Reflex Triode Vircator Research Articles

Electrode shape dependence of output microwave characteristics in reflex triode virtual cathode oscillator

A virtual cathode oscillator (vircator), which can generate a high-power microwave, has been widely studied over the last four decades. This paper describes the dependence of microwaves and electron beams generated in a reflex triode vircator on the electrode shape and material. The reflex triode vircator is driven by a Marx generator with an output voltage of 200 kV and a stored energy of 240 J. Experiments were conducted with aluminum and velvet cathodes and two types of electrode shapes: (1) circular, with a diameter of 46 mm, and (2) elliptical, with an aspect ratio of 2:3. Both shapes have the same emitting area. Experimental results revealed that the electric field strength of the microwave pulse generated with the elliptical area electrode was 1.3 times greater than that with the circular area electrode. In addition, an x-ray pinhole camera with a beryllium window was used to evaluate the cross-sectional shape of the electron beam. It was found from the x-ray images that the shape of the electron beam was nearly the same as that of the electrode and that the cross-sectional area of the elliptical electron beam was larger than that of the circular electrode although both the circular and elliptical electrodes have the same emitting area. These results show that the electrode shape is one of the important parameters for the vircator.

Performances of Velvet Cathodes With Different Geometries in a Reflex Triode Vircator

A frequency tunable vircator, which has the reflex triode geometry, was designed and experimentally tested. The cloth velvet was used as the cathode material. The planar and the Ernst-profiled velvet cathodes, which have diameters of 40, 50, 60, and 70 mm, were used in the experiments. The electric field strength values and the corresponding frequencies of the radiated microwave were measured and analyzed for each of the cathodes. The anode–cathode distances between 4 and 16 mm were swept in the experiments. The planar cathodes display larger electric field strength values, whereas the Ernst-profiled cathodes have wider frequency ranges. The characteristics of the velvet, such as tuft density and fiber dimensions, were investigated in detail using scanning electron microscope images. The condition of the vacuum was also observed during shots.

Development mechanism of cathode surface plasmas of high current pulsed electron beam sources for microwave irradiation generation

AbstractThis paper presents the development mechanism of surface plasmas of carbon-fiber-cathode electron beam source and its effects on the operation of a high-power microwave source, reflex triode vircator powered by about 400 kV, 9 kA, about 350 ns pulsed power accelerator. Based on the current and voltage characteristics of diodes using carbon fiber cathode, the axial expansion velocity is 1.2 cm/μs and the delay time of explosive emission is 2 ns. Further, the comparison of carbon fiber and stainless steel cathodes is made. It was found that the threshold electric field for carbon fiber cathode is about 25 kV/cm, and the delay time of explosive emission and threshold electric field for stainless steel cathode is, respectively, 4.5 ns and 40 kV/cm. The radial expansion velocity of individual emitting centers is estimated to be 1.2 cm/μs, equal to the axial expansion velocity, and this shows the cathode plasma spots spherically expand. In the optimal diode gap for microwave irradiation or at the average current density of 230 A/cm2using carbon fiber cathode, the screening radius was 0.67 cm, the lifetime of cathode emitting centers was about 60 ns, the cathode plasma density was 5 × 1015 cm−3, and the Debye radius of cathode plasma was <3 × 10−5 cm−3. The self-quenching behavior of explosive emission centers occurs, due to the process of cathode surface material release and cooling. The generation and self-quenching of emitting centers, and screening effect of cathode plasmas determine the increase and decrease of cathode emitting area, which is independent of the current density and background pressure. The relation between the lifetime of virtual cathode and background pressure was discussed. It was found, both theoretically and experimentally, that a lower background pressure indicates a longer microwave pulse or a better microwave waveform. It was observed by comparison that the temporary behavior of cathode emitting area is similar to the development process of microwave pulse. The changes of emitting area affects the stability of beam current injected into the virtual cathode region, further leading to the fluctuation of microwave pulse of vircator.

Oscillation Frequency of a Reflex-Triode Virtual Cathode Oscillator

The most fundamental issues in virtual cathode oscillator (vircator)-based high-power microwave devices are efficiency improvement and oscillation frequency control. We present a study of the time-dependent behavior of the oscillation frequency of a reflex-triode vircator for various anode-cathode (AK) gaps. The typical electron-beam parameters were 200 kV, 4 kA, and 300 ns, with a current density of a few hundreds of amperes per square centimeter. Time-dependent frequency analysis is applied to the output signal of the vircator to investigate the time evolution of the emitted frequency spectrum for various AK gaps. We find that the dominant frequency peak is less affected by shot-to-shot fluctuations of diode voltage and current than the other frequency components. In addition, the time of occurrence of the dominant and secondary frequency peaks varies on a shot-to-shot basis. The highest microwave power is emitted when all the power is delivered into a single frequency with minimal mode hopping.

A series of tufted carbon fiber cathodes designed for different high power microwave sources

We report the fabrication technique of tufted carbon fiber cathodes for different microwave sources. Three carbon fiber cathodes were constructed, including a planar cathode, an annular cathode, and a cylindrical cathode for radial emission. Experimental investigations on these cathodes were performed in a reflex triode virtual cathode oscillator (vircator), a backward wave oscillator (BWO), and a magnetically insulated transmission line oscillator (MILO), respectively. The pulse duration of microwave emission from the reflex triode vircator was lengthened by using the planar carbon fiber cathode. In the BWO with the annular carbon fiber cathode, the uniform electron beam with a kA/cm(2) current density was observed. In addition, carbon fiber has great promise as field emitter for MILOs. These results show that the carbon fiber cathodes can be utilized for electron emission in high power diodes with different structures.

A 10-GW Pulsed Power Supply for HPM Sources

A research activity involving the detailed consideration of novel high voltage transformers (HVTs) for pulsed-power applications has recently begun at Loughborough University (LU). Although the main goal is the demonstration of a compact and lightweight unit employing magnetic self insulation under vacuum conditions, the initial stage of the work is directed towards the development of a conventional air-cored HVT as a main component in a compact power supply for HPM sources. In cooperation with the Swedish Defence Research Agency (FOI), the power supply has been tested with a HPM source of the vircator type. The power source for the system uses a 70 kJ/25 kV capacitor bank and an exploding wire array to generate a 150 kV voltage pulse in the primary circuit of the HVT. A pressurised SF6 spark gap in the secondary circuit sharpens the high-voltage output, so that pulses approaching 500 kV and with a rise time below 100 ns are generated on a 20 Omega high-power resistor. The peak power produced by the power supply is in excess of 10 GW. Measurements provided by various diagnostic techniques are analysed with the aid of a detailed numerical code. Experimental results are presented from final testing of the system, where a reflex triode vircator replaces the 20 Omega resistor. Measurements made of the microwave emission using free-field sensors are presented for various electrode configurations. Comments are made with the microwave emission from the same vircator powered by a Marx generator at FOI.