Abstract:
An electrodischarge apparatus has a nozzle that includes a discharge chamber that has an inlet for receiving water and an outlet. The apparatus has a first electrode extending into the discharge chamber that is electrically connected to one or more high-voltage capacitors. A second electrode is proximate to the first electrode to define a gap between the first and second electrodes. A switch causes the one or more capacitors to discharge across the gap between the electrodes to create a plasma bubble which expands to form a shockwave that escapes from the nozzle ahead of the plasma bubble.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority from U.S. Provisional Patent Application 62/105,779 filed Jan. 21, 2015 and from U.S. Provisional Patent Application 62/150,356 filed Apr. 21, 2015. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention relates generally to electric discharge in water and, in particular, to plasma blasting techniques. 
       BACKGROUND 
       [0003]    High-pressure water jet technology is one of the most advanced technologies in the world. Applications of high-pressure continuous water jets vary from mundane operations such as crude cleaning of edifices to highly sophisticated manufacturing of high-precision products. However, for many industrial applications, such as cleaning petro-chemical reactor vessels and mining of hard rocks, the technology, at present, suffers from serious drawbacks. This is because the magnitudes of pressures and powers required by continuous water jets for such applications are prohibitively high (&gt;200 MPa and 250 kW per jet). The notion of using water jet techniques (forced cavitating or pulsed water jets) for such applications is a relatively new one. For example, extensive work conducted by Vijay has shown that forced cavitating and pulsed water jets can be very effective for cutting metals, etc. (Vijay, M. M., “Pulsed Jets: Fundamentals and Applications, Proc 5 th  Pacific Rim International Conference on Waterjet Technology, New Delhi, India,  1998 ). Similarly, when hard rocks are preweakened, the cutting rates will be higher and the operating costs will be lower because of the reduced wear rates and breakdowns of the cutter tools. 
         [0004]    In the context of this specification, a distinction is made between the natural and forced discontinuous jets. The forced water jet concepts are referred to as “novel water jet techniques” in this specification. For example, a stream of high-speed droplets or slugs formed due to break-up of a continuous jet emerging in air can be regarded as a natural pulsed jet. Although natural discontinuous jets are simple to produce, their usefulness is limited because it is not possible to control their intensity and shape of the pulses which are directly related to their performance. In the case of forced pulsed and cavitating waterjets, on the other hand, it is possible to generate well-formed slugs or cavitating bubbles, by modulating a continuous water jet by high-frequency ultrasonic power resulting in enhanced performance (U.S. Pat. No. 7,594,614 B2; U.S. Pat. No. 8,297,540 B1 and U.S. Pat. No. 8,550,873 B2). However, the high-frequency cavitating and pulsed waterjets are not effective in massive fragmentation of hard rocks or rock-like materials, including explosives, such as used in landmines. The purpose of the novel electrodischarge technique disclosed in this application is to generate very powerful low-frequency (of the order of one or more pulses per second) pulsed waterjet with a precursor shock wave and subsequently a vaporous-cavitating waterjet. 
         [0005]    Theoretically, the hydrodynamic phenomena accompanying electric discharges in quiescent liquids at atmospheric pressure have been known for more than a century. An electric discharge in a liquid at atmospheric pressure is known to cause the formation of a strong shock wave and a plasma bubble that could attain a maximum diameter of 10 mm in about 1 μs. The pressure in the plasma bubble can reach 2000 MPa or more depending on the power (voltage and current) of discharge. The interest in the technique for a variety of applications stems from the fact that these shock waves and the bubbles are sources of high power and the processing of materials is clean and can be controlled precisely (a definite advantage compared to explosives). Yutkin, for example, conducted a number of laboratory tests and demonstrated its usefulness in a variety of applications, ranging from metal forming to fragmentation of rocks, without commercial exploitation (Yutkin, L. A. “Electrohydraulic Effect,” Moskva 1955; English Translation by Technical Documents Liaison Office, MCLTD, WP-AFB, Ohio, USA, No. MCL-1207/1-2, Oct. 1961). In at least one embodiment of the present invention, the electrodischarge technique is used to modulate a stream of water flowing through a nozzle, that is, a low-speed waterjet or, in a nozzle filled with quiescent water. According to Huff &amp; McFall (Huff, C. F., and A. L. McFall, “Investigation into the Effects of an Arc Discharge on a High Velocity Liquid Jet,” Sandia Laboratory Report No. 77-1135C, USA, 1977), the arc discharge modulates the stream or quiescent water by three mechanisms: (1) the formation of an initial shock wave, (2) pulsed jet produced by the rapidly expanding plasma bubble and (3) the plasma bubble itself which eventually reverts into a cavitation vapor bubble. As these three hydrodynamic phenomena accompanying the discharge occur at different times, it is possible by a careful design of the nozzle-electrode configurations, as disclosed in this specification, to generate the shock only, the interrupted jet (produced by the rapidly expanding plasma bubble) only or, the cavitating waterjet only or, all the three phenomena in tandem to inflict immense damage on a target material. The nozzles shown in  FIG. 1  and  FIG. 2 , for example, are meant to produce only shock waves. Since the frequency of operation is usually low (≈1.0 Hz), in the interrupted mode, the technique basically functions as a water cannon. 
         [0006]    Generating shock waves in water by electric discharge is disclosed in U.S. Pa. No. 3,364,708 (Padberg). A shock plasma earth drill is disclosed in U.S. Pat. No. 3,679,007 (O&#39;Hare). Various plasma blasting techniques are disclosed in U.S. Pat. No. 5,106,164 (Kitzinger et al.), U.S. Pat. No. 5,482,357 (Wint et al.), U.S. Pat. No. 6,283,555 (Arai et al.), U.S. Pat. No. 6,455,808 (Chung et al.), U.S. Pat. No. 6,457,778 (Chung et al.), and U.S. Pat. No. 7,270,195 (MacGregor et al.). In the foregoing patents, a probe with electrodes (e.g. coaxial electrodes) is inserted into a borehole in the rock formation which is then filled with water or electrolyte. 
         [0007]    Although the prior art provides a qualitative description of the phenomena accompanying the electrical discharge in quiescent water, there is scant information with respect to the discharge in a moving stream of water. Therefore, the inventor has conducted extensive semi-theoretical (computational fluid dynamic analysis) and experimental work on the electrodischarge technique for the conceptual nozzles shown in  FIG. 1  and  FIG. 2 .  FIG. 3 , for example, shows the very high pressures generated by the impact of a shockwave on the target material (Vijay, et al., “Modeling of Flow Modulation following the electrical discharge in a Nozzle,” Proceedings of the 10 th  American Waterjet Conference, August  1999 ). The flow rate through the nozzle was 13 usgal/min at a pressure of 5 kpsi in the vicinity of the electrodes. The orifice (nozzle) diameter was 0.085 in. The magnitude of the electrical energy dumped between the electrodes was 20 kJ and the shock impact was at 81.2:s after the discharge.  FIG. 4  shows the effect of placing a reflector upstream of the electrodes (the tip of the central electrode (d∀) in  FIG. 1  (shown clearly by # 29  and # 29   a  in  FIG. 11 ). The target is placed at 5 in from the nozzle exit. It is seen that at a time (t) of about 30:s, the plasma expands sending a shockwave S 1  towards the nozzle exit and a shockwave S 2  towards the inlet. Shockwave S 1  leaves the nozzle at approximately 50:s and forms a high-speed wave (W 1 ) which accelerates the front F 1  of the original steady jet to F 2 . The front F 2  impacts on the target at 78.2:s producing a peak pressure of 2,600 MPa at 81.2:s as shown in  FIG. 3 . Shockwave S 2 , on the other hand, is reflected as shockwave S 3 . This shockwave on passing through the plasma emerges as shockwave S 4  and ultimately causes another high-speed wave W 2  in the jet impacting the target at 104:s, creating pressure peaks, of the order of 1,700 MPa. These semi-theoretical results show the advantage of using a reflector in the nozzle configuration. 
         [0008]    As illustrated in  FIG. 5A , further computational fluid dynamic analysis has indicated the occurrence of multiple peaks in the impact pressure. This is due to the fact that the discharge voltage, as illustrated in  FIG. 5B , is a decaying sinusoidal wave (Yan, et al., “Application of ultra-powerful pulsed Waterjet generated by electrodischarges,” Proceedings of the 16 th  International Conference on Water Jetting, France, October 2002). Thus, by proper design of the discharge circuit, it is possible to generate multiple shockwaves to impact the target, enhancing the performance of the pulsed waterjet generated by the electrodischarge technique. 
         [0009]    The phenomena accompanying the discharge depend on several operating variables and configurational parameters of the electrode-nozzle assembly. The operating variables are the pressure in the chamber, which could be of the order of 15 kpsi (could be any pressure although a range of 10-20 kpsi provides good results), flow (determined by the orifice diameter, d o , of the orifice, typically of the order of 13 usgal/min although a flow of 10-15 usgal/min provides good results), or quiescent water (depends of the volume of the nozzle chamber, typically of the order of a litre), magnitude the voltage (V) of the capacitor (typically of the order of 20 kV, but could be as high as 100 kV), capacitance (C) of the capacitor, energy (E c ) stored in the capacitor (E c =0.5 CV 2 ). Depending on the capacitance, the energy stored in the capacitor bank could be as high as 200 kJ. Although the energy of discharge can be varied either by varying the voltage or the capacitance, to keep the size of the system compact, it is better to vary the voltage and the duration of discharge (∂), which will depend on the magnitudes of L-C-R (inductance, capacitance and resistance) of the discharge circuit. 
         [0010]    As indicated in  FIG. 1  and  FIG. 2 , the configurational parameters are: the shape (contour) of the nozzle chamber to focus and propagate the shockwaves towards the nozzle exit, the shape (conceptual designs are illustrated in  FIG. 7  and  FIG. 8 ), diameter (d∀), location (k) of the electrodes from the nozzle exit, the gap (ι) between the electrodes. For example, as shown conceptually in  FIG. 7 , the inner contour of the nozzle could be an exponential curve and, in order to obtain smooth flow of water, the outer profile of the electrode would also be exponential, providing generally parallel surfaces. 
         [0011]    As further illustrated in  FIG. 1  and  FIG. 2  and also, in the conceptual configurations shown in  FIG. 7  and  FIG. 8 , there are several different shapes, size and dispositions of the electrodes in the nozzle. These figures also show two possible configurations of the electrodes. Whereas the purpose of the short plasma channel ( FIG. 1 ) is to generate cavitation bubbles in the stream, that of the long channel is to produce a high-speed pulsed water jet (Vijay and Makomaski, “Numerical analysis of pulsed jet foimation by electric discharges in a nozzle,” Proceedings of the 14 th  International Conference on Jetting Technology, 1998). From the standpoint of performance, the most important geometric parameters are (as shown in  FIG. 1  and  FIG. 2 ) the magnitudes of D/do, the distance k, the distance (gap) between the electrodes ι, the inner profile of the nozzle and the shape and disposition of the electrodes. These geometric parameters also deteiniine the operating parameters such as the pressure of the liquid, electrical energy and frequency, etc. As an example, test results are illustrated in the plot of  FIG. 6 . For the given set of operating parameters listed in the legend, the speed of the pulsed waterjet depends considerably on the gap (ι) between the electrodes. The data clearly show that it is possible to increase the speed of the jet by almost a factor of three by simply increasing the gap between the electrodes from 6 to 22 mm. This method affords a simple means to significantly increase the speed of water slug without increasing the input electrical energy. This is very important for many practical applications such as neutralization of landmines where a pulse having a very high speed (≈1000 m/s) is required. 
       SUMMARY 
       [0012]    The following presents a simplified summary of some aspects or embodiments of the invention in order to provide a basic understanding of the invention. This summary is not an extensive overview of the invention. It is not intended to identify key or critical elements of the invention or to delineate the scope of the invention. Its sole purpose is to present some embodiments of the invention in a simplified form as a prelude to the more detailed description that is presented later. 
         [0013]    The present invention, as exemplified by the embodiments disclosed and illustrated in the specification and drawings, is a novel electrodischarge apparatus (or system) that is capable of creating a plasma bubble due to the ionization of water inside a nozzle. A powerful shockwave is generated as a result of the electrodischarge in water. The shockwave emerges from the nozzle to provide a large impact pressure on a target surface. 
         [0014]    An inventive aspect of the present disclosure is an electrodischarge apparatus has a nozzle that includes a discharge chamber that has an inlet for receiving water and an outlet. The apparatus has a first electrode extending into the discharge chamber that is electrically connected to one or more high-voltage capacitors. A second electrode is proximate to the first electrode to define a gap between the first and second electrodes. A switch causes the one or more capacitors to discharge across the gap between the electrodes to create a plasma bubble which expands to form a shockwave that escapes from the nozzle ahead of the plasma bubble. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    Further features and advantages of the present technology will become apparent from the following detailed description, taken in combination with the appended drawings, in which: 
           [0016]      FIG. 1  is a schematic drawing of an electrodischarge apparatus showing the assembly of a capacitor bank with the spark gap switch, water pump and nozzle electrode assembly with a short gap between the electrodes; 
           [0017]      FIG. 2  depicts the same apparatus as shown in  FIG. 1  except with a large gap between the electrodes; 
           [0018]      FIG. 3  is a graphical representation of pressure of the shockwave impacting the surface of a target obtained by numerical study (computational fluid dynamic analysis); 
           [0019]      FIG. 4  is a plot showing the effect of a reflector on the shockwave; 
           [0020]      FIG. 5A  is a plot of impact pressures as a function of time after the electrical discharge; 
           [0021]      FIG. 5B  is a plot showing the decaying voltage as a function of time after discharge; 
           [0022]      FIG. 6  is a plot showing the effect of the gap width on the magnitude of the speed of water pulse; 
           [0023]      FIG. 7  is a schematic drawing showing the design of the nozzle-electrode configuration for producing a short plasma channel; 
           [0024]      FIG. 8  is the same as  FIG. 7  except the electrode is disposed in the axial direction for producing a long plasma channel; 
           [0025]      FIG. 9  is another embodiment of the nozzle-electrode configuration for producing a short plasma channel in a high-speed waterjet; 
           [0026]      FIG. 10  is another embodiment of the nozzle-electrode configuration for producing long or short plasma channels; 
           [0027]      FIG. 11  is an embodiment showing the details of the electrode and a reflector to reflect the shockwave generated by the discharge; 
           [0028]      FIG. 12  is yet another embodiment showing transverse electrodes with the reflector; 
           [0029]      FIG. 13  is the same as  FIG. 12 , except the tips of the electrodes are planar and pointed to enhance the strength of the electric field; 
           [0030]      FIG. 14  is an embodiment showing how the ground and high-voltage electrodes are assembled as a single unit for sliding into and out of the nozzle; 
           [0031]      FIG. 15  is an embodiment in which the position of the reflector with respect to the electrodes can be varied; 
           [0032]      FIG. 16  is yet another embodiment as  FIG. 15  showing the possibility of tracking (unwanted sparking) indicated in the inset; 
           [0033]      FIG. 17  is an embodiment based on the conceptual design illustrated in  FIG. 8 . 
           [0034]      FIG. 18  is an embodiment for improving the alignment of the central electrode in the nozzle; 
           [0035]      FIG. 19  is an embodiment of a highly complex nozzle configuration to confine the cavitation bubble produced by the electric discharge; 
           [0036]      FIG. 20  is an embodiment with the electrode in the nozzle exit for generating sequential discharges; 
           [0037]      FIG. 21  is a conceptual design to enhance the power of the water pulse by the converging shockwaves; 
           [0038]      FIG. 22  is an embodiment that can be placed on the target to be processed, for example, fragmentation of concrete structures such as a nuclear biological shield; 
           [0039]      FIG. 23  is an embodiment having two electrodes to produce a short plasma channel close to the target; 
           [0040]      FIG. 24  is a drawing of the coupling to connect the nozzle to the pump; 
           [0041]      FIG. 25  is yet another embodiment to connect the nozzle to the pump; 
           [0042]      FIG. 26  is an embodiment of the high-voltage electrode and the adaptor to connect it to the cables from the capacitor bank; 
           [0043]      FIG. 27  is another embodiment of the electrode to withstand the high-strength shockwaves produced by the discharge; 
           [0044]      FIG. 28  is yet another embodiment of the high-voltage electrode; 
           [0045]      FIG. 29  is yet another embodiment of the electrode; 
           [0046]      FIG. 30  is yet another embodiment of the electrode assembly; 
           [0047]      FIG. 31  is an embodiment showing a detailed drawing of the insulating material surrounding the high-voltage electrode; 
           [0048]      FIG. 32  is a plot showing the pole height of the defoiined disk as a function of the discharge energy; 
           [0049]      FIG. 32A  is a drawing showing an aluminum disk prior to deformation; 
           [0050]      FIG. 32B  is a drawing showing the intensity of a pulsed waterjet indicated by the deformation of aluminum disk; and 
           [0051]      FIG. 33  is a drawing showing a hybrid system composed of an electrodischarge nozzle and the high-frequency pulsed waterjet for fragmentation of rocks and rock-like materials. 
           [0052]    Since the electrodischarge technique is quite complex, the components and parts shown in the figures are not necessarily drawn to scale and many variations are possible depending on the magnitude of the electrical energy deposited in the nozzle, water parameters, that is, quiescent or flow from the pump and, and various types of applications. 
       
    
    
     DETAILED DESCRIPTION 
       [0053]    In general, and by way of overview, the present invention provides an electrodischarge apparatus and method. 
         [0054]      FIG. 1  is an assembly of a capacitor bank, a water pump to supply a stream of water at pressures of the order of 15 kpsi and the flow rate of the order of 20 usgal/min, a nozzle for producing a high-speed continuous waterjet and an electrode assembly for generating an arc at the rapid discharge of electrical energy stored in the capacitor bank by triggering the spark gap. In some embodiments, the invention also provides a technique for discharging the electrical energy in quiescent water filled in the nozzle. By incorporating a check valve, not shown in  FIG. 1 , it is possible to fill the nozzle after each electrical discharge. When the electrical energy is discharged rapidly between the electrodes, water in the vicinity of the electrodes breaks down to form a plasma which expands at a very high speed forming a shockwave as illustrated in  FIG. 3 . The shockwave moves ahead of the plasma bubble and escapes from the nozzle. The rapidly expanding plasma bubble momentarily interrupts the stream or perturbs the quiescent water forming a slug or pulse of high-speed water. As the plasma cools down, it simply becomes a bubble of water vapor, which is the cavitation bubble. A novel aspect of some embodiments of the invention stems from the fact that by careful design of the electrode nozzle assemblies one can produce each phenomenon (shockwave, interrupted pulsed waterjet or cavitation bubble) discretely or in a sequence one after the other. The objectives of the nozzles disclosed in this specification are either to produce individual effects or all the three effects following the discharge. 
         [0055]    The characteristics of the phenomena accompanying the discharge depend on the electrical circuit parameters of the capacitor bank, configurational parameters and the shape of the nozzle chamber and the operating parameters. As an example of the circuit parameters, the energy, E, stored in the capacitor is a function of the capacitance, C, of the bank and the voltage, V, namely E=½CV 2  and for rapid discharge of the electrical energy in the nozzle, the inductance of the circuit should be as small as possible. 
         [0056]    The fluid parameters are the pressure in the nozzle chamber, of the order of 15 kpsi if the pump is used for the flow which is of the order of 20 usgal/min, or atmospheric pressure if quiescent water is used, the capacity of the nozzle chamber being of the order of 0.25 usgal. 
         [0057]    The configurational parameters of the nozzle electrode assembly are the shape and diameter of the central electrode, de, the chamber diameter, D, the distance between the electrodes, ι, length of the exit channel of the nozzle, k, and the orifice diameter, d o , which is determined by the water flow rate. The shape of the inner surface of the nozzle could be any smooth curve, for example, exponential as shown in  FIG. 7 . The length, k, depends on the desired characteristics of the phenomena accompanying the discharge and is a function of d o , for example, d o ≦k≦100d o . 
         [0058]      FIG. 2  is the same as  FIG. 1 , showing a nozzle configuration with a larger gap width (ι) between the electrodes. A larger gap width (ι) between the electrodes generates more planar shock waves. A shorter gap width (ι) between the electrodes generates more spherical shock waves. The form of the shockwave can thus be varied by varying the gap width (ι) between the electrodes. 
         [0059]      FIG. 3  is a typical appearance of the shock front after the rapid discharge of electrical energy between the electrodes, predicted by computational fluid dynamic (CFD) analysis. 
         [0060]      FIG. 4  shows the benefit of placing a reflector upstream of the electrodes, once again predicted by the CFD analysis. 
         [0061]      FIG. 5A  and  FIG. 5B  show the magnitudes of impact pressures on the target due to the varying (exponentially decaying sinusoidal) voltage after discharge, often called ringing frequency. 
         [0062]      FIG. 6  shows, for the given set of voltage (V), electrical energy (E), duration of discharge (∂) and the orifice diameter (d o ), the influence of the gap width (ι) on the speed of the pulse (slug) of water generated by the electrical discharge in the nozzle. It is remarkable that it is possible to increase the speed of the water pulse from approximately 300 m/s to approximately 1000 m/s, i.e. by a factor of more than three, simply by increasing the gap width from 6 mm to 22 mm. This observation is quite important from the standpoint of designing a robust and reliable nozzle for commercial applications. For instance, while a speed of 1000 m/s may be adequate for neutralizing a landmine, fragmenting a hard rock formation may require a speed of the order of 2000 m/s. As discussed in the Sections on electrodes (for example,  FIG. 26 ), several types of nozzle-electrode assemblies may be required for withstanding the high shock loads after the discharge. The empirical data of  FIG. 6  also show that the speed is linearly proportional to the gap. 
         [0063]      FIG. 7  illustrates a conceptual design for discharging the electrical energy between the axisymmetric central electrode and the circumferential ring electrode. The tip of the central electrode also acts as a reflector for propelling the shock wave downstream towards the nozzle exit. 
         [0064]      FIG. 8  is another conceptual design having a converging section, a throat of constant cross-sectional area and a diverging section. The nozzle includes an insulated central electrode. In this configuration, as the nozzle is grounded, the discharge (spark and arc formation) occurs between the tip of the electrode and the inner surface of the nozzle. In the illustrated configuration, the tip of the central electrode is at the forward end of the constant cross-sectional area throat, i.e. at or near the plane where the throat ends and the diverging portion begins. Therefore, by moving the central electrode forward and backward from the throat of the nozzle, it is possible to vary the gap width (ι). Yet another feature of this configuration is to capture the cavitation bubble formed by the discharge and focus it on the target. The bubble is confined in the annulus (annular stream of water) in the diverging section of the nozzle. 
         [0065]      FIG. 9  shows a first rudimentary configuration investigated by the inventor to observe if the discharge would modulate a stream of high-pressure water to produce a pulsed waterjet (Vijay, et al., “Electro-discharge technique for producing powerful pulsed waterjets: Potential and Problems,” Proceedings of the 13 th  International Conference on Jetting Technology—Applications and Opportunities, October 1996). The configuration has a long cylindrical channel  6  with a high-pressure fitting  1  at the upstream end for connecting a high-pressure hose and the nozzle insert  8  and the electrode assembly  10 . The nuts  3  and  7  are respectively used to connect the high-pressure hose to the cylindrical channel and the nozzle-electrode assembly. Hard O-rings  4  and  5  and the gasket  9  seal the pressurized water flowing through the channel and at the interface between the nozzle and the electrode assembly. The maximum electrical energy discharged from the capacitor bank was of the order of 3.5 kJ, just sufficient to modulate the stream of water. Observations made and the lessons learned from this crude investigation fotm the basis for improvements disclosed herein. Just to cite one example, the strong electromagnetic radiation generated by the high transient current (of the order 50 kA, depending on the magnitude of the voltage) accompanying the high-voltage discharge destroyed most of the sensitive electronic devices in the vicinity of the test facility (Vijay et al., cited above), highlighting the necessity for shielding these devices. 
         [0066]    As will become apparent from this specification, there are several embodiments capable of generating a shock wave, an interrupted jet caused by the expanding plasma bubble and the cavitation bubble which is simply the cooled plasma bubble. However, it is not possible to achieve all these phenomena accompanying the discharge in one nozzle configuration. Furthermore, a particular application dictates whether the electrodes are mounted in the transverse direction, as shown by way of example in  FIG. 9 , or mounted in the axial direction, as illustrated by way of example in  FIG. 10 . 
         [0067]    In the embodiment shown in  FIG. 10 , the insulated electrode  11  is located in the axial direction in the nozzle body  18 . The nozzle body  18  is composed of a lower housing  21  and a curved, hemi-spherical upper housing  13  (which may have another shape). The nozzle body  18  can be connected to a high-pressure pump through the inlet indicated by the 90° elbow  26  or filled with quiescent water using a check valve  23 . Breakdown of water to form a plasma bubble after the discharge occurs due to the high-intensity electric field between the tip of the high-voltage central electrode  11  and the tip of grounded metallic ring  19 . The electric field strength E is determined by V/ι, where V is the magnitude of the applied voltage and ι=gap width, that is, the distance between the tips of the electrodes. Depending upon the physical property of water, e.g. conductive, nonconductive, etc., the electric field strength required for breakdown is of the order of 3.4 kV/mm. By varying the position of the central electrode  11  and/or the grounded metallic ring  19  the required electric field for breakdown of water can be obtained. In the case of flowing water, generally depending upon the pressure, a wake forms downstream of the central electrode  11 . The wake is a bubble composed partially of water vapor, which is actually vaporous cavitation. In this case, the strength of the electric field could be of the order of 1 kV/mm as the water vapor breaks down much more readily to form the plasma than water. In this embodiment, the apparatus also includes spacing rings  12  and  14  to vary the gap width (ι), the metal plug  16  to which a pressure sensor (not shown in the figure) could be attached to measure the pressure exerted by the plasma, a metallic rod  17  to connect the ground electrode to the cables leading to the capacitor, nozzle insert  20  having various diameter orifices (0.5 mm≦d o ≦19 mm), check valve body  22 , nut  24  for fastening the water inlet component to the nozzle body  18 , water inlet part  25 , and the 90° elbow  26  for water inlet tube. The inlet tube is connected to a water pump by a hose  26   a  (which is not depicted in the figure). The tube can also be connected to a water bottle to provide quiescent water in the nozzle chamber. After each discharge, the chamber can be refilled by means of the check valve. Due to the small diameter orifices, the shock and the cavitation bubble most likely decay right inside the nozzle. 
         [0068]      FIG. 11  shows a nozzle configuration with the electrodes mounted in the transverse direction. By suitable design of the electrode assembly, discussed in a subsequent section, the gap width (ι)  28  can be varied from 1 mm to almost 30 mm. The configuration also shows the reflector  29  which also functions as a check valve momentarily stopping the flow of water  33  in the nozzle chamber until the next discharge. The details of one specific embodiment of the reflector are shown in  29   a . The orifice diameters (d o ) in the nozzle insert  30  depend on the flow rates of water and can vary from 0.5 mm to 19 mm. The length of nozzle exit (L 3 ) can be varied by attaching the extensions  31  with the nut  32 . For short lengths, L 3 ≈d o , and large orifice diameters (≧6 mm), the shockwave emerging from the electrode will have a spherical shape. As the lengths are increased, the wave will emerge as a plane wave. Furthermore, confinement of the plasma bubble in the cylindrical sections of the extensions generates a powerful pulse of water. 
         [0069]      FIG. 12  shows an embodiment to modulate a high-speed water stream, that is, a waterjet, to augment its cutting or fragmenting performance. Water from the pump enters through the inlet  33 , flows through the annulus  35   a , indicated by the dotted arrows  33   a , between the centre body  35  (which may be a microtip of an ultrasonic transducer driven by an ultrasonic generator) and the nozzle insert  34 . The centre body, which functions as a reflector, separates the flow and forms a wake (a low-pressure zone) in the gap  36  of the electrodes. In turbulent flow the wake is a stagnant zone composed of a mixture of dissolved gases, water vapor and quiescent water. With the rapid discharge of electrical energy, this mixture breaks down quite readily to form the plasma which travels in the diverging section downstream of the electrodes and in the cylindrical section  34  of the nozzle. The dimension of the annulus depends on the pressure and the flow rate required for a given application. As an example, if the required flow rate is of the order of 15 usgpm at a pressure of 15 kpsi, and for the size of 0.166 in of the cylindrical section of centre body  34 , the dimension of the annulus is of the order of 0.006 in. As stated in section  10 , since the gap width (ι) is of the order of 2 mm, the discharge produces spherical shock waves and plasma bubbles. In the long cylindrical section  34 , the shock waves are transformed into plane waves before impacting the target. The plasma bubbles are confined within the annular flow of water, shown by the dotted arrows  33   b  to implode on the target and generate very high impact pressures enhancing the fragmentation ability of the continuous waterjet. 
         [0070]      FIG. 13  shows another embodiment which is similar to the one illustrated in  FIG. 12 , except that the tip of the grounded electrode is a plane  37  and the tip of the high-voltage electrode  37   a  is pointed like a needle. This configuration of the electrodes focuses the electric field strength for breaking down the water and intensifying the strength of the shock wave and the plasma bubble. 
         [0071]      FIG. 14  is another embodiment for modulating a high-speed waterjet with the electrodischarge technique. The nozzle body is composed of a large inlet section  38  to maintain a fairly low speed of water delivered by the pump  33 , equivalent to quiescent water. The ground electrode  39  and the high-voltage electrode  43  are assembled as one unit (a detachable electrode assembly) so that it can be easily slid into and out of the nozzle body. In addition to the advantage of easy alignment, the current induced by the rapid discharge indicated by the dotted arrow  44  and flowing through the reflector  40  mounted on the ground electrode indicated by the dotted arrow  45  generates a high-intensity electromagnetic force which will provide additional force to increase the speed of the plasma bubble moving towards the nozzle exit. As the electrode assembly can be slid in and out of the nozzle body, the condition of the tips of the electrodes can be readily examined without disconnecting the electrical cables connected to the capacitor bank  1  ( FIG. 1 ). The easily replaceable reflector  40  enhances the strength of the shockwaves as described in  FIG. 4 . The discharge zone  42  can be easily controlled by varying the position of the ground electrode  39 . 
         [0072]      FIG. 15  is an embodiment similar to the one shown in  FIG. 12  except that the space surrounding the electrodes  49  can be varied to reduce the speed of water in the discharge zone, that is, the gap between the electrodes. It is also meant for fairly low pump pressure (≦5 kpsi) and moderate flow of water (≦10 usgal/min) In the embodiment depicted in this figure, the apparatus generates pulses of water by the imploding plasma bubble slightly upstream (≈2 d o ) of the nozzle exit  46 . In the illustrated embodiment, the apparatus includes a large water inlet  33  and a centre body  50  which also functions as a reflector  48 . In addition to functioning as a reflector, it also incorporates a flow straightener  50   c  with vanes  50   f  to smoothen the flow, that is, to reduce the level of turbulence in the flow. In all the embodiments disclosed herein, it is important to reduce the level of turbulence in order to eliminate undesirable sparking (formation of an electric arc), also called tracking from the high-voltage electrode to another part of the nozzle other than the ground electrode. The straightener is mounted on a threaded mandrel  50   d , fabricated from type- 303  stainless steel or similar material. The mandrel  50   d  is held in place by the conical nut  50   a  fabricated from high-strength bronze or similar material and the cone  50   c  with a flat washer  50   b  to absorb the load induced by the shocks. The tip of the mandrel  48  has a shape of a concave hemisphere although in variants it could be parabolic or another suitable shape, to focus and propel the shocks towards the nozzle exit  46 . The discharge zone downstream of the reflector  49  can be controlled by varying the position of the ground electrode tip  47 . The bus bar  51  fabricated from brass or similar material connects the ground cables  51   a  to the capacitor bank and the connector  52  also made of brass or copper or similar material connects the high-voltage cables  53  to the capacitor bank. The number of shielded cables used (which may be≧10) depends on the transient discharge current generated by the energy discharged from the capacitor bank. 
         [0073]      FIG. 16  is the same embodiment as illustrated in  FIG. 15  to highlight the precautions to be taken with high voltages (for example, voltages≧5 kV). The two major issues to address for reliability of the electrodischarge technique are: (1) sealing arrangements in all the embodiments and (2) prevention of undesirable sparks, often called tracking, which could destroy the insulating materials used to separate the ground electrode assembly  51  from the high-voltage electrode  55  (described in the Sections on Electrodes) and other materials. All of the illustrated embodiments of this invention require sealing, e.g. special O-rings  54 ,  56 ,  56   a  ( 4 ,  5  in  FIG. 9 ), gaskets  57  ( 9  in  FIG. 9 ) and washers or any other fluid-tight sealing means to seal against high transient pressures generated by the shocks and the high transient temperatures generated by the plasma bubble. High strength seals (≈90 durometer), such as Viton or similar O-rings may be used in these embodiments. 
         [0074]    For efficient performance, the breakdown of water to form a plasma bubble must happen in the gap between the electrodes. However, the state of the flow (e.g. turbulent flow) and other factors may cause the discharge to take place at other locations, for example from the tip of the high voltage electrode to the inside surface of the nozzle chamber, which will eventually destroy the smooth surface of the nozzle. As illustrated  58 , tracking can also occur between the high-voltage electrode stem  55  and inner surface of the ground casing  51   b  leading to the failure of the insulating material. These problems are overcome with the embodiments described below. 
         [0075]      FIG. 17  shows an embodiment based on the conceptual design illustrated in  FIG. 8 . Water enters through the side port  33 , fills the large volume of nozzle chamber  63  for reducing the speed of the flow and forms a wake downstream of the insulated  64  high-voltage electrode  65 . By moving the electrode axially forward and backward, the discharge zone and length of the arc  61  formed by the discharge can be varied, giving rise to a range of plasma bubbles or plane or spherical shockwaves. The nozzle insert  62  is connected to the chamber  63  by the nut  59 . The lengths of the diverging sections  60  can be varied from zero to any suitable length (≈10 in). 
         [0076]      FIG. 18  shows another embodiment for modulating low water flows (≦2 usgpm/min) at very high pressures (≧20 kpsi). As in the embodiment of  FIG. 17 , high-pressure water enters through an inlet (side port  33 ) from the pump. Since low flows are involved, the annular clearance would be of the order of 0.002 in, forming a long wake downstream of the insulated electrode tip  70 . The flow straightener  50   c  is mounted on a plastic stub  67  for adjusting its position upstream of the annulus. The axially located high-voltage electrode can be moved forward and backward to vary the gap width (ι) between the tip of the electrode and the inside surface of the grounded  70  nozzle attachment  69 . The sleeve  66  fabricated from high-strength plastic holds the other end of the high-voltage electrode for easy movement in the nozzle attachment. The high-voltage cables are connected to the electrode through the adaptor  71 . This embodiment produces pulses of water due to implosion of the plasma bubbles. 
         [0077]      FIG. 19  shows a more complicated design in accordance with another embodiment to confine and focus the cavitation bubble which is, in fact, the plasma bubble when it cools down. In all the embodiments disclosed in this specification a cavitation bubble does indeed form. However, generally as soon as it arrives at the nozzle exit, it has a tendency to ventilate to the atmosphere without doing any useful work. The objective of the embodiment illustrated in  FIG. 19  is to confine and focus the highly energetic cavitation bubble onto the target. 
         [0078]    In the embodiment depicted in  FIG. 19 , the apparatus has a main body  72  to which the main nozzle  74  is connected with the nut  80  sealed with the O-rings  81 . Water from the pump enters into the main body  72  through the port  33  and flows through the annulus between the electrode and the nozzle exit as indicated by arrows  33   a . Electrical discharge occurs in this main flow. Water entering the sheathing nozzle  75  through the port  76  emerges as a sheath (annulus) of water around the main jet as indicated by dashed arrows  76   a . The purpose of this secondary annular jet is to confine and transport the cavitation bubble towards the target to be processed. The port  76  is welded to the ring  78  and sealed with the O-rings  77 . 
         [0079]    Other components of the apparatus in accordance with this embodiment include an insulated central electrode  95 , which is inserted into the guide tube  73  which also acts as a flow straightener ( 50   f ,  FIG. 15 ) to align it with the nozzle exit, a gland  92 , a back-up ring  93 , bushing  94 , cap for holding the high voltage electrode  91 , and another back-up ring  90 , another gland  88 , locking ring  86  for the electrode, electrode nut  85 , stainless steel rod  83  for grounding the main body  72 , and the bracket  82  for securing the nozzle-electrode assembly to a gantry or a robotic manipulator, stem of the high-voltage electrode  89  for connection to the high-voltage cables and O-rings  84  and  87  to seal the electrode against leakage of water. Most of the components illustrated in this embodiment also apply to other embodiments. 
         [0080]      FIG. 20  depicts an apparatus in accordance with another embodiment that is designed for one or several sequential discharges in the diverging exit section of the nozzle  100 . 
         [0081]    As the tips of the ring electrodes  96 , placed circumferentially, are flush with the inner surface of the diverging section of the nozzle, the flow through the nozzle is quite smooth with no disturbances. The apparatus in accordance with this embodiment is meant for low flows (≈1 usgal/min) at low pressures (≈2 kpsi). The ring electrodes  96 , the ground  97  and high voltage stems  101  are encased in silicon rubber  98  as insulating material. For additional safety the ring electrode assembly is embedded in a ceramic plug  99 . A pair of electrodes can be fired once as in other embodiments. Or, they can be fired in sequence, over a delay of a few microseconds, to augment the intensity of the shock and plasma and propel them toward the target. This is possible because the line of spark, indicated by the dotted arrow, is in the same direction as the flow. 
         [0082]      FIG. 21  shows an apparatus according to yet another embodiment for intensifying the strength of shock waves formed in quiescent water in the nozzle. Theoretically, collision and convergence of two shock waves, indicated by the arrows, would increase the speed of the pulsed jet emerging from the nozzle. Ring-type ground electrodes  102  and ring-type high-voltage electrodes  103  are placed above and below the main nozzle  104 . With a check valve, not shown in  FIG. 21 , the flow through inlet (or port)  33  from the pump or a water bottle, fills the nozzle chamber  104   a  and remains momentarily stagnant (quiescent). The expanding spherical shock waves following the plasma channel formation converge at the entry to the nozzle exit  104   b  augmenting the speed of the emerging pulsed waterjet. 
         [0083]    In the embodiment depicted in  FIG. 22 , an apparatus is placed right on the surface  109  to be processed, for example, fragmenting the concrete biological shield of a nuclear power system. In this embodiment, the apparatus is basically the same as the embodiments illustrated in  FIG. 12  and  FIG. 13  with a hemispherical chamber  111  to focus the shock wave, plasma bubble and pulse of water to impact the surface. Water enters through the inlet (or port)  33  into the hemispherical chamber  111  and remains momentarily as quiescent water due to the abutment of the face  111   a  of the chamber against the surface  109 . The reflector assembly is placed in the housing  105 . The high-voltage electrode  107  and the ground shell  106  are assembled as one unit for easy insertion into the hemispherical chamber. The shock absorber  108  fabricated from high-strength elastomers is configured to absorb the high stresses generated by the shock waves. The discharge, as indicated by the arrow  110 , takes place between the tip of the high-voltage electrode  107  and the tip of the ground shell  106 . 
         [0084]      FIG. 23  shows another embodiment similar to the embodiment depicted in  FIG. 22 , except it incorporates separate ground  112  and high voltage electrode  107 , making it possible to vary the gap width (ι). As illustrated in  FIG. 6 , the speed of the pulsed jet can be increased by increasing ι, forming long plasma channel  110  which enhance the efficacy of the electrodischarge technique for inducing fractures (cracks) or fragmentation of very hard rocklike materials. 
         [0085]      FIG. 24  shows an embodiment for connecting nozzle electrode assemblies, disclosed in all the previous sections, to the water pump. As is known in the field of high-voltage engineering (T. Croft and W. I. Summers, “American Electricians Handbook,” 14 th  Edition, McGraw Hill, 2002), extreme precautions need to be taken to ensure safety of the personnel and other equipment. In the case of electrodischarge technique, tracking (that is, undesirable sparking) needs to be eliminated by proper grounding of all the components, to the same ground, for example, a water pipe. The other major problem is to prevent the damage of electronic equipment caused by electromagnetic radiation caused by high transient discharge current, by proper shielding of all cables, etc. 
         [0086]    In the case of a high-pressure water pump, the hose used generally consists of braided metal wire. Therefore, when the hose is connected to the grounded nozzle, the discharge current can also flow through the hose to the pump and may damage electrical components of the pump. The embodiment shown in  FIG. 24  includes an insulated hose coupling to electrically isolate the pump from the nozzle assembly. 
         [0087]    The coupling include a metal part  114  for connecting to the nozzle assembly  33  and the high-pressure fitting  121  fabricated from high-strength stainless steel. Both inner and outer surfaces of the metal part  114  and the fitting  121  are coated with epoxy or similar coating  122  as insulation. Sealing package  123  includes a soft packing  118  made from Teflon or similar material, held in place by high-strength plastic material such as glass-PEEK (Polyether ether ketone)  117 . The parts are assembled and tightened by threaded studs  116  and nuts  120  with metallic washers  119  and a bushing  115  made from glass-PEEK or similar materials. 
         [0088]      FIG. 25  shows yet another coupling for connecting the pump to the nozzle assembly to eliminate grounding problems and which is suitable for low pressures (≈5 kpsi). A high-strength threaded  128  plastic insulator  129  is used to connect the high pressure fitting  124  for water flow  131  from the pump and the fitting  130  leading to the nozzle assembly. Water leakage is prevented by the O-rings  127 . The plastic body was further reinforced from outside by a thermally shrunk metallic sleeve  125 . The whole assembly was enclosed in a flexible plastic tubing  126  to provide additional electrical insulation. 
         [0089]    It is quite clear from the descriptions given in all the previous sections that electrodischarge is a complex phenomenon requiring great deal of attention to design of all components to derive its benefits while preventing damage to personnel and other equipment in the vicinity of the electrodischarge apparatus. It is also clear that, depending on the application, it is possible to manufacture a variety of nozzle configurations (chambers) to optimize the performance of the electrodischarge technique. Each type of nozzle configuration requires a different type of high voltage and ground electrode assembly for efficient deposition of electrical energy in the chamber. This requires that the discharge should occur only between the tips of the electrodes and not anywhere else, that is, tracking (unwanted sparking, as illustrated by the bolded arrow  58  in  FIG. 16 ) must be avoided. This is only possible by paying utmost attention to the design of electrode assemblies and how they are connected to the capacitor bank. In the following sections some of the configurations and the main features are disclosed. 
         [0090]      FIG. 26  shows one embodiment of the electrode assembly and a component to connect it to the cables from the capacitor bank. This embodiment is meant for the nozzles of the type illustrated in  FIG. 12  and  FIG. 13  or similar types. The assembly shows the main body  136  fabricated from stainless steel or similar material connected to the ground bus bar  132 . The central high-voltage electrode  138 , fabricated from tungsten carbide or similar wear-resistant material, is insulated from the grounded main body by the coaxial tubes  135  and  140  fabricated from high dielectric strength plastic materials such as Ultem, PEEK or similar materials. The high-voltage electrode is secured by the main nut  139  made from stainless steel, and the lock nut  137  made from brass or bronze or similar soft metal and the nut  141 . The high-voltage stem  138  is connected to the high-voltage bus bar assembly  142  of high-voltage cables by the coupling  133  made from brass, copper or similar highly conducting metals. The high-voltage bus bar is assembled by the stud  142   a , the plastic nut  133   a , plastic washer  133   b  and the plastic disc  133   c . The high-voltage cables are secured by the set screws. For additional safety, the high-voltage bus bar assembly is enclosed in a plastic tube  134  made from acrylic or similar material. 
         [0091]      FIG. 27  is another embodiment of an electrode assembly  143  for the nozzle configuration illustrated in  FIG. 10  or similar types. The electrode configuration is meant for high static pressure of water (≈20 kpsi) and also high shock loading following the discharge. The front  144  of the high voltage stem  149  is shaped in the form of diverging and converging conical portions for self-sealing. As shown in this embodiment, the tip is a bulbous tip with the converging cone meeting a rear face of the tip to provide an angled annular lip. The entire rod is coated with epoxy  151  or any similar material, capable of withstanding high voltages up to a maximum of 50 kV and which is compatible with water. The high-voltage electrode  149  is inserted into two metallic sleeves  146  and  147  the outer surfaces of which are also coated with epoxy or similar high dielectric strength materials and are glued together with Loctite or similar adhesive. The electrode assembly is connected to the grounded nozzle body with the nut  145 , making provision for changing the gap width (ι) by varying the thicknesses of the washers  148 . Leakage of water is prevented by the O-rings  150  and  152 . 
         [0092]      FIG. 28  is yet another embodiment for use in the nozzle body shown in  FIG. 10  or similar types. The electrode assembly has the same configuration as shown in  FIG. 27  with slight modifications to eliminate tracking (undesirable sparking) between the high-voltage electrode  149  and the grounded nut  145 . The coated high-voltage electrode  155  is surrounded by the inner sleeve  154  fabricated from high strength plastic PEEK or similar material, which is inserted in the metallic sleeve  156 , the inside surface of which is coated with epoxy or similar materials. The electrode assembly is protected by the ring  153  fabricated from soft metal or elastomers. The gap width (ι) can be varied by the washers  157 . Plastic tubing  158  surrounding the rear portion of the electrode  155  prevents any tracking from the electrode to the washer. 
         [0093]      FIG. 29  shows an embodiment of the electrode assembly for the nozzle configuration illustrated in  FIG. 12  or similar types. The high-voltage electrode  149  is insulated from the grounded nut  165  by two plastic sleeves  163  and  164  which may be made from Ultem, PEEK-glass or similar materials. As plastic materials are generally brittle, the sleeves are kept under compression by the nut  162  made from bronze or similar material and the metallic protector  159  made from stainless steel or similar material. The protector is glued or bonded to the sleeve  163  by a strong adhesive, such as Loctite or similar adhesive. The gap (ι) between the electrodes can be varied by using the spacing rings  161  made from Lexan or similar materials. Sealing is achieved by the hard Parker O-rings  166  and  167 . The tip  160  made from tungsten copper or similar material is silver soldered to the front  160   a  of the high-voltage stem  149 . For additional protection the high-voltage stem  149  is inserted into a tubing, e.g. a Tygon® tubing  168 . 
         [0094]      FIG. 30  depicts yet another embodiment of an electrode assembly for use in the nozzle body shown in  FIG. 10  or similar types. It is similar to the electrode assemblies depicted in  FIG. 27  and  FIG. 28  with some additional novel and safety features. The high-voltage electrode  149  includes the tip  174  which is held in place by a pin  173 . When the tip  174  wears off due to ablation caused by the sparks, a new one can be easily inserted to continue the operations where repeated discharges are required. The sleeve surrounding the electrode includes a central insulator  171  made from PEEK or similar material and the front insulator  172  made from elastomers to absorb the shock loads caused by the discharge. The assembly of the electrode and the sleeves are glued to the coated outer metallic sleeve  175 . The assembly is inserted into the nozzle housing  143  and tightened by the grounded nut  145 . The gap width (ι) can be varied by the washers  170 . In order to prevent tracking between the rear part of the nut  145  and the high-voltage cable connector  169  or the stem  149 , an insulator  176 , similar to the undulating or sinusoidal shape used in high-voltage transmission lines, is inserted as shown. 
         [0095]      FIG. 31  illustrates a high-voltage electrode assembly according to another embodiment that can be used for any nozzle configuration for moderate operating pressures (≈10 kpsi) and voltages up to 20 kV. The tip  178  is threaded to the high-voltage stem  179 . In order to prevent tracking between the tip  181  and at any location on the inside surface of the nozzle body, the shoulder  180  is coated with a high-dielectric-strength plasma coating such as aluminum oxide or a similar material. The high-voltage stem  179 , except the threaded part, is also coated with the plasma coating. The curved, hemispherical or any other shape part of the tip  181  can be coated with high ablation resistant metal, such as an alloy of tungsten carbide, chromium and cobalt or similar components, to prolong the life of the electrode. The stem itself can be fabricated from inexpensive metals such as brass or copper. As the tip wears off, a new tip can be easily connected to the threaded electrode stem reducing the downtime. The coated electrode stem is enclosed in a sleeve  177  fabricated from high-strength plastic or a metal coated on all sides with an insulating material same as the shoulder  180 , using plasma or any other coating technique. 
         [0096]      FIG. 32  illustrates the very preliminary results obtained with the electrodischarge technique by the inventor (Vijay, et al., Generating powerful pulsed water jets with electric discharges: Fundamental Study,” Proceedings of the 9 th  American Water Jet Conference, August 1997). Aluminum discs were subjected to the pulsed waterjet emerging from a nozzle of the type illustrated in  FIG. 11 .  FIG. 32A  shows the aluminum disc prior to being subjected to the pulsed waterjet emerging from a nozzle of the type illustrated in  FIG. 11 .  FIG. 32B  is a drawing showing the intensity of a pulsed waterjet indicated by the deformation of aluminum disk. The height of the pole formed by the deformation caused by the impact of the pulsed jet is an indication of the efficacy of electrodischarge technique for industrial applications such as mining of minerals and humanitarian applications such as neutralizing landmines The defoimation is clearly a function of the electrical energy discharged between the electrodes at a gap width of 16 mm. 
         [0097]      FIG. 33  is an illustration of a hybrid system implementing the low-frequency electrodischarge technique and ultrasonically modulated high-frequency pulsed waterjet (Vijay et al., “Ultrasonic Waterjet Apparatus,” U.S. Pat. No. 7,594,614 B2, Sep.29, 2009) for mining of minerals from hard rock formations  188  or similar applications without using environmentally harmful explosives. The method entails first drilling a hole  186  with the ultrasonic rotating nozzle  182 . Some rock formations contain hard minerals such as quartz which are difficult to fracture just with the waterjet. However, such hard minerals being brittle can be easily broken by the carbide bits  183  sintered to the rotating nozzle body. When a certain depth of the hole has been obtained, then the electrodischarge nozzle  184  can be lowered into the hole full with water generating powerful shock waves, pulses and cavitation bubbles  189  resulting in fractures and microfractures in the rock formation  187 . As such fractures weaken the rock formation, the hole diameter  185  and the rate of drilling would increase considerably enhancing the productivity. Thus, such a hybrid system would be extremely beneficial for mining of minerals or in other applications such as, for example, breaking the concrete biological shields in decommissioning operations of obsolete nuclear power stations. 
         [0098]    The embodiments of the invention described above are intended to be exemplary only. As will be appreciated by those of ordinary skill in the art, to whom this specification is addressed, many variations can be made to the embodiments present herein without departing from the scope of the invention. The scope of the exclusive right sought by the applicant is therefore intended to be limited solely by the appended claims. 
         [0099]    It is to be understood that the singular foil is “a”, “an” and “the” include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to “a device” includes reference to one or more of such devices, i.e. that there is at least one device. The terms “comprising”, “having”, “including” and “containing” are to be construed as open-ended turns (i.e., meaning “including, but not limited to,”) unless otherwise noted. All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. The use of examples or exemplary language (e.g., “such as”) is intended merely to better illustrate or describe embodiments of the invention and is not intended to limit the scope of the invention unless otherwise claimed. 
         [0100]    While several embodiments have been provided in the present disclosure, it should be understood that the disclosed systems and methods might be embodied in many other specific forms without departing from the scope of the present disclosure. The present examples are to be considered as illustrative and not restrictive, and the intention is not to be limited to the details given herein. For example, the various elements or components may be combined or integrated in another system or certain features may be omitted, or not implemented. 
         [0101]    In addition, techniques, systems, subsystems, and methods described and illustrated in the various embodiments as discrete or separate may be combined or integrated with other systems, modules, techniques, or methods without departing from the scope of the present disclosure. Other items shown or discussed as coupled or directly coupled or communicating with each other may be indirectly coupled or communicating through some interface, device, or intermediate component whether electrically, mechanically, or otherwise. Other examples of changes, substitutions, and alterations are ascertainable by one skilled in the art and could be made without departing from the scope disclosed herein.