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High‐voltage electric pulse rock breaking has excellent potential for exploiting deep geothermal resources. Numerous researchers have conducted experimental studies on this topic, particularly in rock mechanics, where the breakdown occurs. However, there has been limited scholarly research on drilling fluid. Therefore, the study focuses on the drilling fluid suitable for electric pulse drilling, considering the characteristics of electric pulse rock breaking, which differ from traditional rock breaking. The study focused on the impact of various drilling fluid parameters on the effectiveness of electric impulse rock breaking using red sandstone as the experimental material. This was investigated using the finite element method, and indoor electric rock‐breaking tests were conducted in a drilling fluid environment. The results indicate that the plasma channel mainly grows in the permeable layer of the drilling fluid, resulting in shallow rock breaking depth in the drilling fluid environment. The pore permeated by drilling fluid guides the growth of the plasma channel. The higher the conductivity of the drilling fluid, the closer the ion channel of rock breaking by electric pulse is to the rock surface. This results in a smaller crushing volume and shallower damage depth, which is more detrimental to rock breaking by an electric pulse. The viscosity of drilling fluid can impede the breakdown to some extent. In this paper, the influence of drilling fluid parameters on electro‐pulse rock‐breaking technology is preliminarily studied, which has significant reference value for the selection of actual drilling fluid. [ABSTRACT FROM AUTHOR]

High-voltage electric pulse rock-breaking technology has the potential to improve drilling speed significantly. Numerous mechanism studies and laboratory experiments have laid the foundation for developing high-voltage electric pulse rock-breaking technology. However, the mechanism of medium ion channels growing in three dimensions has yet to be fully understood. Based on this, the paper constructs a prediction model of the electrical breakdown trajectory in three dimensions using the finite difference method. The model considers the dielectric properties of the mineral particles inside the rock. The paper then simulates and analyzes the generation of plasma channels caused by the electrical breakdown of rock in space. This is done using the MATLAB numerical simulation software, in combination with the analysis of the electric rock-breaking circuits and the Probabilistic Development Model (PDM). The laboratory experiment on electric rock breaking verifies the prediction model for the trajectory of electrical breakdown. The research results show that the plasma channel generated by electric breakdown in rocks is a spatial curve that expands between the high-voltage electrode and the ground electrode. The breakdown points generated near the high-voltage electrode are more concentrated than those near the ground electrode, forming the primary fragmentation zone. The area near the ground electrode is the secondary fragmentation zone. The voltage difference between the high-voltage level and the grounding electrode significantly influences the crushing effect. A higher voltage leads to a more pronounced crushing effect at the high-voltage electrode. The research results have definite guiding significance for the engineering application of high-voltage pulse rock-breaking technology and the design of downhole drilling tools. Highlights: In this paper, a three-dimensional trajectory prediction model of electrical breakdown is established to study the electrical breakage of rock. The electric breakdown three-dimensional trajectory prediction model is utilized to simulate electrical breakdown in rock under various voltage amplitudes. The reliability of the three-dimensional trajectory prediction model is indirectly verified by the laboratory experiment with an electric pulse rock breaking. [ABSTRACT FROM AUTHOR]

Because of the low rate of penetration (ROP) and high cost of traditional rotary rock breaking, it is important to explore some unconventional new rock breaking methods. High‐voltage electrical pulse (HVEP) drilling has attracted much attention because of its advantages such as environmental protection, good borehole wall quality, and high rock breaking efficiency. This paper independently designs and builds a complete indoor experimental platform of HVEP electric breakdown, and selects the self‐designed classic coaxial type and cross‐type electrode bits to carry out laboratory experiments of HVEP rock‐breaking, to truly reveal the rock breaking mechanism of HVEP drilling and further promote its industrial application. The influence of the anode structure of electrode bit, liquid insulation medium, drilling fluid circulation condition, and pulse voltage on rock breaking mechanism are investigated. The findings indicate that the bottom‐hole morphology of the rock sample is related to the electrode bit structure. The S‐type electrode bit has a certain discharge blind area, which is not conducive to the rock breaking of HVEP. With the increase of pre‐charging voltage, the effective discharge rate (EDR) of both electrode bits increases gradually, trending towards 100% as the pre‐charging voltage grows. Under non‐circulating working conditions, greater conductivity of liquid medium results in a lower EDR of electrode bits. In all kinds of liquid media, under low voltage (pre‐charging voltage <15 kV), the single pulse penetration depth (SPPD) under the non‐circulating working condition is larger than that of the circulating condition, in contrast, at a higher voltage (pre‐charging voltage >15 kV), the SPPD under the circulating condition is larger than that the non‐circulating condition. To further observe the generation of plasma channel and the rock damage characterization of HVEP, this paper also establishes a three‐dimensional numerical model of dynamic electrical breakdown of red sandstone to reproduce the generation of plasma channel, which can be mutually confirmed by indoor experimental results. The research results are essential for a deeper understanding of the rock breaking mechanism by electric pulse and for providing some theoretical guidance for the industrial application of electric pulse drilling technology. [ABSTRACT FROM AUTHOR]

ABSTRACT The hydraulic‐electric pulsed discharge(HEPD) rock‐breaking technology is a new high‐efficiency technology that generates plasma shock waves to rupture rocks. Since the plasma channel formation mechanism involved in HEPD rock‐breaking technology is difficult to describe, there are fewer theoretical models of this technology. This paper establishes a HEPD plasma model, which integrally considers the mutual coupling between five physical fields. The multi‐physical field model realizes the whole process of plasma channels, breakdown channels, plasma shock waves, and plasma shock wave rock‐breaking during the HEPD. The changes in mass fraction, density, and diffusion flux of relevant elements in the electrochemical reaction equation of the plasma channel are comprehensively analyzed. The obtained plasma multiphysics field model shows that the interpenetration of charged ions forms the breakdown channel and plasma channel. The anion number density is related to the H‐ number density, and the decrease in H‐ number density is due to the fact that the energy in the plasma channel is not sufficient to satisfy the relevant chemical equations to continue the collision reaction. The damage to the rock by the plasma shock wave takes the form of a gradual spreading of the plasma shock wave from the center of the rock to the edges, which leads to rock fragmentation. The model has the potential to establish a link between HEPD rock‐breaking parameters and the efficiency of HEPD rock‐breaking, which could provide a practical way for the development and parameter optimization of hydraulic‐electric pulsed discharge rock‐breaking tools. [ABSTRACT FROM AUTHOR]

In this research, the finite-element numerical software COMSOL Multiphysics is used to simulate the electric pulse rock-breaking process, and the novel numerical model takes into account the multiple fields of electrical, thermal, and mechanical physics. The electric field strength inside the rock under the action of the electric pulse is updated by the full coupling function of COMSOL (version 6.1) software, and the electric damage variable χ is used to describe the process of electric breakdown inside the rock, to simulate the formation of plasma channel in the process of electric pulse rock-breaking. The voltage change curves and plasma channel trajectories in the electric breakdown process of the model in this paper are compared with the literature to verify the accuracy of the model. The formation process of plasma channels captures the temperature and stress changes during the whole process of electric pulse rock-breaking, to reveal the mechanism of the electric pulse rock-breaking process. With the presence of conductive particles inside the rock, the particles can effectively promote the formation of plasma channels, increase the area of electrical damage, and improve rock-breaking efficiency. Before the formation of the plasma channel, the internal temperature of the rock is about 600 K, and the stress is about 10−2 MPa; when the channel is formed, the energy of the electric pulse is mainly concentrated in the plasma channel, and the temperature of the plasma channel rises to 104 K. When the maximum stress exceeds the critical stress of the rock, the rock undergoes fracture. Furthermore, an appropriate increase in the rise time of the electric pulse increases the speed and area of rock-breaking, subsequently improving the efficiency of rock fracture. The results of the numerical modeling in this paper help to better understand the mechanism of the electric pulse rock-breaking process, which is of great engineering significance for the development of new electric pulse rock-breaking technology in the realm of geotechnical engineering. [ABSTRACT FROM AUTHOR]

High‐voltage electric pulse drilling technology has the advantages of high rock‐breaking efficiency, and low rock‐breaking energy consumption, which is one of the most promising rock‐breaking methods in geothermal drilling. However, there is insufficient understanding of the mechanism of high‐voltage electric pulse breaking of HDR under high‐temperature and high‐pressure environments (HTHP). In this paper, a numerical simulation model of electric breakdown under HTHP is constructed, and the whole process of high voltage electric pulse rock‐breaking and plasma channel generation is realized from the seven‐field coupling of circuit field, current field, magnetic field, breakdown field, heat transfer field, solid mechanics field and flow field. The temperature changes in the rock during the electric pulse rock‐breaking process were comprehensively analyzed, and the effects of different initial rock temperatures (200°C, 300°C, 400°C) and surrounding pressure conditions (30, 60, 90 MPa) on the high‐voltage electric pulse rock breaking law were considered. It is found that the thermal conductivity of the rock shows an increasing and then decreasing trend as the initial temperature of the rock increases, and that the thermal conductivity has a tendency to guide the development of plasma channels, and the radius of the plasma channels increases with the increase of the initial temperature of the rock. The heat transfer generated by high‐voltage electric pulse rock breaking is mainly in conduction, and the distribution shape of conduction heat is similar to the shape of the breakdown channel. The research in this paper can provide theoretical and technical guidance for developing electric pulse rock‐breaking technology. [ABSTRACT FROM AUTHOR]

High-voltage electric pulse rock boring (HVEPB) is a high-efficiency rock-breaking drilling technology, which is featured in high rock breaking efficiency and low cost. However, there is no suitable model to predict the high-voltage electric pulses rock-breaking process. First, this paper establishes a grain-based granite model based on Voronoi polygons, which can reflect the real characteristics of granite inhomogeneity. At the same time, based on the improved Wiesmann Zeller model, the electric breakdown channel model is established to simulate the rock electric breakdown discharge channel. Second, a high-voltage electric pulse rock-breaking model is established by combining the shock wave model in the previous research, which can completely predict the HVEPB rock-breaking process and study the influencing factors in the rock-breaking process. Finally, the HVEPB rock-breaking process and the effects of rock composition, electrode bit diameter, and mineral grain size on the rock-breaking are studied by simulation and experiment. The results show that the plasma channel heats up and expands in a quite short time to produce shock waves that act on the surrounding rock, forming crushing fractures and microcracks. Red sandstone is more efficient than granite in breaking rock and is crushed in a shorter period. An increase in drill diameter leads to a decrease in crushing depth and an increase in crushing range and energy utilization. As the mineral grain size decreases, more fractures and smaller grain-size rock debris are produced. The findings are of great importance to guide the commercial application of high-voltage electric pulse rock-breaking. Highlights: A simulation model of the electric breakdown channel is established. A prediction model for the entire process of high-voltage electric pulse rock-breaking is established. The process and the mechanism of high-voltage electric pulse rock-breaking are revealed. The effects of three factors on the high-voltage electric pulse rock-breaking process are studied. [ABSTRACT FROM AUTHOR]

The plasma channel provides the fundamental energy source and acting forces for material removal in electrical discharge machining (EDM). At present, the research on the characteristics of plasma channel is still in the exploratory stage. In this study, to clearly understand the characteristics of plasma channel and its dynamic effects on molten material, the transient mathematical model of plasma channel is first attempted and computed based on magneto-hydrodynamics (MHD). The breakdown process of plasma channel, as well as its inherent characteristics (heat flux, electromagnetic force and expansion pressure) are studied, based on which the dynamic effects of thermal input and force actions on anode molten pool are analyzed. Results show that the plasma channel sequentially experiences diabolo-shaped, lantern-shaped and finally stabilized horn-shaped stages, and the diameter expansion is nearly completed within 2μs which is independent of discharge current. The plasma channel presents time-variant characteristics in terms of heat flux distribution and energy distribution ratio into electrodes and gap dielectric. The electromagnetic force and expansion pressure not only determine the geometry evolution of plasma channel but also provide acting forces, which influence the distribution of hydrostatic pressure in molten pool and shear force on the interface. For preliminary verification purpose, the appearance of plasma channel is observed by high-speed photography, and the dimensions between simulation and experimental craters are compared. The feasibility of established plasma channel model is confirmed to a certain extent by showing an approximated channel appearance, as well as the maximal deviations of 10.8 % and 8.9 % in diameter and depth, respectively. • Breakdown process of plasma channel in EDM is achieved by established transient multi-field coupling model. • The time-variant characteristics of thermal and force effects of plasma channel are revealed. • Plasma channel characteristics influence the distribution of hydrostatic pressure and interface shear force near crater. • Random spatial movement of plasma channel leads to the asymmetry of discharge crater. [ABSTRACT FROM AUTHOR]

High‐voltage electric pulse drilling technology has the advantages of high rock‐breaking efficiency, low energy consumption for rock breaking, and good quality of well wall, which is a new and efficient potential rock‐breaking method. In this paper, we constructed a numerical simulation model of rock electric breakdown and realized the whole process of rock breaking and plasma channel generation by high‐voltage electric pulse. The results of the study showed that the change of magnetic field has a large influence on the formation of plasma channels inside the rock, and under the self‐magnetic field environment, the plasma channels inside the rock show branching, and the maximum penetration depth and maximum damage depth are higher than those under the non‐magnetic field environment. In the external magnetic field environment, the plasma channels gradually tend to develop "downward" (inside the rock), and the temperature distribution of the plasma channels inside the rock is more concentrated. In the presence of a magnetic field, the speed and degree of freedom of charged particles are limited. In an external magnetic field environment, the charged particles cause fewer particles to deviate, and the particles show a "downward" accumulation phenomenon. The research in this paper can provide certain theoretical and technical guidance for the development of electric pulse rock‐breaking technology and provide reference for the development and parameter optimization of electric pulse rock‐breaking tools. [ABSTRACT FROM AUTHOR]

Simulation of the pulse-periodic high pressure cesium discharge is performed on the basis of equations of radiative gas dynamics. It is shown that in the discharge it is possible to implement two different types of structure of the plasma channel. At the beginning of the current pulse, the plasma discharge channel has a centered structure. At the same time, most of the plasma is concentrated near the discharge axis. The concentration of charged particles decreases along the radius. Then, if the current amplitude is large enough, during the plasma heating process, a transformation from the centered to the shell structure of the channel occurs. In this case, most of the plasma is concentrated on the periphery of the discharge and its concentration increases along the radius from the axis to the walls of the tube. It is shown that the transition from one channel structure to another occurs at a time when the specific heat capacity of the plasma near the axis reaches a deep minimum corresponding to a completely single ionized e–i-plasma. [ABSTRACT FROM AUTHOR]

The breakdown process of high-voltage pulse discharge under water involves rapid changes in the plasma channel, which are influenced by various factors, such as time, electric field, and thermal field. Accurately measuring the plasma channel is challenging due to technological limitations. This study proposed the theory of equivalence and equivalent numerical simulation for underwater pulsed discharge to understand the breakdown process and the role of the plasma channel in high-voltage pulse discharge under water. It established the equivalence theory and numerical simulation of the discharge breakdown process by considering the delay characteristics of discharge breakdown and the plasma channel model. The plasma channel was described intuitively and quantitatively, providing a concise and effective analysis of the complex breakdown process. Results indicate that (1) the breakdown channel undergoes an initial plasma channel stage accompanied by heating effects, which then transforms into an arc channel, leading to breakdown and detonation. (2) The equivalent resistance of the plasma channel is approximately 10 Ω, and the equivalent radius of the initial channel is around 0.1 μm. Breakdown is difficult to achieve when the electrode spacing is 0.5 cm and the charging voltage Um is below 5.5 kV, with an estimated minimum charging voltage of approximately 6 kV required for breakdown. (3) Equivalence theory enables effective quantification of the equivalent radius and equivalent resistance in the breakdown process, providing a representative measure of breakdown process stability. It proves to be an effective method for monitoring the operation of discharge equipment in practical engineering. The study provides a reliable reference for assessing the breakdown process of underwater high-voltage pulse discharge and monitoring the performance of pulse discharge equipment. [ABSTRACT FROM AUTHOR]

As a novel rock-breaking technology, the high-voltage electro pulse rock-breaking boring (EPB) technology has a broad prospect for high efficiency and low energy consumption rock-breaking boring. But the mechanism of rock-breaking under the action of high voltage electro pulse is not clear. In this study, First, red sandstone is selected as the boring sample, the high-voltage electro pulse breaking experiment on red sandstone is carried out with an electrode bit with a diameter of 80 mm and the electrode spacing of 35 mm, the red sandstone is broken down by the shock wave from the electric blasting in plasma channels of the red sandstone, and the complete drilling in the red sandstone is realized. Second, the electric current is collected during the experiment, in addition, the parameter of the shock wave is obtained based on the discharge circuit model and the shock wave model developed in the previous study. The parameters of shock wave and discharge energy are obtained based on the electric discharge circuit model and the shock wave model established in the previous study. Finally, the breakdown process of red sandstone by the shock wave is simulated, and the high-voltage EPB mechanism is analyzed, the simulation result is consistent with the experiment result. Experimental and simulation results indicate that the breakdown process of red sandstone under the action of high-voltage electro pulse includes the electric blasting occurrence stage and the energy dissipating stage, the red sandstone is broken down by the resultant mechanisms of shear failure and tension failure, but the shear failure is dominant. Highlights: PFC to model the breakdown process caused by shock wave in red sandstone is presented. The mechanism of high-voltage electro pulse boring red sandstone is studied. The breakdown of red sandstone under the action of high-voltage electro pulse is mainly depended on the shear stress. Experiment of high-voltage electro pulse boring red sandstone is carried out and the specific energy is 197J/cm3 [ABSTRACT FROM AUTHOR]

Our research proposes a unique coupled electro-thermal-mechanical model that takes electric breakdown and heterogeneity into account to show the mechanism of rock fracturing under high-voltage electropulses. Using finite element numerical software, the process of high voltage electrical pulse injection into the rock interior for breakdown is described, and the formation law of plasma channels during the electrical breakdown process is comprehensively analyzed in conjunction with the conductor particles present within the rock. On the basis of electrical, thermal, and mechanical theories, a coupled multi-physical field numerical model of rock failure under the action of high-voltage electrical pulses is developed, and a random distribution model is utilized to simulate the potential occurrence of conductor particles in the rock. Innovative numerical model indicates plasma channel creation in the rock-crushing process. Prior to the formation of the plasma channel, the temperature and stress are approximately 103 k and 10−2 MPa, respectively. Once the plasma channel is formed, the temperature and stress increase abruptly in a short time, with the temperature reaching 104 k and the stress reaching 103 MPa or higher. In addition, it is revealed that the breakdown field strength is the essential factor in plasma channel creation. The heterogeneity of the particles within the rock and the fluctuation in electrode settings are also significant variables influencing the creation of channels. The presented model contributes to a better understanding of the mechanism of rock fragmentation during high-voltage electrical pulses, which has substantial implications for oil exploration and mineral extraction. [ABSTRACT FROM AUTHOR]

The guidance and stable propagation of laser pulses over many Rayleigh lengths are crucial for the plasma electron acceleration in the laser wakefield accelerators. Using plasma channels with specific characteristics can lead to the proper guidance of the laser pulse. Here, quasi-three-dimensional particle-in-cell (PIC) simulations are performed to investigate the guidance of Bessel–Gaussian pulse (BGP) of zeroth order and super-Gaussian pulse (SGP) of 3rd and 4th orders in an axially and radially inhomogeneous plasma channel. The effects of the channel radius and depth, the laser wavelength and initial spot size, and the plasma channel inhomogeneity on the guidance of the laser pulse are also examined. The results indicate that the guidance of a laser pulse in the plasma channel depends on the pulse profile, and under certain conditions, the pulses can be guided with the least variation of spot size in the inhomogeneous plasma channel. It is shown that the channel depth and the initial laser spot size are very effective in pulse guiding, as the values of these parameters increase, the pulse guidance is done better. In addition, the results show that the guidance of laser pulse is dependent on the type of plasma inhomogeneity represented by three different kinds of initial conditions, as considering the nonlinear-axial inhomogeneity in the parabolic plasma channel can lead to more convergence than the axially homogeneous and linear-axially plasma density profiles. [ABSTRACT FROM AUTHOR]

The two-dimensional angular distributions of terahertz radiation at various frequencies emitted from the plasma of a single-color filament are obtained under different experimental conditions. It is shown that the radiation pattern at each terahertz frequency weakly depends on the laser beam numerical aperture and pulse energy. Increasing the laser pulse energy leads to the growth in the terahertz emission energy, including in the multiple filamentation regime. Furthermore, over the entire range of considered pulse energies, the dependence is close to linear for different terahertz frequencies and various laser beam numerical apertures. Our results allow to scale the terahertz emission from single-color filament in spectroscopy and other applications. • For single-color filamentation terahertz emission energy dependence on the laser pulse energy is studied for the first time. • The trend is approximately linear at various terahertz frequencies and for different laser beam focusing conditions. • New technique is used, that allows to take into consideration THz radiation propagating at large angles to the optical axis. • Structure of two-dimensional patterns of terahertz emission depends only on the THz frequency. • The structure is not affected by laser pulse energy and beam numerical aperture. [ABSTRACT FROM AUTHOR]

A four-channel scheme of an experiment aimed at obtaining optical and X-ray spectra and images of radiation generated in optical breakdown plasma channels in Bessel beams formed by focusing electromagnetic radiation by means of axicon-type conoid focusing systems is developed. [ABSTRACT FROM AUTHOR]

Electrical discharge machining (EDM) is known as one of the well-defined advanced manufacturing techniques. The electro-thermal property of EDM enables us to capture the electrical and thermal changes within the plasma channel. Our research has studied the impact of the electro-thermal of the plasma channel on surface integrity. We have analyzed the changes in the plasma channel by considering its physical characteristics based on the EDM's input parameters such as discharge current (I), pulse-on time (Ton), and concentration of added Al powder (Cp). In our experiment, adding Al powder decreased the dielectric resistance of kerosene. Furthermore, our mathematical and physical model demonstrated that adding powder had a positive effect up to 2.5 g/l. Moreover, the uneven distribution of powders into dielectric caused unstable discharge on the surface. We also studied the defects and changes of the plasma channel's features by using field emission scanning electron microscopy (FE-SEM). The consequence of the plasma channel changes led to the crack's disappearance entirely when the Al powder was used. It should be noted that with Ti-6Al-4V, it is hard to machine material, which is usually due to low thermal conductivity and its volume-specific heat. In our study, the fast-cooling rate created a diffusionless acicular martensitic microstructure, which led to a distorted hexagonal close-packed (HCP) crystal structure with the designation α′. This occurred while a high rate of quenching martensitic α′ was formed. The plasma channel creates a recast layer, which was affected by metallurgical changes within the EDM process. By using electron dispersive scanning (EDS) method and elemental mapping, we observed that the cross-section of the workpiece in the HAZ area led to the optimized oxidization, carbonization, and degree of fragility on the surface. Moreover, as the rate of the current intensity increased from 15 to 20-25A, the machining time improved by 16%, 12%, and 7%, respectively. Our experiment is consistent with our proposed mathematical model, and it can have practical applications, especially in medical, aerospace, and automobile industries. To the best of knowledge, our mathematical model that we have proposed for our experiment detects appearance and disappearance of metallurgical defects. [ABSTRACT FROM AUTHOR]

Gas-filled plasma closing switches (PCSs) are essential components of high energy density pulsed power systems used to generate short, high-power (tens MW to several GW) impulses. In practical applications, PCSs are required to operate in high-current (tens kA), high-voltage (10s–100s kV) regimes, to produce fast-rising ns and sub-ns HV impulses. The transient impedance of a PCS affects the rise time of the generated impulses and the power delivered to the load. Thus, the design and optimization of PCSs require detailed information on the dynamic plasma resistance. This article is focused on an investigation of the time-dependent resistance of the plasma channel(s) formed between the electrodes of a multielectrode PCS. Experimentally measured underdamped current and voltage waveforms were used to obtain the transient plasma resistance of the PCS. The PCS was filled with different gases: dry air, CO2, N2, and a 90%/10% Ar/O2 mixture, in the pressure range from 0 up to 10 bar (gauge). It was found that the plasma resistance drops rapidly from a few hundreds of ohms to a few hundreds of milliohms due to the Joule heating of the breakdown channel. The methods proposed by Braginskii and Kushner to model the transient postbreakdown resistance were used to obtain analytical plasma resistances for all of the gases. These resistances have been compared with the transient resistance obtained using a Kirchhoff analysis of the lumped-element, postbreakdown RLC circuit. Based on this comparison, the suitability of the chosen analytical methods to determine the transient breakdown resistance in gases is discussed. The obtained results will help in the optimization of the operational performance of PCSs filled with environmentally friendly, low environmental impact gases. [ABSTRACT FROM AUTHOR]

This work investigates microwave drilling of steel sheets with plasma channel triggered by microwave–metal discharge at the pointed end of a thoriated tungsten tool. The plasma channel was induced inside a multimode domestic microwave applicator operating in atmospheric condition at the frequency of 2.45 GHz. The thermal energy of the microwave–metal discharge-induced plasma channel caused the thermal ablation of the steel sheets by melting and vaporization. Sheets of galvanized steel and stainless steel (thickness ~ 0.5 mm) were drilled to generate through holes. Undersized holes in galvanized steel were observed for the microwave exposure time of 40 s, while an increase in microwave exposure time to 65 s resulted in hole overcut and wider thermal damage. Wider thermal damage zone observed in stainless steel sheets is attributed to lower melting temperature and lower thermal conductivity of stainless steel. Characterization of drilled holes indicated that the quality of holes obtained in both the sheet material is yet to be comparable with other non-traditional drilling methods such as electrical discharge machining and electrochemical machining. A hypothesis of microwave drilling of metals has been proposed based on the experiment results. [ABSTRACT FROM AUTHOR]

The empirical time-varying resistance model of the plasma channel $R_{\mathrm {pl}}(t) = \beta l^{2}/W_{\mathrm {in}}(t)$ in underwater discharge has been widely used, but the internal energy of the plasma channel $W_{\mathrm {in}}(t)$ is difficult to obtain and the discharge coefficient $\beta $ varies widely under different experimental conditions. This article introduces an improved empirical plasma channel resistance model $R_{\mathrm {pl}}(t) = \beta l^{2}/E_{\mathrm {pl}}(t)$ as well as a method for determining the discharge coefficient $\beta $. The effectiveness and the feasibility of the method are verified by comparing the simulation results and test results. The improved plasma channel resistance model and the method to determine the discharge coefficient can help guide the application of electrohydraulic shockwaves. [ABSTRACT FROM AUTHOR]

The spark channel plasma generated in the atmospheric nitrogen trigatron is studied with the laser Mach–Zehnder (M–Z) interferometer. The development of the spark channel plasma is described by analyzing the temporal and spatial evolution of the plasma electron density from the interferograms. The regions of the shock wave and the plasma are distinguished by the phase shift. Different spatial distributions of the anodic and cathodic plasma channels are obtained. The anodic spark channel shows good axial symmetry with constant radial expansion velocity. On the periphery of the main spark channel, a secondary spark channel occurs simultaneously in the cathodic region. After the collision of the two channels, the stagnation forms in the overlapping region and the secondary channel merges with the main one gradually. The leftward expansion velocity of the secondary channel is higher than the rightward velocity of the main channel, but both of them are lower than that near anode. The speculation on the production of the secondary spark channel is given and its influence on the development of the spark discharge in the gap is discussed in this work. [ABSTRACT FROM AUTHOR]

Terahertz (THz) wave generated by femtosecond laser filamentation can spatially constrained inside the plasma channels. Our simulation results show that this constraint to THz wave will gradually weaken with the increasing THz frequency. At the same time, results also indicate that the higher plasma density possesses the stronger constraint. In addition, the model field radius and width of THz wave will become larger with the increasing plasma channel radius. Based on the simulation results we find the changes of refractive index at THz wave play an important role on THz wave propagation in plasma channels. [ABSTRACT FROM AUTHOR]

This paper presents a theoretical investigation of the propagation characteristics of a q‐Gaussian laser beam propagating through a plasma channel created by the ignitor‐heater technique. The ignitor beam creates the plasma by tunnel‐ionization of air. The heater beam heats the plasma electrons and establishes a parabolic channel. The third beam (q‐Gaussian beam) is guided in the plasma channel under the combined effects of density non‐uniformity and non‐uniform ohmic heating of the plasma channel. Numerical solutions of the non‐linear Schrodinger wave equation (NSWE) for the fields of laser beams are obtained with the help of the moment theory approach. Particular emphasis is placed on the dynamical variations of the spot size of the laser beams and the longitudinal phase shift of the guided beam with the distance of propagation. [ABSTRACT FROM AUTHOR]

Abstract High-voltage electrical pulses (HVEP) can potentially be used to increase the yield of low-permeability coalbed methane (CBM) reservoirs. This study highlights the pore change and permeability evolution characteristics of anthracite coal samples using HVEP treatment. An experimental system was developed to study the variation of coal core permeability by subjecting the coal samples to HVEP treatment under confining conditions. Scanning electron microscopy (SEM) and mercury intrusion porosimetry (MIP) analyses were adopted to gain a clearer insight into the changes in pore structure. SEM images revealed that the surface morphologys of the raw coal samples were complete and smooth, whereas those of the coal samples subjected to HVEP developed many pores and cracks. The pores and cracks generated enhanced the permeability of HVEP-treated coal samples. MIP test results showed that the cumulative pore volume of the coal samples subjected to HVEP was significantly greater than that of raw coal samples. Importantly, the HVEP technology had a significant effect on mesopores and macropores, compared with micropores. Because the mesopores and macropores are critical for gas transport, the technology will be helpful for enhancing CBM recovery. Seepage experiments results showed that with the variation of CH 4 injection pressure from 0.4 MPa to 1.2 MPa, the permeability of both raw coal samples and coal samples subjected to HVEP showed a decline at a particular confining stress. The permeability of Yangzhuang (YZ) coal sample subjected to HVEP was 1.6–2.2 times that of YZ raw coal under a constant confining pressure, and the permeability of Guhanshan (GHS) coal sample subjected to HVEP is 1.8–2.4 times that of the GHS raw coal sample. At the same confining pressure and CH 4 injection pressure, the permeability of the coal samples subjected to HVEP was obviously higher than that of the raw coal samples. Graphical abstract Unlabelled Image Highlights • Coal permeability change post-HVEP was analyzed using an experimental system. • Cumulative pore volume of HVEP-subjected coal is greater than that of raw coal. • Permeability of HVEP-subjected coal is higher than that of raw coal. [ABSTRACT FROM AUTHOR]

Abstract Multi-GeV-class laser–plasma accelerating modules are key components of laser–plasma accelerators, because they can be used as a booster of an upstream plasma or conventional injector or as modular acceleration sections of a multi-staged high energy plasma linac. Such a plasma module, operating in the quasi-linear regime, has been designed for the 5 GeV laser–plasma accelerator stage (LPAS) of the EuPRAXIA project. The laser pulse (∼ 150 TW, ∼ 15 J) is quasi-matched into a plasma channel (n p = 1. 5 × 1 0 17 cm−3, L ∼ 30 cm) and the bi-Gaussian electron beam is externally injected into the wakefield. The beam emittance is preserved through the acceleration by matching the beam size to the transverse focusing fields. And a final energy spread of < 1% has been achieved by optimizing the beam loading effect. Several methods have been proposed to reduce the slice energy spread and are found to be effective. The simulations were conducted with the 3D PIC code Warp in the Lorentz boosted frame. Highlights • Scaling laws in the quasi-linear regime derived with the power depletion and self-steepening effect taken into account. • A 5 GeV laser–plasma accelerator designed in a 30 cm long plasma channel and with an accelerating field of 20 GV/m. • The transverse emittance preserved by matching the beam size to the radial fields; the total energy spread of 1% achieved by optimizing the beam loading effect. • The slice energy spread due to beam loading analyzed and improved by reducing the emittance or tuning the plasma density. [ABSTRACT FROM AUTHOR]

Plasma channel characteristics and energy distribution in electrical discharge machining (EDM) were mostly studied by analyzing the geometry parameters of craters caused by a single pulse discharge in previous studies. However, single pulse experiments cannot provide us insights into superposition, migration, abruption, interruption, and other phenomena of the plasma channel which have significant effects on EDM. Besides, EDM itself is a consecutive pulse discharge process. Thus, this paper focuses on the characteristics of plasma channel and the mechanism of material removal based on experimental data from multi-spark pulse discharge machining. The contrastive milling experiments of different parameters in multi-spark pulse discharge machining in high-speed dry EDM by using nickel-based superalloy as workpiece were conducted. The effects of peak current, dielectric type, breakdown voltage, air pressure, and electrode rotation speed on the crater number, crater distance, crater depth, and crater removal volume were studied. The plasma channel characteristics and material removal mechanism in continuous machining of high-speed dry EDM were revealed. [ABSTRACT FROM AUTHOR]

A self-designed experimental system was established to study the effect of NaCl solution on the pore structure of coal body with high-voltage electrical pulse.The anthracite coal samples of Linhua coal mine in Guizhou were broken down by high-voltage electrical pulse after being soaked in distilled water and NaCl solution at different concentrations.Subsequently,the scanning electron microscopy,energy spectrum analysis,and mercury injection experiment were carried out on the coal samples.The results reveal that during the coal soaking in NaCl solution,a large amount of conductive Na+ and Cl-ions entered into the primary pores of coal,effectively improving the conductivity of the coal.Compared with the coal soaked in distilled water,the coal soaked in NaCl solution was more fully broken under the action of the high-voltage electrical pulse.Moreover,the porosity,pore volume,and distribution range of pore size were increased with the increase in the concentration of NaCl.Furthermore,the volumes of mesopore and macropore clearly increased,and the pore connectivity was enhanced,demonstrating that the pore structure was improved. [ABSTRACT FROM AUTHOR]

Multilayer structures in which nanosecond surface microdischarges are generated have been developed, fabricated, and investigated. In these structures, layers are made in the form of thin transparent films, and a plasma discharge channel is formed in thin spacings between the layers. Passage of the discharge channel from one layer into the neighboring layer is implemented via pre-fabricated microholes. Images of microdischarges were obtained which confirmed that their plasma channels are formed according to the route assigned by the holes. The route may follow a fairly complex scheme and have self-intersection points and portions in which the electrons are bound to move in opposition to the electric field. In studying the shape of channels in multilayer strictures, the authors have found a new physical effect which lies in the azimuthal self-orientation of the discharge channel as it passes from one microhole to another. [ABSTRACT FROM AUTHOR]

The molybdenum disulfide (MoS 2) nanosheet is successfully synthesized using a hydrothermal Li-intercalation exfoliation method. We experimentally show that the MoS 2 dispersion solution can be used for optical modulation and all-optical switching based on the plasma channel induced by control pulses. The modulated signal light has a good transmission property in free space. The spatial modulation of the signal laser can be easily adjusted through the power of control lasers and the suspension concentration. The influence of suspension concentration and type on the free-space propagation of modulated signal light is also investigated. Results reveal a potential application of MoS 2 as an all-optical modulator for diversified photoelectric applications. • MoS 2 dispersions can be used for all-optical plasma modulation and optical switching based on plasmas. • Modulations can be adjusted conveniently by control laser power and suspension concentration. • Divergence of modulated signal beams increases with suspension concentration. [ABSTRACT FROM AUTHOR]

Dynamic shock waves induced by high-pulsed current discharge in liquid have wide potential applications in industrial and civil fields, such as rock fragmentation, electrohydraulic cleaning, and oil stimulation. Normally, the interelectrode gap distance is small due to the high electrical breakdown field of the liquid. Most of the electrical energy is consumed by the external circuit due to the low impedance of the plasma channel, and the energy transfer efficiency from the electrical energy to the mechanical energy is very low. In this paper, the optimal interelectrode distance and the effect of the plasma channel length on the intensity of the shock wave are investigated. The voltage across the liquid gap, main discharge current through the gap and the pressure wave, as well as the time-resolved images of the plasma channel development and the bubble formation are observed and presented. Results show that the optimal interelectrode distance can generate the strongest intensity of the shock waves. The plasma channel length can be extended by the plasma channel length regulation (PCLR) method, and the deposited energy into the plasma channel is increased. Besides, the energy transfer efficiency can be improved effectively by the PCLR method, and the requirement of the capacitor and switch can be reduced to obtain the same shock wave intensities. Based on the PCLR method, the induced shock wave was applied to increase the permeability of the specimen. It shows that the energy transfer efficiency can be improved effectively based on the presented method, and the shock wave can increase the permeability effectively. [ABSTRACT FROM AUTHOR]

We experimentally demonstrate spatial light modulation based on graded-index plasma channels induced by femtosecond pulses. The spatial profile of the probe beam can be conveniently controlled by adjusting the intensity distribution of the pump beam or by changing the relative position between pump beam and probe beam. We also show that the modulation depth of the probe beam can be controlled by adjusting the power of the pump beam. [ABSTRACT FROM AUTHOR]

Temporally shaped femtosecond laser pulse is used to generate the air plasma channel. The length of plasma channel is optimized by a genetic algorithm. Compared with the transform-limited pulse, the temporally shaped femtosecond laser produced by the spatial light modulator with the genetic algorithm can lead to a significant increase in length and brightness of plasma channel in atmosphere. In particular, the length of the plasma channel produced by the optimized shaped pulse can be extended by 50%. This method can be especially advantageous in the context of femtosecond laser-induced plasma channel. [ABSTRACT FROM AUTHOR]

The paper deals with the relevance of using electrical discharge technology for construction works in the city. The technical capabilities of the installation for the multi-borehole electro-discharge splitting off and destruction of rocks and concrete are described. The ways to increase the efficiency of the electrical discharge method for the destruction of solids are proposed. In order to augment the discharge circuit energy, the energy storage is separated into two individual capacitor batteries. The throttle with the inductance of 28.6 μH is installed in one of the batteries, which increases the duration of the channel energy release to 400 μs and the efficiency of electrical discharge splitting off of concrete. [ABSTRACT FROM AUTHOR]

Laser wakefield acceleration of electrons in the blowout regime can be controlled by tailoring the laser pulse phase and the plasma target. The 100 nm-scale bandwidth and negative frequency chirp of the optical driver compensate for the nonlinear frequency red-shift imparted by wakefield excitation. This mitigates pulse self-steepening and suppresses continuous injection. The plasma channel suppresses diffraction of the pulse leading edge, further reducing self-steepening, making injection even quieter. Besides, the channel destabilizes the pulse tail confined within the accelerator cavity (the electron density “bubble”), causing oscillations in the bubble size. The resulting periodic injection generates background-free comb-like beams – sequences of synchronized, low phase-space volume bunches. Controlling the number of bunches, their energy, and energy spacing by varying the channel radius and the pulse length (as permitted by the large bandwidth) enables the design of a tunable, all-optical source of polychromatic, pulsed γ-rays using the mechanism of inverse Thomson scattering. Such source may radiate ~ 10 7 quasi-monochromatic 10 MeV-scale photons per shot into a microsteradian-scale observation angle. The photon energy is distributed among several distinct bands, each having sub-25% energy spread dictated by the mrad-scale divergence of electron beam. [ABSTRACT FROM AUTHOR]

The formation of plasma channel with the use of eight spark discharge sources was experimentally investigated in a high current pulsed electron beam (HCPEB) equipment. A triple Langmuir probe system was developed to diagnose the real-time plasma parameters. The spark discharges were initiated by using a high-voltage pulse of −7.0 kV and decoupling resistors of 40 Ω. Displacing the measurement positions, the plasma density distributions in a column of diameter 60 mm and length 175 mm were obtained. The duration of spark discharge is 11.0 μs and the maximum discharge current of 1.4 kA appears at 4.3 μs. Initially, the plasma concentrates in the peripheral region. With the accumulation in radial direction, the plasma density in the central region increases. After 6.6 μs, the maximum plasma density of 8.4 × 1018 m−3 appears at the center of the plane where the spark sources located. Thereafter, the plasma dissipation plays a dominant role. The plasma maintains about 20 μs. Depending on the formation features of plasma channel, the selection of timing control available was illustrated, while the transportation of HCPEB can be realized effectively within a limited distance, i.e. less than 60 mm beyond the source outlet in this case. • Eight spark sources were utilized to form plasma channel in HCPEB equipment. • Triple Langmuir probe system was developed to diagnose the plasma parameters. • Plasma density distributions in the cylindrical space were obtained. • Selection of timing control available was illustrated with experimental results. • HCPEB transportation can be realized effectively within a limited distance. [ABSTRACT FROM AUTHOR]

Electrical discharge machining (EDM) was developed over 70 years ago, but the knowledge of plasma channels that form during this process remain understudied. Therefore, this study systematically investigated the breakdown process and impedance characteristics of plasma channels through singe-pulse discharge experiments. The influence of discharge current and gap distance on the breakdown process and impedance characteristics of plasma channels was also analyzed. The results of this work can contribute to the existing body of knowledge of the physical nature of EDM. [ABSTRACT FROM AUTHOR]

In order to study the feasibility of crushing coal by electric and heat in the process of high-voltage breakdown in air environment, as well as the macroscopic and microscopic characteristics of coal, an high-voltage breakdown testing system of coal is built, and the anthracite coal of Linhua coal mine in Guizhou province was selected for the test. Under the needle electrode-needle electrode, the experimental results show that the air dielectric breakdown field strength from 18.0 to 18.3 kV/cm, the breakdown field strength of coal body is 0.3-0.8 kV/cm, the breakdown voltage of anthracite coal is less than the breakdown voltage of air. Under the same conditions, the breakdown voltage and damage characteristics of coal samples are not the same, the breakdown voltage ranges between 20 kV and 41 kV, and there are three main types of damage on coal samples. In the process of the crushing coal by high-voltage, the coal samples of plasma channel presents a burning state, formed a large number of cracks and pores. At the same time, the oxidation reaction happened in the coal around the plasma channel under the condition of high temperature, formed new oxidation products. [ABSTRACT FROM AUTHOR]

Electrohydraulic forming (EHF) is based upon the electro-hydraulic effect: a complex phenomenon related to the high voltage discharge inside the water filled chamber. The resulting shockwave in the liquid is propagated toward the blank, and the mass and momentum of the water in the shock wave accelerates the sheet metal blank toward the die. Methodology of numerical simulation of EHF processes was developed based upon LS-DYNA commercial code using Arbitrary Lagrange–Eulerian (ALE) Multi-Material formulation. The model incorporates energy deposition inside the plasma channel, expansion of the channel driven by high pressure inside of it, propagation of the pressure pulse through the water filled chamber in contact with the rigid walls of the chamber and with the sheet metal blank being deformed. Comparison of the numerical and experimental results was performed on maximum pressure measured on the wall of the cylindrical chamber employing the membrane method. The model was used to simulate multistage EHF of a complex geometry automotive part. Analysis of the results showed the complex nature of multistage EHF process: a clearly recognizable wave picture during the initial stage of the channel expansion which transitions to almost incompressible water flow during later stages. [ABSTRACT FROM AUTHOR]

This paper describes the chip that is used for generating nanosecond surface micron-size discharges, as well as the results of observations of their interaction with surfaces of living tissues of plants: onion shell, leaves of Canada water weed, and day nettle petals. The interaction was observed for the first time in cellular size range. It was identified that the trajectories of plasma discharge channels follow intercellular walls and have a zigzag shape. It was also identified that a single discharge having <0.5 mJ energy results in destruction of cell wall, which stimulates intensive juice excretion. [ABSTRACT FROM AUTHOR]

In the present paper, laser pulse distortion/breakup and the effect of the plasma channel on the laser propagation through the collisional plasma have been studied by using moment theory approach. Second order nonlinear differential equations of the beam width parameter have been derived for the propagation of the laser through uniform homogenous plasma and preformed plasma channel having parabolic density profile. Differential equations of beam width parameter have been solved numerically using Runge Kutta method. It has been observed from analysis that when the laser pulse propagates through the homogenous plasma, the low intensity front and rear parts of the laser get defocused/diffracted and the high intensity central/main portion of the laser pulse gets self-guided. As a result of this, the laser pulse gets distorted. This distortion of the laser has not been observed when the laser pulse is propagated through the plasma channel having density minimum at the axis and maximum at the edges. The laser pulse is guided as a whole, even the low intensity front and rear parts of the laser are also guided. Therefore, the plasma channel is useful to prevent the distortion/breakup of the laser. [ABSTRACT FROM AUTHOR]