Yanchao Shi's research while affiliated with Northwest Institute Of Textile Science And Technology and other places

The initial experimental results of asymmetric mode competition in an X -band dual-mode relativistic backward wave oscillator (RBWO) operating at the low magnetic field are presented. In the experiment, the use of receiving antennas with different polarization directions allows direct determination of asymmetric mode, and the asymmetric mode with a frequency of 10.27 GHz is excited, which is in good agreement with the particle-in-cell (PIC) simulation results. Judging from that, the competition mode is asymmetric EH $_{\text{21}}$ mode. The asymmetric mode competition mechanism in the device is investigated theoretically. It is shown that the drift length plays a key role in the suppression and excitation of the asymmetric mode, validated by the experiment and the PIC simulation.

We propose an oversized coaxial Cherenkov generator to generate ultrashort pulses with extremely high peak power exceeding 100 GW. The oversize factor of the device is about 5, which ensures sufficient power handling capacity for ultrashort pulses and a very high beam current, and thus the driving power. An optimized profiled slow wave structure (SWS) with squarely increasing corrugation amplitude, rather than linearly increasing function, and a uniform SWS with a slightly larger corrugation period are used to provide better beam-wave interaction and accomplish energy accumulation of electromagnetic wave from the gradually developed bunches precisely and continuously. The operation mode in the coaxial SWS is quasi-TEM mode, which is almost completely reflected by the coaxial reflector and coupled into the coaxial coupler with the mixed TEM mode and coaxial TM <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">01</sub> mode. This kind of single-pass output eliminates the power loss caused by conventional case as the pulse re-enters the SWS. Particle-in-cell simulations show that as the diode voltage is 760 kV and beam current is 78 kA, an ultrashort pulse of 0.4 ns with peak power of 129 GW and frequency range from 9.06-10.66 GHz is generated. The corresponding conversion factor is up to 2.18.

We investigate a klystron-like relativistic traveling wave oscillator (KL-RTWO) with a ridge extractor packaged with a permanent magnet (PM) in simulation and experiment. The result shows that the KL-RTWO with a ridge extractor is more efficient than that with a dual cavity extractor. In particle-in-cell (PIC) simulation, the ridge extractor enhances the beam-wave interaction remarkably by the electric field redistribution and matching the phase between the fundamental current harmonic and RF electric field, and the conversion efficiency is increased by 36%. In experiment, the KL-RTWO with a ridge extractor packaged with a 0.5 T PM generated 1.04 GW microwave power with conversion efficiency of 43% at the diode voltage 520 kV.

To further increase the beam-wave conversion efficiency, a super klystron-like relativistic backward wave oscillator (RBWO) with coaxial-premodulation (CPM) cavity is proposed. Compared with previous dual-premodulation (DPM) cavities, the CPM cavity intensifies the second and the third harmonic components, resulting in a larger fundamental harmonic current both in the slow-wave structure (SWS) and the extraction cavity, and, consequently, leads to an enhanced microwave power generation by the Cherenkov radiation and transition radiation. In the particle-in-cell simulation, an $\textit{X}$ -band microwave with a power of 3.44 GW and record efficiency of up to 80% has been achieved.

Radio frequency (RF) breakdown in the slow-wave structures (SWSs) is a crucial bottleneck restricting relativistic backward wave oscillator (RBWO) to pursuing higher output power and longer pulse width. This paper has experimentally studied the influence of thermal accumulation during repetitive operation on RF breakdown in an X-band RBWO. A method for cooling the SWSs using water flow has been proposed to restrain the temperature rise to some extent. Under different heat dissipation conditions, the operating states of the RBWOs exhibit great differences. The greater the distance between the water-cooling heat transfer channel and the SWSs, the more serious the pulse shortening of high-power microwave. Moreover, the breakdown traces appearing in the SWSs become more obvious with the worse convective heat transfer capacity. The observed experimental phenomena provide a new guideline that helps to enrich the mechanism of RF breakdown and to explore corresponding suppression methods in RBWO.

We propose a novel cross-band high-power microwave (HPM) generator which is based on the relativistic magnetron (RM) with all cavity axial extraction and the radial three-cavity transit-time oscillator (TTO). A foil structure is used to isolate the output microwaves of the RM and the TTO. A regulating ring is introduced before the anode block of the RM to adjust the resonant frequency of the RM. A downstream endcap is added to collect the leakage current from the RM and delay the decrease in the output power. The particle-in-cell simulations show that as the diode voltage is 420 kV, the diode current is 11.4 kA, and the magnetic field is 0.22 T, an output power of 680 MW at the L-band from the RM and 1280 MW at the S-band from the TTO have been achieved, with a total beam–wave conversion efficiency of 41%. Moreover, the RM frequency ranges for about 10% by adjusting the position of the regulating ring, and the TTO frequency bandwidth is up to 68%, which covers the S-and C-band, by changing the cavity width of the radial TTO axially.

In this article, the effect of microwave leakage into the diode region on the suppression of backward current in a relativistic backward wave oscillator (RBWO) operating at low magnetic field is investigated by the theoretical analysis, particle-in-cell (PIC) simulation, and experiments. The theoretical results show that some backward electrons are absorbed by the cathode holder due to microwave leakage, and as the microwave power increases, the microwave frequency increases, and the guiding magnetic field decreases, it will promote the absorption of electrons. The PIC simulations based on the first principle reveal that the leakage microwave partially suppresses the backward current and the suppression is enhanced when the incidence TM <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">02</sub> is dominant. In the ${X}$ -band dual-mode RBWO packaged with permanent magnet experiments, a microwave pulse with the power of 3.9 GW, the frequency of 9.93 GHz, and the efficiency of 33% was measured when the diode voltage was 820 kV and diode current was 14.5 kA.

An oversized Ku-band Cerenkov oscillator with pure TM 01 mode output is proposed. By utilizing a separated slow-wave structure (SSWS), the resonant characteristic of the operating mode is preserved, whereas the resonant characteristics of the high order electromagnetic modes are destroyed. As a result, only the expected mode can be stimulated, and undesired beam–wave interactions are suppressed effectively. In terms of an oversized Cerenkov oscillator, the usage of SSWS shows great potential for obtaining pure operating mode output. By utilizing particle-in-cell simulation, microwaves with an output power of 4.5 GW and frequency of 14.1 GHz are obtained, when the beam voltage is 0.9 MeV, and the beam current is 12.9 kA. The percentage of the operating mode is up to 99.5% and exceeds 99% in a wide range of the beam voltage.

Increasing the dimensions of high power microwave devices is an efficient method to improve the power capacity. However, an overmoded structure usually results in mode competition and a low beam-wave conversion efficiency. In this paper, a multi-mode operation mechanism is used to avoid mode competition and increase the efficiency. The calculation results of nonlinear theory of beam-multimode interaction show that the optimized conversion efficiency is up to 48% when TM 01 mode, TM 02 mode, and TM 03 mode are all considered. As only the TM 01 mode, TM 02 mode, or TM 03 mode is taken into account independently, the corresponding efficiency is 38%, 22%, or 20%. Based on this, a multi-mode relativistic backward wave oscillator is proposed with the ratio of the mean diameter of the slow wave structure (SWS) to the wavelength of the output microwave to be 3.5. The non-uniform SWS is used to increase the beam-wave conversion efficiency, and a combined reflector is adopted to reflect partial of the mixed microwave modes and make the device compact. The particle-in-cell simulations show that as the diode voltage is 1.1 MV, the beam current is 22.8 kA, and the external magnetic field is 0.76 T, the conversion efficiency is 45%, and the output microwave of 11.3 GW is the mixed modes of TM 01 mode, TM 02 mode, and TM 03 modes with the corresponding power ratio of 74%, 7%, and 19%, respectively.

A theory regarding a non-uniform magnetic field with a parallel gradient is presented. The research results show that a proper non-uniform magnetic field can greatly reduce the transverse momentum of an electron beam and even eliminate its gyration motion, and it depends on the gradient of the magnetic field and the phases of electrons entering and leaving the local magnetic field region. Thus, a magnetic field that decreases along the axial direction is proposed to suppress the radial oscillation of the electron beam. However, in the drift tube, the suppression of the radial oscillation is not obvious, because the large phase differences among electrons lead to a mismatch between the electron beam and the non-uniform magnetic field. Further studies found that the non-uniform magnetic field applied in the anode-cathode gap can not only reduce the phase differences among electrons, but also effectively transform the transverse momentum of the electron beam into its axial momentum. The results obtained by PIC simulations and experiments consistently confirm that the non-uniform magnetic field can significantly suppress the radial oscillation of the electron beam in a low-magnetic-field foilless diode.