Patent Number: 042723191
Section: summary

BACKGROUND OF THE INVENTION Field of the Invention The present invention pertains generally to dense plasma heating and more particularly to plasma heating by way of a relativistic electron beam. Plasma heating has, for some time, been of great interest to the scientific community, since heated plasmas can be utilized for a wide variety of functions. A typical use of a hot plasma is the generation of energy in the form of a radiation, neutrons, and alpha particles. Such an energy source can be useful in basic high energy density plasma physics research, with practical application in scientific areas such as controlled thermonuclear fusion, materials studies, and radiography. DESCRIPTION OF THE BACKGROUND OF THE INVENTION Numerous techniques have been proposed in the prior art to produce dense, kilovolt plasmas. One of the more well-known techniques is the compression and heating of the core of a structured pellet by a laser or low voltage electron beam. It has also been suggested that light or heavy ion beams could be utilized to obtain similar compression and heating. Accordingly, the structured pellet and its driving source are directly coupled through classical interactions by heating the outer layer of the structured pellet. Depending upon the characteristics of both the structured pellet and driving source, the outer layer explodes or ablates, leading to compression and heating of the core. Due to the direct coupling of all of these driving sources, preheat of the core has been found to reduce the effectiveness of the compression, thereby, reducing both density and temperature of the pellet core. The use of a laser as a driving source in the above described confinement system has the added inherent disadvantages of low efficiency and associated high development cost in producing lasers with the required power output for a directly driven structured pellet. Also, diffraction limitations and window damage thresholds make it difficult to focus proposed large lasers to millimeter diameters. Low impedance electron and light ion beams also face expensive technological advancement in order to focus these beams to millimeter diameters, and to obtain power levels necessary to achieve the desired compression of the structured pellet. Such sources have the additional disadvantage of limitations in the manner of propagation of the beam to the pellet. Heavy ion sources also require significant technological advancement to produce the desired compression of the structured pellet. In fact, development of heavy ion sources using conventional accelerator concepts appears to be considerably more expensive than the cost associated with the development of lasers. Propagation of the beam to the pellet is also a limitation when employing this concept. Another manner of producing high density, kilovolt plasmas is the use of fast liners. Such devices can be driven by either magnetic forces or high explosives, both of which lead to compression and heating of an interior plasma. Although both of these fast liner techniques have produced energy in the form of radiation, neutrons, and alpha particles, just as the inertially confined laser and low impedance electron beam driven plasmas described above, each technique has its own inherent disadvantage. The primary disadvantage of the high explosive driven liner is that the high explosives have a maximum power density of approximately 10.sup.10 watts/cm.sup.3 and a maximum detonation velocity of 8.8.times.10.sup.5 cm/sec, which limits the liner implosion velocity that can be achieved. Although useful in obtaining scientific data, such a system, would, needless to say, be difficult to develop into a reuseable apparatus. As to magnetically driven liners, the liner forms part of the electrical discharge circuit in which current flowing through the liner creates a large B.sub..theta. field which causes the liner to compress. Since the liner forms part of the electrical circuit, the external circuit resistance and finite liner resistivity lead to ohmic losses which lower the efficiency of converting electrical energy into liner kinetic energy. Also, since the liner must make electrical contact with the circuit, damage to the electrode connection between the moving liner and the electrode limits operability. For liners which essentially remain thin solid shells during the implosion, ohmic heating and magnetic field diffusion limits implosion velocities to approximately 1 cm/.mu.sec. To obtain the desired radiation, neutron, and alpha particle energy at such low implosion velocities, the plasma within the liner must be preionized and complex methods of overcoming heat conduction losses must therefore be incorporated into the system. Although liner implosion velocities exceeding 1 cm/.mu.sec can be achieved, ohmic heating and magnetic field diffusion converts solid liners into plasmas during operation. As a result, the thickness of the liner is increased, which lowers the potential for power multiplication. Even with very thin foils, implosion velocities are limited by the risetime of the driving current and the diffusion of the driving magnetic field through the plasma liner. Lasers have also been used to directly heat a magnetically confined plasma. According to this concept, a laser is used to heat a large volume of plasma to a thermonuclear temperature which is confined by an elaborate magnetic field system. Although the laser provides uniform ionization and rapid heating of a low temperature plasma, the characteristic deposition length increases approximately as T.sup.3/2 for plasma electron temperatures, T&gt;10 eV. This characteristic of the deposition of laser energy in the plasma coupled with the large volume of plasma to be heated, places a total energy requirement for the laser which substantially exceeds present technology. Even if such lasers could be developed, the inherent low efficiencies associated with lasers would result in a large capital investment in such a system. A similar system incorporates a light or heavy ion beam to deposit its energy in a magnetically confined plasma. Since such beams are nonrelativistic, they exhibit a very low coupling efficiency and lack versatility obtainable by the relativistic interaction. The concept of using an intense relativistic electron beam to heat a confined plasma has been investigated experimentally for a number of years. Prior art experiments have concentrated primarily on heating a large volume of plasma to a thermonuclear temperature with the electron beam, while maintaining the plasma with an external magnetic field. A typical configuration of a prior art experimental apparatus is shown in FIG. 1. A cathode 10 is positioned within a vacuum chamber 12 which is separated from the plasma chamber 14 by an anode foil 16. A series of dielectric spacers are separated by a series of metal plates 20 which function to prevent breakdown between the cathode 10 and the diode support structure 22. A solenoidal or mirror magnetic field configuration 24 is produced by an external source along the axial direction of the device. In operation, a relativistic electron beam 26 is formed by charging the cathode 10 with a fast risetime high voltage pulse, causing electrons to be emitted from the cathode 10 penetrating the anode foil 16 so as to enter the plasma chamber 14 as a relativistic electron beam 26. As the relativistic beam propagates through the plasma along the externally applied axial magnetic field 24, the plasma is heated by the following methods: (a) relaxation heating due to relativistic streaming instabilities (two-stream and upper-hybrid instabilities), and PA0 (b) anomalous resistive heating due to the presence of a plasma return current (ion-acoustic and ioncyclotron instabilities). Typically, devices such as klystrons, magnetrons, vacuum tubes, etc., which are based upon electron bunching have been considered very efficient devices with respect to energy utilization. Therefore, the process of heating a plasma by electron bunching, i.e., method (a) by generating the two-stream and upper-hybrid instabilities, was initially expected to be an efficient technique for producing a thermonuclear plasma. Although all early experiments observed anomalous (nonclassical) coupling of the beam energy to the plasma resulting from the presence of the streaming instabilities, the coupling efficiency was only on the order of 15% at plasma densities of .apprxeq. 10.sup.12 electrons/cm.sup.3 and dropped rapidly to less than a few percent as the plasma density approached 10.sup.14 electrons/cm.sup.3. These results were obtained with anode foils having thicknesses on the order of 25 to 50 .mu.m and conventional electron beams available during this period which typically had relatively low voltages, i.e., 1 MeV or less. This combination of relatively thick anode foils and low voltage beams, caused classical anode foil scattering which prevented the relativistic streaming instabilities from efficiently coupling the beam energy to the plasma. In other words, although unknown to the experimentalists and theoreticians during the period 1970-1975, the foil thickness and low voltages of the electron beam used in the experiments caused the electron beam to scatter in a manner which prevented substantial electron bunching in the beam. This, in turn, produced the observed rapidly decreasing energy absorption efficiencies as the plasma density approached 10.sup.14 electrons/cm.sup.3. As a result of these low observed efficiencies, scientific attention shifted toward investigation of the resistive heating mechanism which was known to have several scientifically interesting properties. One property of the resistive heating mechanism is its ability to place a substantial fraction of the beam energy into plasma ions. This differs from the streaming instabilities which primarily heat the plasma electrons. Since the ions must eventually be heated in magnetically contained plasmas, direct heating of the ions eliminates an energy conversion step. Furthermore, when energy is initially deposited into plasma electrons rather than the ions, heat conduction is enhanced due to the initially elevated electron temperature, so that achievable plasma confinement time is shortened. Consequently, increased magnetic field strengths are required to produce comparable confinement. Another property of the resistive heating mechanism is its ability to heat a large volume of plasma in a uniform manner, rather than depositing energy in a small localized region, as is characteristic of the optimized streaming instability mechanism. The ability to directly heat a large volume of plasma in a uniform manner by resistive heating thus avoids problems of heat redistribution within the plasma. Moreover, the potential for developing a plasma heating system which could also be used in conjunction with devices requiring preheated plasmas and which, additionally have high political priority such as tokamaks, renders the resistive heating mechanism even more attractive. For these reasons, experimental attention was directed from the onset of plasma heating experiments by relativistic electron beams towards producing resistive heating in plasmas. Consequently, experimental apparatus to optimize resistive heating effects, such as low voltage electron beams with high .nu./.gamma. outputs, were utilized in ongoing experiments of relativistic electron beam heated plasmas. Here, .gamma. is the beam relativistic factor which is nearly proportional to the beam particle voltage. The ratio .nu./.gamma. is basically a measure of the beam magnetic energy to beam particle kinetic energy. The increased use of high .nu./.gamma. beams is more graphically shown in FIGS. 2 and 3 which illustrate the decrease in maximum beam voltage and increase in maximum .nu./.gamma. for relativistic electron beam experiments between 1970 and 1975. Thus the prior art experiments have, from the beginning, concentrated on high .nu./.gamma., low voltage, beams for optimizing the resistive heating mechanism, virtually ignoring the effect of streaming instabilities. In so doing, prior art experiments, have clearly pointed out the limitations of resistive heating, i.e., that resistive heating does not scale to higher density plasmas, but, to the contrary, is absolutely limited by self-stabilization within the plasma. More particularly, the experiments have shown that above a certain electron temperature, depending on the density of the plasma, low frequency instabilities which are responsible for resistive heating, are stabilized. Consequently, only classical resistivity, which is inadequate to couple significant energy to the plasma from the relativistic electron beam, has any effect in resistively heating the plasma. In addition to this inherent stabilization limitation, the technique of resistive heating has several other disadvantages in producing kilovolt plasmas. First, even if the experiments had shown that resistive heating was effective at high plasma density, the required .nu./.gamma. for efficient coupling would be at least an order of magnitude higher than that achievable by present day technology. Second, since resistive heating is not effective at high plasma densities, the mechanism is only suitable for low plasma densities which require long confinement times, dictated by external magnetic field strengths achievable within strength of material limitations. Additionally, such plasmas are very large in volume and the total energy required to heat said plasma would again be at least an order of magnitude beyond the total beam energy achievable by present technology standards. As a result of these limitations, and the belief by prior art theoreticians and experimentalists that resistive heating dominated anomalous energy deposition in a plasma, the relativistic electron beam plasma heating program in the United States was virtually abolished in 1975 without any further investigation into the streaming instability heating mechanism. SUMMARY OF THE INVENTION The present invention overcomes the disadvantages and limitations of the prior art by providing a novel method and device for relativistic electron beam heating of a high density plasma. The present invention utilizes streaming instabilities to locally heat a small volume of plasma to kilovolt temperatures. This is accomplished by utilizing .nu./.gamma. .perspectiveto. 1 relativistic electron beams having high voltages in conjunction with thin foils to reduce foil scattering effects, so as to enhance the streaming instabilities. In this manner, energy from the relativistic electron beam is deposited in the plasma with very high coupling efficiency due to the anomalous effects of the streaming instabilities. OBJECTS OF THE INVENTION It is therefore an object of the present invention to provide an improved method and device for relativistic electron beam heating of a high density plasma. It is also an object of the present invention to provide an improved method and device for relativistic electron beam heating of a high density plasma which is efficient in operation. Another object of the present invention is to provide a method and device for relativistic electron beam heating of a high density plasma which is relatively inexpensive to implement and simple in operation. Another object of the present invention is to provide a method and device for relativistic electron beam heating of a high density plasma which utilizes devices which are available according to present day or near term technology. Another object of the present invention is to provide a method and device for relativistic electron beam heating of a high density plasma which is capable of producing energy in the form of radiation, neutrons and alpha particles. Another object of the present invention is to provide a method and device for relativistic electron beam heating of a high density plasma which requires relatively low capital investment. Another object of the present invention is to provide a method and device for relativistic electron beam heating of a high density plasma utilizing relativistic streaming instabilities. Other objects and further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. The detailed description, indicating the preferred embodiment of the invention is given only by way of illustration since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from the detailed description. The abstract of the disclosure is for the purpose of providing a nonlegal brief statement to serve as a searching and scanning tool for scientists, engineers, and researchers and is not intended to limit the scope of the invention as disclosed herein, nor is it intended to be used in interpreting or in any way limiting the scope or fair meaning of the appended claims.