A hybrid simulator with novel bicone-wire grid antenna.

The simulator is an indispensable device for the study of high-altitude electromagnetic pulse (HEMP) effects. In order to improve the quality of the electric field waveforms of the HEMP simulator, we carried out simulation and experimental studies for the biconical-wire grid antenna. A new bicone-wire grid antenna HEMP simulator was designed by using skeletonized cones instead of solid cones, which can substantially improve the mobility of the simulator. The effects of different aspect ratios and different cone structures of the simulator on the waveform and distribution characteristics of the electric field were analyzed. The accuracy of the simulation results was verified by testing the bicone-wire grid antenna simulator with the new structure. The results obtained are as follows: First, the simulator can provide a HEMP environment compliant with the International Electrotechnical Commission (IEC) standard in an area of 5 × 6 × 2 m3. Second, the simulator possesses optimal waveform quality and working space among simulators with different aspect ratios. Finally, the influence of using skeletonized cones instead of solid cones on the simulator's electric field waveforms is negligible, and the mobility of the new simulator is significantly improved compared to the original simulator.

Experimental investigation and theoretical analysis of the breakdown time delay and jitter of multi-gap gas switch gaps.

The gas gap of a multi-gap gas switch can be classified as trigger and self-breakdown gaps based on the breakdown condition. A two-gap gas switch consisting of a trigger gap and a self-breakdown gap is developed to independently study the breakdown characteristics of these two types of switch gaps. Trigger experiments for the switch are conducted under various trigger voltage rise rates and different working coefficients. The experimental results indicate that the trigger gap has significantly more jitter than the self-breakdown gap, and the overall performance of the gas switch is determined primarily by the trigger gap. A novel pre-ionization structure with disks is implemented into the two-gap gas switch, considerably decreasing the breakdown delay of the trigger gap and reducing the jitter to a quarter or even less compared to that without pre-ionization. A calculation model of the breakdown time delay for the trigger gap is provided based on the foundational development of the avalanche. The probability distribution of the time required for the initial electron generation is derived in the absence of pre-ionization. The calculated breakdown time delay agrees well with the experimental results in cases with and without pre-ionization under most trigger settings. The method and principle of calculating the breakdown time delay can analyze the collapse of a gas gap with different electrode configurations (quasi-uniform or uniform electrical fields) and various gas media under a nanosecond pulse voltage.

The effect of initial electrons generation approaches on nanosecond pulsed breakdown characteristics.

In this paper, the effects of initial electrons generation approaches on nanosecond pulsed breakdown characteristics are analyzed. Based on the numerical simulations with a 3D PIC-MCC model, the impacts of field-enhancement factor and initial electron concentration on nanosecond pulsed breakdown characteristics are investigated. Three types of switches are designed and subjected to testing under pulse voltages with rise times of 40, 70, and 120 ns, respectively. The results can be summarized as follows. First, the field-enhancement factor and initial electron concentration have significant influences on the development of the discharge channel. Second, the cathode-grooved self-triggered switch exhibits lower breakdown time delay jitter than the hemispherical self-breakdown switch at low pressure, while the differences in jitter between the two switches become negligible at high pressure. Third, the cathode-grooved self-triggered switch shows a lower breakdown time delay jitter compared to the pre-ionization self-triggered switch for pulse voltages with rise times of 40 and 70 ns. Conversely, this trend reverses for pulse voltage with a rise time of 120 ns. Finally, the breakdown time delay jitter for both the cathode-grooved self-triggered switch and the pre-ionization self-triggered switch has been reduced, and both switches are suitable for different operating requirements and conditions.

Study on discharge characteristics of six-gap gas switch with corona assisted triggering.

To improve the triggering characteristics of the gas switch used for linear transformer driver, a method of corona assisted triggering based on the pre-ionization in switch gaps is proposed and applied to a six-gap gas switch. The principle is demonstrated by electrostatic field analysis and verified by the experimental study on the discharge characteristics of the gas switch. The results indicate that when the gas pressure is 0.3 MPa, the self-breakdown voltage remains about ±80 kV, and its dispersivity is lower than 3%. The effect of corona assisted triggering on the triggering characteristics increases with the higher permittivity of the inner shield. The positive trigger voltage of the switch with the proposed method can be reduced from 110 to 30 kV at a charging voltage of ±80 kV when the jitter is equal to that of the original switch. There are no pre-fire or late-fire when the switch operates continuously for 2000 shots.

Stress measurement system for underwater electro-explosive platforms.

As a new method for generating strong underwater shock waves with rapid repetition frequency, the use of underwater electrical-wire explosion (UEWE) to drive insensitive energetic materials has attracted increasing research attention in recent years. Accordingly, equipment based on this new method have been developed. The ability to measure the stress produced by an UEWE on a device plays a very important role in optimizing the device performance. However, in conventional stress measurements, the spatial electromagnetic interference (EMI) produced by the discharge can affect the measurement accuracy or even damage the experimental instruments. In this study, a novel system for measuring stress in a strong electromagnetic field, based on a piezoelectric film and a conditioning circuit, was debugged and evaluated. Shielding was used to eliminate the intense EMI due to the strong electromagnetic field. Our simulation and experimental results demonstrate that the stress generated can be quickly determined by measuring the output voltage of the conditioning circuit. The new system can be used to measure the stress at the fluid-solid interface under a strong electromagnetic field environment.

Compact pulse generator with sub-nanosecond rise time based on inductance of order of several nH.

Pulse generators with a sub-nanosecond rise time are typically used to calibrate measurement probes in electromagnetic pulses. However, the technological dilemma between high voltage and low inductance has not been adequately addressed in this context. In this paper, the authors investigate the effects of the circuit and structural parameters on the generator. To reduce the rise time of the output voltage of the generator to a few hundred picoseconds, the inductance of its structure and the spark gap needs to be strictly controlled. We use SF6 at 1 MPa as an insulating gas for the spark gap to reduce the inductance of the capacitor and the switch to the order of several nH. The results of theoretical calculations and simulations were used to design and test two generators that used a coaxial ceramic capacitor and three plate ceramic capacitors, respectively. The experimental results showed that a double-exponential pulse voltage with a sub-nanosecond rise time could be obtained in a 50 Ω transmission line in both generators. The generator with the coaxial ceramic capacitor had better characteristics than the one that used three plate ceramic capacitors with a rise time of 630-860 ps when the peak output voltage was in the range of 5-30 kV.

Modeling and tests of nested transmission lines for current adding on a four-stage linear transformer driver.

Linear transformer driver (LTD) technology allows a pulsed-power generator to be transportable due to its salient features in compactness and modular design. To further reduce the footprint of an MA-class pulsed-power generator, nested transmission lines were designed and tested for current adding in a four-stage gas-insulated LTD module. The current adder assembly contained two modules that were charged in opposite polarities. Each module held two LTD cavities that shared a common electrode of the nested transmission line with deionized water insulation. Post-hole convolutes were installed for the aggregation of the output current of different modules. More specifically, numerical simulations were conducted to calculate the nested line inductance, which revealed that the total system inductance was ∼10 nH in the nested geometry. Experimentally, testing on the four-stage LTD prototype showed that the LTD module can deliver a 1.2 MA current peak with a rise time of 140 ns to a short circuit load under the charging voltage of ±50 kV, which validated the applicability of using nested lines for current adding in an MA-class LTD module.

Simulation of primary current distribution in Tesla-type pulse generators.

In a Tesla-type pulse generator, self-inductance of the primary coil is a crucial parameter to determine the final oscillating condition. However, the accurate value of this inductance might be changed due to the uneven primary current distribution caused by practical configuration of the primary side. Consequently, in order to precisely design the transformer, it is helpful to evaluate the primary inductance based on electromagnetic simulation instead of conventional approximate calculation. In this paper, a simulation model based on the finite integration technique is established to solve the uneven primary current problem. A primary coil with multiple contacting points is designed, and hexahedral mesh generation of the coil is also discussed. Hence, a series of verification tests using different primary structures are performed to support the results of simulation. Both results of the simulation model and the verification test presented that the variation of the primary inductance will affect the performance of the generator, and the number of contacting points is the main cause to determine the maximum current density of the coil.

Measurement of magnetic field distribution produced by high-current pulse using Zeeman splitting of Na emission distributed by laser ablation.

Measurement of the magnetic field distribution in Z-pinch experiments remains an ongoing challenge. We present a method of measuring the radial distribution of the magnetic field around a copper rod using Zeeman splitting of sodium (Na) emission lines, in which an Na layer is formed by the laser ablation of NaCl crystals on a load surface. The load consists of a copper rod of 2 mm diameter and is pre-covered on its surface by the NaCl crystals. An 8 ns pulsed laser with an energy of 1 J and wavelength of 532 nm is focused on the crystals. The Na plasma is produced and expands from the surface of the copper rod into a vacuum. After applying a pulsed current with a peak value of 375 kA to the load, the Na 3s-3p doublet displays significant Zeeman splitting patterns. The self-luminosity of the Na plasma is recorded by a spectrometer coupled with an intensified charge-coupled device camera from an end-on view to eliminate the effects of different observing angles and Doppler shifts. We determine the magnetic field by fitting the measured spectra with the calculated results of the Voigt profile. The measurable range of radial position is 5-7 mm, and the corresponding magnetic field is 5-15 T. The averaged error of curve fitting is less than 12%.

Self-integrating current sensor for fast pulsed current monitor in transmission line.

The fast linear transformer driver (FLTD) utilizes a water-insulated transmission line as its secondary. To monitor the fast pulsed current and locate the fault, a compact self-integrating current sensor is developed. Print circuit board (PCB) coils and PCB integrating resistors are used to form the current sensor. By soldering a large number of chip resistors, PCB integrating resistors with various resistance and low inductance can be obtained. The current sensor is designed in a coaxial structure to reduce its inductance and size and can be installed on the inner conductor of the FLTD's secondary water-insulated transmission line with a small opening. The principle and matching schemes for the current sensor are theoretically analyzed with the circuit principle. Both simple matching and two-stage division can be used to obtain signals without oscillation. The time constant of the two schemes is the same. However, the sensitivity of the current sensor with two-stage division is lower than that with simple matching. A 100 kV step pulse generator is used as the pulsed high current generator to verify the properties of the current sensor. The designed current sensor could respond to the step current pulse with the rise time of 4 ns. The matching scheme is verified to be effective with step response experiments. Finally, the influence of the parameters, which are the coil type, the angle between the PCB coil and the magnetic flux, the resistance of the integrating resistor, and the length of the measuring cable, on the output signal of the current sensor is studied.

A gas-insulated mega-ampere-class linear transformer driver with pluggable bricks.

This paper presents the design and test of a gas-insulated linear transformer driver (LTD) cavity aimed at the Z-pinch experimental device CZ-34. The LTD cavity has a diameter of 2290 mm and a height of 346 mm. It consists of 23 main bricks and 1 trigger brick. Each main brick is comprised of two 100 nF capacitors connected electrically in series with a field-distortion gas switch. The trigger brick is comprised of two 50 nF capacitors connected in series with a compact multi-gap gas switch. All bricks are placed in the cavity filled with compressed SF6 and are pluggable like drawers. The trigger pulse generated by the trigger brick passes through an azimuthal transmission line to the trigger ring and makes the main bricks discharge synchronously. The LTD cavity can deliver ∼1 MA current pulse with a rise time of 115 ns to 0.08 Ω liquid resistance load when the charging voltage is ±100 kV, which is in good agreement with the circuit simulation results. Experimental results demonstrate the successful application of using gas insulation and pluggable bricks. The technical feasibility of the charging configuration, triggering method, and isolation resistors is verified. There is little difference in output performance as return-current rods replaced the outside metal cylinder, which provides a new path for the design of LTD cavities in series.

A 80 kV gas switch triggered by a 17 µJ fiber-optic laser.

A gas switch triggered by µJ laser pulses was developed. The switch employed a 10 mm-gap GaAs photoconductive semiconductor switch (PCSS) mounted in parallel with one of its two gaps. The dark current-voltage characteristic of the PCSS was obtained, and the gas switch characteristics at different bias voltages were experimentally investigated. The results indicate that the switch can be triggered reliably by using a 17 µJ laser pulse, and the jitter is less than 3 ns at the bias voltage of 80 kV and 60% of the self-breakdown voltage.

Capacitive sensor for fast pulsed voltage monitor in transmission line.

To monitor fast pulsed voltage in the transmission line of the fast linear transformer driver, a compact and self-integrating capacitive sensor is developed. A printed circuit board (PCB) with multi-layer ceramic capacitors (MLCCs) serves as a low-voltage arm. Given the reflow structure of the PCB and a parallel arrangement of MLCCs, the residual inductance of the PCB low-voltage arm can be effectively reduced. Moreover, its capacitance can be easily adjusted. The capacitive sensor is designed using a coaxial structure, suitable for installation on the inner high voltage electrode of the secondary water-insulated transmission line. Three matching schemes for the measuring cable are discussed, showing that two-stage division with matching at the end of the cable is more suitable for fast pulsed voltage measurement due to its high dividing ratio and flat frequency response. A square wave with a rise time of ∼1.5 ns and a pulse width of ∼200 ns is used to verify the properties of the capacitive sensor. The results show that the rise time of the designed capacitive sensor can reach 1.5 ns when a short cable is used. Increasing capacitance of the low-voltage arm or resistance at the cable entrance will increase the time constant of the capacitive sensor, which improves its low-frequency properties. A long cable will significantly reduce the high-frequency response and the magnitude of the signal from the capacitive sensor.

Note: Erosion of W-Ni-Fe and W-Cu alloy electrodes in repetitive spark gaps.

A pair of W-Ni-Fe and W-Cu electrodes were tested under 100 kA level pulsed currents for 10 000 shots, respectively. Surface roughness and morphology characteristics of the two pairs of electrodes were obtained and compared. Experimental results indicated cracks divided the W-Cu electrode surface to polygons while the W-Ni-Fe electrode surface remained as a whole with pits and protrusions. Accordingly, the surface roughness of W-Ni-Fe electrodes increased to ∼3 µm while that of W-Cu electrodes reached ∼7 µm at the end of the test. The results reveal that the W-Ni-Fe alloy has a better erosion resistance and potential to be further applied in spark gaps.

A platform for exploding wires in different media.

A platform SWE-2 used for single wire explosion experiments has been designed, established, and commissioned. This paper describes the design and initial experiments of SWE-2. In summary, two pulsed current sources based on pulse capacitors and spark gaps are adopted to drive sub-microsecond and microsecond time scale wire explosions in a gaseous/liquid medium, respectively. In the initial experiments, a single copper wire was exploded in air, helium, and argon with a 0.1-0.3 MPa ambient pressure as well as tap water with a 283-323 K temperature, 184-11 000 µS/cm conductivity, or 0.1-0.9 MPa hydrostatic pressure. In addition, the diagnostic system is introduced in detail. Energy deposition, optical emission, and shock wave characteristics are briefly discussed based on experimental results. The platform was demonstrated to operate successfully with a single wire load. These results provide the potential for further applications of this platform, such as plasma-matter interactions, shock wave effects, and reservoir simulations.

Note: Novel trigger pulse feed method for mega-volt gas switch.

It is difficult to feed the trigger pulse into an electrically triggered mega-volt switch, and the present note presents a novel trigger pulse feed method. The trigger pulse is introduced via a damping resistor, which is mounted between the inner and outer cylindrical electrodes of the pulse transmission line. The mega-volt pulse is damped because the voltage is resistively divided by the resistor and trigger cable arrangement. Both the complex breakdown processes of the switch and its insulation issues are experimentally studied. The function and the beneficial effects of the damping resistor, installed together with an additional inductor, are discussed. Finally, the parameters of these two damping components are set to 500 Ω and 2 µH values for which the switch has been demonstrated to work successfully at over 2.3 MV.

Design and experiment studies of a 2.6-MV diverter system.

Malfunctions of the Marx pre-fire or in the event that the main switch does not close were analyzed. Principles of the diverter system for protection of those events were introduced in detail. A 2.6 MV diverter system, consisting of an oil trigger switch and a Marx-coupled trigger generator, was developed. Based on "JianGuang-I" facility, a diverter-system test stand was established. And experiments with 2.3-MV working voltages were carried out to study the performance of this diverter system. Experiment results show that the time delay of this diverter system (from the beginning of the Marx erection to the time that the diverter-switch closes) is about 320 ns and its jitter (standard deviation) is about 8.9 ns. This diverter system has been tested more than 180 shots, and no problem has been encountered yet.

Development of a Marx-coupled trigger generator with high voltages and low time delay.

Coupled by the Marx of the "JianGuang-I" facility, a high voltage, low time-delay trigger generator was developed. Working principles of this trigger generator and its key issues were described in detail. Structures of this generator were also carefully designed and optimized. Based on the "JianGuang-I" Marx generator, a test stand was established. And a series of experiment tests were carried out to the study performance of this trigger generator. Experiment results show that the output voltage of this trigger generator can be continuously adjusted from 58 kV to 384 kV. The time delay (from the beginning of the Marx-discharging pulse to the time that the output pulse of the trigger generator arises) of this trigger pulse is about 200 ns and its peak time (0%∼100%) is less than 50 ns. Experiment results also indicate that the time-delay jitter of trigger voltages decreases rapidly with the increase in the peak voltage of trigger pulses. When the trigger voltage is higher than 250 kV, the time-delay jitters (the standard deviation) are less than 7.7 ns.

Signal Analysis and Waveform Reconstruction of Shock Waves Generated by Underwater Electrical Wire Explosions with Piezoelectric Pressure Probes.

Underwater shock waves (SWs) generated by underwater electrical wire explosions (UEWEs) have been widely studied and applied. Precise measurement of this kind of SWs is important, but very difficult to accomplish due to their high peak pressure, steep rising edge and very short pulse width (on the order of tens of µs). This paper aims to analyze the signals obtained by two kinds of commercial piezoelectric pressure probes, and reconstruct the correct pressure waveform from the distorted one measured by the pressure probes. It is found that both PCB138 and Müller-plate probes can be used to measure the relative SW pressure value because of their good uniformities and linearities, but none of them can obtain precise SW waveforms. In order to approach to the real SW signal better, we propose a new multi-exponential pressure waveform model, which has considered the faster pressure decay at the early stage and the slower pressure decay in longer times. Based on this model and the energy conservation law, the pressure waveform obtained by the PCB138 probe has been reconstructed, and the reconstruction accuracy has been verified by the signals obtained by the Müller-plate probe. Reconstruction results show that the measured SW peak pressures are smaller than the real signal. The waveform reconstruction method is both reasonable and reliable.

A novel design of Rogowski coil for measurement of nanosecond-risetime high-level pulsed current.

In pulsed power systems, pulsed currents with risetimes from nanosecond to microsecond can be effectively measured by self-integrating Rogowski coils. Appropriate design of the structure and the integrating resistor is crucial to the high-frequency response of a coil. In this paper, several novel designs of Rogowski coil's integrating resistors were proposed and tested. Experimental results showed that the optimized coil could response square waves with fronts of ∼1.5 ns and had a sensitivity of ∼0.75 V/kA. The maximal peak current was designed as 100 kA.