Jiawei Wu

Generation of Electrohydraulic Shock Waves by Plasma-Ignited Energetic Materials: I. Fundamental Mechanisms and Processes

Shock waves in water have many applications, such as lithotripsy, sterilization, weapons, and so on. Underwater electrical wire explosion can be utilized to produce shock waves with fast front and short duration time efficiently. In order to amplify the energy of shock waves, energetic materials (EMs) were considered. In this paper, Al, Cu, Mo, and W wires with EM covers were designed and tested. The results showed that wire explosion ignited the EMs, which in turn affected the process of wire explosion. As a result, the duration time of shock waves was elongated.

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.

Generation of Electrohydraulic Shock Waves by Plasma-Ignited Energetic Materials: III. Shock Wave Characteristics With Three Discharge Loads

As an important plasma-assisted technology to generate shock waves (SWs), underwater pulsed discharge has drawn much attention in recent years for its complex physical process. Based on three discharge loads, the water gap (WG) load, the electrical wire (EW) load, and the energetic material (EM) load, the discharge processes are briefly introduced and the characteristics of the associated SWs are analyzed. First, the experimental setups were built and typical structures of the three loads were presented. Second, the inherent characteristics of SWs under the three loads, such as their peak pressure, impulse, and time duration of positive pressure and power spectral density (PSD), were studied and compared. Finally, a cracking effect experiment is carried out to study the SW fracturing characteristics. The results show that SWs generated with the WG load have the lowest peak pressure, impulse, and power density, SWs generated with the EW load have a better energy conversion efficiency and the largest peak pressure, and SWs generated with the EM load have the maximum impulse and power density. Furthermore, SW fracturing characteristics are mainly affected by its inherent characteristics. The peak pressure and impulse determine the shock number of fracturing, and the fracture pattern is significantly affected by the PSD.

Generation of Electrohydraulic Shock Waves by Plasma-Ignited Energetic Materials: II. Influence of Wire Configuration and Stored Energy

Electrohydraulic shock waves (SWs) have been used for shale gas exploitation in recent years. The traditional method to generate SWs is underwater pulsed discharge with a water gap load or an electrical wire (EW) load, but the SW energy is limited heavily by the stored energy. To obtain stronger SW, the EW explosion ignited energetic material (EM) load is proposed here. To obtain a good ignition performance, the influence of capacitor charging voltage, stored energy, wire diameter, and wire material (Cu and Mo) were studied. The results show that the higher the charging voltage is, the more effective the ignition is; liquidation and vaporization will affect the discharge but will not ignite the EM explosion effectively; the EMs given in this paper are more sensitive to the arc plasma. Besides, the nonrefractory copper is more suitable for the EM ignition than the refractory molybdenum.

X-pinch radiography for the radiation suppressed tungsten and aluminum planar wire array

X-pinch radiography experiments were carried out on the 1 MA QiangGuang-1 facility to investigate the wire core behaviors of the tungsten and aluminum planar wire array. An axial quencher and over-massed loads were used to suppress keV radiation from the planar wire array. Two-wire 30 μm Mo and/or 25 μm W X-pinches were used as backlighters. The x-ray point-projection images showed quite uneven characteristics of the dense wire cores and the current distribution in the linear array. For the W single planar wire array (SPWA), Wire core diameter profiles are likely to reveal that the initial current distributed inductively among the wires with the same diameter in the array, and both inductively and resistively among the wires with different diameters. For the Al SPWA, wires in different positions were in quite different ablation processes. No correlations of stratifications or plasma jets between adjacent wires were observed.

Characteristics of exploding metal wires in water with three discharge types

This paper presents the characteristics of underwater electrical wire explosion (UEWE) with three discharge types, namely, Type-A, Type-B, and Type-C. Experiments were carried out with copper and tungsten wires (4 cm long and 50–300 μm in diameter) driven by a microsecond time-scale pulsed current source with 500 J stored energy. A time-integrated spectrometer and a photodiode were used to measure the optical emission of UEWE. A Polyvinylidene Fluoride probe was adopted to record the pressure waveforms. Experimental results indicate that from Type-A to Type-C, more energy deposits prior to the voltage peak and the first peak power increases drastically. This variation of energy deposition influences the optical emission and shock wave generation process. Specifically, the light intensity decreases by more than 90% and the peak of continuous spectra moves from ∼400 nm to ∼700 nm. In addition, the peak pressure of the first shock wave increases from ∼2 MPa to more than 7.5 MPa.

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 coils 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.

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.

Experimental verification of the vaporization's contribution to the shock waves generated by underwater electrical wire explosion under micro-second timescale pulsed discharge

This paper studies pressure waves generated by exploding a copper wire in a water medium, demonstrating the significant contribution of the vaporization process to the formation of shock waves. A test platform including a pulsed current source, wire load, chamber, and diagnostic system was developed to study the shock wave and optical emission characteristics during the explosion process. In the experiment, a total of 500 J was discharged through a copper wire load 0.2 mm in diameter and 4 cm in length. A water gap was installed adjacent to the load so that the current was diverted away from the load after breakdown occurred across the water gap. This allows the electrical energy injection into the load to be interrupted at different times and at different stages of the wire explosion process. Experimental results indicate that when the load was bypassed before the beginning of the vaporization phase, the measured peak pressure was less than 2.5 MPa. By contrast, the peak pressure increased significantly ...

A Novel Compact Repetitive Frequency Square-Wave Generator Based on Coaxial Pulse Forming Lines and Coupled Magnetic Switches

In this paper, a novel repetitive frequency square-wave generator named pulse forming line-based repetitive square-wave generator (PFLRSG)-based coaxial pulse forming lines (PFLs) and coupled magnetic switches (MSs) is proposed. The generator is capable of amplitude multiplication, high-frequency operation, and rise-time compression. The operation principle is to charge PFLs in parallel via coupled inductors when MSs are unsaturated and discharge in series when MSs saturate to form square-wave voltage pulses across the matched load, with the common diode arrays as the last stage to sharpen the rise-time. The amplitude of the output pulse is dependent on the charging voltage and impedance matching between the PFLs and the load. The pulsewidth is determined by the transmission time of the PFLs. With the principle, circuit topology of a two-stage PFLRSG is presented. Test stand yields an output square-wave pulse with frequency of 1-10 kHz, amplitude of 8.6 kV, rise-time of 60 ns, and half width of 500 ns into a 150-Ω matched load. Furthermore, design rules are proposed so that topologies of PFLRSGs with stages other than two can be conveniently derived. As an example, a 4-stage PFLRSG is proposed.