Abstract:
A method for inducing nuclear fusion and a reactor for inducing nuclear fusion involve positioning a bubble containing fusionable nuclei at the center of a liquid filled spherical vessel and generating a spherically symmetric positive acoustic pulse in the liquid. The acoustic pulse surrounds and converges toward the center of the vessel to compress the bubble, thereby providing energy to and inducing nuclear fusion of the atomic nuclei.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims priority from U.S. patent application Ser. No. 60/363,401 filed 12 Mar. 2002. 
     
    
     TECHNICAL FIELD  
       [0002]     This invention relates to nuclear fusion. Specific embodiments of the invention relate to nuclear fusion reactors and methods for generating energy by promoting nuclear fusion.  
       BACKGROUND  
       [0003]     Nuclear fusion reactions between atomic nuclei can produce large amounts of energy. Fusion reactions involve bringing together atomic nuclei against their mutual electrostatic repulsion and fusing pairs of nuclei together to make heavier nuclei. Energy is released in this process. Isotopes of light elements (i.e. elements having a relatively small number of protons) are the easiest to fuse, because the electrostatic repulsion between the nuclei of light elements is smaller than that of heavier elements.  
         [0004]     Fusion reactions involving the nuclei of such light elements could be used to produce energy with significantly reduced radioactivity than comparable fission reactions. The easiest fusion reactions to produce include: 
 
 D+D=&gt;   3 He+ n+ 3.6 MeV;   (i) 
 
 D+T=&gt;   4 He+ n+ 17.6 MeV; and   (ii) 
 
 D+   3 He=&gt; 4 He+H+18.3 MeV   (iii) 
 
 where n is a neutron, H is a hydrogen atom having a single proton, D is deuterium (i.e. a hydrogen isotope with 1 proton and 1 neutron), T is tritium (i.e. a hydrogen isotope with 1 proton and 2 neutrons), and  x He is a helium ion having x neutrons and protons. The number of MeV is the energy released by the fusion reaction. 
 
         [0005]     Inducing nuclear fusion reactions is difficult, because of the energies required to accelerate the nuclei to speeds fast enough to overcome their mutual electrostatic repulsion and because the nuclei are so small that the chance that two passing nuclei will interact with one another in a manner which results in fusion of the nuclei is small.  
         [0006]     The energy efficiency of a reactor is the ratio of the energy output to the energy input. In a fusion reactor, the energy output is largely determined by the number of fusion reactions that are induced in the reactor and the amount of capturable energy released. The energy input in a fusion reaction is largely determined by the amount of energy required to accelerate the nuclear reactants to thermonuclear speed and to confine the nuclear reactants in a space that allows them to interact.  
         [0007]     In order to make a fusion reactor commercially viable, the energy output must be sufficient to offset the financial cost of manufacturing, installing and operating the reactor. The inventor is unaware of any nuclear fusion method or nuclear fusion reactor which can successfully produce energy in a controlled, reproducible and commercially viable manner.  
         [0008]     One method for achieving controlled nuclear fusion involves heating a plasma of light nuclei to such a temperature that the thermal speed of the particles in the plasma is sufficient to produce fusion reactions between the nuclei. The plasma may be made of a deuterium-tritium mixture, for example. Containing the heated plasma while producing a sufficient number of fusion reactions to provide a commercially viable reactor has so far presented insurmountable difficulties.  
         [0009]     Various means have been proposed to contain the plasma. Such proposals include the use of intense magnetic fields in a variety of configurations. Specific drawbacks with magnetic containment systems include: limited magnetic field strength; energy loss from plasma instabilities, heat losses and particle drift across the magnetic fields; and difficulties related to pumping sufficient amounts of energy into the plasma.  
         [0010]     Inertial confinement techniques for implementing fusion reactors, involve quickly heating a solid pellet of fusionable material with one or more energy beams and letting the pellet explode, so that a sufficient number of fusion reactions occur before the temperature drops. Most inertial confinement experiments have been conducted using laser beams to heat the fusionable material, but ion beams and electron beams have also been proposed. A number of expensive apparatus have been developed to produce nuclear fusion reactions via inertial confinement techniques. The principal drawbacks of inertial confinement relate to costs and technological difficulties associated with generating the high-power beams used to heat the fusionable pellet. Although improvements have been made to inertial confinement devices, the inventor is currently unaware of any commercially viable inertial confinement power generation devices. No such devices have been able to produce power at a price competitive with other power sources.  
         [0011]     Another technique proposed for producing fusion reactions involves providing a gaseous bubble of fusionable material in a liquid and then collapsing the bubble by creating an acoustic wave (i.e. a pressure wave) in the liquid. It has been suggested that upon the collapse of a bubble, fusionable material within the bubble can be compressed and heated to thermonuclear conditions (i.e. the fusionable nuclei are accelerated to sufficient speeds such that fusion reactions may occur). In  Science,  March 2002, “Evidence for Nuclear Emissions During Acoustic Cavitation”, Taleyarkhan et al. have proposed a basic apparatus for inducing nuclear fusion by bubble compression and claim to have detected neutrons and tritium produced during the collapse of bubbles of fusionable gas. The teachings of Taleyarkhan et al. are hereby incorporated by reference.  
         [0012]     Flynn (U.S. Pat. No. 4,333,796), Putterman et al. (U.S. Pat. No. 5,659,173) and Pless (U.S. Pat. No. 5,968,323) have proposed techniques for collapsing gaseous bubbles of fusionable material using substantially sinusoidal acoustic waves produced by ultrasound generators.  
         [0013]     Sinusoidal acoustic waves have periods of relatively high pressure (i.e. during the peaks of the sinusoidal pressure waves) and periods of relatively low pressure (i.e. during the troughs of the sinusoidal pressure waves). A liquid in tension will cavitate. Because of cavitation, the pressure in a liquid can not be reduced significantly below zero, even during the troughs of an applied sinusoidal acoustic wave. For this reason, the peak pressure achievable using a sinusoidal acoustic wave is limited to approximately twice the static pressure of the liquid. The static pressure of the liquid is limited by the strength of the vessel used to contain the liquid.  
         [0014]     There exists a need for apparatus and methods for nuclear fusion reactors that ameliorate at least some of the aforementioned disadvantages of the prior art.  
       SUMMARY OF THE INVENTION  
       [0015]     In accordance with the invention, a method for inducing nuclear fusion is disclosed. The method comprises: positioning a bubble containing atomic nuclei at a location within a liquid filled vessel; generating a positive acoustic pulse in the liquid which surrounds and converges toward the bubble; and, allowing the acoustic pulse to compress the bubble to provide energy to the atomic nuclei and to thereby induce nuclear fusion therebetween.  
         [0016]     In another aspect of the invention a nuclear fusion reactor is disclosed. The reactor comprises a vessel filled with liquid and a plurality of pistons positioned outside of the vessel. The pistons are actuable to strike an outer surface of the vessel and to thereby generate a positive acoustic pulse in the liquid. A bubble containing atomic nuclei is positionable at a location within the vessel, such that the acoustic pulse surrounds and converges toward the bubble to compress the bubble. Compression of the bubble provides energy to the atomic nuclei and thereby induces nuclear fusion therebetween.  
         [0017]     Further features and applications of the invention are described below. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]     In drawings which depict non-limiting embodiments of the invention:  
         [0019]      FIG. 1  is cross-sectional view of a spherical nuclear fusion reactor according to a particular embodiment of the invention;  
         [0020]      FIG. 2  is a cross-sectional view of a spherical nuclear fusion reactor according to a second embodiment of the invention;  
         [0021]      FIG. 3  is a diagram showing the radial distribution of pressure in an acoustic pulse provided in accordance with the invention;  
         [0022]      FIG. 4  schematically depicts a piston control system used to control the movement of a piston in the  FIG. 2  reactor; and  
         [0023]      FIG. 5  schematically depicts a bubble tracking system and a bubble positioning system which may be used in the reactors of  FIGS. 1 and 2 . 
     
    
     DETAILED DESCRIPTION  
       [0024]     Throughout the following description, specific details are set forth in order to provide a more thorough understanding of the invention. However, the invention may be practiced without these particulars. In other instances, well known elements have not been shown or described in detail to avoid unnecessarily obscuring the invention. Accordingly, the specification and drawings are to be regarded in an illustrative, rather than a restrictive, sense.  
         [0025]     This invention provides methods and apparatus which implement a “bubble compression” fusion reactor. The invention involves creating a spherically symmetric positive acoustic pulse in a liquid which contains a spherical bubble of fusionable material. The acoustic pulse compresses and collapses the bubble to correspondingly increase the pressure and temperature of the fusionable material contained therein. At the surface of the bubble, the peak pressure of the acoustic pulse is significantly greater than that achievable using a sinusoidal or other continuously oscillating acoustic waveform. Consequently, the temperature and pressure imparted on the fusionable material is greater. This facilitates nuclear fusion reactions in the compressed fusionable material within the collapsing bubble.  
         [0026]      FIG. 1  depicts a nuclear fusion reactor  10 A in accordance with a particular embodiment of the invention. Reactor  10 A includes a spherical vessel  12  filled with a liquid  14 . Spherical vessel  12  may generally be of any size. In a particular preferred embodiment, the diameter of spherical vessel  12  is in the range of about 0.6 m to about 2 m and may be approximately 1 m.  
         [0027]     Liquid  14  may comprise any of a variety of substances that provide desirable characteristics, such as acoustic wave transfer speed, relatively low melting point, low vapour pressure at temperatures at or above the melting point of the liquid, and relatively high thermal conductivity and capacity. Liquid  14  may comprise a molten metal such as molten lithium or molten sodium, for example. As explained in greater detail below, lithium has the added advantage that it may react with neutrons produced in the fusion reaction to generate tritium which may be reused as a fusionable material. Further, lithium has a reasonably high cross-section for absorbing energy from neutrons and can serve as a shielding medium and also as a medium for converting energy of neutrons produced by fusion into heat.  
         [0028]     Liquid  14  may contain one or more additives. The additives may include: 
        Isotopes which can interact within a neutron released by a fusion reaction to yield two or more lower energy neutrons. A particular example of such an isotope is  11 B. Those skilled in the art will appreciate that other neutron multiplying isotopes may be used;     Isotopes which absorb neutrons. The addition of such isotopes provides better shielding for the walls of reactor  10 A; and,     Additives which increase a density of liquid  14  to provide a better match of acoustic impedance at the interface between liquid  14  and the wall of reactor  10 A.        
 
         [0032]     As shown in  FIG. 1 , reactor  10 A may include a pressure control system  16 . Pressure control system  16  functions to maintain liquid  14  at a suitable operating pressure. Pressure control system  16  may comprise any of a wide variety of pneumatic and/or hydraulic elements. In the illustrated embodiment, pressure control system  16  comprises a piston  18  driven by a solenoid  20 . Solenoid  20  is preferably controlled by controller  116 . Controller  116  may comprise one or more suitably programmed computers or other data processors. Controller  116  may also comprise one or more analog electronic circuits configured to perform the functions described herein. Controller  116  controls solenoid  20  to maintain a hydrostatic pressure within reactor  10 A at a desired level. Pressure control system  16  may also comprise a pressure sensor (not shown) which provides a pressure feedback signal to controller  116 . The pressure at which liquid  14  is maintained may vary between specific embodiments of the invention. When liquid  14  comprises lithium, a suitable pressure may be in the range of 70 bar to 200 bar and may be approximately 100 bar to 125 bar, for example.  
         [0033]     Within practical limits, maintaining liquid  14  at a higher pressure is desirable, since higher pressure reduces the sizes of bubbles of fusionable material and therefore both increases the initial density of the bubbles and makes the shape of the bubbles more nearly perfectly spherical.  
         [0034]     Reservoir  22  contains fusionable material  24 . Preferably, fusionable material  24  is maintained in gaseous form and comprises one or more isotopes of light elements, such as deuterium, tritium,  3 He or some combination thereof. It is expected that the best results can be achieved where fusionable material  24  comprises a mixture of deuterium and tritium. For example, fusionable material  24  may comprise a 50/50 mixture of deuterium and tritium.  
         [0035]     In operation, valve  26  is controlled to release gaseous bubbles  28  of fusionable material  24  into liquid  14  through aperture  15 . Preferably, valve  26  is a pulsed valve, but valve  26  may comprise any suitable valve(s) including any of a wide variety of commercially available valves. Valve  26  is preferably controlled by controller  116 . Preferably, bubble  28  is spherical in shape and is released by valve  26  at the center of the bottom of vessel  12 . When released into liquid  14  at the bottom of vessel  12 , the buoyancy of bubble  28  causes it to rise toward the center of vessel  12 .  
         [0036]     When a spherical bubble travels through a liquid, it may be distorted in shape by forces related to viscosity and/or fluid dynamics. Preferably, bubble  28  is relatively small when first released at the bottom of vessel  12  and bubble  28  remains small when it is travelling through liquid  14 . For example, the diameter of bubble  28  may be on the order of 100 μm. Because the size of bubble  28  is maintained relatively small during its movement through liquid  14 , the distortions introduced to the spherical shape of bubble  28  during its travel through liquid  14  may be reduced or minimized.  
         [0037]     In reactor  10 A of  FIG. 1 , pressure control system  16  rapidly reduces the pressure of liquid  14  at or near the time that bubble  28  reaches the center of vessel  12 . When the pressure of liquid  14  is reduced, the size of bubble  28  increases correspondingly. In the illustrated embodiment of the invention, pressure control system  16  rapidly reduces the pressure of liquid  14  by retracting piston  18 . The amount of pressure reduction and the amount of increase in size of bubble  28  may vary between embodiments of the invention. In one specific example embodiment, the pressure of liquid  14  is maintained at approximately 125 bar and bubble  28  is introduced into liquid  14  with a diameter of approximately 100 μm. When bubble  28  reaches the center of vessel  12 , the pressure is rapidly reduced to approximately 1 mbar, resulting in a corresponding increase in the diameter of bubble  28  to approximately 5 mm.  
         [0038]     At or near the time of the decrease in pressure of liquid  14  and the corresponding increase in size of bubble  28 , a high pressure, spherically symmetrical positive acoustic pulse  40  (see  FIG. 3 ) is introduced into liquid  14 . Acoustic pulse  40  may comprise a shock wave propagating in liquid  14 . As shown in  FIG. 3 , acoustic pulse  40  converges on bubble  28 .  
         [0039]     Preferably, acoustic pulse  40  is introduced soon after bubble  28  has increased in size (i.e. when bubble  28  is still located at or near the center of vessel  12 ). The timing of acoustic pulse  40  is important. To obtain nuclear fusion, it is necessary to maintain a sufficiently high degree of spherical symmetry of bubble  28  as it is collapsed. If buoyant bubble  28  continues to travel through liquid  14  for a significant time after it has increased in size, there may be distortions to the shape of bubble  28 . If such distortions occur before bubble  28  is collapsed by acoustic pulse  40 , then they could interfere with the required symmetry.  
         [0040]     In general, spherically symmetric acoustic pulse  40  may be introduced by a wide variety of apparatus. In reactor  10 A of  FIG. 1 , pulse generation system  30  comprises a pneumatic-mechanical system made up of valve  42 , compressor  44  and a plurality of air guns  32 , each of which actuates a corresponding piston  36 . Air guns  32  and pistons  36  are positioned spherically symmetrically about the outside of spherical vessel  12 . Air guns  32  are loaded with compressed air (or other gas) which may be introduced by valve  42  and pressurized by compressor  44 . In reactor  10 A, air guns  32  are controlled by their respective latches  34 . When it is desired to create spherically symmetric acoustic pulse  40 , latches  34  are triggered (preferably by controller  116 ), causing air guns  32  to accelerate pistons  36  to high speeds. Pistons  36  then strike outer surface  12 A of the wall of vessel  12 .  
         [0041]     The time at which each individual piston  36  strikes outer surface  12 A of vessel  12  and the speed (i.e. kinetic energy) with which each individual piston  36  strikes outer surface  12 A of vessel  12  should be as close as possible to the same (i.e. within the minimum possible tolerances). Such consistent kinetic energy and timing ensures that acoustic pulse  40  produced in liquid  14  is spherically symmetric. In reactor  10 A of  FIG. 1 , controller  116  controls the timing and the speed of pistons  36  by controlling the time at which latches  34  are triggered. In addition, the characteristics of air guns  32  may be fine tuned. Thus, after latches  34  are triggered, pistons  36  run “open loop” (i.e. without feedback).  
         [0042]     The impact speed (i.e. kinetic energy) with which pistons  36  strike outer surface  12 A of vessel  12  is preferably made to be as high as practical without causing unacceptable damage to pistons  36  or vessel  12 . When pistons  36  impact vessel  12  with a higher impact speed, resultant acoustic pulse  40  has a higher peak pressure which results in a greater potential fusion yield. If the impact speed of pistons  36  can be made to be sufficiently high, high-pressure acoustic pulse  40  can become a shock wave.  
         [0043]     When pistons  36  strike the outside of vessel  12 , spherically symmetric acoustic pulse  40  propagates through liquid  14 , and converges toward bubble  28 . Pistons  36  are preferably triggered at a time, such that when acoustic pulse  40  converges, it will converge at the point where bubble  28  will be located.  
         [0044]      FIG. 3  schematically depicts a pressure profile of the propagation of acoustic pulse  40  along the radial dimension of vessel  12  at-various different points in time (A through I). At time A, bubble  28  is close to the center of vessel  12  and pulse generation system  30  triggers latches  34  to release pistons  36 . In between times A and B, pistons  36  strike outer surface  12 A of vessel  12  and generate spherically symmetric acoustic pulse  40 .  
         [0045]     In general, the amplitude of a spherically converging waveform increases as l/R where R is the instantaneous radius of the wave. As shown in  FIG. 3  at times B, C and D, the amplitude of spherically converging, spherically symmetric acoustic pulse  40  grows as I/R. When spherically symmetric acoustic pulse  40  reaches bubble  28  (i.e at time E), its amplitude is given by:  
               P   peak     ∝       P   o     ⁡     (       R   v       R   b       )               (   1   )             
 
 where P o  is the initial pressure at the interior wall of vessel  12 , R b  is the radius of bubble  28  and R v  is the radius of vessel  12 . 
 
         [0046]     In practice, the peak pressure of acoustic pulse  40  is limited by the precision and symmetry of vessel  12 , bubble  28  and acoustic pulse  40 . In addition, the initial pressure P o  cannot be made arbitrarily high, because increasing the kinetic energy with which pistons  36  strike vessel  12  will cause damage to vessel  12 , pistons  36  or both. Typically, high strength steel can handle impact pressures on the order of 10 kbar without incurring substantial structural damage. By way of example, if the symmetry of vessel  12 , bubble  28  and acoustic pulse  40  are within tolerances of approximately 3%, then the ratio R v /R b  may be around 30. Assuming that P o  is approximately 10 kbar, then the peak pressure of acoustic pulse  40  at the surface of bubble  28  (i.e. time E of  FIG. 3 ) will be approximately P peak =300 kbar. If the symmetry of vessel  12 , bubble  28  and acoustic pulse  40  are within tolerances of approximately 1%, then the peak pressure of acoustic pulse  40  at the surface of bubble  28  will be approximately P peak =1 Mbar. These examples illustrate the importance of symmetry to reactors according to the invention.  
         [0047]     Acoustic pulse  40  produced by pulse generation system  30  of reactor  10 A is a positive waveform that is focused at the center of spherical vessel  12 . With good symmetry, the peak pressure of acoustic pulse  40  at the surface of bubble  28  may be 100 times (or more) greater than the pressure that could be withstood by the walls of vessel  12 . The peak pressure of acoustic pulse  40  represents a significant improvement over the peak pressure attainable using sinusoidal or otherwise oscillating continuous acoustic waves. As discussed above the maximum pressure that such waves can produce is limited to roughly twice the hydrostatic pressure. The hydrostatic pressure is limited to the pressure that can be withstood safely by the walls of vessel  12 .  
         [0048]     When acoustic pulse  40  converges on bubble  28 , the peak pressure of pulse  40  is sufficient to cause bubble  28  to undergo a violent collapse. When this collapse occurs, the fusionable material  24  contained in bubble  28  is adiabatically compressed and heated to pressures and temperatures high enough to induce thermonuclear fusion in fusionable material  24 .  
         [0049]     When fusion reactions occur, energy is released. This energy is captured by liquid  14  in the form of heat. The amount of heat absorbed by liquid  14  depends on the nature of the fusion reaction that occurs. Preferably, any neutrons produced in the fusion reactions are absorbed in liquid  14  and do not reach the walls of vessel  12 . This absorption of neutrons in liquid  14  may prevent neutron activation and degradation of vessel  12  and the other reactor components. The heat energy absorbed by liquid  14  may be extracted from liquid  14  using various energy conversion techniques that are well known in the art of energy production. Liquid  14  preferably comprises a material which acts as a moderator to slow neutrons having energies in the range of 14 MeV to thermal energies.  
         [0050]      FIG. 2  depicts a fusion reactor  10 B according to a second embodiment of the invention. Reactor  10 B of  FIG. 2  is similar to reactor  10 A of  FIG. 1 , in that it comprises a spherical vessel  12  filled with liquid  14 . However, reactor  10 B of  FIG. 2 , comprises a fluid flow circuit  50 . Fluid flow circuit  50  comprises an input port  52 , through which fluid  14  flows into vessel  12 , an output port  54 , through which fluid  14  flows out of vessel  12 , and a pump  56  for directing the flow of fluid  14 . Fluid flow circuit  50  preferably comprises a flow control valve (not shown) and is controlled by controller  116 .  
         [0051]     Reservoir  22 , which contains fusionable material  24  (preferably in gaseous form), is in communication with fluid flow circuit  50 . A controllable valve (not shown) may be provided at the junction between reservoir  22  and fluid flow circuit  50 . As with reactor  10 A of  FIG. 1 , fusionable material  24  preferably comprises one or more light element isotopes, such as deuterium, tritium,  3 He or some combination thereof.  
         [0052]     When desired, a bubble  28  of fusionable material  24  is released from reservoir  22  arid flows through input port  52  into vessel  12 . The flow of liquid  14  through vessel  12  from input port  52  to output port  54  carries bubbles  28  relatively rapidly to the center of vessel  12 . In some embodiments, the pressure of liquid  14  in reactor  10 B is not changed and there is no corresponding change in the size of bubble  28 .  
         [0053]     Where liquid  14  is flowing inside vessel  12 , bubbles  28  containing fusionable materials- 24  are preferably encapsulated in spherical capsules (i.e. micro-balloons). Such micro-balloons may be rigid to minimize any deformations to bubbles  28 . The walls of the micro-balloons may be fabricated, for example, from glass, plastic or other suitable materials. Micro-balloons of this type are used, for example, in inertial confinement experiments. The micro-balloons may be injected into the liquid  14  which enters vessel  12  at port  52 .  
         [0054]     At or near the time that bubble  28  reaches the center of vessel  12 , a spherically symmetric positive acoustic pulse  40  (see  FIG. 3 ) is introduced into liquid  14 . As with reactor  10 A, spherically symmetric acoustic pulse  40  may be introduced by a wide variety of apparatus. In the illustrated reactor  10 B of  FIG. 2 , acoustic pulse  40  is generated by pulse generation system  70  which includes impactors which strike outer surface  12 A of vessel  12 .  
         [0055]     Pulse generation system  70  contains the same basic components as the pulse generation system  30  of reactor  10 A. Such components include: valve  42 , compressor  44 , a plurality of air guns  32 , and a corresponding plurality of pistons  36  positioned spherically symmetrically about the outside of spherical vessel  12 . As described further below, the times at which air guns  32  are fired, the kinetic energies with which pistons  36  impact vessel  12  and times at which pistons  36  impact vessel  12  may be controlled to cause acoustic pulse  40  to converge at the location of bubble  28 , even if bubble  28  is not perfectly centered with vessel  12 .  
         [0056]     The pulse generation system  70  of reactor  10 B comprises a piston control system  71  associated with each of its pistons  36  (see  FIG. 4 ). As shown in  FIG. 4 , each piston control system  71  includes a position feedback mechanism  72  and a servo loop  74 . Servo loops  74  may be digital and may be connected to controller  116 .  
         [0057]     Each piston control system  71  uses its associated servo loop  74  and position feedback mechanism  72  to control the movement of its corresponding piston  36 . More specifically, piston control systems  71  control the speed (i.e. kinetic energy) with which each of pistons  36  strikes outer surface  12 A of vessel  12  and the time at which each of pistons  36  strikes outer surface  12 A of vessel  12  to determine various characteristics of acoustic pulse  40 . Piston control systems  71  may control the amplitude of acoustic pulse  40  and/or the location at which acoustic pulse  40  will converge. In preferred embodiments, acoustic pulse  40  is as large as possible without causing excessive damage to pistons  36  or vessel  12  and will converge on a bubble  28  that is located at or near the center of vessel  12 .  
         [0058]     In the embodiment of  FIG. 4 , each position feedback mechanism  72  comprises an optical fiber interferometer  76 , which measures the position of its associated piston  36 . Preferably, the position measurement accuracy of optical fiber interferometer  76  is greater than 1 μm. Other types of position sensors, such as acoustic, optical and/or capacitive position sensors could be used to implement position feedback mechanism  72 .  
         [0059]     In the illustrated embodiment of  FIG. 4 , each piston  36  comprises a permanent magnet  78  and each associated air gun  32  is surrounded by a coil  80 . Coil  80  is schematically depicted in  FIG. 4  as having a plurality of loops  80 A- 80 G. Those skilled in the art will appreciate that coil  80  may have a different number of loops. Each one of a plurality of fast transistors  82 A- 82 G is associated with a corresponding one of loops  80 A- 80 G. The fast transistor  82 A- 82 G associated with each loop  80 A- 80 G is configured to limit and thereby control the amount of current that is allowed to pass through its corresponding loop  80 A- 80 G. The motion of piston  36  causes magnet  78  to pass through coil  80  and to induce current flow in each loop  80 A- 80 G. Current flowing in loops  80 A- 80 G of coil  80  generates magnetic fields which interact with the magnetic fields of magnet  78 . The energy necessary to provide current in each loop  80 A- 80 G comes from the kinetic energy of piston  36 . Accordingly, fast transistors  82 A- 82 G may be controlled by servo loop  74  to rapidly adjust the current which may be induced in loops  80 A- 80 G and to correspondingly control the movement of piston  36 .  
         [0060]     The servo loop  74  of each piston control system  71  controls the movement of its corresponding piston  36  based, at least in part, on position information obtained from its associated position feedback mechanism  72  (i.e. optical fiber interferometer  76 ). For example, if it was desired to have an acoustic pulse  40  (see  FIG. 3 ) converge precisely at the center of vessel  12  (i.e. if bubble  28  was going to be located at the center of vessel  12  when acoustic pulse  40  converges), then each servo loop  74  would control the movement of its corresponding piston  36 , such that each of the plurality of spherically symmetric pistons  36  strikes outer surface  12 A of vessel  12  with precisely the same speed (i.e. kinetic energy) at precisely the same time.  
         [0061]     When pistons  36  strike outer surface  12 A of vessel  12 , they generate spherically symmetric acoustic pulse  40 . The propagation of acoustic pulse  40  in reactor  10 B is substantially similar to that in reactor  10 A and is shown schematically in  FIG. 3 . Because pulse generation piston  36  strikes vessel  12  may be controlled to adjust the location at which acoustic pulse  40  will converge. For example, if bubble tracking system  110  predicts that bubble  28  was going to be located some distance away from the center of vessel  12  at the time that acoustic pulse  40  would converge, then piston control systems  71  may control the movement of their corresponding pistons  36 , such that pistons  36  located at different positions around vessel  12  strike outer surface  12 A of vessel  12  at different times. In this manner, the location at which pulse  40  converges may be moved away from the center of vessel  12  toward a predicted location of bubble  28 .  
         [0062]     Reactor  10 B may also comprise a bubble positioning system  118  (see  FIG. 5 ) which controls the position of bubble  28  and directs bubble  28  toward the center of vessel  12 . Bubble positioning system  118  may comprise a first pair of jets  120 A,  120 B located on opposing sides of vessel  12  along a first axis  122  and a second pair of jets  124 A,  124 B located on opposing sides of vessel  12  along a second spaced apart axis  126 . Each of first axis  122  and second axis  126  are preferably mutually orthogonal to one another and to the axis defined by input port  52  and output port  54 . For the sake of clarity in the illustration of  FIG. 5 , second axis  126 , which is orthogonal to the page, and jet  124 B are not explicitly shown.  
         [0063]     Jets  120 ,  124  may cause flow of liquid  14  inwardly from the walls of vessel  12  along their respective axes  122 ,  126 . Individual jets  120 ,  124  may be controlled by controller  116 . Preferably, controller  116  is connected to receive measured position information relating to bubble  28  from bubble tracking system  110  and, based at least in part on the measured position of bubble  28 , to determine a flow required from each of jets  120 ,  124  to move bubble  28  toward the center of vessel  12 . As shown in  FIG. 5 , controller  116  may also provide the actuation signals for jets  120 ,  124 .  
         [0064]     When acoustic pulse  40  converges on bubble  28 , the peak pressure of pulse  40  is sufficient to cause bubble  28  to undergo a violent collapse. When this collapse occurs, the fusionable material  24  contained in bubble  28  is adiabatically compressed and heated to pressures and temperatures sufficient to cause fusionable material  24  to undergo a thermonuclear fusion reaction.  
         [0065]     When fusion reactions occur, energy is released. This energy is captured by liquid  14 . This heats liquid  14 . The amount of heat absorbed by liquid  14  depends on the nature of the fusion reaction that occurs. In the embodiment of  FIG. 2 , reactor  10 B comprises an optional heat exchanger  90  which extracts heat energy from liquid  14 . Any suitable heat exchanger may be used. Heat exchangers are well known in the art of energy production. Pump  56  of fluid flow circuit  50  causes liquid  14  to flow out of vessel  12  through output port  54 , which leads to conduit  96  in heat exchanger  90 . As heated liquid  14  flows through conduit  96 , the heat from liquid  14  boils water  94 , turning water  94  into pressurized steam  92 . In turn, pressurized steam  92  turns turbine  98  and electric alternator  100 , converting the heat energy into electrical energy. After conversion of the heat energy into electrical energy, condenser  102  completes the thermal cycle of steam  92  back into water  94 . In the illustrated embodiment, turbine  98  also drives compressor  44  of pulse generation system  70 .  
         [0066]     Heat exchanger  90  is not required in all embodiments of the invention. In some embodiments, heat energy produced in liquid  14  may be used directly. Other forms of heat energy conversion equipment may be used and may convert the heat to electrical energy or into other forms of energy.  
         [0067]     In operation, bubbles  28  (which may be encased in micro-balloons) are released into liquid  14  contained in vessel  12 . Bubbles  28  may carried toward the center of vessel  12  by the flow created by fluid flow circuit  50 . During their travel towards the center of vessel  12 , the position of bubbles  28  may be tracked by bubble tracking system  110  and may be adjusted by bubble positioning system  118 . Pulse generation system  70  creates a positive spherically symmetric acoustic pulse  40  by striking outer surface  12 A of vessel  12  at a plurality of spherically symmetric locations. Pulse generation system  70  may create pulse  40  using a plurality of pistons  36  actuated by air guns  32 . The kinetic energy and timing of each piston  36  may be controlled by a piston control system  71  which may use information from the bubble tracking system to control characteristics of pulse  40 , such as the location of convergence of pulse  40 . Acoustic pulse  40  converges on bubble  28 , compressing and heating the fusionable material contained therein to thermonuclear pressures and temperatures sufficient to promote fusion reactions. The fusion reactions release heat energy which is captured in liquid  14  and which may be converted to electrical energy (or other forms of energy) by heat exchanger  90 .  
         [0068]     As will be apparent to those skilled in the art in the light of the foregoing disclosure, many alterations and modifications are possible in the practice of this invention without departing from the spirit or scope thereof. For example: 
        Those skilled in the art will appreciate that elements of reactor  10 A ( FIG. 1 ) may be combined with elements of reactor  10 B ( FIG. 2 ). For example, piston control systems  71  may be incorporated into pulse generation system  30  of reactor  10 A to control and minimize any differences in the speed and timing of pistons  36 . In other examples, fluid flow system  50  and or the heat exchanger  90  of reactor  10 B may be introduced into reactor  10 A.     The spherical vessels  12  of reactors incorporating the concepts of the invention may generally be of any size. As discussed above, the currently preferred vessel  12  is approximately 1 m in diameter. If vessel  12  is made larger, then the peak pressure of acoustic pulse  40  may be higher (see equation 1). However, if vessel  12  is made larger, then it may be more difficult to control the symmetry of the vessel itself, it may be more difficult to control the timing, velocity and symmetry of the associated pulse generation systems. These trade-offs related to the size of vessel  12  are a matter of engineering and the size of vessel  12  may be selected to achieve a desired result.     Reactors incorporating the concepts of the invention and, in particular pulse generation systems  30 ,  70 , may be designed to recapture some of the energy expended to create acoustic pulse  40 . After bubble  28  collapses (see  FIG. 3E ), waveform  40  continues to propagate and diverges radially towards the inner wall of vessel  12  as shown in  FIGS. 3F, 3G  and  3 H. When diverging waveform  40  reaches the inner surface of the wall of vessel  12 , it may accelerate pistons  36  radially outwardly and thereby recompress the air (or other gas) in air guns  32 . This recompression of air guns  32  may help to improve the efficiency of reactors incorporating the invention by minimizing the energy expenditure to produce subsequent acoustic pulses  40 . Typically, some energy will be lost as acoustic pulse  40  propagates through vessel  12 . As such, an acoustic pulse  40  will not supply all of the energy required to reset air guns  32  to supply another acoustic pulse  40 . The air (or other gas) in air guns  32  may be replenished to the desired level by compressor  44  and valve  42  which may be controlled by controller  116 .     The pulse generation systems  30 ,  70  described above incorporate a plurality of impactors that are spherically symmetrically distributed about the outside of vessel  12 . In the illustrated embodiments, the impactors comprise pistons  36 . The exact number of pistons  36  may vary. Pulse generation systems  30 ,  70  may comprise over  50  pistons  36  and preferably comprises over  100  pistons  36 . As described above, each piston  36  also has a corresponding air gun  32 . The mass and speed of pistons  36  depends on the number of pistons and the total amount of kinetic energy required from the pulse generation system  30 ,  70 . For example, a pulse generation system comprising  100  pistons, each of which weighs 0.5 kg and travels at 200 m/s when impacting vessel  12  can provide a total of 1 MJ of kinetic energy.     Pulse generation systems  30 ,  70  may be implemented by a variety of other techniques which generate spherically symmetric high-pressure acoustic pulses. For example, pulse generation systems  30 ,  70  may be implemented using electrical components, such as electrical actuators, electrical energy storage and/or electrical switches. Similarly, pulse generation systems  30 ,  70  may be implemented using chemical energy components, such as components based on exploding energetic compounds.     The rate at which energy is produced in fusion reactors incorporating the concepts of the invention depends on the period of time required to move successive bubbles  28  to the center of vessel  12  and to apply successive acoustic pulses  40  to collapse bubbles  28 . This period may vary significantly, depending on the buoyancy of bubble  28  (reactor  10 A), the flow rate of fluid flow circuit  50  (reactor  10 B), and the speed of acoustic waves in liquid  14 . It is expected that reactors incorporating the concepts of the invention may be designed to be pulsed at rates of over 2 Hz and, preferably, at rates of 4 Hz or higher.     The rate of fluid flow through circuit  50  of reactor  10 B depends on a number of factors, including the rate of heat energy production in reactor  10 B and the upper limit of the speed at which bubble  28  may be transported through liquid  14  without distorting the spherical shape of bubble  28 . A stable flow pattern is preferably maintained. In a preferred embodiment, a toroidal flow pattern is maintained, wherein liquid  14  rises along a central axis of reactor  10 B and travels downwardly around the circumference of reactor  10 B. Under such a toroidal flow pattern, the currently preferred flow rate of fluid flow circuit  50  is approximately 0.25 m 3 /s for a vessel  12  having a 1 m diameter and a pulse rate of 4 Hz. As more heat energy is produced, the flow rate of circuit  50  may be increased to extract the heat energy from liquid  14  in heat exchanger  90 . However, the flow rate of circuit  50  may not be increased indefinitely, because higher flow rates tend to distort the shape of bubbles  28 , reducing the fusion yield.     When the fusion reaction produces a fast neutron and liquid  14  is lithium, then there is a possibility that the liquid lithium will react with the fast neutron to produce tritium ( 6 Li+n=&gt;T+ 4 He 4.6 MeV). This tritium may be extracted and re-used by the system as fuel (i.e. fusionable material  24 ).     As shown in  FIG. 2 , compressor  44  may use some of the energy produced by reactor  10 B to replace the air (or other gas) in the air guns  32  of pulse generation system  70 . Compressor  44  may be driven by turbine  98 . In alternative-embodiments, the pressurized steam  92  produced in heat exchanger  90  may be used directly to power air guns  32  of pulse generation system  70 .     Bubble tracking system  110  may comprise a larger number of position detectors to improve the accuracy of the measured position of bubble  28 . Bubble tracking system  110  of  FIG. 5  may include more than three ultrasonic position detectors  112 .     Bubble positioning system  118  may comprise more than two pairs of jets  120 ,  124 . Those skilled in the art will appreciate that additional jets (not shown) may be used to improve the accuracy with which bubble positioning system  118  may position bubble  28 .        
 
         [0080]     Accordingly, the scope of the invention is to be construed in accordance with the substance defined by the following claims.