Tao Xun

Compact Rep-Rate GW Pulsed Generator Based on Forming Line With Built-In High-Coupling Transformer

In this paper, a compact rep-rate GW pulsed power generator is developed. First, its three key subsystems are theoretically analyzed, engineering designed, and experimentally investigated, respectively. The emphases are put on these four problems: the theoretical analysis of the voltage distribution across the conical secondary windings of the high-coupling transformer, the investigation of the high energy storage density dielectric used in the pulse forming line, the choice of the gas flow velocity of the gas blowing system, and theoretical analysis of the passive stability of the pulsed power generator operated in rep-rate mode. Second, the developed pulsed power generator is described in detail. It has a 0.2-m diameter, a 1.0-m length, and a 20- Ω wave impedance. Across a 100- Ω resistive dummy load, it can steadily operate at a 300-kV output voltage in 50-Hz rep-rate and 250 kV in 150 Hz without gas blowing system. The average power is ~ 1 kW. Finally, the pulsed power generator is applied to drive a relativistic backward-wave oscillator, generating a high-power microwave with peak output power of 200 MW and duration (full-width at half-maximum) of 5 ns in 150-Hz rep-rate. These efforts set a good foundation for the development of a compact rep-rate pulsed power generator and show a promising application for the future.

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...

Influence of different illumination profiles on the on-state resistances of silicon carbide photoconductive semiconductor switches

Characteristics of a silicon-carbide (SiC) photoconductive switch under different illumination profiles are presented. We triggered a V-doped semi-insulated 6H-SiC switch with lateral geometry using a laser beam of 532-nm wavelength. Photoconductivity tests for different spot profiles and locations show that such switches achieve a minimum on-state resistance when the switching gap is illuminated. The differences between on-state resistances are small for various partial illuminations of the switching gap. Semiconductor modeling is used to simulate the electric field and current profiles for different partial illuminations. The simulation results show poor on-state switch performance when partially illuminated. Based on these results, a more revealing circuit model for the switch matches well with experimental results for partial illuminations.

Hydrodynamic Loading of Ceramic Components Due to Pulsed Discharge in Water

Pulsed discharge in water produces transient pressure waves. For one kind of high-current electron accelerators composed of a water pulse-forming line and a ceramic-insulated vacuum diode, the mechanical stability of the water-vacuum interface should be taken into account during operations. In this paper, by combining empirical formulas of a plasma-driven water-shock theory with a self-consistent underwater explosive approach, a finite element model was introduced to investigate the shock-wave behaviors. The pressure-time history and ceramic mechanical response to pressure waves were presented. In order to get the pressure profile and verify the calculation models, the arc pressure test, including ldquopoint-planerdquo electrode system, was carried out based on a ten-stage Marx generator. Peak pressures of shock waves were measured by the piezoelectric sensor, and related results have a close approximation to the predictions. The relationship between peak pressure and shock-wave energy under nanosecond time-scale pulsed-discharge condition was also obtained in the experiment. These results may be helpful for the estimation of a water shock when designing structural components of pulsed-power machines.

Outgassing rate analysis of a velvet cathode and a carbon fiber cathode

In this paper, the outgassing-rates of a carbon fiber array cathode and a polymer velvet cathode are tested and discussed. Two different methods of measurements are used in the experiments. In one scheme, a method based on dynamic equilibrium of pressure is used. Namely, the cathode works in the repetitive mode in a vacuum diode, a dynamic equilibrium pressure would be reached when the outgassing capacity in the chamber equals the pumping capacity of the pump, and the outgassing rate could be figured out according to this equilibrium pressure. In another scheme, a method based on static equilibrium of pressure is used. Namely, the cathode works in a closed vacuum chamber (a hard tube), and the outgassing rate could be calculated from the pressure difference between the pressure in the chamber before and after the work of the cathode. The outgassing rate is analyzed from the real time pressure evolution data which are measured using a magnetron gauge in both schemes. The outgassing rates of the carbon fibe...

A vacuum-sealed, gigawatt-class, repetitively pulsed high-power microwave source

A compact L-band sealed-tube magnetically insulated transmission line oscillator (MILO) has been developed that does not require bulky external vacuum pump for repetitive operations. This device with a ceramic insulated vacuum interface, a carbon fiber array cathode, and non-evaporable getters has a base vacuum pressure in the low 10−6 Pa range. A dynamic 3-D Monte-Carlo model for the molecular flow movement and collision was setup for the MILO chamber. The pulse desorption, gas evolution, and pressure distribution were exactly simulated. In the 5 Hz repetition rate experiments, using a 600 kV diode voltage and 48 kA beam current, the average radiated microwave power for 25 shots is about 3.4 GW in 45 ns pulse duration. The maximum equilibrium pressure is below 4.0 × 10−2 Pa, and no pulse shortening limitations are observed during the repetitive test in the sealed-tube condition.

Thermal Properties of an Intense Relativistic Electron Beam Collector Under Repetitive Pulse Operation

With the increase in power and miniaturization in a size of repetitively operated high-power microwave (HPM) sources, such as relativistic backward-wave oscillators, the thermal control and cooling methods for a collector have the great effects on efficient operations of these devices. In this paper, the thermal properties of an intense relativistic electron beam collector under repetitive operation were studied. The bombardment area caused by energetic electrons was experimentally quantified, and the pulsed thermal loading for a 700-keV and 7-kA beam current was calculated. With an ANSYS multiphysics package, the typical results of temperature history of the collector under 10-100-Hz operation were presented. The dependence of peak temperatures on forced cooling conditions at different pulse repetition rates was also obtained. Comparison results show that the cooling process is influenced by collector thermophysical parameters, interface thickness, and convective factor of the forced water cooling. Finally, the hydraulic system necessary to achieve appropriate cooling fluid flow was discussed. These results may supply an efficient reference to the optimization and reasonable use of a repetitively operated beam collector in the HPM tubes.

Performance of a SiC-nanowire-based explosive-emission pulsed plasma electron source

Large-scale, well-aligned SiC nanowires (SiCNWs) were developed and their performance as an explosive-emission cathode driven by a high-voltage pulse modulator (450 kV, 120 ns) is reported. The current density was on the order of 1.2 kA/cm2 at an electric field of 90 kV/cm. By comparison with polymer velvets, the SiCNW-based cathodes showed a larger current amplitude, a smaller ignition delay, more uniform surface plasmas, and constant diode impedances during the pulse flattop. The SiCNWs also showed a very small variation in current density in the shot-to-shot mode, which suggests an outstanding advantage in terms of cathode lifetime.

Preliminary experimental results on water breakdown in tens nanoseconds region

Deionized water is a good dielectric material. In this paper, preliminary investigations are carried out on water breakdown in tens nanoseconds region via a switch with “point-plane” electrodes. The switch is set up on the output port of a wave erection Marx generator, which can deliver a minus high voltage pulse with an amplitude of 0∼300 kV, and a pulse width of 40∼50 ns to a matched load. According to the experimental results, the breakdown electric field is calculated to be about 1.2 MV/cm by Martins formula. A circuit model for the whole system is built utilizing PSpice software, and the simulation results agree with the experimental ones basically.