Chengwei Yuan

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

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 progress of the improved magnetically insulated transmission line oscillator

The improved magnetically insulated transmission line oscillator (MILO) is a gigawatt-class L-band high power microwave tube driven by a 550 kV, 57 kA, 50 ns electron beam. It has allowed us to generate 2.4 GW pulse of 22 ns duration. The recent progress of the improved MILO is presented in this paper. First, a field shaper cathode is introduced into the improved MILO to avoid the cathode flares in the triple point region. The experimental results show that the cathode flares are avoided, so the lifetime of the velvet cathode is longer than that of the taper cathode. Furthermore, the shot-to-shot reproducibility is better than that of the taper cathode. Second, In order to prolong the pulse duration and increase the radiated microwave power, a self-built 600 kV, 10 Omega, 80 ns pulser: SPARK-03 is employed to drive the improved MILO. Simulation and experimental investigation are performed. In simulation, when the improved MILO is driven by a 600 kV, 57 kA electron beam, high-power microwave is generated with output power of 4.15 GW, frequency of 1.76 GHz, and relevant power conversion efficiency of 12.0%. In experiments, when the diode voltage is 550 kV and current is 54 kA, the measured results are that the radiated microwave power is above 3.1 GW, the pulse duration is above 40 ns, the microwave frequency is about 1.755 GHz, and the power conversion efficiency is about 10.4%.

Experimental Investigation of an Improved MILO

The magnetically insulated line oscillator (MILO) is an attractive high-power microwave source. It is a compact lightweight gigawatt-class coaxial crossed field device that needs no externally applied magnetic field to insulate electron flow in a slow-wave structure. An improved MILO model has been presented by Fan, Yuan and Zhong. A novel beam dump, a one-cavity RF choke section, and a novel mode-transducing antenna are introduced into the improved MILO. In simulation, high-power microwave of TEM mode is generated with peak power of 4.2 GW, frequency of 1.76 GHz, and peak power conversion efficiency of 12% when the voltage is 600 kV and the current is 52 kA. The TEM mode from the extractor gap is converted into a coaxial TE11 mode and radiated directly by the mode-transducing antenna. The direction of the radiated microwave agrees with the axis of the MILO. The antenna gain is 17.6 dBi at 1.76 GHz in simulation. The experiments have been carried out on the improved MILO device, which had been fabricated in accordance with the optimized configuration. The detailed experimental results are discussed in this paper. The improved MILO is driven by a self-built 600-kV, 10-Omega, 50-ns pulser: SPARK-04, a capacitor- and transformer-driven coaxial-water-line machine in our laboratory. The radiated microwave was detected with crystal detectors in the far-field region. The improved MILO has been extensively investigated by experiments. In the experiments, the measured microwave frequency ranges from 1.74 to 1.78 GHz, with a peak power level of above 2.4 GW, when the diode voltage is 550 kV and the current is 57 kA. The pulse duration (full-width at half-maximum) of the radiated microwave is 22 ns. The cold test and hot test results of the mode-transducing antenna are in good agreement with the simulational results. The mode of the radiated microwave is TE11 mode, and the direction of the radiated microwave overlaps with the axis of the MILO device. The antenna gain is about 17.4 dBi at 1.76 GHz. The 3-dB beam widths are 21.2deg in E-plane and 26.3deg in H-plane, respectively. No obvious breakdown appeared in the region of the mode-transducing antenna and the region of the interface of the vacuum-air in the experiments. The experimental results confirm the ones predicted by simulation.

Repetition rate operation of an improved magnetically insulated transmission line oscillator

In order to investigate the performances of repetition rate (rep-rate) operation of an improved magnetically insulated transmission line oscillator (MILO), a series of experiments are carried out on the improved MILO device, which is driven by a 40 Ω, 50 ns rep-rate pulser: TORCH-01. Polymer velvet and graphite cathodes are tested respectively in the experiments, whose diameters and lengths are the same. The results of experimental comparison between them are presented in the paper. Both cathodes are tested at electric field strengths of about 300kV/cm. The applied voltage has 60 ns duration with a rise time of 10 ns. This paper focuses on the performance of the voltage and current characteristics, the shot-to-shot reproducibility, the pressure evolution of the diode, and the lifetime of the cathodes, not upon the radiated microwave power. The experimental results show that the graphite cathode is superior to the velvet cathode in the lifetime and the shot-to-shot reproducibility during the rep-rate operation, and it is a promising cathode for the MILO device under the rep-rate conditions.

Simulation Investigation of an Improved MILO

Magnetically insulated line oscillator (MILO) is a gigawatt-class high-power microwave source whose behavior has been investigated experimentally and numerically. This paper presents an improved MILO model. A novel beam dump, a one-cavity RF choke section and a novel mode-transducing antenna are introduced into the improved MILO. The improved MILO is investigated in detail with particle-in-cell method (KARAT code). In simulation, high-power microwave of transmission electron microscopy (TEM) mode is generated with peak power of 4.2 GW, frequency of 1.76 GHz, and peak power conversion efficiency of 12%, when the voltage is 600 kV and the current is 52 kA. A novel plate-inserted mode-transducing antenna, which is composed of a plate-inserted mode converter and a coaxial horn, is introduced into the improved MILO. The TEM wave generated by the MILO propagates down the section of coaxial waveguide and is transformed into the TE11 mode by the novel plate-inserted mode converter, and then radiated by the coaxial horn antenna into air. The direction of the radiated microwave agrees with the axis of the MILO

Asymmetric-mode competition in a relativistic backward wave oscillator with a coaxial slow-wave structure

The initial experimental results of an L-band relativistic backward wave oscillator with a coaxial slow-wave structure are presented. The asymmetric-mode-competition mechanism in the device is investigated theoretically and experimentally. It is shown that the diode voltage, guiding-magnetic field, and concentricity play a key role in the suppression and excitation of the asymmetric-mode (coaxial quasi-TE11 mode). In the experiments, the asymmetric-mode with a frequency of 2.05 GHz is suppressed and excited, which is in good agreement with the theoretical results.

Generation of gigawatt level beat waves

The initial experimental results of generating gigawatt level beat waves with two microwave sources are presented. The detailed measures for enhancing the power handling capacities of the applied devices are explained. In the experiments, the operation frequencies of the microwave sources are 9.41 GHz and 9.59 GHz, respectively. The microwave sources are driven by a single accelerator capable of producing dual electron beams to make sure that they are operating simultaneously. The beat frequencies, measured by the oscilloscope directly, are about 180 MHz, and the peak powers of the pulsed beat waves are about 4.3 GW with durations of about 40 ns.

Investigation of a 1.2-GHz Magnetically Insulated Transmission Line Oscillator

A 1.2-GHz magnetically insulated transmission line oscillator (MILO) is investigated numerically and experimentally in this paper. Simulation optimization is performed with the particle-in-cell code KARAT. When the diode voltage and current are 680 kV and 53 kA, the output microwave power is 4.15 GW, the microwave frequency is 1.169 GHz, and the power efficiency is 11.5%. In order to radiate a boresight peak pattern, a TEM-TE11 mode-converting antenna is introduced into the MILO device. In the TEM-TE11 mode-converting antenna, four metal plates are inserted into the coaxial waveguide to convert the TEM into TE11 mode, and then, the coaxial TE11 mode is converted into circular TE11-like mode and radiated by a conical horn antenna, which generates a boresight peak pattern in a far-field region. The gain of the mode-converting antenna is 16.3 dBi, and the aperture efficiency of the conical horn antenna is 79% at 1.169 GHz. The 3-dB beamwidths are 24° in E-plane and 32° in H-plane, and the sidelobes of the radiation patterns are both less than -21 dB. In the experiments, the MILO device is driven by a 590 kV 49 kA electron beam. The measured results show that the peak microwave power is about 2.9 ± 0.3 GW, the pulse duration is above 20 ns, the microwave frequency is about 1.20 GHz, and the power conversion efficiency is about 10%. The experimental results validate the simulation prediction.

Combining microwave beams with high peak power and long pulse duration

The beam combining results with a metal dichroic plate illuminated by the S/X band gigawatt level high power microwaves are presented. According to the previous experiments, the microwave breakdown problem becomes obvious when the peak power and the pulse duration increase, thus, several methods for enhancing the power handling capacity have been considered, and the metal dichroic plates are redesigned to handle the S/X band high power microwaves. Then the design, fabrication, and testing procedure are discussed in detail. The further experimental results reveal that, operated on the self-built accelerator Spark-04, the radiated powers from the S and X band sources have reached 1.8 GW with pulse durations of about 80 ns, and both beams have been successfully operated on the selected dichroic plate without microwave breakdown.

A metal-dielectric cathode

In order to improve the pulse repetition rate and the maintenance-free lifetime of an improved magnetically insulated transmission line oscillator (MILO) previously developed in our laboratory, a metal-dielectric cathode is investigated experimentally. It consists of three components: a stainless steel base, bronze foils, and double-sided printed boards. The experimental results show that the shot-to-shot reproducibility of the diode voltage and current is very good and the performances of the improved MILO are steady. In addition, no observable degradation could be detected in the emissive characteristic of the metal-dielectric cathode after 350 shots. The experimental results prove that the metal-dielectric cathode is a promising cathode for repetitively pulsed MILO operation. However, the leading edge of the radiated microwave pulse is gradually delayed during the repetition rate. A likely reason is that high pressure results in gas ionization in the beam-microwave interaction region, and plasma format...