Yuqun Deng

Power combiner with high power capacity and high combination efficiency for two phase-locked relativistic backward wave oscillators

To realize power combination of two phase-locked relativistic backward wave oscillators (RBWOs), a compact power combiner is designed and investigated by 3-D particle-in-cell (PIC) simulation and experiment. The power combiner consists of two TM01-TE11 serpentine mode converters with a common output. When the two incident ports are fed with TM01 modes with a relative phase of 180° and power of 2.5u2009GW at each port, the conversion efficiency from the incident TM01 modes to the combined TE11 mode is 95.2% at 9.3u2009GHz, and the maximum electric field in the combiner is 714u2009kV/cm. The PIC simulation shows that the output power from the common port is 4.2u2009GW when the power combiner is connected to the two RBWOs with input signals, both producing 2.2u2009GW microwave, corresponding to a combination efficiency of 95.4%. In the high power microwave test, a method is proposed to obtain the combination efficiency without breaking the vacuum, which is 94.1% when the two phase-locked RBWOs output 1.8u2009GW and 2.2u2009GW. The powe...

Mechanism of phase control in a klystron-like relativistic backward wave oscillator by an input signal

Theoretical analyses and particle-in-cell (PIC) simulations are carried out to understand the mechanism of microwave phase control realized by the external RF signal in a klystron-like relativistic backward wave oscillator (RBWO). Theoretical calculations show that a modulated electron beam can lead the microwave field with an arbitrary initial phase to the same equilibrium phase, which is determined by the phase factor of the modulated current, and the difference between them is fixed. Furthermore, PIC simulations demonstrate that the phase of input signal has a close relation to that of modulated current, which initiates the phase of the irregularly microwave during the build-up of oscillation. Since the microwave field is weak during the early time of starting oscillation, it is easy to be induced, and a small input signal is sufficient to control the phase of output microwave. For the klystron-like RBWO with two pre-modulation cavities and a reentrant input cavity, an input signal with 100 kW power and 4.21 GHz frequency can control the phase of 5 GW output microwave with relative phase difference less than 6% when the diode voltage is 760 kV, and beam current is 9.8 kA, corresponding to a power ratio of output microwave to input signal of 47 dB.

An X-band overmoded relativistic klystron

An X-band overmoded relativistic klystron is proposed, the operation mode of which is the TM02 mode. The drift tube could not cut off the TM01 mode; isolating the buncher cavity from the input cavity is achieved by introducing a sectional RF lossy material. Microwaves are extracted from the modulated electron beam using a cylindrical waveguide, rather than a coaxial waveguide; thereby, the output structure is significantly simplified. Particle-in-cell simulations show that microwaves with power of 1.28u2009GW and frequency of 9.30u2009GHz can be obtained, corresponding to an efficiency of 32% and relative bandwidth of about 8%.

Axial motion of collector plasma in a relativistic backward wave oscillator

In this paper, it is proposed that plasma formed at the collector may drift back to the cathode and cause pulse shortening of the relativistic backward wave oscillator. Theoretical analysis shows that the axial drift velocity of plasma ions can be up to 5u2009mm/ns due to the presence of space charge potential provided by an intense relativistic electron beam. Particle-in-cell simulations indicate that the plasma electrons are initially trapped around the collector surface. With the accumulation of the plasma ions, a large electrostatic field forms and drives the plasma electrons to overcome the space charge potential and enter the beam-wave interaction region along the magnetic field lines. As a result, the beam current modulation is disturbed and the output microwave power falls rapidly. The plasma ions move in the beam-wave interaction region with an average axial velocity of 5–8u2009mm/ns. After the plasma ions reach the diode region, the emitted current at the cathode rises due to the charge neutralizations ...

Particle-in-Cell Demonstration of the Effect of Voltage Rise Time on Phase Synchronization in Two Parallel Relativistic Backward-Wave Oscillators

This paper demonstrates the effect of voltage rise time on phase synchronization in two parallel relativistic backward-wave oscillators (RBWOs) through 3-D particle-in-cell simulations. With a short rise time, the axially symmetrically mode is excited, the frequency spectrum is pure for both RBWOs, and phase synchronization between the two microwaves is realized. As the rise time increases, asymmetric modes appear in one or two channels, mode competition occurs, and phase synchronization cannot be achieved. This can be explained as the transient excitations in the RBWOs initiated by the current variation in beam head, whose magnitudes are inversely proportional to the rise time. It is suggested that phase synchronization can be obtained at a larger voltage rise time in the RBWOs with stronger resonant characteristics. Furthermore, an external signal can suppress the asymmetric mode and control the relative phase of output microwaves in the two RBWOs.

Influence of voltage rise time on two parallel klystron-like relativistic backward wave oscillators

The influence of voltage rise time on two parallel klystron-like relativistic backward wave oscillators (RBWOs) is investigated through 3-D particle-in-cell (PIC) simulations. With a short voltage rise time, the axially symmetrically mode is excited, the frequency spectrum is pure, and phase locking is realized for both RBWOs. As the rise time increases, asymmetric modes appear in one or two channels, modes competition occurs, and the output power reduces. This can be explained as the transient excitation in the resonant reflector initiated by the current change in beam head, which generates an induced voltage with magnitude inversely proportional to the rise time. Furthermore, the induced voltage caused by the beam head is equivalent to an externally injected radio frequency signal. At a large rise time, an external signal can also suppress the asymmetric modes and lock the two RBWOs.

Generation of powerful microwave pulses by channel power summation of two X-band phase-locked relativistic backward wave oscillators

We demonstrate both theoretically and experimentally the possibility of the generation of powerful microwave pulses by channel power summation of two X-band phase-locked relativistic backward wave oscillators (RBWOs). A modulated electron beam induced by an external signal can lead the microwave field with an arbitrary initial phase to the same equilibrium phase, which is determined by the initial phase of the external signal. A high-current dual-beam accelerator was built to drive the two RBWOs. An external signal was divided into two channels with an adjusted relative phase and injected into the two RBWOs through two TE10-TEM mode converters. The generated microwaves were combined with a power combiner consisting of two TM01-TE11 serpentine mode converters with a common output. In the experiments, as the input power for each channel was 150u2009kW, the two RBWOs output 3.1u2009GW and 3.7u2009GW, respectively, the jitter of the relative phase of two output microwaves was about 20°, and the summation power from the p...

Research on origination of oscillations and microwave growth in weakly resonant RBWOs

The origination of oscillations and microwave growth in weakly resonant relativistic backward wave oscillators (RBWOs) are investigated theoretically. First, we investigate the electromagnetic radiation of electrons during the oscillation starting process in RBWOs. It is found that the initial microwave power in the X band RBWO is at the level of 1u2009μW, and the X band RBWO needs about 6.0–16u2009ns when the output power reaches up to 1.0u2009GW theoretically. Furthermore, based on the nonlinear circuit, we simulate the microwave growth and obtain the analytical relationship between the starting time and quality factor of the RF cavity in the weakly resonant RBWO. It suggests that, for the weakly resonant RBWO, the effect of the quality factor on the starting process is more prominent. It can be estimated that, for the X band RBWO, the suitable quality factor of the RF cavity may be in the range of 20–40. The theoretical results agree well with particle-in-cell results.

Influence of voltage rise time on phase locking by priming effect in weakly resonant relativistic backward wave oscillators

Phase locking is the key point of coherent power combination, which is very important for the development of high power microwave sources. In this paper, theoretical analysis and particle-in-cell simulations investigate the influence of the diode voltage rise time on phase locking by the priming effect in a weakly resonant relativistic backward wave oscillator (RBWO). When the diode voltage rise time becomes long and the final output frequency remains unchanged, the initial operation frequency may fluctuate around a value which is not equal to the final output frequency. Moreover, this state may last for several nanoseconds and then jumps to the final output frequency, which is very important for phase locking. Besides, it is suggested that, due to the weak resonance of the RF cavity without the electron beam, the microwave signal with frequency which is much lower than the final output frequency is usually excited at the beginning of the starting process. Finally, it is found that, when the injected freq...

A relativistic backward wave oscillator for directly generating circularly polarized TE11 mode

A relativistic backward wave oscillator for directly generating circularly polarized TE11 mode is proposed. In the device, the electrodynamics structures are divided into two groups in azimuth, each group consisting of two opposite 90° sectors, to produce two orthogonal TE11 modes. The axial position of the two groups is shifted to each other with a quarter of slow wave structure period to achieve a 90° phase difference between the two orthogonal TE11 modes. In particle-in-cell simulation, a circularly polarized TE11 mode with 1.5u2009GW power has been demonstrated. The amplitude ratio between the two orthogonal TE11 modes is smaller than 0.5u2009dB, and the phase difference is close to 90°.