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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]

Abstract In rock engineering, high-voltage pulse technology has attracted attention because it offers environmental protection, controllable energy, and repeatable discharge. It is necessary to study the fracture behavior of rock under high-voltage pulse discharge (HVPD) for the parametric design of rock breaking thereby. HVPD experiments were conducted in red sandstone samples with the plasma channel spacing ranging from 26 to 66 mm at intervals of 10 mm. The stress wave generated by HVPD was obtained from the current waveform measured by Rogowski coils. In combination with numerical simulations, the distribution characteristics, propagation process, and formation mechanism of fractures were analyzed. The results showed that after two applications of HVPD at different positions, the sample was both broken down and two plasma channels and radial fractures centered around them were formed within. The stress wave decays exponentially with the increase of the distance from the plasma channel. When the spacing between plasma channels is less than or equal to 46 mm, fracture coalescence occurs between the two plasma channels; thereafter, the fractures formed by the second HVPD face resistance to propagation towards the fracture area formed by the first HVPD. In addition, numerical simulation results indicate that the second HVPD will generate significant tensile stress in the middle region of the two plasma channels, leading to near-horizontal fracture coalescence. When the spacing between plasma channels increases to 56 mm and 66 mm, the tensile stress induced by the second HVPD in the middle region of the sample is small, and it is difficult to form fracture coalescence between the two channels.

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]

Goal. Development of the generalized physical principle of development of plasma channel of a high-voltage electrical pulse spark discharge in the homogeneous dielectric of the different aggregate state. Methodology. Basis of physical optics, theoretical electrical engineering, electrophysics bases of technique of high-voltage and large pulse currents, bases of high-voltage pulse technique and measuring technique. Results. Development of physical principle of development of plasma channel of an electric pulse spark discharge is executed in a homogeneous gas dielectric on the applied example of the use in calculations and experiments of the double-electrode discharge system (DEDS) with a long air interval, testing action of standard interconnect аperiodic pulse of high-voltage of temporal shape of Tm/Тd≈200 μs/1990 μs of positive polarity. The generalized formula is got for the calculation of total length of lc of the real way of development of an pulse spark discharge in an air dielectric, which allowed to formulate the offered physical principle in the following kind: «The plasma channel of an pulse spark discharge in a gas dielectric spreads from one of its points to other after a way length of lc, providing the least falling on it of electric voltage of Uc». It is shown that this principle in the first approaching can be applied and to the homogeneous liquid and hard dielectrics. Comparison of the developed physical principle of distribution of plasma channel of an electrical spark discharge is executed in a dielectrical environment with fundamental Fermat physical principle (a law) for distribution of light in an optically transparent environment, which specifies on mathematical likeness and closeness on destiny of these physical principles. Calculation estimations of falling of electric voltage of Uc on total length of lc of the real zigzag way of development in the air dielectric of DEDS a «edge-plane» with the least length of its discharge interval of lmin=1,5 m is presented, that a value Uc does not exceed 9 % from the experimental level of aggressive voltage of Umd≈611,6 кV in this DEDS for the аperiodic pulse of voltage of Tm/Тd≈200 μs/1990 μs. It is set that the estimated time of td advancement of leader channel of electric pulse discharge in air DEDS (lmin=1,5 m) on its real way total length of lc≈1,53 m makes td≈15,3 μs, and experimental duration of cut of Tdc of the indicated аperiodic impulse of voltage utilized in experiments, characterizing time of short circuit by the plasma channel of discharge of air interval in DEDS, appears equal Тdc≈td≈17 μs. Originality. The generalized physical principle of development of plasma channel of a high-voltage electrical pulse spark discharge is first developed in the homogeneous dielectric of the different aggregate state. Practical value. Application in electrical engineering practice and high-voltage pulse technique of the offered principle of distribution in the dielectrics of plasma channel of an pulse spark discharge will allow to develop both new and to perfect the existent methods of computer design of electro-discharge processes in the gas, liquid and hard insulation of different high-voltage electrical power engineering and electrophysics devices, directed on the increase of reliability of their operation.

High-voltage electric pulse(HVEP)drilling has become a new and efficient rock breaking method, which is also the research focus in the field of drilling speed increase. To probe into rock breaking mechanism of high voltage electric pulse, this study establishes a two-dimensional numerical model of multi-physical field coupling electric breakdown of single pair of electrodes. The model reproduces the generation of plasma channels in homogeneous red sandstone from the coupling of current field, electric breakdown field and circuit field. This paper analyzes the effect of electrode pair angle, voltage and electrode spacing on rock electrical breakdown(that is, the formation of plasma channel in rock). The model includes the circuit structure parameters of pulse tool, the occurrence and development of electrical breakdown, and the relationship between electrical breakdown intensity and time. Results indicate that the plasma channel begins to sprout from the partial area near the top of the discharge electrode and develops towards the partial weak dielectric strength. With the voltage value of loading pulse increasing gradually, the time of electrical breakdown decreases gradually; comparatively, the equivalent failure volume of rock model increases gradually, and there is a positive correlation between them. On the precondition that the rock can be electrically broken, increasing the electrode spacing can improve the rock breaking efficiency of high voltage electric pulse. The equivalent failure volume of rock shows significant fluctuations during the gradual increase of electrode inclination angle of discharge electrode, and its extreme value mostly appears in the range of electrode inclination angle of 35°~ 55°. To further promote the industrial application of high-voltage electric pulse rock breaking, this paper proposes a three-dimensional numerical model of multi-physical field coupling dynamic electric breakdown of red sandstone based on two-dimensional model, reproducing the appearance of the fracture crater in the rock during the rock breaking process with electrode bit. At the same time, the self-designed coaxial electrode bit is selected for experiments of electric breakdown, and the laboratory experimental results of electric breakdown confirm the simulation experimental results.

This work investigates micro-electro discharge machining (EDM) performance involving deionized and tap water. The chosen machining regime was semi-finishing, where open voltage (from 100 to 130 V) and current values (5–10 A) were applied using a 0.5 µs pulse-on time and a frequency of 150 kHz, i.e., a duty cycle of 25%. First, numerical analyses were performed via COMSOL Multiphysics and used to estimate the plasma channel distribution and melted material, varying the current, sparking gap, electrical conductivity, and permittivity of the two fluids. Then, experimentally, the micro-EDM of holes and channels in hardened thin steel plates were replicated three times for each considered fluid. The material removal rate (MRR), tool wear ratio (TWR), radius overcut, and surface roughness were plotted as a function of open voltage and electrical conductivity. The study proves that as voltage and current increase, the MRR and TWR decrease with electrical conductivity. Nonetheless, for higher electrical conductivity (tap water), the process did not proceed for lower open voltages and currents, and the radius overcut was reduced, contrary to what is commonly acknowledged. Finally, the crater morphology and size were evaluated using a confocal microscope and compared to simulated outcomes. [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]

Goal. Development of the generalized physical principle of development of plasma channel of a high-voltage electrical pulse spark discharge in the homogeneous dielectric of the different aggregate state. Methodology. Basis of physical optics, theoretical electrical engineering, electrophysics bases of technique of high-voltage and large pulse currents, bases of high-voltage pulse technique and measuring technique. Results. Development of physical principle of development of plasma channel of an electric pulse spark discharge is executed in a homogeneous gas dielectric on the applied example of the use in calculations and experiments of the double-electrode discharge system (DEDS) with a long air interval, testing action of standard interconnect аperiodic pulse of highvoltage of temporal shape of Tm/Тd≈200 μs/1990 μs of positive polarity. The generalized formula is got for the calculation of total length of lc of the real way of development of an pulse spark discharge in an air dielectric, which allowed to formulate the offered physical principle in the following kind: «The plasma channel of an pulse spark discharge in a gas dielectric spreads from one of its points to other after a way length of lc, providing the least falling on it of electric voltage of Uc». It is shown that this principle in the first approaching can be applied and to the homogeneous liquid and hard dielectrics. Comparison of the developed physical principle of distribution of plasma channel of an electrical spark discharge is executed in a dielectrical environment with fundamental Fermat physical principle (a law) for distribution of light in an optically transparent environment, which specifies on mathematical likeness and closeness on destiny of these physical principles. Calculation estimations of falling of electric voltage of Uc on total length of lc of the real zigzag way of development in the air dielectric of DEDS a «edge-plane» with the least length of its discharge interval of lmin=1,5 m is presented, that a value Uc does not exceed 9 % from the experimental level of aggressive voltage of Umd≈611,6 кV in this DEDS for the аperiodic pulse of voltage of Tm/Тd≈200 μs/1990 μs. It is set that the estimated time of td advancement of leader channel of electric pulse discharge in air DEDS (lmin=1,5 m) on its real way total length of lc≈1,53 m makes td≈15,3 μs, and experimental duration of cut of Tdc of the indicated аperiodic impulse of voltage utilized in experiments, characterizing time of short circuit by the plasma channel of discharge of air interval in DEDS, appears equal Тdc≈td≈17 μs. Originality. The generalized physical principle of development of plasma channel of a high-voltage electrical pulse spark discharge is first developed in the homogeneous dielectric of the different aggregate state. Practical value. Application in electrical engineering practice and high-voltage pulse technique of the offered principle of distribution in the dielectrics of plasma channel of an pulse spark discharge will allow to develop both new and to perfect the existent methods of computer design of electro-discharge processes in the gas, liquid and hard insulation of different high-voltage electrical power engineering and electrophysics devices, directed on the increase of reliability of their operation. References 25, figures 5. [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]

It is shown by numerical simulations that, if a laser pulse from the eXawatt Center for Extreme Light Studies (Sarov) is used as a driver for a laser wakefield accelerator, an electron bunch with a charge of 50 pC can be accelerated to energy of 100 GeV with an energy spread of less than 1%. To this end, it is necessary to form a plasma channel 70 m long with a characteristic radius of 200 μm and a plasma density of 3 × 1015 cm–3 on the axis. In a denser plasma, the acceleration rate is higher, but the acceleration length and the resulting energy are smaller. The accelerator parameters can be numerically optimized using a quasistatic model describing the laser pulse in terms of its envelope, which reduces the computation time by several orders of magnitude as compared to complete models. [ABSTRACT FROM AUTHOR]

The spectrum of terahertz radiation generated in plasma of a single-color laser filament is observed, using a new technique based on obtaining two-dimensional angular distributions at different frequencies. It is shown that the maximum of the spectrum occurs in the low-frequency region for different laser pump focusing conditions. It is demonstrated that with the initial beam numerical aperture growth the generation of terahertz radiation at high frequencies increases more intensely compared to low frequencies. [ABSTRACT FROM AUTHOR]

The phase velocity of a laser-driven wakefield can be efficiently controlled in a plasma channel. A beatwave of two long laser pulses is used. The frequency difference between these two laser pulses equals the local plasma frequency, so that the slow resonant excitation of the plasma wave is possible. Because the driver energy is spread over many plasma periods, the interference pattern can run with an arbitrary velocity along the channel and generate the wakefield with the same phase velocity. This velocity is defined by the channel radius and the structure of laser transverse modes excited in the channel. The wake velocity can be matched exactly to the witness velocity. This can be the vacuum speed of light for ultra-relativistic witnesses, or subluminal velocities for low-energy, weakly relativistic witnesses such as muons.

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]

This paper describes the plasma channel formation during the micro-electrical discharge machining (micro-EDM) at various voltage and capacitance settings. A numerical model using COMSOL Multiphysics finite element is developed to estimate the plasma parameters, such as the plasma channel diameter and temperature distribution in the radial and axial directions. The numerical model also predicts the heat flux distribution responsible for material melting and subsequent removal. Additionally, the coupled numerical model explains the plasma-electrode interactions via different heat transfer mechanisms such as conduction, convection, radiation and thermionic effect. Experimental plasma diameters obtained from high-speed imaging of the discharge process were compared with the simulation results and were in close agreement, with an error between 5% and 14.5%. The simulated molten diameters at various input parameters were compared with experimental crater diameters of single craters and observed to follow a similar trend. [ABSTRACT FROM AUTHOR]

A study of the generation of terahertz (THz) radiation by propagation of a circularly polarized laser pulse in a parabolic plasma channel is presented. The laser–plasma system is embedded in a uniform axial magnetic field. Transverse electric and magnetic wakefields are evaluated using a perturbation scheme and quasi-static approximation. Nonlinear plasma electron velocities arise along the longitudinal and transverse directions as a result of interaction with the laser pulse. This results in the generation of an electromagnetic wave oscillating at THz frequency. The frequency of the obtained THz radiation can be controlled using the plasma channel parameters and can be tuned by varying the transverse position of observation. The THz radiation amplitude is enhanced in the presence of the parabolic channel compared to the homogeneous plasma configuration. The possibility of obtaining a tunable range of THz frequencies is presented. The analytical results have been validated using three-dimensional particle-in-cell simulations. [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]

This work investigates micro-electro discharge machining (EDM) performance involving deionized and tap water. The chosen machining regime was semi-finishing, where open voltage (from 100 to 130 V) and current values (5–10 A) were applied using a 0.5 µs pulse-on time and a frequency of 150 kHz, i.e., a duty cycle of 25%. First, numerical analyses were performed via COMSOL Multiphysics and used to estimate the plasma channel distribution and melted material, varying the current, sparking gap, electrical conductivity, and permittivity of the two fluids. Then, experimentally, the micro-EDM of holes and channels in hardened thin steel plates were replicated three times for each considered fluid. The material removal rate (MRR), tool wear ratio (TWR), radius overcut, and surface roughness were plotted as a function of open voltage and electrical conductivity. The study proves that as voltage and current increase, the MRR and TWR decrease with electrical conductivity. Nonetheless, for higher electrical conductivity (tap water), the process did not proceed for lower open voltages and currents, and the radius overcut was reduced, contrary to what is commonly acknowledged. Finally, the crater morphology and size were evaluated using a confocal microscope and compared to simulated outcomes.

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]

In this paper, guiding of Gaussian laser pulse in plasma channel in the presence of Kerr effect is studied numerically. We assume that the plasma channel has radial variation. We obtain matched condition to guide intense laser pulse through plasma channel in presence of Kerr effect. Using the SDE method, four coupled equations for the pulse amplitude, phase, spot size and wave front curvature will be derived. We apply two numerical methods for solving these equations, Runge–Kutta and extrapolation methods. We give the functional form of the nonlinear initial value problem and obtain some properties about the convergence of the methods. Since these are pure mathematical considerations, they are mentioned in Appendix section. The solutions obtained by the methods are compared together and we see that the solutions are the same. Hence, the proposed methods are applicable and have the sufficient accuracy for the current system of equations. [ABSTRACT FROM AUTHOR]

The spectrum of terahertz radiation generated in plasma of a single-color laser filament is observed, using a new technique based on obtaining two-dimensional angular distributions at different frequencies. It is shown that the maximum of the spectrum occurs in the low-frequency region for different laser pump focusing conditions. It is demonstrated that with the initial beam numerical aperture growth the generation of terahertz radiation at high frequencies increases more intensely compared to low frequencies.

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 angular distributions of various spectral components of terahertz radiation generated by filament plasma are measured in a wide range of numerical apertures NA of the laser beam from 0.003 to 0.1. It is shown that propagation angles of terahertz radiation are inversely proportional to the square root of the terahertz frequency. It is shown that an increase in the numerical aperture of the laser beam leads to an increase in propagation angles of terahertz radiation in the entire range of observed frequencies. [ABSTRACT FROM AUTHOR]

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In this paper the electrokinetic characteristics of helium low-voltage beam discharge plasma in operating conditions of a three-electrode device with a hot cathode are studied. A method and a device are proposed to ensure effective voltage stabilization in a range up to 110 V by controlling the electron velocity distribution function using the plasma channel external boundaries. [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]

In a cancer radiation treatment, Hadron therapy plays an important role as a technique with a high accuracy providing treatment results better than conventional radiation therapy methods such as chemotherapy. The laser-based Hadron therapy techniques are attractive due to their compactness in producing miniaturized particle accelerators and supporting a more intense electric field compared to other methods. A good-quality laser beam focusing and guiding in a cold plasma channel leads to an accurate and precise laser spot after focus, which is an important necessity in medical aims and occurs in μm -wavelength (ultra-short pulse) laser types. A method used to level up this treatment approach has been emerged by using high-power lasers along with plasma channels as a source for high energy to more effectively perform the Hadron therapy. Additionally, cancer treatment by using ions as the particle transferor is preferred over other methods for Hadron therapy due to some physical properties such as maximum energy deposition to the cancerous targets providing a better dose delivery to the tumor. In this paper, multiple simulations around the dose delivery procedures were presented for efficient dosimetric evaluations. Moreover, the effects of the electric field on the particle energy enhancement were evaluated in the presence of the plasma channel. Finally, it was demonstrated that the use of Oxygen ion in the presence of plasma channel is the appropriate approach due to its minor energy dissipation through matter on the way to the tumor for a better dose delivery which was impressed with radially polarized laser accelerator simultaneously. The proposed research demonstrated that the Hadrons can reach the maximum accelerated energy and convey the maximum path through the body to the cancerous target and finally deliver the sufficient dose to the tumor by using Oxygen ion beam in a cold plasma channel, which is low in density, with appropriate laser power. Consequently, Hadron therapy by using Oxygen ionization in the laser-plasma based accelerator is an impressive and efficient method for precise cancer treatment. Lastly, Laser-Plasma accelerators have three important limitation factors of “Energy Spread”, “Dephasing Length”, and “Pump Depletion”, which can limit the energy increasing. In this paper, facing up to these limitations was a critical challenge in Hadron therapy for better Hadron acceleration and dose delivery.

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]