Liangwen Qi

In magnetic confinement fusion devices, plasma-facing materials are required to withstand severe high thermal loads, and their damage behavior significantly impacts the stability of the fusion plasma and the safety of the device. For the expected Type I edge-localized modes in future long-pulse steady-state fusion devices, the damage effects of multiple irradiations of transient thermal loads with low-medium energy density on tungsten materials are simulated using a coaxial plasma gun. At constant initial conditions (pulse frequency, gas-prefilled), the energy density is changed by altering the charging voltage to 0.1 MJ m ⁻² and 0.2 MJ m ⁻² . Combining the discharge and plasma evolution characteristics of coaxial plasma gun, the temperature rise of the tungsten target under different heat loads and the damage effect on the tungsten material are studied. The results indicate that the types of W target damage mainly include roughening, micro-cracks, crack networks, and crack melting. The number of thermal shocks dominates heat accumulation and damage evolution. In the initial stage, microcracks dominate. During the intermediate stage, molten droplets play a role in repairing the crack network. In the later stage, molten droplets form a brittle recrystallization layer. Energy density determines the intensity of a single thermal shock. Under low-energy thermal shocks, micro-cracks caused by thermal stress are predominant, while high-energy thermal shocks promote crack propagation through grain loss and molten W droplets. The results of this simulation experiment provide valuable insights into the interaction mechanisms between low to medium energy density plasmas and first-wall materials in future tokamaks.

Evolution of current harmonics excited by the third harmonic series resonance between the external circuit and plasma under varying discharge conditions was experimentally investigated in atmospheric pressure pulsed radio frequency (RF) discharges. Fast Fourier Transform analysis of voltage and current waveforms revealed the following: (1) At a fixed pulse repetition frequency, reducing the duty cycle caused the quasi-steady-state third harmonic resonance point at the tail of the pulse plateau to shift toward the high-power region; (2) under fixed pulse parameters, when the RF power at the trailing edge of the pulse plateau exceeds the resonance threshold, the transient harmonic resonance point during the plateau phase exhibits a progressive shift from the trailing edge toward the leading edge with increasing power; (3) during the pulse rising edge phase, the amplitude and position of the harmonics exhibited complex changes based on the pulse-off time. These results indicate that these changes in resonance points are attributed to the ability of the discharge capacitor to achieve the values required for harmonic series resonance under various operating parameters.

High-entropy oxides are a new type of material that consists of five or more principal elements in an equimolar or nearly equimolar ratio. They have many excellent properties and are rapidly becoming a hotspot for the development of new high-performance materials. In this study, electrical explosion is used for the first time to synthesize high-entropy oxide nanopowders with different crystal structures. (FeCoNiCrCu)O is the rock salt structure, (FeCoNiCrTi)O is the spinel structure, and (CoNiTiCuZn)O contains the two phases. According to the TEM and EDS results, the distribution of the five metal elements in the electrical explosion products is comparatively homogeneous, and the particle size of the products is concentrated in 20–40 nm. Elements such as Ti are prone to the formation of spinel structure, and the element Cu is prone to the formation of rock salt structure. The study shows that the electrical explosion of wires is a new method for the synthesis of high-entropy oxide nanopowders.

The formation and evolution of unconfined counter-helicity spheromaks merging have been experimentally investigated by using a magnetized coaxial plasma gun. By comparing the time-dependent photodiode signals and plasma radiation images of counter-helicity spheromaks merging and plasma jets merging, it is found that the field-reversed configuration (FRC) plasma formed by counter-helicity spheromaks merging has a distinct contour and a long maintenance time. For plasma jets merging, the resulting plasma has no discernible contours and a shorter lifetime. In addition, it is inferred from these data that stagnation heating and magnetic reconnection events occur during the counter-helicity spheromaks merging, causing a rapid rise in plasma pressure at the merging midplane and sharp kinks in the field lines near the merger region. By changing different operating parameters and observing the impact on the merger characteristics, it is suggested that the qualitative dynamics of the FRC plasma depends on the balance between the plasma pressure and the magnetic pressure. The high discharge voltage breaks the equilibrium in the merged body, while the large gas-puffed mass just weakens the compression effect of the merged body. These results give us an intuitive understanding of the counter-helicity spheromak merger process and its dependence on discharge parameters, and also provide a distinct perspective for the optimal design of FRC.

The effects of discharge parameters on the discharge process and plasma transport characteristics of a coaxial gun in the gas-prefilled mode are studied. The plasma optical intensity and ejection velocity are measured by photodiodes, the optical emission spectrum is taken by a spectroscopic system, and the plasma evolution in the transport process is captured by a high-speed camera. The plasma acceleration characteristics under different discharge parameters show that the velocity and electron density of the ejection plasma are mainly determined by the pre-filled pressure and discharge current, which is consistent with the snowplow model. The kinetic energy of ejection plasma can be significantly increased by reducing the outer loop inductance, which is conducive to increasing the energy utilization efficiency. The time-varying images of plasma radiation and the plasma density at different transport locations illuminate the transmission characteristics of coaxial gun discharge plasma. The results show that the snowplow effect continues to play a role in the plasma transport process, and the plasma accumulation is induced by the combination of shock wave compression. The current-driven magneto hydrodynamics instability occurs during the transport process, and the luminous signal of the plasma current sheet oscillates periodically. In addition, the plasma impact effect is obvious and the gas retarding effect is enhanced with the increase in the gas pressure. These results give us a more comprehensive view of the coaxial gun discharge process and plasma transport and provide a certain reference for optimizing the parameters selection and physical design of coaxial gun discharge plasma characteristics.

Phononic crystals are a kind of artificial acoustic metamaterial whose mass density and elastic modulus are periodically arranged. The precise and efficient design of phononic crystals with specific bandgap characteristics has attracted increasing attention in past decades. In this paper, an improved adaptive genetic algorithm is proposed for the reverse customization of two-dimensional phononic crystals designed to maximize the relative bandwidth at low frequencies. The energy band dispersion relation and transmission loss of the optimal structure are calculated by the finite-element method, and the effective wave-attenuation effect in the bandgap range is verified. This provides a solution for the custom-made design of acoustic metamaterials with excellent low-frequency bandgap sound insulation or other engineering applications.

The effects of bias magnetic field on the discharge characteristics of a coaxial gun and the parameters of ejected plasma are studied. At constant initial conditions (charging voltage, gas-puffed), magnetic probe data at different axial positions reveal the influence of the external bias magnetic field on the current distribution within the annular flow channel. Both photodiode signals and plasma radiation images show the plasma ejection characteristics. It is found that either applying or not applying the bias magnetic field results in two types of plasma with distinct ejection characteristics: spheromak or jet. By comparing the plasma parameters with and without bias magnetic field, it is found that the variation of jets velocity and momentum with discharge conditions are consistent with the snowplow model, whereas the spheromak plasma is subjected to a retarding force − J ϕ B r z ⇀ during the ejection, which decelerates the spheromak. Irrespective of the bias magnetic field, an increase in the discharge current amplitude leads to an elevation in electron density and temperature. The density of the spheromak is lower than that of the jet, but the temperature is higher. This is due to the fact that the electromagnetic focusing mechanism in the jet leads to a further increase in density, while the magnetic field lines stretched during the formation of the spheromak inhibit the focusing effect and expand freely after ejection. In addition, the magnetic field stretching process consumes part of the axial kinetic energy from the plasma and converts it into magnetic energy. The subsequent magnetic reconnection event causes the magnetic energy to be dissipated resistively by heating the plasma and then be converted into thermal energy. These results give us a better understanding in the interaction between plasma and magnetic field.

Coaxial gun can produce high-speed and high-density plasma jet and has some potential applications in many research areas such as space thruster, space debris impact simulation, nuclear fusion, and material processing. The coaxial gun is usually composed of a pair of coaxial cylindrical and hollow electrodes. The pulsed discharge of coaxial gun has two discharge modes, i.e., deflagration mode and pre-fill mode. Compared with the pre-fill mode, deflagration discharge mode can induce a plasma jet with few impurities, high collimation, and fast speed. In this paper, the effect of gas injection mass and discharge voltage on the discharge characteristic of deflagration mode are studied with electrical and optical diagnosis including the emission spectrum, plasma velocity and discharge current measurements. The experimental results show that when the gas injection mass is relatively low, such as 1.4 mg, many plasma clusters eject from the muzzle. As the gas flowing into the coaxial gun bottom increases, the plasma density increases and the jet velocity decreases. Eventually, when the gas injection mass increases to 2.6 mg, one cluster of plasma is found and ejects from the muzzle of the gun. In the discharge process, as a small quantity of gas flows into the bottom of the coaxial gun through the electromagnetic valve continuously, new current paths will be generated at the bottom of the coaxial gun and move forward. This results in the observation of multiple plasma jet at the exit of the coaxial gun. It is noted that the plasma densities are different for different gas mass flowing into coaxial gun bottom, but the currents have little effect in the first discharge half cycle due to the small plasma inductance in discharge circuit. Meanwhile, the plasma characteristics under different voltages with the fixed gas mass of 2.6 mg flowing into the coaxial gun bottom are experimentally measured. The results show that the plasma density and speed increase with voltage increasing, which is attributed to the stronger discharge current and larger self-induced Lorentz force. More neutral particles can be ionized into plasma with discharge voltage increasing, and the transport speed becomes faster under the enhanced force. In addition, the multiple ionization phenomena are observed again when the discharge voltage increases from 5 kV to 8 kV. This study provides an insight into how to better apply the coaxial gun discharge plasma to practical engineering field. The article further verifies the phenomenon of multiple discharges at the bottom of the coaxial gun by changing the charging capacitance and analyzing the magnetic probe signals.