Hanwu Yang

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.

Analysis and improvement of an X-band magnetically insulated transmission line oscillator

An X-band magnetically insulated transmission line oscillator has been investigated theoretically and experimentally in our laboratory. However, severe pulse shortening and electrode erosion are observed in the experiments. The theoretical analyses show that anode plasma formation in the load region is the essential reason for the pulse shortening and electrode erosion. In order to eliminate or at least minimize anode plasma formation in the load region, an improved beam dump is presented. The theoretical analyses show that anode plasma formation can be eliminated or at least minimized in the improved beam dump.

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.

Frequency agile characteristics of a dielectric filled relativistic magnetron with diffraction output

This paper reports the investigations of a dielectric filled relativistic magnetron with diffraction output (MDO) on frequency agile. The mechanism of frequency agile is theoretically analyzed. Particle-in-cell simulations and preliminary experiments prove the analytics. In experiments, under the working conditions, 605 kV and 0.3 T, a microwave with 1.98 GHz, 200 MW is radiated from an A6 type MDO when the 95% Al2O3 ceramics with the total thickness of 0.9 cm are filled. Compared with the microwave of 3.72 GHz, 240 MW obtained without the ceramics filled, the frequency agile from S band to L band is achieved.

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...

Experimental demonstration of a compact high efficient relativistic magnetron with directly axial radiation

A compact relativistic magnetron with axial microwave radiation is experimentally investigated. Under the modified magnetic field distribution, only connecting a special horn antenna in the axial direction, the relativistic magnetron can stably radiate high power and high efficiency microwaves. The total length of the device is ∼0.3 m, and the volume is ∼0.014m3. In such working condition that the applied voltage is 539 kV and the magnetic field is ∼0.38 T, the output microwave power is ∼1.24 GW. Correspondingly, the total efficiency is about 34.1%. The radiating frequency is 2.35GHz, which is in agreement with the π mode frequency of the theoretical expectation.

Effects of the transparent cathode on the performance of a relativistic magnetron with axial radiation

Experimental investigation of the transparent cathode used in a relativistic magnetron with axial radiation is reported in this paper. The transparent cathode is composed of six separate stalks with the diameter of 6 mm. Under the working condition of 549 kV and ∼0.38 T, the relativistic magnetron with the transparent cathode experimentally produces a 550 MW microwave. The radiation mode is TE(11) at the frequency of 2.35 GHz. The total efficiency is 16.7%. The variations of the relative positions between the separate stalks and the anode blocks can perform the maximum difference of 4 ns in microwave duration. Compared with the conventional solid cathode, the transparent cathode provides faster startup time of 12 ns, relatively wider pulse duration of 35% and relatively higher efficiency of 10.6%.

Characteristics of a velvet cathode under high repetition rate pulse operation

As commonly used material for cold cathodes, velvet works well in single shot and low repetition rate (rep-rate) high-power microwave (HPM) sources. In order to determine the feasibility of velvet cathodes under high rep-rate operation, a series of experiments are carried out on a high-power diode, driven by a ∼300 kV, ∼6 ns, ∼100 Ω, and 1–300 Hz rep-rate pulser, Torch 02. Characteristics of vacuum compatibility and cathode lifetime under different pulse rep-rate are focused on in this paper. Results of time-resolved pressure history, diode performance, shot-to-shot reproducibility, and velvet microstructure changes are presented. As the rep-rate increases, the equilibrium pressure grows hyperlinearly and the velvet lifetime decreases sharply. At 300 Hz, the pressure in the given diode exceeded 1 Pa, and the utility shots decreased to 2000 pulses for nonstop mode. While, until the velvet begins to degrade, the pulse-to-pulse instability of diode voltage and current is quite small, even under high rep-rate...

Breakdown characteristics of niobate glass-ceramic under pulsed condition

On the road to the portable pulsed power generators, application of the ceramic capable of high energy density in energy storage for pulse forming line has aroused great enthusiasm in the research group of pulsed power technology. The niobate glassceramic investigated in this paper is promising for applications in parallel-plate, stacked Blumleins, which represents an alternative to achieve the required compactness for portable devices. Experiments for testing its breakdown characteristics have been performed on the platform named ARC-01 with the ability to output repetitive pulses. A circuit simulation with the same parameters of the testing platform has been presented to predict the output pulse of the test. Test of breakdown characteristics under repetitively pulsed condition with various frequencies and pulse numbers has been carried out. The results indicate that the configuration design of the electrode has a significant effect on the performance of the ceramic breakdown strength. To eliminate the electric-field (E-field) enhancement points and find an optimized configuration of the electrode, some different electrodes have been studied. The influence of the testing environment has also been investigated with different dielectrics, i.e. air, transformer oil, and glycerin. Based on the testing results of breakdown properties and static simulations, the design of a 2-stage parallel-plate, stacked Blumlein is presented.