Patent Publication Number: US-8981261-B1

Title: Method and system for shockwave attenuation via electromagnetic arc

Description:
FIELD 
     The disclosure relates to methods and systems for shockwave attenuation, and more particularly to methods and systems for attenuating shockwaves by rapidly heating air to interpose an intermediate medium between the shockwave and a protected region. 
     BACKGROUND 
     Explosive devices are being used increasingly in asymmetric warfare to cause damage and destruction to equipment and loss of life. The majority of the damage caused by explosive devices results from shrapnel and shockwaves. Shrapnel is material, such as metal fragments, that is propelled rapidly away from the blast zone and may damage stationary structures, vehicles, or other targets. Damage from shrapnel may be prevented by, for example, physical barriers. Shockwaves are traveling discontinuities in pressure, temperature, density, and other physical qualities through a medium, such as the ambient atmosphere. Shockwave damage is more difficult to prevent because shockwaves can traverse an intermediate medium, including physical barriers. 
     Damage from shockwaves may be lessened or prevented by interposing an attenuating material between the shockwave source and the object to be protected. This attenuating material typically may be designed or selected to absorb the energy from the shockwave by utilizing a porous material that distorts as the energy of the shockwave is absorbed. 
     U.S. Pat. No. 5,394,786 to Gettle et al. describes a shockwave attenuation device that utilizes an absorbing medium. That assembly includes porous screens that form an enclosure filled with a pressure wave attenuating medium. This attenuating medium may be an aqueous foam, gas emulsion, gel, or granular or other solid particles. However, as shown and described in the drawings of that patent, the shockwave attenuating assembly must be positioned before the explosion occurs and surround the area to be protected. For example, the assembly may be positioned on the side of a vehicle to prevent damage to the vehicle or passengers within. 
     A similar shockwave attenuation device is described in U.S. Patent Publication No. 2007-0006723 to Waddell, Jr. et al. That device includes a number of cells filled with an attenuating material, such as aqueous foams. However, like the device described in Gettle et al., the pressure-attenuating material and device must be positioned on a structure, surface, or person desired to be protected by the system before the explosion occurs. 
     One feature common among prior art shockwave attenuation systems is that they require an intermediate medium or structure that acts to attenuate the force of the shockwave by absorbing the energy of the shockwave. Although only a portion of the shockwave may pass through the medium, the energy of the shockwave is nevertheless significantly reduced by the intermediate medium. However, because these systems are structural, they must be fixed in place before a shockwave is created. Further, these shockwave attenuation systems may not protect an entire vehicle or person. For example, attenuating panels are not transparent and therefore cannot be placed over windows or used as facemasks in helmets. They also may be bulky and heavy, and therefore negatively impact the performance of a vehicle on which they are mounted. 
     Such prior art shockwave attenuation systems may not be effective to protect highly mobile land assets for which an incoming threat may be in the form of a ballistic shell, rocket, IED, or landmine, or waterborne assets for which an incoming threat may be in the form of a torpedo, ballistic shell, bomb or a naval mine. Therefore, a need exists for a shockwave attenuation device that is capable of dynamically interposing a medium between an explosion source and a protected asset. There is also a need for an intermediate medium that effectively attenuates the energy from a shockwave and that allows for protection of a protected asset in a marine environment. 
     SUMMARY 
     Presented is a system and method for attenuating a shockwave propagating in a first medium by detecting a shockwave-producing event, determining a direction of the shockwave relative to a protected asset, and interposing a second, transient medium, different from the first medium, between the shockwave and the protected asset such that a shockwave produced by the event contacts the second medium and is attenuated in energy thereby prior to reaching the protected asset. The second medium may be formed by rapidly heating a region of the first medium so that the second medium differs from the first medium in at least one of temperature, density and composition. 
     In one embodiment, a system for attenuating a shockwave propagating in a first medium may include a sensor for detecting a source of the shockwave and generating a detection signal, an arc generator in communication with the sensor and configured to receive the detection signal therefrom, and in response thereto create an electromagnetic arc to heat a selected region of the first fluid medium rapidly to create the second, transient medium, different from the first medium, interposed between the shockwave and the protected region such that the shockwave contacts the second, transient medium. The arc generator may be configured to heat the selected region by generating an electromagnetic arc, such as an electric arc or a laser or microwave arc, between the protected region and the incoming shockwave. In one embodiment, the arc generator may include a power supply for generating the arc and may provide a conducting path. 
     In embodiments, the arc generator may be configured to generate a focused microwave beam or a focused laser beam. In each case, the beam may rapidly heat the fluid medium in the selected region to create the second medium. In one embodiment, in which the fluid medium is atmospheric air, the focused beam rapidly heats the air in the selected region and changes its temperature, density and composition, the latter the result of the creation of free electrons. 
     In other embodiments, the arc generator may be adapted to develop and drive a large electric current through the fluid medium rapidly. In the embodiment in which the medium is atmospheric air, the second medium may differ from ambient air in temperature, density and composition. With these embodiments, the arc generator may be adapted to create a conducting path for the electric current. Accordingly, the arc generator may be configured to generate one or more of a laser-induced plasma channel (LIPC) from converging laser beams, ionizing tracer pellets fired along converging paths, and projectiles trailing fine electrical wires fired along converging paths. In each of these embodiments, an electric arc may be generated to travel along a conducting path created by dielectric breakdown of ionized ambient air at the selected region. 
     In still other embodiments, the arc generator may include a sacrificial conductor that may not rely on current travel along a path of ionized air. Rather, the arc generator may include a power supply that applies current to a conductor in the form of one or more strips or wires of conductive material. The high current flowing through the conductor from the power supply may vaporize the conductor, forming a conductive channel of vapor that may rapidly heat and ionize the air in the selected region to create a rapidly expanding second medium. An advantage of this embodiment is that the sacrificial conductor may be attached directly to the protected asset, such as a vehicle. 
     Such embodiments as described above may reduce the energy density of the shockwave by creating a second medium in the path of the advancing shockwave that reflects, refracts, absorbs and deflects at least a portion of the shockwave. This may result from creating a second medium that differs from the ambient medium (e.g., atmospheric air) in density, temperature and/or composition. Such differences may change the index of refraction of the wave front as it contacts the second medium, causing at least some of the shockwave to be reflected from the surface of the second medium, to diverge as the shockwave travels through the medium, and to be reflected and diverge further as the shockwave contacts the rearward boundary of the second medium. The second medium, some embodiments, acts to absorb the energy of the shockwave as the medium may be increased in temperature. 
     In yet other embodiments, the arc generator create the second medium by magnetic induction. The arc generator may be adapted to create channels or ionized air. When contacted by an advancing shockwave, the conducting channel may be deformed as ionized air is pushed inward. This movement does mechanical work, which removes energy from the shockwave, making it weaker. In one aspect, the channels may not be rigid, and may be pushed by the shockwave against the ambient air that the channels are displacing, which may transfer energy from the shockwave to kinetic energy of displaced air. In another aspect, parts of the conductive channels are not rigid, but the channels of ionized air pushed ahead of the magnetic flux are disrupted and broken until they form new channels through the air that heat the air. At least a portion of the shockwave energy may be transformed to ionization energy of air ions and into pressure-volume-temperature energy of expanded hot air. 
     In still another aspect, the conductive channels of magnetic flux are rigid may not be deformed by contact with the advancing shockwave. Instead, the flux may be compressed by contact with the shockwave, which may increase the electric current in the conductive channels. This increased current results from energy lost by the shockwave. The arc generator may be configured such that the excess current may bypass the power supply and be shunted to heat a resistive load, or charge a capacitor where it may be used later to power the arc generator. 
     With such embodiments, the advancing shockwave is diminished in force as the energy of the shockwave is converted either into mechanical energy, as when it deforms magnetic induction channels, or into electrical energy, as when it interacts with rigid magnetic flux channels. It is within the scope of this disclosure to provide a system and method in which combinations of the foregoing embodiments are deployed to defend a protected asset, or in which an embodiment is deployed multiple times against an incoming threat. 
     The features, functions, and advantages that have been discussed can be achieved independently in various embodiments of the present invention or may be combined in yet other embodiments further details of which can be seen with reference to the following description, drawings and claims. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic diagram of the elements of a disclosed system, according to an embodiment; 
         FIG. 2  is an elevational, schematic view of the disclosed system mounted on a protected asset generating an arc to encounter a shockwave from an explosion; 
         FIG. 3  is an elevational, schematic view of the disclosed system configured to utilize converging laser or microwave beams; 
         FIG. 4  is an elevational, schematic view of the disclosed system configured to utilize pellets leaving conductive trails; 
         FIG. 5  is an elevational, schematic view of the disclosed system configured to utilize projectiles trailing conductive wires; 
         FIG. 6  is an elevational, schematic view of the disclosed system configured to utilize a sacrificial conductor; 
         FIG. 7  is an elevational, schematic view of the system of  FIG. 6  surface mounted on a protected asset; 
         FIGS. 8A and 8B  are schematic views of an embodiment of the heating element of the disclosed system configured to generate an arc using flexible electrodes; 
         FIGS. 9A and 9B  are schematic views of an embodiment of the arc generator of the disclosed system configured to generate an arc using relatively rigid electrodes; and 
         FIGS. 10 and 11  are diagrams showing some of the effects of a shockwave impacting the second, fluid medium. 
     
    
    
     DETAILED DESCRIPTION 
     The disclosed shockwave attenuation method and system may utilize a second, transient fluid medium that may be dynamically deployed in a first fluid medium between an explosion and a protected asset within a protected region. When contacted by an advancing shockwave produced by the explosion traveling through the first fluid medium, the second fluid medium may attenuate the energy from the shockwave through several vectors, rather than simply by absorbing the energy of the shockwave. While the following discussion describes various embodiments of the disclosed system and method as operating in a first fluid medium of atmospheric air, it is to be understood that the first fluid medium may be water, such that the disclosed system and method may be deployed in a marine environment. 
     As shown in  FIG. 1 , in one embodiment, the system for attenuating a shockwave via electromagnetic arc, generally designated  10 , may include a sensor  12  including or incorporating a computer  14 , and an arc generator, generally designated  16 . The sensor  12  and computer  14  may be mounted on or incorporated in a protected asset, such as a vehicle  18 , which may be a military vehicle as shown in  FIG. 2 , or they may be physically separate from the vehicle. Although  FIG. 2  shows vehicle  18  as a military vehicle, it is to be understood that the depicted vehicle  18  also may represent other types of land vehicles, such as automobiles, armored vehicles such as tanks and armored personnel carriers; fixed structures such as encampments, bunkers, revetments, buildings or portions of buildings such as balconies; marine vessels such as surface ships, submarines, a hovercraft or air-cushion vehicles, offshore platforms, and other structures that operate in, under or adjacent a body of water; aircraft including fixed wing aircraft, vertical takeoff and landing (VTOL) craft and helicopters; and people and animals. Each of the foregoing would be placed in a protected region  20 . 
     Referring to  FIG. 1 , the sensor  12  may be selected to provide measurements that enable the computer  14  to estimate the location and time of an explosion  22 , either before or after it occurs, and direct the arc generator  16  to respond. In one embodiment, the sensor  12  is configured to detect an explosion  22  that has occurred, but before the shockwave  24  caused by the explosion, traveling through a first fluid medium  26 , which in this embodiment is atmospheric air at ambient temperature and pressure, reaches the protected region  20 . This type of sensor  12  may be configured to detect any one or more of the electromagnetic signals emitted during chemical detonations, including microwave bursts, flashes of infrared radiation, flashes of visible light, flashes of ultraviolet light, and x-ray bursts. In an embodiment a sensor  12  may be configured to detect two or more of the foregoing types of electromagnetic radiation, which may result in the sensor detecting fewer false positives. In a variation of such an embodiment, the sensor  12  may be in the form of two discrete sensors, each configured to detect a different type of electromagnetic radiation. In an embodiment in which the sensor  12  is selected for use in a marine environment (e.g., to detect underwater explosions from naval mines or torpedoes), the sensor  12  may be configured to detect electromagnetic radiation at wavelengths for which water is substantially transparent, such as visible light near-ultraviolet light and near-infrared light. 
     In some embodiments, once the sensor  12  has detected an explosion  22 , the sensor may be configured to estimate one or more of the magnitude, distance, and azimuthal position of the explosion and provide the estimates to the computer  14  or to the arc generator  16 . In some embodiments, the computer  14  may measure the signature of the explosion  22  and compare it with stored known explosion signatures of a plurality of different known explosive devices. The computer  14  may then determine a probability that the explosion is from one of the known explosive devices. The computer  14  may then calculate an estimated probability distribution function (p.d.f.) of explosion magnitudes and locations relative to the protected asset  18  or protected region  20  based on at least one of stored data about the type of explosive device, location of the explosion, and the shape, relative orientation and relative motion of the protected asset, and make a determination to counter the explosion  22 , including determining when and where to activate the arc generator  16 , or not counter the explosion, based on one of stored data, models of vulnerability of the protected asset to shockwaves, and data from at least one of data and models of performance of the arc generator  16  with respect to attenuating shockwaves from at least one of an estimated explosion magnitude and location. Such calculations and estimates may prevent the deployment of the arc generator  16  in the event that the explosion  22  is too far away or too weak to generate a shockwave  24  that damages the protected region  20  significantly. 
     In another embodiment, the sensor  12  may be configured to detect an incoming threat  28  containing an explosive device or devices, such as a ballistic shell, bomb, torpedo, depth charge, naval mine or bomb-laden surface vessel. In such an embodiment, the sensor  12  may be configured to use radar, visible or infrared light, passive or active acoustic sensors, or other threat-detection method known to those skilled in the art, as well as trajectory tracking and prediction methods known to those skilled in the art. 
     In yet another embodiment, the sensor  12  may be configured to detect both the incoming threat  28  and the explosion  22  from the threat. In one embodiment, two systems  10  may be deployed on a protected asset  18  in which one system is configured to detect an incoming threat  28  and the other system is configured to detect an explosion  22 . In yet another variation of such an embodiment, the sensor  12  may be in the form of two discrete sensors: one configured to detect an incoming threat  28 , and the other configured to detect an explosion  22  from that incoming threat. 
     In some embodiments, the computer  14  may receive measurements from the sensor  12 , estimate where and when an incoming threat  28  will detonate, or has already detonated, and directs the system  10  to deploy. When used with a sensor  12  that may detect an explosion that already has occurred, the computer  14  may be configured to receive information from the sensor pertaining to one or more of the direction, location, time, distance and magnitude of the explosion  22 , the computer determines when and where to activate the arc generator  16 . 
     In an embodiment wherein the sensor  12  is configured to detect an incoming threat  28  before explosion  22  has occurred, the computer  14  may be configured to compare the signature of the incoming threat  28  with stored known signatures of various threats (e.g., particular missiles). The computer  14  then estimates how probable each type of threat is, and, based on the stored data about the type of warhead for each threat, the measured motion of the incoming object (with associated uncertainty), and the shape, trajectory, orientation, speed and motion of the protected asset  18  in the protected region  20 , the computer estimates a probability distribution function (p.d.f.) of explosion magnitudes and locations relative to the protected asset. 
     Based on data or models of vulnerability of the protected asset  18  to shockwaves  24  of various magnitudes from various directions (including crew injuries likely to result from shockwaves) stored in a database (either locally or available over a network), together with data or models of what the arc generator  16  can do to attenuate shockwaves in what positions and in what time interval, the computer  14  may then form a firing plan to counter the threat at minimum cost. Cost may include not only the cost to operate the arc generator  16 , but also the probable cost of damage from the attenuated shockwave. In cases where the probable explosion yield is small and the probable distance of the explosion  22  from the protected region is large, the lowest cost plan may be not to deploy the system  10 . 
     As shown in the embodiments of  FIGS. 1 and 2 , the arc generator  16  may be configured to create a transient second medium  30  between the advancing shockwave  24  resulting from the explosion  22  and the protected region  20 , which may contain a protected asset  18 . As shown in  FIG. 2 , generally speaking, the sensor  12  may generate a detection signal in response to detecting at least one of the explosion  22  or explosive device such as incoming threat  28 . The detection signal is received by arc generator  16 , and in response, may create an electromagnetic arc through the first medium  26  which rapidly heats the medium to change one or more of its composition and temperature to create the second, transient medium  30 , different from the first medium  26 . The location of the second medium  30  may be in a selected region calculated by the computer  14  of the sensor  12  to be interposed between the shockwave  24 , or predicted shockwave path, and the protected region  20 . Thus, the shockwave  24  contacts the second, transient medium  30  and is attenuated in energy density before it reaches the protected region  20 . The arc generator  16  shown schematically in  FIG. 1  may take the form of multiple, discrete arc generators, each connected to and controlled by the sensor  12 . 
     In one embodiment, shown schematically in  FIG. 1 , the arc generator  16  is configured to direct a beam  34  of electromagnetic energy to focus in a selected region where the second medium  30  may be created. The beam  34  may be in the form of a relatively powerful microwave beam or laser beam. The electric field of the intense beams  34  may cause dielectric breakdown of the ambient air at the focal point of the focused beams, in which the electrons are separated from the molecules in the air. The free electrons are accelerated by the electric field and strike other molecules to knock other electrons loose, creating a cascade of electrons and ions. 
     As the density of free charge carriers in the air increases, the air becomes opaque to the incoming electromagnetic beams and rapidly absorbs energy from the beams as heat, which raises the temperature of the air as well as its density and composition, thus creating the second medium  30  to intercept the advancing shockwave  24 . The composition changes of the second fluid medium  30  may include adding free electrons, which have a relatively low molar mass, ionization of molecules so that they interact more strongly and therefore propagate shockwaves at higher speeds, and breaking diatomic molecules such as molecular oxygen into single atoms, which reduces the average molar mass. 
     In some embodiments, the beam  34  may be a microwave beam. The arc generator  16  may include a vacuum tube amplifier (e.g., a magnetron) and focused by a static focusing device (e.g., a dish antenna or a Fresnel plate), or produced and focused by other means of combinations apparent to those skilled in the art. In other embodiments, the beam  34  is a laser beam. The laser beam may be a single beam focused to a point by optics, or multiple beams converging to a common point. In either case, the beam  34  may create single or multiple arcs  32  ( FIG. 2 ). In some embodiments, the arc generator  16  may use many converging beams  34  to create one arc  32 . In other embodiments one beam  34  may be used multiple times in quick succession to create many arcs  32 . The beams may be fixed or steerable. 
     As shown in  FIG. 3 , in another embodiment of the system  10 A the arc generator  16  ( FIG. 1 ) may be configured to generate an electric arc  36  through a volume of ambient air  26  to create the second medium  30 . Electrical resistance to the current heats the air, changing its temperature, density and composition to create the second medium  30  to attenuate the shockwave  24 . The change in composition may include adding free electrons, which have very low molar mass, ionization of molecules so they interact at longer distance and therefore propagate shockwaves at higher speed, and breaking diatomic molecules like oxygen into single atoms, which reduces the average molar mass of the fluid in the second medium  30 . 
     The arc generator  16  ( FIG. 1 ) of the system  10 A of this embodiment may include a power supply  38  configured to initiate a large electric current quickly—typically on the order of a few milliseconds or less. Such a power supply  38  may include a capacitor, a superconducting storage coil, and an explosive flux compression generator. The first two examples may require fast-acting, high-current switchgear, such as a gas-insulated switch, to turn on a large current quickly (for the capacitor) or to divert it quickly from a shunt to the electric arc (for the storage coil). 
     In one aspect of this embodiment, the arc generator  16  creates an electrically conductive path from the power supply  38  to the selected region between the shockwave  24  and the protected region  20  to establish the arc, and back to the power supply. In one embodiment, shown in  FIG. 3 , the arc generator  16  creates a laser-induced plasma channel (LIPC). The arc generator  16  fires two high-intensity laser beams  40 ,  42  along converging paths. The laser wavelengths and intensities are selected to ionize the air along their paths, thereby forming the plasma channels. The power supply  38  applies voltage across the terminals at the bases of the two channels  40 ,  42 , and the voltage is sufficient to form the arc  36  through the air  26  where the beams are close to each other. Current flows through the channels of the LIPC and induces a magnetic field. The current interacts with the magnetic field and produces an outward force (J×B). This force acts to widen the area enclosed by the current loop. 
     As shown in  FIG. 4 , in another embodiment of the system  10 B the arc generator  16  ( FIG. 1 ) is a gun, which may have at least two or multiple barrels  44 ,  46  that fire two pellets  48 ,  50  along converging paths. The pellets  48 ,  50  are configured to create trails  52 ,  54  of ions, for example, by burning silver iodide, in the manner of tracer bullets. The two trails  52 ,  54  form conductive channels through the air (the first fluid medium), and the power supply  38  applies voltage across terminals at the bases of the two conductive channels, preferably the barrels  44 ,  46 . The voltage is sufficient to form an arc  56  through the air  26  between the two trails  52 ,  54  (i.e., the conductive channels) where they are at their closest. The arc  56  heats the air  26  to create the second transient fluid medium  30 . In other embodiments, the pellets  48 ,  50  may be selected to melt or burn up completely, thereby avoiding creating a hazard for by standers. 
     As shown in  FIG. 5 , in another embodiment of the system  10 C the arc generator  16  ( FIG. 1 ) is a projectile launcher configured to launch projectiles  58 ,  60  trailing conductive wires  62 ,  64  much like a Taser®. The power supply  38  applies voltage across the two wires sufficient to form an arc  66  through the air  26  where the paths of the projectiles  58 ,  60  are closest to each other. The arc  66  heats the air  26 , forming the second fluid medium  30  in a selected region between the advancing shockwave  24  and the protected asset  18  in the protected region  20 . As electrical current flows through the wires  62 ,  64 , which form conductive channels, the current further heats and ionizes the air  26 . In some embodiments, the current is sufficient to vaporize the wires  62 ,  64 , thereby heating additional nearby air  26 . 
     As shown in  FIGS. 6 and 7 , in another embodiment the system  10 D includes a arc generator  16  ( FIG. 1 ) in the form of one or more sacrificial conductors  68  connected to the power supply  38 . The heating mechanism of this embodiment does not rely on dielectric breakdown of the air  26  to create a current channel. The sacrificial conductor  68  may be in the form of strips or wires made of electrically conductive material. In some embodiments, the conductor  68  may be mounted on the protected asset  18 . 
     To deploy the system  10 D, the arc generator  16  actuates the power supply  38  to supply current to one of more of the conductors  68 , first vaporizing it, then ionizing it to form a conductive channel of vapor  70 . Due to both the resultant magnetic field (J×B) force and the expansion of the hot vapor created by vaporizing the conductor, the ionized vapor moves outward from its initial position and heats and ionizes the nearby air to create the second fluid medium  30 . 
     In one embodiment, shown in  FIG. 7 , the sacrificial conductor  68  may be attached to a protected asset  18 , which may be the land vehicle shown. The conductor  68  may be attached to a wider insulating strip  72  that in turn is mounted on the asset  18 . With such a placement of the conductor  68 , the conductor  68  may be protected from routine vehicle operations, and may be further protected by overlaying it with a coat of paint. When energized by the power supply  38  ( FIG. 6 ), the conductor  68 , which may be a metallic strip, vaporizes, and the heated and ionized air may expand from the vehicle surface and move away from the vehicle to create the second medium  30  in the selected region. 
     In yet another embodiment, the sacrificial conductor  68  may be made partly or entirely of lithium. Lithium has a very low molecular weight, and consequently a higher shock speed and lower effective index of refraction than other metallic vapors. Further, it disperses into nearby air  26  more quickly, which helps to heat the air more rapidly. 
     In other embodiments, the arc generator  16  may be configured to attenuate the advancing shockwave by magnetic induction. As shown in  FIG. 8A , the arc generator  16  may be powered by the power supply  38  to create an electric arc  72  that forms a conducting channel between two electrodes  74 ,  76  that are not rigid, such as thin rod antennae. The conducting channel  72  interacts with its own magnetic field to produce an outward force F=J×B (where F is the outward force, J is the current density and B is the magnetic field flux). 
     When the shockwave  24  created by an explosion  22  reaches the arc  72 , as shown in  FIG. 8B , the shockwave deforms the arc in directions perpendicular to the conductive channels. Specifically, the shockwave  24  pushes the ionized air created by or making up the arc  72  inward, and the current tends to flow where the air  26  is ionized. This movement by the shockwave  24  may do mechanical work against the magnetic field force F and remove energy from the shockwave, making it weaker. Although not shown, the movement by the shockwave  24  also may deform the electrodes  74 ,  76 , which also represents physical work that drains energy from the shockwave  24 . 
     In the embodiment shown in  FIGS. 8A and 8B , because the electrodes  74 ,  76  may be relatively non-rigid, these parts and the conductive channel  72  push against the air  26  they are displacing. This may transform energy from the shockwave  25  into kinetic energy of displaced air. In some cases it may be possible for the conductive channels  72  and electrodes  74 ,  76  to push the air so fast that they may create new shockwaves moving away from the protected region  20 . 
     In a variation, the parts of the conductive channels  72  and electrodes  74 ,  76  may not be rigid, but the channels of ionized air pushed ahead of the magnetic flux are disrupted (broken) when they flow rapidly through the air  26 . Each time a channel  72  breaks, the electric current briefly stops. When it stops, the trapped magnetic flux creates an electromotive force strong enough to ionize a new channel through the air  26  and then heat that air. This may transform energy from the shockwave  24  into ionization energy of air ions and into pressure-volume-temperature energy of expanded hot air. 
     In the embodiment shown in  FIGS. 9A and 9B , the arc generator  16  may be configured to have rigid electrodes  78 ,  80  joined by a rear wall  82  to form three sides of a box. An arc  84  may be formed by the power supply  38  of the arc generator  16  between electrodes  78 ,  80 . When the arc  84  is contacted by the shockwave  24 , the magnetic flux (represented by the crossed circles  86 ) is deflected inward but movement is constrained by the electrodes  78 ,  80  and wall  82 . Instead, the flux  86  is compressed and increases the electric current in the electrodes  78 ,  80 . In one embodiment, the power supply  16  is configured so that this additional electric current bypasses the power supply and instead is directed to a resistive load  88 . Resistive load  88  also may take the form of a capacitor, which may receive a charge from the additional electric current. Thus, with the embodiments of  FIGS. 8 and 9 , magnetic induction is used to remove energy from the advancing shockwave by the additional mechanism of converting the shockwave energy to mechanical energy or converting the shockwave energy to current. 
     A system  10  may be comprised of multiple copies of each embodiment. In an embodiment, a system  10  may include a single sensor  12  connected to and controlling multiple, discrete arc generators  16 , each mounted on the protected asset  18 . In some applications, an embodiment may produce a relatively narrow, substantially linear arc of hot, ionized air. Multiple copies of each embodiment may be used to increase the frontal area as desired. Alternatively, a single copy of an embodiment may be used multiple times in rapid succession, producing multiple arcs that collectively cover the desired protected region  20 . Further, these embodiments may be combined. For example, a system  10  may utilize ionizing tracer pellets to protect an asset  18  from explosions  22  relatively far away, and employ sacrificial conductors to protect the same asset from explosions at short range. 
     With each of the embodiments discussed, the system  10  is deployed to attenuate the energy of an advancing shockwave  24  form an explosion  22  by creating a second fluid medium  30  that differs from the first fluid medium  26 , which may be ambient air, positioned so that it interacts with the shockwave. As shown in  FIG. 10 , as the shockwave contacts the interface  90  between the first fluid medium  26  and the second fluid medium  30 , the difference in refractive index reflects a fraction of the incoming energy toward the explosion  22 , as indicated by arrows A. This partial reflection occurs a second time as the shockwave passes through the second fluid medium  30  and contacts the interface  92  between the second medium and the ambient  26  as it exits the second medium. All gradients or discontinuities in the medium provide a reflection point for the incoming shockwave  24 . For example, if the second medium  30  is non-uniform, reflection will occur at each of many places within the medium. 
     As shown in  FIG. 11 , shockwaves  24  obey Fermat&#39;s theory of least time and therefore an effective refractive index for the shockwave can be defined that is inversely proportional to the shock speed. The properties or composition of the second medium  30  are chosen such that the effective refractive index of the second medium  30  differs from the first medium  26  in at least one of temperature, molecular weight and composition. As the shockwave passes into or out of the second medium  30 , the difference in effective refractive index refracts the wave, as shown by lines B, diverting it and defocusing it away from the protected asset  18 . In the disclosed embodiments, the second medium  30  is created such that the shockwave travels faster in the second medium  30  than in the first medium  26 , so the refractive index of the second medium is less than that of the first medium. Further, the second medium is created to have a convex shape and therefore acts as a divergent lens, so that the energy of the shockwave  24  spreads out, as shown by lines C, so its intensity drops as it approaches the protected asset  18 . 
     In addition, the second medium  30  may absorb some shock energy as the shock travels through it. Factors contributing to the absorption of energy include energy retained in the molecules of the second medium itself (e.g., enhanced rotational energy, excited molecular bonds, excited electrons, molecular decomposition, and ionization) and shock energy converted to electromagnetic energy through blackbody emission from hot particles or photon emission from de-exciting various excited states. 
     A further mechanism for attenuating the energy density of the shockwave  24  is momentum exchange. If the second medium  30  is moving relative to the first medium  26 , then it will exchange momentum with the shockwave  24 . The result is a combination of reflection, slowing, and redirection of the shockwave. Any or all of the foregoing mechanisms may operate in a given embodiment. The composition, temperature, speed and location of the second medium  30  may be chosen or created to create any one or all of the aforementioned mechanisms. 
     While the method and forms of apparatus disclosed herein constitute preferred aspects of the disclosed shockwave attenuation apparatus and method, other methods and forms of apparatus may be employed without departing from the scope of the invention.