https://aip.scitation.org/doi/pdf/10.1016/j.mre.2016.04.001

Research progresses on Cherenkov and transit-time high-power microwave sources at NUDT

All-metal beam steering antenna based on the concept of Risley prism is presented in this communication. The antenna is composed of a feed antenna and two metal lenses. Mechanically rotate two lenses along the same axis independently can steer the radiation beam in both azimuth and elevation planes. Simulations and measurements were carried out in X-band to verify its performance, and the results agree each other well, which are also in agreement with the theoretical results. Furthermore, the power handling capability of the lens antenna with a dimension of $1~\text {m} \,\, \times \,\, 1 \,\, \text {m}$ is estimated about 4 GW, which indicates the all-metal beam steering antenna has the application potentials for high-power microwave applications.

All-Metal Beam Steering Lens Antenna for High Power Microwave Applications

Even after 50 years of development, narrowband high-power microwave (HPM) source technologies remain the focus of much research due to intense interest in innovative applications of HPMs in fields such as directed energy, space propulsion, and high-power radar. A few decades ago, the main aim of investigations in this field was to enhance the output power of a single HPM source to tens or hundreds of gigawatts, but this goal has proven difficult due to physical limitations. Therefore, recent research into HPM sources has focused on five main targets: phase locking and power combination, high power efficiency, compact sources with a low or no external magnetic field, high pulse energy, and high-power millimeter-wave generation. Progress made in these aspects of narrowband HPM sources over the last decade is analyzed and summarized in this paper. There is no single type of HPM source capable of excellent performance in all five aspects. Specifically, high pulse energy cannot be achieved together with high power efficiency. The physical difficulties of high power generation in the millimeter wave band are discussed. Semiconductor-based HPM sources and metamaterial (MTM) vacuum electron devices (VEDs) are also commented on here. Semiconductor devices have the advantage of smart frequency agility, but they have low power density and high cost. MTM VEDs have the potential to be high power efficiency HPM sources in the low frequency band. Moreover, problems relating to narrowband HPM source lifetime and stability, which are the important determinants of the real-world applicability of these sources, are also discussed.

Progress in narrowband high-power microwave sources

There is a continuous interest in research of electron sources which can be used for generation of uniform electron beams produced at E≤105 V/cm and duration ≤10−5 s. In this review, several types of plasma electron sources will be considered, namely, passive (metal ceramic, velvet and carbon fiber with and without CsI coating, and multicapillary and multislot cathodes) and active (ferroelectric and hollow anodes) plasma sources. The operation of passive sources is governed by the formation of flashover plasma whose parameters depend on the amplitude and rise time of the accelerating electric field. In the case of ferroelectric and hollow-anode plasma sources the plasma parameters are controlled by the driving pulse and discharge current, respectively. Using different time- and space-resolved electrical, optical, spectroscopical, Thomson scattering and x-ray diagnostics, the parameters of the plasma and generated electron beam were characterized.

Pulsed Plasma Electron Sources

This paper describes a new design of a relativistic Cherenkov oscillator with strong modulation of the e-beam current at the input of two-sectioned slow-wave structure, as well as numerical and experimental results on generation of few gigawatt-level $X$ -band pulses with pulse duration of up to 35 ns. Due to special phase shift between two sections and electrodynamic coupling between $E_{01}$ surface wave and $E_{02}$ volume mode, the first one is locked at the upper boundary of the passband and the second one gains more transparency. An electron beam can interact simultaneously with two waves with higher power conversion efficiencies, which in simulations amount up to 47% and 30% for high (~4 T) and low (~1 T) guiding magnetic fields, correspondingly. The tests of the geometry optimized in simulations were performed on the base of SINUS-7 high-current electron accelerator in the diode voltage range of 400–850 kV and current from 7 to 16 kA. The repeated and full-pulsewidth generation of about 1.5-GW microwave power is achieved only in the low magnetic field. The pulse shortening and the mode competition took place in the strong magnetic field. The results are considered as preliminary for creation of the efficient relativistic millimeter-wave oscillators at magnetic fields below the cyclotron resonance.

Two-Wave Cherenkov Oscillator With Moderately Oversized Slow-Wave Structure

Recent experimental results of three kinds of long-pulse high-power microwave (HPM) sources operating in S-, C-, and X-bands are reported. The difficulties in producing a long-pulse HPM for the O-type Cerenkov HPM source were analyzed theoretically. In S- and C-bands, single-mode relativistic backward-wave oscillators were designed to achieve long-pulse HPM outputs; in X-band, because of its shorter wavelength, an O-type Cerenkov HPM source with overmoded slow-wave systems was designed to increase power capacity. In experiments, driven by a repetitive long-pulse accelerator, both S- and C-band sources generated HPMs with power of about 2 GW and pulse duration of about 100 ns in single-shot mode, and the S-band source operated stably with output power of 1.2 GW in 20-Hz repetition mode. The X-band source generated 2 GW microwaves power with pulse duration of 80 ns in the single-shot mode and 1.2 GW microwave power with pulse duration of about 100 ns in the 20-Hz repetition mode. The experiments show good performances of the O-type Cerenkov HPM source in generating repetitive long-pulse HPMs, especially in S- and C-bands. It was suggested that explosive emissions on surfaces of designed eletrodynamic structures restrained pulse duration and operation stability.

Recent Advance in Long-Pulse HPM Sources With Repetitive Operation in S-, C-, and X-Bands

Effects of vacuum pressures on the performance of a velvet cathode under repetitive high-current pulse discharges

A repetitive ten-stage wave erection Marx generator is developed to investigate the electrical characteristics of such compact devices and potentially provide an economical approach to realize the miniaturization of intense electron beam accelerators. Compact design has been made for the generator in order to achieve a proper stray capacitance of the spark gap electrode with respect to the ground in each stage because these proper grounded stray capacitances are critical for obtaining a good wave erection process. This generator is initially resistively isolated for single-shot tests and then changed to inductively isolated for repetitive operation. In single-shot experiments, the generator is tested to be able to deliver a high-voltage pulse of 210 kV and a rise time of about 5 ns on a 90-Ω dummy load at a charging voltage of 40 kV. This result agrees basically with that of the PSpice circuit simulation, which adopts a self-breakdown spark gap model. The preliminary experimental results of repetitive operation show that at a charging voltage of 30 kV, the generator can operate at 8.5 Hz without gas blow-off from the internal spark gaps, producing an output pulse of 150 kV and a rise time of less than 20 ns. Differences in the output pulse waveforms between resistively and inductively isolated configurations are analyzed.

Development of a Repetitive Wave Erection Marx Generator

A repetitive ten-stage wave erection Marx generator is developed to investigate the electrical characteristics of such compact devices and potentially provide an economical approach to realize the miniaturization of intense electron beam accelerators. Compact design has been made for the generator in order to achieve a proper stray capacitance of the spark gap electrode with respect to the ground in each stage because these proper grounded stray capacitances are critical for obtaining a good wave erection process. This generator is initially resistively isolated for single-shot tests and then changed to inductively isolated for repetitive operation. In single-shot experiments, the generator is tested to be able to deliver a high-voltage pulse of 210 kV and a rise time of about 5 ns on a 90-Omega dummy load at a charging voltage of 40 kV. This result agrees basically with that of the PSpice circuit simulation, which adopts a self-breakdown spark gap model. The preliminary experimental results of repetitive operation show that at a charging voltage of 30 kV, the generator can operate at 8.5 Hz without gas blow-off from the internal spark gaps, producing an output pulse of 150 kV and a rise time of less than 20 ns. Differences in the output pulse waveforms between resistively and inductively isolated configurations are analyzed.

Experimental studies on a designed pulsed power modulator with a compact Tesla transformer in single and repetitive modes from low voltages to high voltages are experimentally carried out in turn The experimental results are presented and analyzed The conclusions are shown as follows: (1) the modulator's highest output voltage across a 100 Ω resistive load in single mode is obtained to be 330 kV with 7 ns pulse duration and 2 ns rise-time; (2) the cooling system and gas blowing system are unnecessary and not adopted experimentally in repetitive mode in the case that the number of pulses in a batch is less than or equal to 300, as a result, the volume and weight of the modulator system is reduced; (3) in the range of 300 pulses in a batch, with the increase of the number of pulses in repetitive mode, the average value of the output voltage declines and the pulse-to-pulse instability of the output voltage decreases, at the same time, the decline degree of the output voltage increases and reaches to be stable when the number of pulses in a batch is more than 50; (4) with the increase of the repetition rate, the pulse-to-pulse instability of the output voltage increases, and the average value of the output voltage declines, especially after the repetition rate exceeds 40 Hz; (5) pressurization can significantly improve the characteristics in repetitive mode and the improved level of the characteristics is almost the same for different pressurization level; (6) the pulsed power modulator can steadily output 310 kV for 300 pulses in 40 Hz repetition rate

Jiande Zhang

Papers

Recent Advance in Long-Pulse HPM Sources With Repetitive Operation in S-, C-, and X-Bands

Effects of vacuum pressures on the performance of a velvet cathode under repetitive high-current pulse discharges

Development of a Repetitive Wave Erection Marx Generator

Development of a Repetitive Wave Erection Marx Generator

Experimental studies on a repetitive pulsed power modulator with a compact Tesla transformer