Aici Qiu

The Influence of spot size on the expansion dynamics of nanosecond-laser-produced copper plasmas in atmosphere

Laser produced copper plasmas of different spot sizes in air were investigated using fast photography and optical emission spectroscopy (OES). The laser energy was 33 mJ. There were dramatic changes in the plasma plume expansion into the ambient air when spot sizes changed from ∼0.1 mm to ∼0.6 mm. A stream-like structure and a hemispherical structure were, respectively, observed. It appeared that the same spot size resulted in similar expansion dynamics no matter whether the target was located in the front of or behind the focal point, although laser-induced air breakdown sometimes occurred in the latter case. Plasma plume front positions agree well with the classic blast wave model for the large spot-size cases, while an unexpected stagnation of ∼80 ns occurred after the laser pulse ends for the small spot size cases. This stagnation can be understood in terms of the evolution of enhanced plasma shielding effects near the plasma front. Axial distributions of plasma components by OES revealed a good confinement effect. Electron number densities were estimated and interpreted using the recorded Intensified Charge Coupled Device (ICCD) images.

Infrared nanosecond laser-metal ablation in atmosphere: Initial plasma during laser pulse and further expansion

We have investigated the dynamics of the nanosecond laser ablated plasma within and after the laser pulse irradiation using fast photography. A 1064 nm, 15 ns laser beam was focused onto a target made from various materials with an energy density in the order of J/mm2 in atmosphere. The plasma dynamics during the nanosecond laser pulse were observed, which could be divided into three stages: fast expansion, division into the primary plasma and the front plasma, and stagnation. After the laser terminated, a critical moment when the primary plasma expansion transited from the shock model to the drag model was resolved, and this phenomenon could be understood in terms of interactions between the primary and the front plasmas.

Experimental Study on Multigap Multichannel Gas Spark Closing Switch for LTD

In this paper, a multigap multichannel gas spark closing switch (MMCS) has been modified to improve the synchronous closing characteristics of ten switches designed for linear transformer driver (LTD). Convexo-convex discal electrodes have been used instead of the plane toroidal electrodes to improve the electric field across the gaps. A new endo-grooved plane bracing structure has been designed for fixing the electrodes instead of the small ball anchoring method to ensure the length of the gap uniformity. In order to observe the discharge process of the MMCS, the transparent lucite insulation chambers instead of the nylon ones have been used. A test circuit comprising a single discharge circuit called LTD brick has been established to investigate the self-breakdown and triggered breakdown characteristics of ten switches. The results show that when switches are filled with pure nitrogen of 0.14 MPa and charged to plusmn60 kV, the synchronous discharge of ten MMCSs has output a current pulse with an amplitude of about 100 kA and a rise time of 100 ns.

Transforming dielectric coated tungsten and platinum wires to gaseous state using negative nanosecond-pulsed-current in vacuum

With the help of thin dielectric coatings, corona free explosions were achieved in the region of about half a wire length (2 cm) for tungsten wires and nearly the whole wire length for platinum wires under a fast rising (46–170 A/ns) negative polarity current in vacuum. Expansion velocity of the tungsten gas was over 10 km/s. Current waveforms from exploding coated wires were similar to those from bare wires in the air including a current pause stage. Coated wires with different coating parameters had a similar joule energy deposition before voltage collapsed, but a quite different scenario in the region near the electrodes. The axial field under negative current was the main reason for the axial inhomogeneity of coated tungsten wires. Tungsten or platinum gases in the vaporized region were tightly encompassed by the dielectric coating, while gaps or probably low density gases, were observed between the coating and the edge of the dense wire core in the core-corona structure region.

Simulation Analysis of Transmission-Line Impedance Transformers With the Gaussian, Exponential, and Linear Impedance Profiles for Pulsed-Power Accelerators

Based on the transmission-line code, a 1-D circuit model for the transmission-line impedance transformer with an arbitrary impedance profile is developed, and the simulation results are compared with those of the existing literatures. By using this model, the power efficiencies and pulse compression ratios of the transformers (with Gaussian, exponential, and linear impedance profiles) were quantified as functions of Ψ (the ratio of the output impedance to the input impedance of the transformers) and Γ (the ratio of the pulsewidth to the one-way transit time of the transformers). The simulation results indicate that the exponential transformers have the maximum power efficiencies and pulse compression ratios. However, in the limit h <; 0.05 (where h is the Gaussian parameter), the Gaussian transformers almost have the same power efficiencies and pulse compression ratios as the exponential ones. Finally, the performances of a radial transformer line in smoothing the arrival time variations in the forward-going power pulses were studied. The simulation results show that, for the cases considered, the rise time (0-1.0) of the output pulse is prolonged by about half of the maximum difference in the time at which the pulses are launched, and the reduction of the peak output power is less than 2% when the maximum difference is 30 ns.

Study of the shock waves characteristics generated by underwater electrical wire explosion

A model is proposed to simulate the generation and propagation of the shock wave (SW) produced by underwater electrical wire explosion in microsecond timescale, with the assumption that the exploding wire instantly turns into uniform discharge plasma channel (DPC) after the onset of explosion. To describe the interaction between the DPC and the surrounding water medium, the initial temperature of DPC is obtained by fitting calculated pressures with experimental data, and the injected energy of DPC is provided by the measured discharge current after wire explosion. To attenuate the high frequency oscillations generated by the discretization, the method with the double artificial viscosity parameters is proposed to calculate the SW propagation characteristics, and the input parameter is the above-calculated DPC boundary trajectory. Based on the proposed model, the DPC and SW properties of an underwater copper wire explosion are analyzed. The results show that the estimated initial temperature of DPC is about 15 000 K, the attenuation of peak pressure can be characterized by a law of the radial propagation distance r to the power of −0.74, and the efficiency transferred from stored electrical energy to the exploding wire and the generated water flow are ∼71.5% and ∼10%, respectively.

Study of nanosecond laser-produced plasmas in atmosphere by spatially resolved optical emission spectroscopy

We investigate the evolution of the species from both the target and the air, and the plasma parameter distribution of the nanosecond laser-produced plasmas in atmospheric air. The technique used is spatially resolved optical emission spectroscopy. It is argued that the N II from the air, which is distributed over a wider region than the target species in the early stages of the discharge, is primarily formed by the shock wave. The ionized species have a larger expansion velocity than the excited atoms in the first ∼100 ns, providing direct evidence for space-charge effects. The electron density decreases with the distance from the target surface in the early stages of the discharge, and both the electron density and the excited temperature variation in the axial direction are found to become insignificant at later stages.

Spatial confinement in laser-induced breakdown spectroscopy

The spatial confinement of plasma produced by a nanosecond laser is investigated using time resolved spectroscopy, fast imaging, interferometry, and numerical computation. The dynamics of the plasma, depending on shock waves, laser power, and wall distances, are studied. Experimental results confirm that the plasma is constricted by the reflected shock associated with a temperature and density gradient. The peak laser power determines the initial plasma parameters which affect the spectral intensities and the velocity of the reflective shock waves. The wall distance determines the reflection time of the shocks, which in turn influences the time delay of the collision between the two reflective shocks. The numerical results reveal a fast propagation process surrounding the reflective shocks, which indicates that the velocity of the reflective shock wave is influenced by the density of the plasma. The maximum enhancement factor ~5.2 is realized at a delay time of 11.7 µs under a pulse laser energy of 180 mJ and a wall distance of 9 mm.

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.

Transient Earth Voltage Measurement in PD Detection of Artificial Defect Models in

This paper mainly discussed the utilization of transient Earth voltage measurement (TEVM) in partial discharge (PD) detection of gas insulation switchgear. Four typical artificial defect models in SF6 such as spikes fixed on high voltage (HV) conductor, suspended metals, voids in dielectric, and metal particles on the surface of dielectric had been used as test objects in ac PD experiments with the aid of the synchronized PD signals sampled from a transient Earth voltage sensor, photomultiplier, Rogowski monitor, and PDCheck system. The measuring results of TEVM results, which included the phase distribution of PD, the relative amplitude, and the pulse number of different voltage thresholds, showed good consistency with three other methods and had better sensitivity of detection in spark or corona PD patterns. For further study, the frequency distributions of the three PD patterns were analyzed and discussed for understanding the different measurement sensitivities showed in different typical defects in SF6 . In addition, TEVM was applied to detect PD under oscillating impulse voltage. As an example, the needle-plane electrode with short gap was adopted under 613.2-kHz oscillating impulse, which showed the effectiveness in this kind of detection.