Jinchuan Ju

Simulation of a gigawatt level Ku-band overmoded Cerenkov type oscillator operated at low guiding magnetic field

We present the simulation results of a Ku-band overmoded Cerenkov type high power microwave oscillator. A guiding magnetic field as low as 0.6 T has been operated in the device. Overmoded slow wave structures with gradually tapered vanes are used in order to increase power capacity and the efficiency of beam-wave interaction. The drift cavity is adopted to enhance the beam-wave interaction of the device. After numerical optimization, the designed generator with an output microwave power of 1.2 GW, a frequency of 13.8 GHz, and a power conversion efficiency as high as 38% can be achieved, when the diode voltage and current are, respectively, 540 kV and 5.8 kA. The power compositions of TM{sub 0n} modes of the output microwave have been analyzed, the results of which show that TM{sub 01} mode takes over almost 95% of the power proportion.

An improved X-band magnetically insulated transmission line oscillator

An improved Ku-band magnetically insulated transmission line oscillator (MILO) was designed and presented in this paper. To eliminate the electrode erosion in the load region, an improved limited load is presented. Theoretical calculations show that both the current density and the energy deposition density of the improved load are much smaller than those of the traditional plane diode load, so the electrode erosion can be eliminated or greatly reduced. An improved coaxial extractor with a gradient inner conductor is introduced to achieve efficient microwave extraction and a single coaxial TEM output. Simulation results show that the right amount of microwave reflection generated by the gradient inner conductor helps to strengthen the oscillation of the resonant cavities and the beam-wave interaction, then enhancing the microwave output efficiency. The improved Ku-band MILO is optimized numerically with a particle-in-cell code. Typical simulation results show that when the input voltage is 450 kV, the beam current is 48 kA, and high-power microwave is produced with an average power of 2.7 GW, an efficiency of 12.5%, and a frequency of 12.5 GHz.

A Novel Dual-Frequency Magnetically Insulated Transmission Line Oscillator

In this paper, a novel dual-frequency magnetically insulated transmission line oscillator (MILO) is presented and investigated to generate two separate, stable, and pure high-power microwaves (HPMs) in high-frequency bands. The proposed device is derived from the L-band complex MILO put forward by Fan According to the operation principle, the dual-frequency MILO is divided into two MILOs (MILO-1 and MILO-2). The MILO-2 (X-band MILO) is studied first, where a new load is introduced to keep it from disruption by anode plasma in the load region. Then, the dual-frequency MILO model is overall analyzed and optimized. Results of particle-in-cell simulation show that when the dual-frequency MILO is driven by an electron beam with 610 kV and 82 kA, two HPMs are generated with a total power of 5.9 GW, and power conversion efficiency is about 11.8%. HPM of MILO-1 falls in C-band of 7.6 GHz with a power of 3.2 GW, and that of MILO-2 lies in X-band of 9.26 GHz with a power of 2.7 GW. Power difference between the two HPMs is about 0.7 dB. Time-frequency analysis shows that no frequency interference between MILO-1 and MILO-2 occurs. The results in this paper verify the feasibility of a high-efficiency dual-high-frequency MILO. Correlative experiments are being prepared in our laboratory.

A high efficiency Ku-band radial line relativistic klystron amplifier

To achieve the gigawatt-level microwave amplification output at Ku-band, a radial-line relativistic klystron amplifier is proposed and investigated in this paper. Different from the annular electron beam in conventional axial relativistic klystron amplifiers, a radial-radiated electron beam is employed in this proposed klystron. Owing to its radially spreading speciality, the electron density and space charge effect are markedly weakened during the propagation in the radial line drift tube. Additionally, the power capacity, especially in the output cavity, is enhanced significantly because of its large volume, which is profitable for the long pulse operation. Particle-in-cell simulation results demonstrate that a high power microwave with the power of 3 GW and the frequency of 14.25 GHz is generated with a 500 kV, 12 kA electron beam excitation and the 30 kW radio-frequency signal injection. The power conversion efficiency is 50%, and the gain is about 50 dB. Meanwhile, there is insignificant electron beam self-excitation in the proposed structure by the adoption of two transverse electromagnetic reflectors. The relative phase difference between the injected signals and output microwaves keeps stable after the amplifier saturates.

Suppression of the asymmetric modes for experimentally achieving gigawatt-level radiation from a Ku-band Cerenkov type oscillator.

We present the analysis and suppression of asymmetric modes in a Ku-band Cerenkov-type oscillator numerically and experimentally. The asymmetric modes generated in the initial experiments were identified to be HE11, HE21, and HE31 modes, respectively, by analyzing of the dispersion relationships, the simulation results and the experiment phenomenon. The factors, such as the cathode emission uniformity, the diode voltage, guiding magnetic field, and the concentricity play key roles in the excitation and suppression of these asymmetric modes. In the improved experiments, the asymmetric modes were suppressed effectively. In the improved experiments the asymmetric modes are suppressed effectively, and the designed TM01 mode microwave is generated at a frequency of 13.76 GHz with a power of 1.1 GW, which is in good agreement with numerically predications.

Asymmetric modes decomposition in an overmoded relativistic backward wave oscillator

Most of the investigated overmoded relativistic backward wave oscillators (RBWOs) are azimuthally symmetric; thus, they are designed through two dimensional (2-D) particle-in-cell (PIC) simulations. However, 2-D PIC simulations cannot reveal the effect of asymmetric modes on beam-wave interaction. In order to investigate whether asymmetric mode competition needs to be considered in the design of overmoded RBWOs, a numerical method of determining the composition of both symmetric and asymmetric modes in three dimensional (3-D) PIC simulations is introduced in this paper. The 2-D and 3-D PIC simulation results of an X-band overmoded RBWO are analyzed. Our analysis indicates that the 2-D and 3-D PIC simulation results of our device are quite different due to asymmetric mode competition. In fact, asymmetric surface waves, especially EH11 mode, can lead to serious mode competition when electron beam propagates near the surface of slow wave structures (SWSs). Therefore, additional method of suppressing asymmetric mode competition, such as adjusting the reflections at both ends of SWSs to decrease the Q-factor of asymmetric modes, needs to be utilized in the design of overmoded RBWOs. Besides, 3-D PIC simulation and modes decomposition are essential for designing overmoded RBWOs.

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.

Gigawatt-class radiation generated by a Ka-band overmoded Cherenkov-type high power millimeter wave generator

Particle simulation and experimental results are presented about a Ka-band overmoded Cherenkov-type high power millimeter wave generator in this paper. The relativistic electron beam with peak current of 8.4 kA was generated by a pulsed power accelerator working at the voltage of 625 kV, which was guided by an axial magnetic field of 1.05 T and transported through the beam-wave interaction structures. After careful calibration, the microwave power radiated in the far field was as high as about 500 MW, with a frequency of 32.1 GHz and a pulse width of 20 ns. The radiation mode was well controlled to be TM(0n) mode.

Characterization of Cesium Iodide-Coated Carbon-Fiber Aluminum Cathode for an

Carbon-fiber-aluminum (CFA) cathode is characterized to generate high-power microwave (HPM) from a high-efficiency three-cavity virtual-cathode oscillator (vircator). Plasma expansion velocity associated with the cesium iodide (CsI)-coated CFA cathode is determined in the experiment. It is found that the stationary plasma expansion velocity is diminished by CsI-coating to about 3 cm/μs when driving by a 300-kV electrical pulse. Particle-in-cell (PIC) simulations indicate that the proposed vircator is capable of generating 550-MW S-band HPM at a central frequency of 2.1 GHz, when the diode voltage and current are 420 kV and 9.2 kA, respectively. The corresponding power conversion efficiency is as high as 14%. The generated HPM is with the TE10 mode of rectangular waveguide, which can give rise to an on-axis radiation pattern without requiring mode converter.

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In this paper, a modified numerical method is used to investigate the mode composition of a Gigawatt-class Ka-band overmoded Cerenkov oscillator, which has been proposed and studied in our previous experiment. In the experiment, the measured angular distribution of radiation did not fit a single TM01 mode. So the particle in cell code calculations and the antenna radiation calculations are carried out, which show a consistent picture: the dominant modes are the TM01 mode and the TM03 mode, and their phase relationship is constant with time; therefore, a steady radiation pattern is produced, which matches the experimental data. As a conclusion, the comprehensive analysis shows that the existing modes of the output microwave in our experiment are the first five TM0 n modes (n = 1–5), with corresponding power ratios of 36.64%, 0.78%, 56.26%, 5.70%, and 0.52%, and relative phase differences of 0°, 146°, 54°, 169°, and 133°.