Drill head having an intermediate electrode

Abstract

The invention relates to a drill head (1) for a drilling tool, the drill head (1) extending along a longitudinal axis (Z) and comprising a drilling end (10) and an attachment end (11), wherein the attachment end (11) is configured to be fitted to the drilling tool, and wherein the drilling end (10) comprises a centre (10- A) and at least one pair of electrodes (100), wherein the at least one pair of electrodes (100) comprises a high-voltage electrode (100-1) and a ground electrode (100-2), wherein the high-voltage electrode (100-1) and the ground electrode (100-2) are radially aligned with the centre (10-A) of the drilling end (10), and wherein the drilling end (10) comprises at least one intermediate electrode (102) mounted between the high-voltage electrode (100-1) and the ground electrode (100-2), wherein the intermediate electrode (102) is mounted on an insulating support (104).

Description

Intermediate electrode drill head

[0001] The present invention relates to the field of rock fracturing and more particularly concerns an intermediate electrode drill head.

[0002] In the field of drilling, rotary drilling tools are commonly used to bore through rock. In a typical setup, this type of tool consists of a drill bit with teeth that rotate to bore through the earth or rock. For deep drilling, the drill bit is configured so that a drilling fluid, which can be water- or oil-based, can flow through the bit to remove rock cuttings.

[0003] Such purely mechanical rotary drilling can prove to be particularly time-consuming and costly, especially for hard rocks, and even more so at great depths.

[0004] Another known solution involves using a high-power pulsed tool that generates electric arcs in the rock from a high-voltage electrode and a ground electrode subjected to a high potential difference, for example, on the order of tens to hundreds of kilovolts. However, this solution is limited in terms of drilling speed and is detrimental to the lifespan of the components subjected to the high voltage.

[0005] US8172006B2 relates to a drilling tool whose bit incorporates electro-grinding electrodes. In one embodiment described in Figure 5 of this document, the bit comprises both teeth and a high-voltage electrode surrounded by an annular ground electrode. The electrical pulses are therefore concentrated only at the center of the bit head, which can limit its effectiveness and is thus a drawback. Furthermore, the electrodes can be damaged because they are directly exposed to the rock during bit rotation. In another embodiment described in Figure 6 of this document, the bit comprises rows of teeth, a high-voltage electrode, and a remote electrode positioned in place of a row of teeth.Furthermore, it is not certain that the electrical pulses are generated efficiently, as this depends on the distance between the high-voltage electrodes and the ground electrodes, which must be maximized for a given voltage in order to maximize the efficiency of the discharge.

[0006] Therefore, there is a need for a simple and effective solution to address at least some of these drawbacks.

[0007] To this end, the invention first relates to a drill head for a drilling tool, said drill head extending along a longitudinal axis and comprising a drilling end and a mounting end, said mounting end being configured to be mounted on said drilling tool, said drill end comprising a center and at least one pair of electrodes, said at least one pair of electrodes comprising a high-voltage electrode and a ground electrode, said high-voltage electrode and said ground electrode being radially aligned with the center of the drill end, said drill end comprising at least one intermediate electrode mounted between the high-voltage electrode and the ground electrode, said intermediate electrode being mounted on an insulating support, said insulating support being able to be attached either to the ground electrode or to the high-voltage electrode,either at both the ground electrode and the high-voltage electrode.

[0008] The insulating support allows for simple and efficient mounting of the intermediate electrode. This drill head configuration creates two successive electric arcs in series between the intermediate electrode, positioned between the two electrodes, and the electrode pair. These arcs propagate through the rock material, generating two dynamic pressure waves that fracture the rock. The insulating support design prevents the discharge from occurring in the drill head rather than in the rock material, such as granite, and ensures that the fracturing is enhanced by the double discharge and effectively penetrates the rock.

[0009] Preferably, with the mounting end having a peripheral edge, the high-voltage electrode is mounted at said peripheral edge, and the grounding electrode is mounted radially between the high-voltage electrode and the center of the drill bit, or at the center of the drill bit. This configuration keeps the high-voltage electrode away from electrically grounded elements that could create discharges outside the rock material.

[0010] Alternatively, with the fixing end having a peripheral edge, the ground electrode is mounted at said peripheral edge and the high-voltage electrode is mounted radially between the ground electrode and the center of the drill end or at the center of the drill end.

[0011] Preferably, the drill bit end defines an internal cavity opening to the outside between the high-voltage electrode and the ground electrode, and the insulating support is placed within this internal cavity. During drilling, the internal cavity allows the passage of drilling fluid, which flushes out cutting debris and prevents electrical arcs between the electrodes from developing anywhere other than in the rock material to be fractured.

[0012] Advantageously, the drill head is configured so that drilling fluids, particularly water-based fluids, flow through it between the drill bit and the mounting end. The drilling fluid flows around the insulating support, which, in the case of a water-based fluid, increases the chamber resistance by reducing the surface area of ​​the opposing electrodes. The system's efficiency is thus increased by allowing the use of greater inter-electrode distances.

[0013] In the case of an oil-based drilling fluid, its resistance being high, the design of the intermediate electrode, its insulating support and the internal cavity must be such that the parasitic capacitance between the intermediate electrode and ground is greater than that between the high-voltage electrode and the intermediate electrode, for example at least twice greater, in order to retain the majority of the voltage across the terminals of the high-voltage electrode and the intermediate electrode before breakdown and thus maximize the inter-electrode distance and therefore the efficiency of the device.

[0014] Advantageously, at least one intermediate electrode has an outer end, oriented outwards from the drill bit end and extending between the high-voltage electrode and the grounding electrode, and an inner end, opposite the outer end and extending into the inner cavity. At the outer end, which is the portion of the intermediate electrode intended to come close to or against the rock material, the distance between the high-voltage electrode and the intermediate electrode, and the distance between the intermediate electrode and the grounding electrode, are less than the distances between these same elements and the inner end.

[0015] Preferably, the external end has protruding shapes to enhance the electric field, ensuring that the electric arc between the high-voltage electrode and the intermediate electrode and the electric arc between the intermediate electrode and the ground electrode develop well in the rock material to be fractured and not inside the drill head, which would significantly reduce the effectiveness of the device.

[0016] Advantageously, the intermediate electrode has a central section between its outer and inner ends, and the inner end is connected to the insulating support at this central section. The insulating support is thus easy to machine, and the intermediate electrode can be made in two parts that are assembled during manufacturing on either side of the insulator, for example, by screwing or welding.

[0017] Preferably, the inner end of the intermediate electrode has a rounded, preferably spherical, shape. The absence of sharp angles resulting from the spherical shape helps to limit the electric field in the inner end of the intermediate electrode that could trigger a discharge in the drill head rather than in the rock, thus significantly reducing the device's efficiency.

[0018] According to one aspect of the invention, the insulating support is disposed in the internal cavity at a predetermined non-zero distance from the drill bit tip, preferably at least 3 mm, and even more preferably 4 mm, so as to create a space between said drill bit tip and the insulating support. The space between the insulating support and the drill bit tip allows sufficient passage of drilling fluid during drilling to prevent electric arcs between the electrodes from developing anywhere other than in the rock to be fractured.

[0019] Alternatively or in addition, the insulating support has a recess, preferably rounded or spherical, extending from the drill bit end between the intermediate electrode and the high-voltage electrode. This recess ensures that the electric arc between the high-voltage electrode and the intermediate electrode develops within the rock material to be fractured and not along the insulating support, creating an insulating space filled with drilling mud composed of drilling fluid and rock fragments.

[0020] Advantageously, the drill bit as presented comprises a plurality of electrode pairs and a plurality of intermediate electrodes, preferably the same number. Electrical fracturing of the rock is therefore more efficient because the electrodes, and thus the electric arcs, are distributed over the entire area of ​​the drill bit.

[0021] Preferably, the drill bit comprises a plurality of drill bits, each drill bit including a blade extending outward from the outer surface of the drill bit, and a plurality of drill teeth extending from said blade, for example, arranged side-by-side between a first drill tooth located in the central part of the drill bit and a last drill tooth located at the periphery of the drill bit. Drilling the rock material is therefore all the more efficient when the drill bits allow for mechanical cutting, which is facilitated by electrical fracturing.

[0022] In a preferred embodiment, the drill head as presented is a drill bit, said drill bit comprising a drill bit body having on one side the drilling end and on the other side the end for fixing the drill bit to a rotor assembly of the drilling tool.

[0023] The invention also relates to a rock fracturing process implemented by a drill head as described, said drill head being connected to a high-power pulsed generator configured to supply voltage to the high-voltage electrode, and being in contact with or in the immediate vicinity, for example less than 20 cm, of a rock material, said process comprising the steps of:

[0024] - generation, by the high-power pulsed generator, of a voltage between the high-voltage electrode and the intermediate electrode,

[0025] - generation in the rock material of a first electric arc bypassing the insulating support between the high-voltage electrode and the intermediate electrode, due to the difference in electrical potential between the high-voltage electrode and the intermediate electrode,

[0026] - transfer of potential from the high-voltage electrode to the intermediate electrode via the first electric arc,

[0027] - generation in the rock material of a second electric arc bypassing the insulating support between the intermediate electrode and the ground electrode, due to the potential difference between the intermediate electrode and the ground electrode,

[0028] - increase, by the pulsed high-power generator, of the current flowing in the first electric arc and the second electric arc,

[0029] - fracturing of the rock material by the expansion of the plasma from the first electric arc and the second electric arc generated and the propagation of the resulting dynamic pressure waves.

[0030] Other features and advantages of the invention will become apparent upon reading the following description. This description is purely illustrative and should be read in conjunction with the accompanying drawings, in which:

[0031] The diagram schematically illustrates a radial cross-sectional view of one embodiment of the drill head according to the invention.

[0032] The diagram schematically illustrates an intermediate electrode as used in the forming head according to the invention.

[0033] The diagram schematically illustrates a drilling device such as that present on the drill head according to the invention.

[0034] The diagram schematically represents the rock fracturing process carried out by the drill head according to the invention.

[0035] The diagram schematically illustrates an example of a drill head 1 according to the invention in contact with or very close to a rock material 2 to be drilled.

[0036] Drill head 1

[0037] The drill head 1 comprises a drill end 10 and a mounting end 11. The drill head 1 is configured to rotate about a longitudinal axis Z during drilling operations.

[0038] As is known, the drill head 1 is configured to allow the flow of drilling fluid intended to remove debris from the drilled rock material 2. For clarity, the drilling fluid lines are not shown in the figures.

[0039] Drilling end 10

[0040] With reference to the, the drill end 10 comprises a center 10-A, a pair of electrodes 100, an intermediate electrode 102, an insulating support 104, an internal cavity 106 and a drill member 108.

[0041] Lare represents a radial section of the drill head 1 extending from its longitudinal axis Z. Preferably, the entire drill head 1 comprises a plurality of pairs of electrodes 100 and intermediate electrodes 102, with one intermediate electrode 102 for each pair of electrodes 100.

[0042] Pair of electrodes 100

[0043] Each pair of electrodes 100 comprises a high-voltage electrode 100-1 and a ground electrode 100-2.

[0044] As shown in the figure, the high-voltage electrode 100-1 is located near the edge of the borehole end 10 of the drill head 1.

[0045] The ground electrode 100-2 is located inside the drill end 10 of the drill head 1 so as to be radially aligned with the corresponding high-voltage electrode 100-1 of the electrode pair 100.

[0046] The high-voltage electrode 100-1 has a salient angle in contact with or very close to the rock material 2 and directed towards the ground electrode 100-2.

[0047] The ground electrode 100-2 has a salient angle in contact with or very close to the rock material 2 and directed towards the high-voltage electrode 100-1.

[0048] The high-voltage electrode 100-1 and the ground electrode 100-2 are connected to a pulsed high-power generator 110 by an electrical circuit 111, which allows an electrical charge to circulate between the electrode pair 100 and the pulsed high-power generator 110.

[0049] Intermediate electrode 102

[0050] As shown in the figure, the intermediate electrode 102 is located inside the drill end 10 between the high-voltage electrode 100-1 and the ground electrode 100-2 of the corresponding electrode pair 100.

[0051] As shown in the figure, the intermediate electrode 102 comprises an outer end 102-1, an inner end 102-2 and a middle portion 102-3.

[0052] The intermediate electrode 102 is made of an electrically conductive material, for example a metallic material.

[0053] The outer end 102-1 has salient angles, directed in the direction of the high-voltage electrode 100-1 and the ground electrode 100-2 as shown in the.

[0054] The inner end 102-2 has a rounded shape without salient angles, for example a spherical shape as shown in figures 1 and 2.

[0055] Preferably, the intermediate electrode 102 is in two parts, one corresponding to the outer end 102-1 and the middle part 102-3, and the other corresponding to the inner end 102-2.

[0056] These two parts are mounted on either side of the insulating support 104 so as to fix the intermediate electrode 102 to them, for example by screwing the two parts together. In this configuration, the middle part 102-3 is threaded into the insulating support 104.

[0057] Insulating support 104

[0058] As shown in the figure, the insulating support 104 is fixed in the body of the drill head 1 and extends into the internal cavity 106.

[0059] The insulating support 104 is made of an electrically insulating material, for example a plastic material.

[0060] The insulating support 104 extends so as to be in contact or very close to the high-voltage electrode 100-1 and the intermediate electrode 102 so as to electrically isolate the intermediate electrode 102 from the high-voltage electrode 100-1.

[0061] Alternatively or in addition, the insulating support 104 can be in contact with the ground electrode 100-2.

[0062] Preferably, the insulating support 104 includes a hole to receive the middle part 102-3 of the intermediate electrode 102.

[0063] Internal cavity 106

[0064] The internal cavity 106 is formed in the drill end 10 and opens into a space 106-1 formed between the insulating support 104 and the rock material 2 when the drill end 10 is supported against said rock material 2, as illustrated in the.

[0065] Preferably, several internal cavities 106 are formed in the drill end 10 (one cavity 106 around each intermediate electrode 102).

[0066] Alternatively, a single internal cavity 106 extending fully circularly in the drill end 10 around the longitudinal axis Z can be formed so as to encompass all the intermediate electrodes 102 present in the drill head 1.

[0067] The space 106-1 allows the flow of the mixture of crushed rock material 2 and the drilling fluid injected into the drilling tool (water or oily liquids) which flows during drilling.

[0068] As shown in the figure, the space 106-1 opens into a recess 106-2 formed in the insulating support 104 in the vicinity of the high-voltage electrode 100-1. Preferably, the recess 106-2 is rounded or spherical in shape.

[0069] Drilling unit 108

[0070] As shown in Figure 1, each drill bit 108 comprises a blade 108-1 and a plurality of drill teeth 108-2. Figure 1 represents a cross-sectional view of a drill bit 108, where a blade 108-1 and two drill teeth 108-2 are shown.

[0071] The drilling elements 108 are configured to mechanically crush the rock material 2 during the rotation of the drill head 1 around the longitudinal axis Z.

[0072] The blade 108-1 advances from the drill end 10 and comprises a plurality of drill teeth 108-2 (for example made of Polycrystalline Diamond Compact pellets) which extend from the blade 108-1.

[0073] The 108-2 drill teeth mechanically grind the rock material 2 by friction during the rotation of the drill head 1.

[0074] Fixing end 11

[0075] The fixing end 11 allows the drill head 1 to be attached to a drilling tool.

[0076] Advantageously, the high-power pulsed generator 110 and the drill head rotation mechanism 1 are located outside the drill head 1 in the drill tool.

[0077] The fixing end 11 is thus traversed by the electrical circuit 111 connecting the pulsed high power generator 110 to the electrode pair 100.

[0078] The fixing end 11 is also crossed by the pipes which allow the drilling fluid to be conveyed to the level of the rock material 2 to be drilled.

[0079] High-power pulsed generator 110

[0080] The high-power pulsed generator 110 generates an electrical voltage intended to create electric arcs in the rock material 2 between the pair of electrodes 100 and the corresponding intermediate electrode 102 in order to fracture it.

[0081] Preferably, the 110 pulsed high power generator generates a high voltage, between a few tens and a few hundred kilovolts, in a pulsed manner (500 kHz – 25 MHz) in order to create electric arcs at a high frequency.

[0082] The pulsed high power generator 110 is connected to the high-voltage electrode 100-1 and to the ground electrode 100-2 by the electrical circuit 111.

[0083] rock material 2

[0084] Rock material 2 consists of rocks through which the drill head 1 drills a well or a pipeline for various geoengineering applications, such as drilling geothermal, oil or mineral exploration wells, or drilling pipelines or tunnels.

[0085] Example of implementation

[0086] When drilling is carried out using the drill bit on which the drill head 1 is mounted, the drill head 1 is rotated by the drill bit to mechanically drill through the rock material 2. The drilling is then performed primarily by the drilling components 108.

[0087] Drilling fluid is injected through drill head 1 to flush out debris resulting from crushing rock material 2.

[0088] To add electrical fracturing to mechanical fracturing, the high-power pulsed generator 110 generates, in a first stage E1, an electrical voltage in a pulsed manner.

[0089] With the high-voltage electrode 100-1 electrically connected to the pulsed high-power generator 110, the electrical potential of the high-voltage electrode 100-1 increases in a step E2.

[0090] Due to the shape of the high-voltage electrode 100-1, the electric field accumulates mainly at the salient angle directed towards the intermediate electrode 102.

[0091] When the electric field between the high-voltage electrode 100-1 and the ground electrode 100-2 is sufficiently large, a first electric arc is created in the rock material 2 between the salient angle of the high-voltage electrode 100-1 and the intermediate electrode 102 in a stage E3.

[0092] The shape of the high-voltage electrode 100-1 and the intermediate electrode 102, with a salient angle, makes it possible to increase the electric field in the rock material 2 and thus ensure even more the creation of the first electric arc in the rock material 2.

[0093] The insulating support 104 isolates the intermediate electrode 102 from the high-voltage electrode 100-1 to prevent the first electric arc from occurring inside the drill head 1.

[0094] The first electric arc generates a mixture of rock fragments and drilling fluid in cavity 106 and space 106-1. This mixture has insulating properties which also ensure and enhance the creation of the first electric arc of the subsequent discharge in rock material 2.

[0095] The recess 106-2 also ensures the creation of the first electric arc in the rock material 2 rather than on the surface of the insulating support 104 by creating a space filled with the mixture of rock fragments and liquid which increases the insulation between the insulating support 104 and the high-voltage electrode 100-1.

[0096] In step E4, the electric potential is transferred into the intermediate electrode 102 via the first electric arc and the electric potential of the intermediate electrode 102 increases.

[0097] Because of the salient angle shape of the outer end 102-1 and the ground electrode 100-2, the electric field is amplified at the salient angles, especially the one facing the ground electrode 100-2.

[0098] Due to the convex shape of the inner end 102-2, the electric field is reduced in the inner end 102-2, which prevents an electric arc from being generated inside the drill head 1, which would prevent the electrical fracturing of the rock material 2.

[0099] The difference in electrical potential between the intermediate electrode 102 and the ground electrode 100-2 and the salient angles on the side of the opposing electrodes make it possible to create sufficiently high electric fields to generate in a step E5 a second electric arc between the intermediate electrode 102 and the ground electrode 100-2.

[0100] The shape of the ground electrode 100-2, with a salient angle very close to the rock material 2, helps to promote the creation of the second electric arc in the rock material 2.

[0101] In stage E6, once the second electric arc is created, the high-power pulsed generator 110 increases the generated current, which increases the pressure in the arc.

[0102] In a step E7, the pressure generated in the first electric arc and the second electric arc generates a dynamic pressure wave propagating in the rock material 2, causing cracking or fracturing of the rock material 2 if the pressure level and the energy dissipated in the arcs are sufficiently high.

[0103] In an E8 step, the current decreases due to the dissipation of electrical energy in the circuit 111, for example in resistive elements, and in electric arcs.

[0104] The high pulsed power generator 110 generates an electrical voltage at a determined high frequency to ensure the electrical fracturing of the rock material 2 by high pulsed power.

[0105] The fragments thus generated both mechanically by the rotation of the drill head 1 and electrically with the electric arcs between the pair of electrodes 100 and the intermediate electrode 102 are evacuated by the flow of the drilling fluid.

Claims

Drill head (1) for a drilling tool, said drill head (1) extending along a longitudinal axis (Z) and comprising a drill tip (10) and a mounting tip (11), said mounting tip (11) being configured to be mounted on said drilling tool, said drill tip (10) comprising a center (10-A) and at least one pair of electrodes (100), said at least one pair of electrodes (100) comprising a high-voltage electrode (100-1) and a ground electrode (100-2), said high-voltage electrode (100-1) and said ground electrode (100-2) being radially aligned with the center (10-A) of the drill tip (10), said drill tip (10) comprising at least one intermediate electrode (102) mounted between the high-voltage electrode (100-1) and the ground electrode (100-2), said intermediate electrode (102) being mounted on an insulating support (104), said insulating support (104) being able to be attached either to the ground electrode (100-2),either to the high-voltage electrode (100-1), or to both the ground electrode (100-2) and the high-voltage electrode (100-1). Drill head (1) according to claim 1, in which, the drill end (10) delimiting an internal cavity (106) opening to the outside between the high-voltage electrode (100-1) and the ground electrode (100-2), the insulating support (104) is disposed in said internal cavity (106). Drill head (1) according to the preceding claim, wherein the drill head (1) is configured so that drilling fluids flow through said drill head (1) between its drill end (10) and its mounting end (11). Drill head (1) according to any one of claims 2 or 3, wherein at least one intermediate electrode (102) has an external end (102-1), oriented outwards from the drill end (10) and extending between the high-voltage electrode (100-1) and the ground electrode (100-2), and an internal end (102-2), opposite the external end (102-1) and extending into the internal cavity (106). Drill head (1) according to the preceding claim Drill head (1) according to the preceding claim, wherein the external end (102-1) has protruding shapes. Drill head (1) according to the preceding claim, wherein the intermediate electrode (102) having a middle portion (102-3) between its outer end (102-1) and its inner end (102-2), the inner end (102-2) is connected to the insulating support (104) at the level of said middle portion (102-3). Drill head (1) according to the preceding claim, wherein the inner end (102-2) of the intermediate electrode (102) has a rounded shape, preferably spherical. Drill head (1) according to any one of the preceding claims, wherein the insulating support (104) is disposed in the internal cavity (106) at a predetermined distance from the drill end (10) so as to create a space (106-1) between said drill end (10) and the insulating support (104). Drill head (1) according to any one of the preceding claims, wherein the insulating support (104) has a recess (106-2), preferably of rounded shape, which extends from the drill end (10) between the intermediate electrode (102) and the high-voltage electrode (100-1). Drill head (1) according to any one of the preceding claims, wherein the drill tip (10) comprises a plurality of electrode pairs (100), and a plurality of intermediate electrodes (102). Drill head (1) according to any one of the preceding claims, wherein the drill tip (10) comprises a plurality of drill elements (108), each drill element (108) comprising a blade (108-1), projecting outward from the external surface of the drill tip (10), and a plurality of drill teeth (108-2) projecting outward from said blade (108-1). Drill head (1) according to any one of the preceding claims, wherein the mounting end (11) has a peripheral edge, the high-voltage electrode (100-1) is mounted at said peripheral edge and the ground electrode (100-2) is mounted radially between the high-voltage electrode (100-1) and the center of the drill end (10) or at the center of the drill end (10). Drill head (1) according to any one of the preceding claims, wherein the mounting end (11) has a peripheral edge, the ground electrode (100-2) is mounted at said peripheral edge and the high-voltage electrode (100-1) is mounted radially between the ground electrode (100-2) and the center of the drill end (10) or at the center of the drill end (10). Drill head (1) according to any one of the preceding claims, said drill head (1) being a drill bit comprising a drill body having on one side the drilling end (10) and on the other side the end for fixing the drill bit to a rotor assembly of the drilling tool. A rock fracturing process implemented by a drill head (1) according to any one of the preceding claims, said drill head (1) being connected to a high-power pulsed generator (110) configured to supply voltage to the high-voltage electrode (100-1), and being in contact with or in the immediate vicinity of a rock material (2), said process comprising the steps of: - generation (E1), by the high-power pulsed generator (110), of a voltage between the high-voltage electrode (100-1) and the intermediate electrode (102), - generation (E3) in the rock material (2) of a first electric arc bypassing the insulating support (104) between the high-voltage electrode (100-1) and the intermediate electrode (102), due to the difference in electrical potential between the high-voltage electrode (100-1) and the intermediate electrode (102),- transfer (E4) of the potential from the high-voltage electrode (100-1) to the intermediate electrode (102) via the first electric arc, - generation (E5) in the rock material (2) of a second electric arc bypassing the insulating support (104) between the intermediate electrode (102) and the ground electrode (100-2), due to the potential difference between the intermediate electrode (102) and the ground electrode (100-2), - increase (E6), by the pulsed high-power generator (110), of the current flowing in the first and second electric arcs, - fracturing (E7) of the rock material (2) by the expansion of the plasma from the first and second generated electric arcs and the propagation of the resulting dynamic pressure waves.

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