which is only 0.35 ‰ of the maximum backward power in the buncher cavity. The TEM leakage power from the output cavity to the bucnher cavity reduces to about 2.5MW, which is 0.2 % of the maximum backward power in the output cavity. The results reveal the TEM mode isolation effect with reflectors is remarkable.4. The generation mechanism of the asymmetric mode competition in the TKA is analyzed. The results indicates the asymmetric mode is generated in the three-gap buncher cavity and amplified for the coaxial wavguide between the buncher cavity and the input cavity providing a positive feedback channel. A coaxial TE mode reflector and a slotted coaxial waveguide are respectively proposed to suppress the asymmetric mode competition.5. The influence of the high-frequency characteristics on the phase shift is studied by theoretical and PIC simulation methods. The results indicate that the eigen frequency of the buncher cavity with high Q0 factor can significantly affect the phase shift of the output microwave in the TKA.6. The performance characteristic of the X-band TKA is simulated by a PIC code. A microwave with a power of ~ 1 GW is generated by an IREB with a voltage of 570 k V and a current of 6.5 k A. The frequency and power of the input microwave are 100 k W and 9.375 GHz, respectively. The extraction efficiency is over 29 % and the gain is about 40 d B. The fluctuation range the phase does not exceed 3°. The influences of the parameters of the structure, the input microwave, the beam and the strength of the coil magnetic field on the output characteristics of the device are investigated. The results indicate that these parameters would not affect the performance of the device except the axial offset between the inner and outer conductors of the device.7. The designed X-band TKA is investigated by experiments. The cold test results indicate that the frequency and the Qe factor of the input cavity are 9.375 GHz and 120, respectively. The frequency and the Q0 factor of the bucnher cavity are 9.42 GHz and 493, respectively. The frequency and the Qe factor of the one-gap output cavity are 9.375 GHz and 45, respectively. When the power and the frequency of the input microwave are 90 k W and 9.37 GHz, respectively, a microwave with a power of ~ 240 MW and pulse duration of ~ 100 ns is generated. The difference of the output powers between the experimental and simulation results mainly results from the significant resistive loss in the input cavity. By an improved input cavity with eigen frequency of 9.37 GHz and Qe factor of 309, a microwave with a power of ~ 920 MW and pulse duration of ~ 52 ns is generated. The power and the frequency of the input microwave are 50 k W and 9.38 GHz, respectively. The gain and extraction efficiency are 42.6 d B and 23 %, respectively. The frequency of the experimental output microwave is same with that of the input microwave. The phase jitter within ±10° is gained from shot to shot.