Yu-Wei Fan

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.

An L-band coaxial relativistic backward wave oscillator with mechanical frequency tunability

The initial experimental results of an L-band coaxial relativistic backward wave oscillator with mechanical frequency tunability are presented. The key effects of the inner-conductor contributing to the mechanical frequency tunability are investigated theoretically and experimentally. In the experiments, the L-band microwave with frequency of 1.58 GHz is radiated when the inner-conductor radius is 1.5 cm. Meanwhile, the S-band microwave with frequency of 2.31 GHz is generated after removing the inner-conductor. In addition, the frequency tuning within 4% is realized by mechanically altering the radius of the inner-conductor at a half power level.

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

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.

A double-band high-power microwave source

In order to increase the power conversion efficiency of a magnetically insulated line oscillator (MILO), an axially extracted virtual cathode oscillator (VCO) is introduced to utilize the load current in the MILO, so it is called the MILO-VCO. In this device, the MILO and VCO are operated synchronously and generate high-power microwaves. The MILO-VCO is investigated in detail with particle-in-cell (PIC) methods (KARAT code). In simulation, the diode voltage is 640 kV and the current is 50 kA. The total peak power of the MILO-VCO is 5.22 GW and the corresponding power conversion efficiency is 16.3%. In the MILO-VCO, the peak power of the MILO is 3.91 GW and its frequency is 1.76 GHz; the peak power of the VCO is 1.33 GW and its frequency is 3.79 GHz.

Experimental investigation of a Ka band high power millimeter wave generator operated at low guiding magnetic field

An overmoded slow wave type Ka band generator is investigated experimentally to produce high power millimeter waves in this paper. The experiments were carried out at the TORCH-01 accelerator. The produced microwave frequency was measured by dispersive line method, and the power was estimated by integrating over the radiation pattern at far field. With relatively low guiding magnetic field of 0.8 T and diode voltage and beam current of 590 kV and 5.2 kA, respectively, a 33.56 GHz millimeter wave with an output power of 320 MW was generated, and the microwave mode was quasi-TM01 mode.

Complex magnetically insulated transmission line oscillator

A magnetically insulated transmission line oscillator (MILO) is a crossed-field device designed specifically to generate microwave power at the gigawatt level, which is a major hotspot in the field of high-power microwaves (HPM) research at present. It is one of the major thrust for MILO development to improve the power conversion efficiency. In order to improve the power conversion efficiency of MILO, a complex MILO is presented and investigated theoretically and numerically, which comprises the MILO-1 and MILO-2. The MILO-2 is used as the load of the MILO-1. The theoretical analyses show that the maximum power conversion efficiency of the complex MILO has an increase of about 50% over the conventional load-limited MILO. The complex MILO is optimized with KARAT code (V. P. Tarakanov, Berkeley Research Associates, Inc., 1992), and the simulation results agree with the theoretical results.