Patent Number: 042723191
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT OF THE INVENTION The present invention, in contrast to prior art plasma heating techniques, takes advantage of the natural characteristics of two extremely powerful microinstabilities, i.e., the two-stream and upper-hybrid instabilities, to locally heat a small volume of plasma to kilovolt temperatures. Essentially, the instabilities are created by the relative drift between the relativistic beam electrons and target plasma electrons. Although a large number of parameters influence this collective interaction, the dominant factors in determining the strength of the instabilities are (1) beam temperature along a stream line and (2) the wavelength of the instabilities relative to the radial dimension of the target plasma. Beam temperature along a stream line occurs primarily from the passage of the relativistic electrons of the beam through the foil dividing the low density plasma from accelerator vacuum, and the low density plasma from the high density plasma target. The effect of the foil can be made negligible by (1) increasing the electron energy, (2) reducing the thickness of the foil, or (3) reducing the effective Z of the foil material. As a result, a high voltage, i.e., exceeding 3 MeV, electron beam can penetrate a number of foils and still deposit its energy efficiently to the high density plasma. By utilizing plasmas of high density, the wavelength of the instabilities are small compared to the radial dimensions of the plasma. Thus, although the instantaneous deposition rate can vary, the nonlinear evolution of the instability functions to relax the beam distribution in both angle and energy, resulting in an efficient coupling of beam energy to the plasma. The characteristic nonuniform energy deposition of the collective interaction, i.e., two-stream and upper-hybrid instabilities, is illustrated in FIG. 4. This nonuniform deposition property is utilized to concentrate energy deposited into the plasma from the relativistic electron beam, rather than allowing the energy to dissipate its explosive character by expansion into a large volume of plasma. The initial deposition of beam energy is into plasma electrons which, depending upon the parameters of the device, results (1) in heat conduction which is used propitiously to obtain power multiplication or (2) in current multiplication and confinement of the plasma. In this manner, the disadvantages of preferential heating of plasma electrons associated with magnetically confined plasmas is advantageously employed in the present invention. The potential efficiency of relativistic beam energy deposition into a dense plasma via the streaming instability mechanism has heretofore been unknown in the prior art. In contrast, FIG. 5 illustrates results of recent experiments performed according to the present invention in which energy deposition is plotted against plasma density for the application of a relativistic beam through anode foils having various thicknesses. As is apparent from the data of FIG. 5, a reduction in the anode foil thickness causes a great increase in deposition energy into the plasma. These results therefore indicate that the basic efficiency of the streaming instability deposition varies in the following approximate manner: EQU Efficiency=.alpha.S[1-exp (-.alpha.S/F)]/(1+.alpha.S) where S.ident.(.gamma.-.gamma..sup.-1)(n.sub.b /2n.sub.e).sup.1/3 is the strength parameter, F is a function depending upon the foil thickness and material, n.sub.b is the beam density, n.sub.e is the plasma electron density, and .alpha.=1.0-1.5 is a parameter associated with beam premodulation. It is therefore apparent from the efficiency equation that if either the beam voltage (.gamma.) is increased or the foil function (F) reduced by decreasing the foil's effective Z or thickness, the factor exp (-.alpha.S/F) approaches zero, such that the efficiency increases in direct proportion to the strength parameter to approximately S.perspectiveto.0.60. Thus, for high voltage beams, the coupling efficiency approaches 60% for high density plasma targets. Moreover, these coupling efficiencies can be obtained with little or no advancement in the current technology of electron beams since relativistic electron beams presently exist with voltage parameters sufficiently high to practice the present invention. As a result, currently available high voltage relativistic electron beams are capable of achieving high energy deposition due to the ability of the high voltage beams to penetrate the foil with reduced electron beam scattering. Thus, for equal energy relativistic electron beams, beams with .mu./.gamma..perspectiveto.1 achieve much higher coupling efficiencies via the streaming instabilities than beams designed with .mu./.gamma.&gt;&gt;1 to optimize the resistive heating mechanism in the high density plasma target regime. FIG. 6 is a block diagram illustrating the major components of the preferred embodiment of the invention. The relativistic electron beam generator 42 produces a high voltage, i.e., 3 MeV or greater, high current density, i.e., 1 kA/cm.sup.2 or greater, relativistic electron beam. A foil or foilless diode can be used to produce a solid or annular electron beam in the conventional manner, or, alternatively, a conventional high impedance generator can be used as an injector to a radial pulse line accelerator to increase the electron energy to the 100 MeV range. The relativistic electron beam 44 produced by generator 42 is propagated along the vacuum drift tube 46 to a modulator 48. A solenoidal magnetic field generator 50 generates a magnetic field along the vacuum drift tube to ensure beam equilibrium throughout the drift tube 46 and modulator 48. Modulator 48 constitutes a portion of the vacuum drift tube and is formed by a periodic structure in the direction of the beam propagation. The periodic structure of the modulator 48 causes the beam electrons to be bunched longitudinally which allows a more stable propagation through low density gas chamber 50 and enhanced deposition in target plasma 52. The low density gas chamber 50 provides isolation between replaceable target plasma chamber 62 and modulator 48, drift tube 46 and accelerator 42. The electron density (n.sub.e) of the ionized low density gas 58 is typically close to the electron beam density (n.sub.b), whereas the target plasma electron density is 4 to 6 orders of magnitude above the beam density. The low density gas 58 typically comprises H.sub.2, He, Ar, N.sub.2 or residual gas associated with the previous operation of the device. The high density target plasma 52 comprises DT, DD, hydrogen boron or similar gas with a fully ionized electron density (n.sub.e) of 10.sup.17 to 10.sup.20 cm.sup.-3. Foil 54 consists of a metal, e.g., Ti, Al, Be, etc. or plastic, e.g., mylar or kapton, support and a layer of plastic impregnated with high Z atoms. The foil 54 retains the vacuum in the drift tube 46 and modulator 48 and converts a portion of the rising beam impulse into Bremsstrahlung radiation which is directed predominantly along the axis of the electron beam. The Bremsstrahlung radiation creates a low density plasma channel 60 for beam propagation through the low density gas 58. Foil 66 may also be constructed similar to foil 54 to generate Bremsstrahlung radiation and provide ionization of the target plasma 52 to assist or replace preionizer 56. In the ionized low density plasma channel 60 and target plasma 52, the self fields of the beam are shorted out so that an external magnetic field is not required to achieve beam equilibrium, although overall efficiency of the device may be enhanced by the presence of a magnetic field. The beam can therefore be ballistically guided through the low density plasma channel 60 to the plasma target 52. An external magnetic field source 64 can also be used in conjunction with the preferred embodiment providing further stabilization of the relativistic electron beam within the low density gas chamber 50. The preionizer 56, which fully ionizes the target gas in target chamber 62, constitutes any one of a number of devices for creating a fully ionized gas such as a discharge device, an exploding wire or wires, various lasers, a microwave generator, or various low energy particle beam generators. Where necessary, such as with a laser preionizer, windows formed from sapphire, salt, or other appropriate materials are formed in the plasma container 62 to allow the laser light to enter and ionize the target plasma 52. For a fully ionized density of 10.sup.19 to 10.sup.20 electrons/cm.sup.3 of the plasma 52, a 0.1 to 2 .mu.s, 0.2 to 10 kJ HF laser can be used for the ionization source 56. A 0.1 to 2 .mu.s, 0.2 to 5 kJ CO.sub.2 laser can be used for the ionization source 56 when the fully ionized density of the plasma 52 is below 10.sup.19 electrons/cm.sup.3. In operation, the beam passes through chamber 62 where convective wave growth is initiated such that the waves e-fold until saturated through nonlinear trapping. The presence of a thin foil located at 66 defines a starting point for the wave growth, thus ensuring that the beam energy is deposited at a specified location within the target plasma 52. Since energy is being transferred from relativistic electrons in the electron beam 44 to nonrelativistic electrons within the target plasma 52, conservation of energy and momentum require that the beam both heat and drive a large axial current in the plasma. The presence of a large axial current, in turn, initiates additional plasma heating and confinement. FIG. 7 is a schematic illustration of one arrangement of the preferred embodiment. As shown, relativistic electron beam generator 42 produces a relativistic electron beam 46 which is propagated through the vacuum drift tube 44 and adjacent modulator 48. The electron beam 46 penetrates foil 54 which separates the low density gas chamber 50 from the vacuum drift tube 46 and modulator 48. A low density plasma channel 66 is formed in the low density gas chamber 50. Windows 68 and 70 allow the preionizer 56 to penetrate the low density gas chamber 50 and target chamber 62 respectively. Preionizer 56 functions to ionize the target plasma 52 such that the relativistic electron beam deposits its energy in an anomalous fashion in the target plasma 52 according to the present invention. FIG. 8 is an alternative arrangement in which two preionizers 74 and 76 apply dual preionization beams 78 and 80 transverse to the axis of the electron beam 46. Windows 82 and 84 in the low density gas chamber and 86 and 88 in the target plasma chamber allow passage of the preionization beam to the target plasma 52. FIG 9 is a schematic end view of an additional alternative arrangement utilizing three preionization sources 92 through 96 which produce three ionization beams 98 through 102 penetrating windows 104 through 108 of the low density gas chamber structure 110 through 114. The target plasma chamber comprises support structure 124 through 128 and windows 118 through 122 which provide ionization beams 98 through 102 access to the target plasma. Sapphire or salt is used as the window material for both the low density gas chamber and target plasma chamber when HF or CO.sub.2 lasers are utilized as preionization sources. The advantage of the arrangement shown in FIG. 9 is that a multiplicity of preionization sources can be respectively arranged in an off-axis position such that their beams are not directed at each other causing possible damage. Of course, the ability to utilize a mulitiplicity of preionizers reduces capital costs by reducing the individual output of each preionizer required to fully preionize the target plasma 130. Referring again to the modulator 48 which is schematically illustrated in FIGS. 7 and 8, the purpose of the modulator is to provide improved stability of the relativistic electron beam 46 within the low density gas chamber 50, as well as improved coupling efficiency to the target plasma 52. As shown in FIG. 10, the characteristic growth rate (dashed line) and the characteristic change in beam velocity (solid line) .delta..beta.=2(.beta..sub.0 -.omega./kc) resulting from waves propagating along the relativistic electron beam axis due to the streaming instability are shown as a function of the wave number k=2.pi./.lambda.. The initial beam velocity is v.sub.0, the wavelength of unstable electrostatic oscillations is .lambda., .omega./k is the phase velocity associated with the electrostatic oscillation, .beta..sub.0 =v.sub.0 /c, and c is the speed of light. Note that the growth rate is relative to the target plasma frequency .omega..sub.p. For an unmodulated beam the nonlinear evolution of the streaming instability is determined by the fastest growing wave, which occurs for example, at kv.sub.0 /.omega..sub.p =1.01 in FIG. 10. The beam energy loss .delta..gamma.=.gamma..sup.3 .delta..beta./(1+.gamma..sup.2 .beta..delta..beta.) is thus determined by the unmodulated .delta..beta. shown in FIG. 10. However, by modulating the beam at a wavelength and phase velocity slightly shorter and slightly lower than the fastest growing wave, as shown in FIG. 10, the beam energy loss .delta..gamma.=.gamma..sup.3 .beta..delta..beta./(1+.gamma..sup.2 .beta..delta..beta.) is then determined by the modulated .delta..beta.. Comparing the unmodulated and modulated .delta..beta., the increase in the coupling efficiency is .alpha.=1.0-1.5 due to the modulator. FIG. 11 illustrates the modulated beam as it appears in phase space. The spatial distance is equal to one wavelength of the instability in the dense plasma. In contrast, FIG. 12 illustrates the beam phase space that appears in the low density gas chamber. As is apparent, the relatively short wavelength modulation in the low density interaction establishes an effective beam temperature which retards the development of the instability in the low density gas chamber, in contrast to FIG. 10 wherein the characteristic .delta..beta. of the instability is enhanced to provide improved energy deposition from the relativistic electron beam. FIG. 13 is a block diagram of an alternative embodiment utilizing two relativistic electron beams which deposit energy in the target plasma 140. As shown, two preionizers 136 and 138 are applied transverse to the axis of the electron beams of generators 132 and 134. In the arrangement of FIG. 13, the relativistic electron beams are designed to deposit their energy in the target plasma 140 prior to intersecting one another. The present invention therefore provides a unique method and device for relativistic electron beam heating of a high density plasma. The present invention optimizes the extremely powerful streaming instabilities to heat a high density plasma requiring minimal confinement time. Unlike prior art experimentation, .nu./.gamma..apprxeq.1 beams having high voltages are utilized to enhance the streaming instabilities. As a result, interaction of the plasma is initiated within a small localized region of energy deposition of the electron beam to provide energy in the form of radiation, neutrons, and alpha particles. Obviously many modifications and variations of the present invention are possible in light of the above teachings. For example, various applications of the present invention may not require the use of the external magnetic field, preionizer, low density gas chamber, modulator, or drift tube, either alone, or in combination. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.