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United States Patent m [11] 4,263,095Thode [45] Apr. 21, 1981

[54] DEVICE AND METHOD FOR IMPLODING AMICROSPHERE WITH A FAST LINER

[75] Inventor: Lester E. Thode, Los Alamos, N.Mex.

[73] Assignee: The United States of America asrepresented by the United StatesDepartment of Energy, Washington,D.C.

[21] Appl. No.: 9,702

[22] Filed: Feb. 5, 1979

[51] Int. Cl.’ ................................................ G21B 1/00[52] U.S. Cl. .............. .............................. 176/1; 176/5[58] Field of Search ........................ 250/402, 499-502;

313/61 R, 61 S; 328/233-238, 256; 176/1, 3, 5,9

[56] References Cited

PUBLICATIONS

Phys. of Fluids, vol. 19, No. 6, (6/76), pp. 83 I-848,Thode, Plasma Heating by Relativistic Electron Beams.NucIear Fusion Suppl., (1977), vol. 2, p. 543, Thode 11.J. Appl. Phys., VO].44, No. 11, (1 1/73), pp. 4913-4919,Mather et al.Sov. Tech. Phys. Lett., vol. 2, No. 1, (1/76), pp. 20-22,Kiselev et al.Phys. of Fluids, vol. 20, No. 12, (12/77), pp. 2121-2127,Thode 111.

Primary Examiner—S. A. Cangialosi,4tforney, Agent, or Firm—James E. Denny; Paul D.Gaetjens; William W. Cochran, H

[57] ABSTRACT

A device and method for relativistic electron beam

heating of a high-density plasma in a small localizedregion. A relativistic electron beam generator or accel-erator produces a high-voltage electron beam whichpropagates along a vacuum drift tube and is modulatedto initiate electron bunching within the beam. The beamis then directed through a low-density gas chamberwhich provides isolation between the vacuum modula-tor and the relativistic electron beam target. The rela-tivistic beam is then applied to a high-density targetpIasma which typically comprises DT, DD, hydrogenboron or similar thermonuclear gas at a density of 1017to 1020electrons per cubic centimeter. The target gas isionized prior to application of the electron beam bymeans of a laser or other preionization source to form aplasma. Utilizing a relativistic electron beam with anindividual particle energy exceeding 3 MeV, cIassicalscattering by relativistic electrons passing through iso-lation foils is negligible. As a result, relativistic stream-ing instabilities are initiated within the high-densitytarget plasma causing the relativistic electron beam toefficiently deposit its energy and momentum into asmall localized region of the high-density plasma target.Fast liners disposed in the high-density target plasmaare explosively or ablatively driven to implosion by aheated annular plasma surrounding the fast liner gener-ated by an annular relativistic electron beam. An azi-muthal magnetic field produced by axial current flow inthe annular plasma, causes the energy in the heatedannular plasma to converge on the fast liner to drive thefast liner to implode a microsphere.

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4,263,0951

DEVICE AND METHOD FOR IMPLODINGMICROSPHERE WITH A FAST LINER

BACKGROUND OF THE INVENTION

A

The present invention pertains generally to denseplasma heating and more particularly to plasma heatingby way of a relativistic electron beam.

Plasma heating has, for some time, been of greatinterest to the scientific community, since heated plas-mas can be utilized for a wide variety of functions. Atypical use of hot plasmas is the generation of energy inthe form of radiation, neutrons, and alpha particles.Such an energy source can be useful in basic high-en-ergy density plasma physics research, with practicalapplication in scientific areas such as controlled thermo-nuclear fusion, material studies, and radiography.

Numerous techniques have been proposed in theprior art to produce dense, kilovolt plasmas. One of themore well-known techniques is the compression andheating of the core of a structured pellet by a laser orlow-voltage electron beam. It has also been suggestedthat light- or heavy-ion beams could be utilized to ob-tain similar compression and heating. According to thistechnique, the structured pellet and its driving sourceare directly coupled through classical interactions byheating the outer layer of the structured pellet. Depend-ing upon the characteristics of both the structured pelletand driving source, the outer layer explodes or ablates,leading to compression and heating of the core. Due tothe direct coupling of all of these prior art drivingsources, preheat of the core has been found to reducethe effectiveness of the compression, thereby reducingboth density and temperature of the pellet core.

The use of a laser as a driving source in the abovedescribed confinement system has the added inherentdisadvantages of low efficiency and associated high-development cost to produce lasers with the requiredpower output for a directly driven structured pellet.Also, diffraction limitations and window damagethresholds make it difflcuh to focus proposed largelasers to millimeter diameters.

Low-impedance electron and light-ion beams alsoface expensive technological advancement to enablethese beams to be focused to millimeter diameters, andto obtain power levels necessary to achieve the desiredcompression of the structured pellet. Low-impedanceelectron and light-ion sources are additionally limited inthe manner of propagation of the beam to the pellet.

Heavy-ion sources also require significant technolog-ical advancement to produce the desired compressionof the structured pellet. In fact, development of heavy-ion sources using conventional accelerator conceptsappears to be considerably more expensive than the costassociated with the development of lasers. Beam propa-gation is also a limitation when employing heavy-ionsources.

High-density, kilovolt plasmas can also be producedby fast liners. Such devices can be driven by eithermagnetic forces or high expolsives, both of which leadto compression and heating of a confined plasma. Al-though both of these fast liner techniques have pro-duced energy in the form of radiation, neutrons, andalpha particles, each technique has its own inherentdisadvantage. The primary disadvantage of the highexplosive driven liner is that the high explosives have amaximum power density of approximately 10IOwatts/cmJ and a maximum detonation velocity of

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L8.8 X 105 cm/see, which limits achievable liner implo-sion velocity. Althrough useful in obtaining scientificdata, such a system would be difficult to develop into areuseable apparatus,

Magnetically driven liners are fabricated such thatthe liner forms part of the electrical discharge circuit inwhich current flowing through the liner creates a largemagnetic field causing the liner to compress. Since theliner forms part of the electrical circuit, the externalcircuit resistance and finite liner resistivity lead toohmic losses which lower the efficiency of convertingelectrical energy into liner kinetic energy. Also, sincethe liner must make electrical contact with the circuit,damage to the electrode connection between the mov-ing liner and the electrode limits operability.

For liners which essentially remain thin solid shellsduring the implosion, ohmic heating and magnetic fielddiffusion limits implosion velocities to approximately 1cm/psec. To obtain the desired radiation, neutron, andalpha particle output at such low implosion velocities,the plasma within the liner must be preionized and com-plex methods of overcoming heat conduction lossesmust be incorporated into the system.

Although liner implosion velocities exceeding 1cm/psec can be achieved, ohmic heating and magneticfield diffusion converts solid liners into plasmas duringoperation. As a result, the thickness of the liner is in-creased, which lowers the potential for power multipli-cation. Even with very thin foils, implosion velocitiesare limited by the risetime of the driving current anddiffusion of the driving magnetic field through theplasma liner,

Lasers have also been used to directly heat a magneti-cally confined plasma. According to this concept, alaser is used to heat a large volume of plasma confinedby an elaborate magnetic field system to thermonucleartemperatures. Although the laser provides uniform ioni-zation and rapid heating of a low-temperature plasma,the characteristic deposition length increases approxi-mate] y as T3/z for plasma electron temperatures T22 10eV. This characteristic of the deposition of laser energyin the plasma, coupled with the large volume of plasmato be heated, places a total energy requirement for thelaser which substantially exceeds present technology,Even if such lasers could be developed, the inherentlow eftlciencies associated with generation of laser en-ergy would result in a large-capital investment for sucha system.

A similar system incorporates a light- or heavy-ionbeam to deposit its energy in a magnetically confinedplasma. Since such beams are nonrelativistic, they ex-hibit a very low coupling efficiency and lack versatilityobtainable by the relativistic interaction.

The concept of using an intense relativistic electronbeam to heat a confined plasma has been investigatedexperimentally for a number of years. Prior art experi-ments have concentrated primarily on heating a largevolume of plasma to thermonuclear temperatures withan electron beam, while maintaining the plasma with anexternal magnetic field. A typical configuration of aprior art experimental apparatus is shown in FIG. 1. Acathode 10 is positioned within a vacuum chamber 12which is separated from the plasma chamber 14 by ananode foil 16, A series of dielectric spacers 18 are sepa-rated by a series of metal plates 20, which functiontogether to prevent breakdown between the cathode 10and the diode support structure 22. A solenoidal or

3mirror magnetic field configuration 24 is produced byan external source.

In operation, a relativistic electron beam 26 is formedby charging the cathode 10 with a fast risetime high-voltage pulse, causing electrons to be field emitted fromthe cathode 10 penetrating the anode foil 16 so as toenter the plasma chamber 14 as a relativistic electronbeam 26. As the relativistic beam propagates throughthe plasma along the externally applied axial magneticfield 24, the plasma is heated by the following methods:

(a) relaxation heating due to relativistic streaminginscabililies (two-stream and upper-hybrid bunchinginstabilities); and,

(b) amomalous resistive heating due to the presence ofa plasma return current (ion-acoustic and ion-cyclotroninstabilities).

Typically, devices such as klystrons, magnetrons,vacuum tubes, etc., which are based upon electronbunching according to method (a) have been consid-ered very efficient devices with respect to energy utili-zation. Therefore, the process of heating a plasma byelectron bunching, i.e., by generating the twe-streamand upper-hybrid instabilities according to method (a),was initially expected to be an efficient technique forproducing a thermonuclear plasma. Although all earlyexperiments observed anomalous (nonclassical) cou-pling of the beam energy to the plasma resulting fromthe presence of the streaming instabilities according tothe method (a), the coupling efficiency was only on theorder of 15V0 at plasma densities of approximately 1012electrons/cmj, and dropped rapidly to less than a fewpercent as the plasma density approached 101’r elec-trons/cmq. These results were obtained with anode foilshaving thicknesses on the order of 25 pm to 50pm andconventional electron beams available for experimentsduring this period which typically had relatively lowvoltages, i.e., 1 MeV or less. This combination of rela-tively thick anode foils and low-voltage beams resultedin classical anode foil scattering of the beam whichprevented the relativistic streaming instabilities fromeftlciently coupling the beam energy to the plasma. Inother words, although unknown to the experimentalistsand theoreticians during the period 1970– 1975, the foilthickness and low voltage of the electron beams used inthe experiments caused the electron beam to scatter in amanner which prevented substantial electron bunchingin the beam. This, in turn, produced the observed rap-idly decreasing energy absorption efficiencies as theplasma density approached 10I4 electrons/ems. As aresult of these low observed efficiencies, scientific at-tention shifted toward investigation of the resistiveheating mechanism according to method (b), which wasknown to have several scientifically interesting proper-ties.

One property of the resistive heating mechanism ofmethod (b) is its ability to place a substantial fraction ofthe beam energy into plasma ions. This differs from thestreaming instabilities which primarily heat the plasmaelectrons. Since the ions must eventually be heated in amagnetically contained plasma, according to conven-tional magnetic confinement systems, direct heating ofthe ions eliminates an energy conversion step. Further-more, when energy is initially deposited into plasmaelectrons rather than the ions, heat conduction is en-hanced due to the initially elevated electron tempera-ture, so that achievable plasma confinement time isshortened. Consequently, increased magnetic field

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4strengths are required to produce comparable confine-ment.

Another property of the resistive heating mechanismis its ability to heat a large volume of plasma in a uni-form manner, rather than depositing energy in a smalllocalized region, as is characteristic of the optimizedstreaming instability mechanism. The ability to directlyheat a large volume of plasma in a uniform manner byresistive heating thus avoids problems of heat redistri-bution within the plasma. Moreover, the potential fordeveloping a plasma heating system which could also beused in conjunction with devices requiring preheatedplasmas, such as tokamaks which has received substan-tial funding, renders the resistive heating mechanismeven more attractive. For these reasons, experimentalattention was directed from the onset of plasma heatingexperiments using relativistic electron beams towardsproducing resisfive heating in plasmas according tomethod (b). Consequent y, experimental apparatus tooptimize resistive heating effects, such as low-voltageelectron beams with high v/y outputs, were utilized inongoing experiments of relativistic electron beamheated plasmas. Here, -y is the beam relativistic factorwhich is nearly proportional to the beam particle volt-age. The ratio v/y is basically a measure of the beamself-magnetic field energy to beam particle energy. Theincreased use of high v/y beams is more graphicallyshown in FIGS. 2 and 3 which illustrate the decrease inmaximum beam voltage and increase in maximum v/yfor relativistic electron beam experiments between 1970and 1975. Thus the prior art experiments have, from thebeginning, concentrated on high v/y, low-voltagebeams for optimizing the resistive heating mechanismaccording to method (b), virtually ignoring the effect ofstreaming instabilities produced according to method(a).

In so doing, prior art experiments, have clearlypointed out the limitations of resistive heating accord-ing to method (b), i.e., that resistive heating does notscale to higher density plasmas, but, to the contrary, isabsolutely limited by self-stabilization within theplasma. More particularly, the experiments have shownthat above a certain electron temperature, depending onthe density of the plasma, low-frequency instabilitieswhich are responsible for resistive heating, are stabi-lized. Consequently, only classical resistivity, which isinadequate to couple significant energy to the plasmafrom the relativistic electron beam, has any effect inresistively heating the plasma,

In addition to this inherent stabilization limitation, thetechnique of resistive heating has several other disad-vantages. First, even if experiments had shown thatresistive heating according to method (b) was effectiveat high plasma density, the required v/y for efficientcoupling would be at least an order-of-magnitudehigher than that achievable by present day technology.Second, since resistive heating is only suitable for lowplasma densities which are very large in volume, thetotal energy required to heat such a plasma wouldagain, be at least an order-of-magnitude beyond thetotal beam energy achievable by present technologystandards.

As a result of these limitations, and the belief by priorart theoreticians and experimentalists that resistive heat-ing dominated anomalous energy deposition in plasmas,the relativistic electron beam plasma heating programin the United States was virtually abolished in 1975

4,263,0955

without any further investigation into the streaminginstability heating mechanism.

SUMMARY C)F THE INVENTION

The present invention overcomes the disadvantagesand limitations of the prior art by providing a deviceand method for electron beam heating of a high-densityplasma to drive a fast liner to implode a structuredmicrosphere. The present invention utilizes an annularrelativistic electron beam to heat an annular plasma tokilovolt temperatures through streaming instabilities inthe plasma. Energy deposited in the annular plasmathen converges on a fast liner to explosively or abla-tively drive the liner to convergence to implode thestructured microsphere.

It is therefore an object of the present invention toprovide a device and method for generating a hotplasma to dirve a fast liner to implode a structuredmicrosphere.

It is also an object of the present invention to providea device and method for driving a fast liner to implodea structured microsphere which is eftlcient in operation.

Another object of the present invention is to providea device and method for power density multiplication.

Another object of the present invention is to providea device and method for generating a hot plasma,

Another object of the present invention is to providea device and method for generating energy in the formof radiation, neutrons, and/or alpha particles.

Another object of the present invention is to providea device and method for generating energy which re-quires relatively low-capital investment.

Another object of the present invention is to providea device and method for generating high-intensity radi-ation, neutrons, and/or alpha particles utilizing cur-rently available technology.

Other objects and further scope of applicability of thepresent invention will become apparent from the de-tailed description given hereinafter. The detailed de-scription, indicating the preferred embodiment of theinvention is given only by way of illustration sincevarious changes and modifications within the spirit andscope of the invention will become apparent to thoseskilled in the art from the detailed description. Theforegoing abstract of the disclosure is for the purpose ofproviding a nonlegal brief statement to serve as asearching and scanning tool for scientists, engineers,and researchers and is not intended to limit the scope ofthe irwention as disclosed herein, nor is it intended to beused in interpreting or in any way limiting the scope orfair meaning of the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of a typical prior artrelativistic electron beam plasma heating device,

FIG. 2 is a graph of maximum experimental relativis-tic eiectron beam voltages utilized from 1970 to 1975,

FIG. 3 is a graph of maximum experimental v/y ofrelativistic electron beams utilized from 1970 to 1975.

FIG. 4 is a graph illustrating the characteristic rela-tionship between the relativistic electron beam andplasma ions and electrons for resistive heating accord-ing to method (b). The graph illustrates the componentof velocity along the direction of beam propagationV [1(axis) versus the distribution function fa (V 11)(ordi-nate).

FIG. 5 is a graph illustrating the characteristic rela-tionship between the relativistic electron beam and

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6plasma ions and electrons for relaxation heating accord-ing to method (a) of the present invention. The graphillustrates the component velocity along the direction ofbeam propagation V Ii (axis) versus the distributionfunction fa (V It) (ordinate).

FIG. 6 is a graph illustrating the characteristic nonu-niform energy deposition (ordinate) along the directionof beam propagation (axis) associated with the stream-ing instabilities of method (a). A one-dimensional inter-action is represented by the solid line while the dashedline represents a two-dimensional interaction.

FIG. 7 is a graph of the experimental scaling ofplasma heating in joules (ordinate) versus the plasmaparticle density np in electrons/cmj for three differentanode foil thicknesses. Theoretical predications areindicated by solid curves.

FIG. 8 is a graph illustrating experimental results ofbeam energy transmitted to a calorimeter (ordinate)versus anode foil thickness for three different anode-cathode gap spacings.

FIG. 9 is a table of the foil scattering function F forseven different materials having various thicknessesmeasured in microns.

FIG. 10 is a graph of the dimensionless parameter ~(ordinate) versus the relativistic factor -y(axis) for givenvalues of plasma electron density in electrons/cmJ.

FIG, 11 is a schematic diagram illustrating the pri-mary components of a system utilizing high-energydensity plasma as a direct source for radiation, neutrons, -. .—and/or alpha particles.

FIG. 12 is a schematic diagram illustrating the pri-mary components of a system which utilizes a high-en-ergy density plasma to drive a fast liner according to thepreferred embodiment of the invention.

FIG. 13 is a schematic illustration of a two annularbeam system providing cylindrical symmetry.

FIG. 14 is a schematic illustration of a two annularbeam system providing spherical symmetry.

FIG. 15 is a schematic illustration of a four annularbeam system which also provides spherical symmetry ina multi-megajoule system.

FIG. 16 is a schematic diagram illustrating the rela-tive sizes of various relativistic electron beam genera-tors relative to a 1.83 meter individual.

FIG. 17 is a graph ilh.rstrating the approximate costper joule delivered (ordinate) as a function of total gen-erator cost in thousands of dollars (axis).

FIG. 18 is a schematic illustration of the base compo-nents of a high-impedence relativistic electron beamgenerator.

FIG. 19 is a schematic illustration of the electricalequivalent of a Marx stage.

FIG. 20 is a schematic illustration of the electricalequivalent of a Blumlein and diode.

FIG. 21 is a schematic illustration of a multigap accel-erator.

FIG. 22 is a graph of the characteristic growth rateand velocity change (ordinate) as a function of wavenumber for the streaming instabilities (axis).

FIG. 23 is a schematic illustration of an anomalouspinch,

FIG. 24 is a schematic illustration of a device forproducing an anomalous pinch utilizing a single laserpreionizer,

FIG. 25 is a schematic illustration of a device forproducing an anomalous pinch utilizing two laser pre-ionizers.

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FIG. 26 is a schematic illustration of the end view ofa device for producing an anomalous pinch utilizingthree laser preionizers.

FIG. 27 is a schematic illustration of the basic geome-try of a device for driving a fast spherical liner with anannular relativistic beam.

FIG. 28 is a schematic illustration of the basic geome-try for driving a fast cylindrical liner with an annularrelativistic electron beam.

FIG. 29 is a schematic cross-sectional view of aspherical liner configuration with dual ionizationbeams.

FIG. 30 is a schematic illustration of a cross-sectionalview of cylindrical liner configuration with dual ioniza-tion beams.

FIG. 31 is a schematic illustration of a cross-sectionalview of a fast spherical liner.

FIG. 32 is a schematic illustration of a cross-sectionalview of a fast cylindrical liner.

FIG. 33 is a schematic illustration ofa cross-sectionalview of an alternative fast liner device.

FIG. 34 is a schematic illustration of the target geom-etry utilizing two annular relativistic electron beams todrive a spherical liner, in the manner indicated in FIG.14.

FIG. 35 is a schematic illustration of the baise geome-try for fast spherical liner implosion of a structuredmicrosphere.

FIG. 36 is a schematic illustration of the basic geome-try for fast cylindrical liner implosion of a structuredmicrosphere.

FIG. 37 is a schematic illustration of a cross-sectionalview of a fast spherical liner and a structured micro-sphere.

FIG. 38 is a schematic illustration of a cross-sectionalview of a fast cylindrical liner and a structured micro-sphere.

DETAILED DESCRIPTION OF THEPREFERRED EMBODIMENT OF THE

INVENTION

Central to the concept of the preferred embodimentof the invention is the rapid heating of a 1017–2020elec-trons/cmJ, 3 cms to 50 cmj volume of plasma by anintense, high-voltage relativistic electron beam. Effi-cient coupling is achieved through optimization andcontrol of a very powerful co] Iective wave interaction,which occurs naturally when a directed stream of elec-trons passes through a plasma.

The anomalous transfer of relativistic electron beamenergy and momentum into thermal and directedplasma energy, respectively, is nonclassical and, there-fore, the strength of the nonlinear state of the microin-stabilities depends upon a large number of factors. Thecharacteristic nonuniform energy deposition of the col-lective interaction is utilized to concentrate the energyin the plasma, In fact, the optimized relativistic electronbeam-plasma interaction is a power density multiplica-tion process. Since energy is being transferred fromrelativistic beam electrons to nonrelafivistic electrons inthe plasma, conservation of energy and momentumrequire that the interaction both heat and drive a local-ized axial current in the plasma. The driven axial cur-rent, in turn, generates an azimuthal magnetic field.

If the relativistic beam is solid, the physical configu-ration is similar to a nonuniform denze Z pinch in whichthe azimuthal magnetic field provides confinement.However, in contrast to a classical Z pinch, the heating

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8and confinement are anomalous in character. For anannular relativistic electron beam, the azimuthal mag-netic field leads to a directed heat flow towards the axisof the device. In this configuration, the kilovolt plasmais used to drive a hierarchy of inertial confinement de-vices according to the present invention.

Early theory by R. V. Lovelace and R. N. Sudan,Phys. Rev. Letter 27, 1256 (1971), indicated that resis-tive heating according to method (b) was a very efficient process for beams with v/y> >1. As pointed outabove, v/y is a measure of the beam self-magnetic fieldenergy to the beam particle energy. Defining N as theline density of beam electrons and rc as the classicalelectron radius, v=Nre for a solid, constant densitybeam. The relativistic factor y= (1 –~2)– iand /3= v/care, in this manner, related to the beam velocity v andspeed of light c. The basic idea behind anomalous resis-tive heating is that a v/y> 1 beam cannot propagatesince its self-magnetic field energy exceeds its particleenergy. But, when such a beam is injected into a plasma,it neutralizes this characteristically large self-magneticfield energy by inducing a plasma return current. Therelationship between the plasma and beam species invelocity space for a magnetically neutralized beam isshown in FIG. 4. Due to the relative drift between theplasma electron and ion species, ion-acoustic and/orion-cyclotron waves are generated, as illustrated inFIG. 4 by the dashed lines. Such microturbulence isknown to manifest itself as anomalous resistance. Thus,the plasma is heated at a rate

d Wp/dt=q”Jp2, (1)

where WP is the plasma energy density, T* is the anoma-lous resistivity, and JP is the plasma return current den-sity. At the same time, the macroscopic electric fieldwhich maintains the return current, in order for thebeam to propagate, removes energy from the beam. Inthis fashion, energy is transferred from the beam anddeposited into plasma electrons and ions.

In contrast to resistive heating described above, re-laxation heating according to method (a) results fromthe relative drift between the relativistic beam electronsand plasma electrons. Optimally, these instabilities takethe form of electron bunching at a wavelength of

As(l-4)[l@/nc{c!m– 3)]~ym (2)

and a frequency of

f=[n,.(cm-3)/1016]~THz, (3)

where neis the plasma electron density. The characteris-tic relationship between the plasma and beam speciesfor optimized relaxation heating is illustrated in FIG. 5.Locally, the net current In,, within the beam channelcan exceed the beam current l& in contrast to the mag-netically neutralized beam where I,,C,GO within thebeam channel. As stated, this current muhiplication is aconsequence of momentum conservation, and is a verylocalized phenomenon. The location of the unstablespectrum for these instabilities is indicated by dashedlines in FIG. 5.

The present invention, in contrast to prior art plasmaheating techniques takes advantage of the natural char-acteristics of two extremely powerful microinstabilities,i.e., the two-stream and upper-hybrid instabilities illus-trated in FIG. 5, to locally heat a small volume of

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plasma in the form of an annulus to kilovolt tempera-tures. Essentially, the instabilities are created by therelative drift between the relativistic beam electronsand target plasma electrons. Although a large numberof parameters influence this collective interaction, thedominant factors in determining the strength of theinstabilities are (1) beam temperature along a streamline,and (2) the wavelength of the instabilities relative to theradial dimension of the target plasma.

In prior art experimentation, beam temperature alonga streamline occurs primarily from the passage of therelativistic electrons of the beam through the foil divid-ing the low-density plasma and diode vacuum. Theeffect of the foil can be made negligible by (1) increas-ing the electron energy, (2) reducing the thickness offoil, or (3) reducing the effective Z of the foil material.As a result, a high-voltage, i.e., exceeding 3 MeV, elec-tron beam can, in fact, penetrate a number of foils andstill deposit its energy efficiently in the high-densityplasma.

By utilizing plasmas of high-density, the wavelengthof the instabilities are small compared to the radial di-mensions of the plasma. Thus, although the instanta-neous deposition rate can vary, the nonlinear evolutionof the instabilityy functions to relax the beam distributionin both angle and energy, resulting in an efficient cou-pling of beam energy to the plasma.

The characteristic nonuniform energy deposition ofthe collective interaction, i.e., two-stream and upper-hybrid instabilities along the direction of beam propaga-tion, is illustrated in FIG. 6. A one-dimensional interac-tion is represented by the solid line while the dashed linerepresents a two-dimensional interaction. This nonuni-form deposition property is utilized to concentrate en-ergy deposited into the plasma from the relativisticelectron beam, rather than allowing the energy to dissi-pate its explosive character by expansion into a largevolume of plasma. The initial deposition of beam energyis into plasma electrons which, depending upon theparameters of the device, results, in (1) heating conduc-tion which is used propitiously to obtain power multi-plication, or in (2) current multiplication and confine-ment of the plasma. In this manner, the disadvantages ofpreferential heating of plasma electrons associated withmagnetically confined plasmas is advantageously em-ployed in the present invention.

The potential etllciency of relativistic beam energydeposition into a dense plasma via the streaming insta-bility mechanism has heretofore been unknown in theprior art. FIG. 7 illustrates results of recent experimentsperformed according to the present invention in whichenergy deposition is plotted against plasma density forthe application of a relativistic beam through anode foilshaving various thicknesses. As is apparent from the dataof FIG. 7, a reduction in the anode foil thickness causesa great increase in deposition energy into the plasma.These results show that the basic coefficient of couplinga of the streaming instability deposition varies in thefollowing manner:

a =XSII –exp (–xS/F)]/(1 +x.S9 (4)

where S=@y (nb/2nJ* is the strength parameter, F is afunction depending upon the foil thickness and material,nb is the beam density, ne is the plasma electron density,and x = 1.0- 1.3 is a parameter associated with beampremodulation.

It is therefore apparent from efficiency equation (4)that if either the beam voltage (y) is increased, or the

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10foil function (F) reduced by decreasing the foil’s effec-tive Z or thickness, the factor exp( – @/F) approacheszero, such that the efficiency increases in direct propor-tion to xS/( 1+xS). Thus, the coupling eftlciency islarge for high-density plasma targets when high-voltagebeams are utilized. Moreover, these coupling efficien-cies can be obtained with little or no advancement inpresent relativistic electron beam technology sincebeams with voltage parameters sufficiently high topractice the present invention presently exist. As a re-sult, currently available high-voltage relativistic elec-tron beams are capable of achieving high energy deposi-tion due to the ability of the high-voltage beams topenetrate the anode foil with reduced electron beamscattering. Thus, beams with v/y S 1 achieve muchhigher coupling efficiencies via the streaming instabili-ties than beams with v/y> >1 which are designed tooptimize the resistive heating mechanism when usinghigh-density plasma targets.

FIG. 8 illustrates results of an additional experimentshowing propagation distance in a high-density plasmawith various foil thicknesses and anode-cathode gapspacing. In this experiment a 7 MeV beam was injectedinto a 43 cm Iong, 0.4 torr H2 gas target. No externalmagnetic field was present. The beam energy transmit-ted to a calorimeter located 43 cm from the anode foilwas measured as a function of the anode-cathode gapspacing and anode foil thickness. Anode foils of 25.4pmkapton and 25.4 pm, 76.2 pm, 127.0 pm, and 304.8 pmtitanium were used. FIG. 8 shows a strong experimentaldependence of the transmitted beam energy on theanode thickness and anode-cathode gap spacing. Tencentimeter long witness plates starting at the anode foilon the bottom of the gas container showed significantdamage when the kapton foil was used, but showedlittle or no damage when the thicker titanium foils wereused. The distortion of the anode foil was also found todepend dramatically upon foil thickness. Independentof foil thickness, the center region through which thebeam passed was completely gone. Howe~-~e ob-served debris protruded in the direction of beam propa-gation for the thicker titanium foils, while the kaptonfoil showed debris protruding in the opposite direction.These results indicate formation of hot plasma in thevicinity of the thin foils and severe disruption of beampropagation as foil scattering is reduced due to a mecha-nism which depends upon microscopic properties of thebeam distribution function. Furthermore, the distanceover which such a disruption occurs is approximately 5 -to 10 cm with the kapton foil, while the classical rangefor a 7 MeV electron in 0.4 torr Hz gas is approximately1(Y meter. These observations, as well as the scalingwith the anode-cathode gap, ill ustrate the effect of thestreaming instability.

The basic dependence among the beam relativisticfactor y =(1 –62)– i, beam particle density nb, andplasma electron particle density n,, is given by thestrength parameter S= ~zy (nb/2nJ~. The potentiallylarge coupling eftlciency associated with the relativisticstreaming instabilities is a consequence of the relativisticdynamics, the strength of which depends upon S. Spe-cifically, if an electron undergoes a change in velocity8/3= 8v/c, its change in energy is8Y =ys~f@/(1 + y2&3@. For the streaming instabili-ties, the characteristic change in velocity incurred dur-ing bunching is 8&y- I (nb/2n~)ifl. It follows that

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8y/yd/(I+s) (5)

which can be of order unity.A more detailed one-dimensional analysis indicates

that not all the beam electrons act coherently during thebunching process, since their individual responses varywith energy. Basically, this is due to phase mixing. De-noting a as the coupling coefficient, the one-dimensional analysis yields

a=l.5.S/( 1+1.5 S)s/2, (6)

which maximizes at Ss0.45 with asO, 19. This ratherhigh optimum efficiency for a one-dimensional analysisis still believed to be the maximum efficiency obtainableby the great majority of the plasma physics community.

In reality, the assumption that the nonlinear state isone-dimensional as shown by the solid line in FIG. 6, isphysical] y incorrect for an optimized interaction andmore clearly resembles the dashed line of FIG. 6, whichis the result of a two-dimensional analysis. Since therelativistic electron beam will relax strongly in bothenergy and angle, it is necessary to carry out a self-con-sistent, two-dimensional, fully relativistic nonlinearcalculation to determine the coupling coefficient. Suchcalculations can only be carried out using advancedparticle code techniques. Because such codes are expen-sive to run and cannot be employed for all physicalparameter regimes of interest, an analytical procedureor model has been developed which determines the

.\_. magnitude of various parameters for optimal interac----tions which is disclosed in Los Alamos Scientific Re-port LA-7215-MS (April 1978) by Lester E. Thodeentitled “Preliminary Investigation of Anomalous Rela-tivistic Electron Beam into a 1017to 1020cm– 3 DensityPlasma” available at the Library of Congress. From thismodel and extensive numerical particle simulations, thephase mixing present in the one-dimensional analysiscan be overcome by the angular relaxation of the beam,and an optimal coupling efficiency of

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12sustain a high coupling efficiency over the entire beampulse, as pointed out above.

Beam temperature along a streamline can result fromthe random motion associated with the temperature ofthe cathode surface. However, transverse temperaturesof 300-lCQO eV at the emission surface are requiredbefore this source of random motion begins to degradethe interaction. Due to the high-voltage applied to thecathode, electrons are field emitted with typical trans-verse energies of 1 to 20 eV. Thus, this source of ran-dom motion is negligible in the invention.

A possibly more serious source of random motion iselectron emission from the cathode shank and lack ofbeam equilibrium at the emission surface. However, byshaping the cathode and anode surfaces properly, andby simultaneously applying an external magnetic fieldto the diode region, this source of random motion canalso be reduced to a negligible level.

In fact, beam temperature along a streamline seems toresult primarily from the passage of relativistic elec-trons through thin foils. Extensive analysis has shownthat the effect of such a foil on the interaction can bemade” negligible. The effect of a foil is to reduce thefraction of beam electrons An/nG which can act coher-ently during the development of the instability. Thisfraction is determined as follows:

&/nb= 1–LXp(–xs/~. (8)

Tyical values for the foil scattering function (F) aregiven in the table of FIG. 9. It follows that increasing -yand decreasing the foil’s effective thickness results inthe factor exp(– xS/F) approaching zero. Thus, thebeam can penetrate a closed container and retain a highcoupling efficiency to the enclosed target plasma.

It- has- generally~been argued that th~ transverse mo-tion associated with the beam self-fields comprises aneffective temperature. If no external magnetic field is

in present and the beam is injected into a plasma in order. .aopfimd=W 1+s) (7)

appears achievable.The factors which influence the coupling coefficient

include the following: 45(1) beam relativistic factor,(2) beam particle density,(3) plasma particle density,(4) beam temperature along a streamline,(5) Larmor radius effects resulting from radially de-

pendent ordered transverse motion,(6) wavelength of instability relative to radial size of

the beam and plasma,(7) radial plasma gradients,(8) externally applied magnetic field strength,(9) plasma temperature,(10) electron-ion and eletron-neutral collision rate,(11) ionization state of plasma and ionization gradi-

ents,(12) plasma hydrodynamic gradients,(13) beam pinching resulting from current multiplica-

tion,(14) premodulation, and(15) time dependence of the electron beam power.It has been found that the electron random motion or

temperature along a streamline and the wavelength ofthe instabilities relative to the radial size of the plasmaprimarily determines the ability of the interaction to

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to obtain equilibrium, such ordered motion can evolveinto random motion. However, for the optimized inter-action, the coherence length of the beam is long relativeto the deposition length. Thus, high-voltage, low v/ybeams in a focused flow configuration can interactstrongly with a plasma, provided the plasma begins atthe anode foil and hr/n&l.

If the target plasma is also high-density, the wave-length associated with the streaming instabilities is veryshort compared to the radial dimensions of the beamand plasma, Eq. (2). Under these conditions, the optimalnonlinear evolution of the instability is highly two-di-mensional, and once initiated is extremely difficult todegrade. The formation of plasma hydrodynamic gradi-ents and beam pinching due to current multiplicationresults in the instantaneous deposition rate varying intime. Such a time variation is not monotonic, however.

The distance over which the relativistic electronbeam can deposit upwards of S/(1+ S) of its kineticenergy is

L,\f=10y(llc./~b)iC/lOp (9)

where tofl is the target plasma frequency and c is thespeed of light. This is orders of magnitude shorter thanthe classical range of megavolt electrons in a 1017– 1020cm– ~ density plasma. For example, if n~(-y) is deter-mined from one-dimensional, relativistic foil diode re-

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suit, such as disclosed by H. R. Jory and A. W. Trivel-piece, J. Appl. Ph ys. 40, 3924 (1969),

L,v=r(y)(d2/M)~cm. (lo)

in E/q. (10) the diode gap spacing is d and the adiabaticcompression ratio is M, The dimensionless parameterr(y) is shown for given values of the plasma electrondensity n,. in FIG. 10. Because wave e-fold from noise,most of the beam energy is actually deposited over a!ength shorter than Ljvby a factor of 2 to 3. The charac-teristic nonuniform energy deposition of the collectivemechanisms, two-stream and upper-hybrid instabilities,is shown in FIG. 6, for both the one- and two-dimen-sional interactions. According to the present invention,this nonuniform deposition property is utilized to con-centrate energy deposited into the plasma from therelativistic electron beam, as opposed to prior art exper-imentation wherein energy is allowed to dissipate itsexplosive character by expansion into a much largervolume of plasma.

Two basic approaches for utilizing a relativistic elec-tron beam driven, high-energy density plasma to pro-duce radiation, neutrons, and/or alpha particles arepossible.

The first approach is a direct use of the plasma as thesource by confining its energy for a sufficient length oftime as disclosed in copending application Ser. No.882,024 entitled “Device and Method for ElectronBeam Heating of a High Density Plasma” tiled Feb. 28,1978 by Lester E, Thode. According to this approach,a solid relativistic electron beam penetrates a 3 cm3 to50 cmJ gas filled container, and transfers a fraction of itsenergy and momentum to the enclosed gas. Conserva-tion of energy and momentum requires that the beamboth heat the plasma and drive a large axial plasmacurrent. The presence of the large axial current, in turn,initiates additional plasma ion heating and confinement.This configuration is similar to a dense Z-pinch. At highplasma density the option for predominantly heatingelectrons or heating both electrons and ion exists. Thisis possible because the classical equipartition time be-tween the plasma species and the anomalous elecronand ion heating rates can be varied significant] y. FIG.11discloses a schematic diagram of the major compo-nents of a device which uses the high-energy densityplasma as a source.

According to the present invention, an annular rela-tivistic electron beam is utilized to penetrate a 3 cmJ to50 cms gas filled container, and to transfer a fraction ofits energy and momentum to the enclosed gas. Again,conservation of energy and momentum causes the beamto both heat the plasma and drive a large axial plasmacurrent. Since the heated plasma is annular, the largeaxial current leads to directed heat flow towards theinterior of the annular region, where a fast liner is dis-posed which is engulfed and driven inwardly by hotelectrons to implode a structured microsphere. The fastliner functions as a power multiplier, which is cylindri-cal, spherical, or ellipsoidal in shape. By adjusting theelectron heating rate and plasma density, the liner canbe driven by either ablation or exploding pusher toimplode the structured microsphere. Also, control ofthe driving electron temperature and distribution isaccomplished by varying the plasma density and magni-tude of the external magnetic field. A conceptual dia-gram of the major components of a system to utilize a

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14high-energy density plasma to drive power multiplica-tion conversion devices is shown in FIG. 12.

Many modifications and variations of the configura-tions shown in FIGS. 11 and 12 are possible. For exam-ple, various applications of the concept do not requirethe use of a low-density gas chambers 52 and 94, modu-lators 38 and 80, drift tubes and adiabatic compressors36 and 78, or multigap accelerators 32 and 74. Withadvances in relativistic electron beam technology, ex-ternal magnetic field sources 70 and 110, preionizers 62and 64, 104 and 106, and windows 54 to 60, and 96 to102 can be eliminated. With annular beams, multiplebeam systems are possible, as depicted in FIGS. 13through 15. For multiple beam systems, the energydeposition regions do not overlap, allowing such sys-tems to drive larger power multiplication devices.

To practice the present invention, a high-voltage,high-current density relativistic electron beam is re-quired for the reasons set forth above. Presently, a num-ber of high-impedance generators are in use, such as thePI23-1OO, PI1 5-90, P114-80, and P1950 which are sche-matically illustrated in FIG. 16. Here, PI refers to thePhysics International Company, the first number is thediameter of the Blumlein in feet, and the second numberis the number of stages in the Marx generator. As shownin FIG. 16, the generators are relatively compact in sizefor the energy delivered. Also, the time to design andbuild such generators is relatively short. For example,the P114-80 was recently designed and built in eightmonths. As shown in FIG. 17, the cost of the technol-ogy is relatively inexpensive. State of the art generatorsproduce a 16 to 20 MeV, 400 to 800 kA electron beamwith a pulse width of approximate] y 100 ns. The overallelectrical efficiency for such a generator is approxi-mately 40%%to 459%. If the energy remaining in theMarx generator is recovered, the energy efficiency ofsuch a generator is 80% to 90Y0.

As shown in FIG. 18, high-impedance generators arecomposed of five basic components. A dc chargingsystem 116 is used to charge the Marx generator 118,which is the primary energy storage component. TheMarx generator 118 consists of a large number of stageswhich are charged in parallel and discharged in seriesusing spark gap switches. FIG. 19 schematically illus-trates the electrical equivalent of a Marx stage whichconsists of two capacitances 126 and 128 connected inseries with a center ground to allow positive and nega-tive dc charging.

The Marx generator is then used to charge a Blumlein120 such as schematically illustrated in FIG. 20. ABlumlein 120 is essentially two coaxial transmissionlines 130 and 131 connected in series with the diodeimpedance 134 ZD. Physically, the Blumlein appears asthree concentric, annular conductors. This folded con-figuration is used to reduce the spatial dimension of theBlumlein. In operation, the center conductor 132 ischarged through an inductor 138 having an inductanceL. which appears as a short. Once charged, the switch136 is closed and the transmission line 131 begins todischarge with a pulse propagating toward the diode134. When the pulse hits the impedance discontinuity(,Z~) of diode 134, a voltage appears across the diode134. As opposed to the shorted transmission line 131,which has an impedance Z/, the transmission line 130,which impedance ZO, is open. Thus, for a properlymatched configuration (ZO= Z]= ZD/2), a voltageequal to charge voltage on the inner conductor 132appears across the diode 134 for a period twice the

4.263.095,—-15

propagation time down any one of the transmission lines130 or 131. The inductor 138 appears as an open circuitduring the Blumlein discharge. For high voltages theBhrmlein 120uses transformeroi1 as a dielectric.

Due to the physical configuration of the Blumlein120, it is difftcult to design the transmission lines 130 and131 such that Zr=ZO. Asaresuh, there is typically avery small, but non-negligible, voltage that appearsacross the diode 134during the Blumlein charge, due tostray capacitances andinductances which is referred toas a prepulse. From the standpoint of proper operationof a high-current density diode, this prepulse must besuppressed. Significant progress inprepulse suppressionhas occurred in the past few years. Through the use ofprepulse switches 122 combined with careful design ofthe feed and diode region, a prepulse of less than 50 kVhas been demonstrated fora9 Blumlein charge. Withthis advance in prepulse suppression, beam particledensities exceeding 1014 electrons/cm3 have been ob-tained in a focused flow configuration. More recently,however, a technique utilizing water as a dielectric,rather than oil in a Blumlein configuration, has beendeveloped by Maxwell laboratories of San Diego,Calif., which reduces prepulse voltage to less than 1 kVfor multi-megavolt beams. This very low preptdse volt-age provided by the Maxwell Laboratories configura-tion appears to be the preferred method of operation.

The final component is the diode 124, which can beeither foil or foilless. Foil diodes suffer rapid impedancecollapse when the current density exceeds 20 kA/cm2.The physics of this problem has not, however, beenconsidered in a systematic fashion and current densitiesup to 100 kA/cm2 should be obtainable with improvedvacuum systems.

Foilless diodes are naturally suited for the device ofthe present invention since annular beams are readilyproduced at high current densities. However, the opera-tion and flow characteristics of such diodes could besignificantly advanced. A detailed discussion of thepotential of the foilless diode is disclosed in Los AlamosScientific Report LA-7169-P by Lester E. Thode enti-tled “A Proposal for Study of Vacuum Adiabatic Com-pression of a Relativistic Electron Beam Generated bya Foilless Diode. ”

Pulsed high-current electron beams with particleenergy exceeding 20 MeV can be produced with a mul-tigap accelerator, as schematically y illustrated in FIG.21. The mukigap accelerator is basically a linear accel-erator with radial transmission lines or Blumleins pro-viding energy to the accelerating gaps 146. Radial lines140 are composed of coaxial disk or cone conductorswhich are stacked in series. As a result, the acceleratoris amenable to mass production, probably at a cost ofless than $5/joule delivered. In addition, the develop-ment time of a 200 to 800 kJ, 5 to 20 TW, 10 to 100 cycleper second prototype accelerator is less than five years.The injector 144 for such an accelerator can be thehigh-current electron beam generator disclosed infra, orthe first accelerating stage of the accelerator. Fabrica-tion of such accelerators is disclosed by A. I. Pavlovskiiet al., Sov. Phys. - Dokl. 20, 441 (1975), in an articleentitled “Muhielement Accelerators Based on RadialLines. ”

Referring again to FIGS. 11 and 12, the relativisticelectron beams 34 and 76 propagate along the vacuumdrift tube and adiabatic compressors 36 and 78 to themodulators 38 and 80. External solenoidal magneticfield sources 40 and 82 generate a magnetic field in the

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16generator diode, accelerator, drift tube, and modulatorregions to ensure a laminar flow beam equilibrium, Inthe vacuum drift tubes 36 and 78, the strength of theexternal magnetic field can be increased along the direc-tion of beam propagation to produce adiabatic beamcompression. Modest compression ratios can reduce thebeam radius a factor of 2 to 3, while preserving a lami-nar flow equilibrium, provided the compression is car-ried out in vacuum. Vacuum systems 42 and 84 maintainthe required vacuum.

Modulators 38 and 80 constitute an inner portion ofthe vacuum drift tubes 36 and 78 and are formed by aperiodic structure or dielectric layer along the directionof beam propagation. Alternatively, a rippled magnetictieId could be utilized to weakly bunch the beam, Thepurpose of modulators 38 and 80 is to provide an en-hanced narrow band noise level (very weak modula-tion) at a wavelength and phase velocity slightly belowthe natural wavelength and phase velocity of the insta-bility in the target plasma.

The underlying idea behind this weak modulation isincreased coupling efllciency. For waves propagatingalong” the relativistic electron beam axis, the characteris-tic growth rate 8/cop and characteristic change in beamvelocity fi~ = 2(j3 —co/kc) for the streaming instabilityas a function of the wave number k =2w/1. is shown inFIG. 22. Here oJ/k is the phase velocity associated withthe electrostatic spectrum, and v is the initial beamvelocity. The growth rate is normalized to the plasmafrequency tiP. For an unmodulated beam the nonlinearevolution of the streaming instability is determined bythe fastest growing wave, which occurs at kv/oP= 1.1in this example, The beam energy loss is determined by

8y/y=y2/38fl(unmodulated) /[1 + y2/3 M3(unmodulated)] (11)s .s/(1 + s)

as described previously.By enhancing the noise level at a wavelength and

phase velocity slightly shorter and slightly lower thanthe fastest growing wave, the beam energy loss is deter-mined by flti(modulated) shown in FIG, 22, The cou-pling efficiency is then increased to

8y/y = y2/3 8fl(modulated)/[1 + Y2B 6~(modulat.cd)] (12)=y.s/(I + x.sl,

where x=1.0 to 1.3 based upon analysis of the mod u-lated interaction, Physically, the modulation leads to anenhanced strength parameter. The modulation alsoreduces the effect of foil scattering and collisions on theinteraction.

The low-density gas chambers 52 and 94 provideisolation between the replaceable target plasma contain-ers 66 and 108 and modulators 38 and 80, drift tube andadiabatic compressors 36 and 78, accelerators 32 and 74,and generators 30 and 72 of FIGS, 11 and 12, respec-tively. The electron density in the ionized low-densityplasma channels 46 and 88 is typically close to the rela-tivistic electron beam density, whereas in the targetplasmas 68 and 112 the electron density is 4 to 6 ordersof magnitude above the beam density. The low-densitygases 50 and 92 comprise either Hz, He, Ar, N2 or resid-ual gas associated with the previous operation of thesystem.

4.263.09517

Foils 44 and 86 provide isolation between the vacuummodulators 38 and 80 and the low-density plasma chan-nels 46 and 88, and convert a small fraction of the risingbeam impulse into Bremsstrahlung radiation which isdirected predominantly along the direction of beampropagation. The isolation function is provided by alayer of metal (titanium, aluminum, or beryllium),graphite, or plastic, such as mylar (C1O H8 04), kapton(C22 Hlo N2 0s), or polycarbonate. A layer of plasticimpregnated with high Z atoms, a fine mesh high Zwire screen with a very high optical transparency, or ahigh Z aperturing layer can be used to provide theBremsstrahlung radiation. Bremsstrahlung radiationgenerated in this manner, aids in the creation of low-density plasma channels 46 and 88 for beam propagationthrough the low-density gases 50 and 92. With advancesin relativistic electron beam technology, foils 44 and 86can be eliminated in favor of strong differential pump-ing of the modulator regions 38 and 80.

Foils 48 and 90, provide isolation between the low-density plasma channel 46 and the dense target plasmaand are constructed in a fashion similar to foils 44 and86. Foils 48 and 90 also act to initiate the collectiveinteraction and generate Bremsstrahlung radiation forpartial ionization of the dense plasma target to assist orreplace preionizers 62 and 64, 104 and 106,

In the ionized low-density plasma channels 46 and 88and in the target plasma, the self-fields of the beam areshorted out so that an external magnetic field is notrequired to achieve beam equilibrium. Thus, the beamcan be ballistically guided through low-density plasmachannels 46 and 88 to the plasma target. However, theoverall efficiency of the system is enhanced by the pres-ence of external magnetic field sources 70 and 110.Also, external magnetic field sources 70 and 110 pro-vide increased stabilization of the relativistic electronbeam within low-density plasma channels 46 and 88,respective] y.

Preionizers 62 and 64, 104 and 106 provide full ioniza-tion of target plasmas 68 and 112, respectively, Anynumber of devices for creating a fully-ionized gas, suchas discharge tubes, channel forming wires, various la-sers including electron beam driven free electron lasers,plasma guns, microwave generators, or low-energyparticle beams, can be used. The laser, however, is thebest device for creating a low-temperature, fully-ion-ized plasma in the 101T to 1020 electrons/cmJ densityregion,

With a laser preionizer, windows 54 to 60 and 96 to102, formed from sapphire, salt, or other appropriatematerials, are positioned in the low-density gas cham-bers 52 and 94 and target plasma containers 66 and 108,respectively. For a fully-ionized target density of8X 101Sto 1020electrons/cmj, a 0.1 ps to 2.0 ps, 0.2 kJto 10 kJ HF laser, or a number of smaller HF lasers, canbe used as preionizers 62 and 64, 104 and 106. A 0.1 psto 2.0 ps, 0.2 kJ to 3 kJ C07, laser, or a number of smallerC02 lasers, are appropriate for fully ionizing gases withdensities less than 8 x 10Ig electrons/cmJ.

The combination of Bremsstrahlung radiation pro-duced at foils 48 and 90, direct impact ionization by thebeam, avalanche, and the initial collective interaction iscapable of fully ionizing the target plasma. However,the beam requirements are more stringent when therelativistic beam provides both ionization and heating ofthe target plasma, Use of preionizers 62 and 64, 104 and106, therefore lowers the relativistic electron beamtechnology requirements.

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18The anomalous pinch, as disclosed in the above refer-

enced copending application and illustrated in FIG. 11,is the simplest mode of operation and provides a basisfor employment of the fast liner device comprising thepresent invention. The relativistic electron beam targetis a simple gas-filled container of DT, DD or HB. As aneutron source, the anomalous pinch intrinsically re-quires a very high-density plasma of at least 1019 elec-trons/cm3.

As an alternative approach to a pulsed neutronsource, the anomalous pinch can be operated as a targetfor an intense deuterium beam generated by the rapidlydeveloping pulse power light-ion beam technology.Operating with a plasma density of approximately 101selectrons/cmj, the plasma electron temperature can beelevated sufficiently to reduce the cross section fordeuterium beam energy absorption by target plasmaelectrons. Thus, the probability of survival of trappedenergetic deuterium ions to undergo fusion with theplasma deuterium and tritium ions is significantly en-hanced. Although this two component concept is old,intense neutron pulses can be produced using presentpulse power technology.

Moreover, a moderate Z gas or a mixture of H2 andhigh Z gas with an electron density of 1017– 10I9 cm–Jcan be used as the target plasma 68 of the device of FIG.11 to produce radiation. In the radiation mode, beryl-lium windows in the target plasma container 66 are usedand the low-density gas chamber S2 is eliminated. Sucha tunable radiation source is suitable for a variety ofapplications.

The device of FIG. 11 operates by applying the rela-tivistic electron beam 34 to low-density gas chamber 52such that beam 34 penetrates foil 48 with negligiblescattering and initiates convective wave growth suchthat the waves e-fold until saturated through nonlineartrapping of the beam electrons. Since the nonlinearwaves are not normal plasma modes, they are absorbedinto the plasma very rapidly through nonlinear modebeating. Actually, this nonlinear mode beating actsthroughout the entire interaction and keeps the level ofelectric field energy relatively low compared with theenergy transferred from the beam to the plasma. Thepresence of the foil 48 thus ensures that the beam energyis deposited at a specified location within the targetplasma container 66, as opposed to moving upstream.

Since energy and momentum are being transferredfrom relativistic electrons 34 to nonrelativistic electronswithin the target plasma 68, the beam both heats anddrives an axial current in the target plasma. The pres-ence of the axial current, in turn, initiates plasma energyconfinement through the generation of a azimuthalmagnetic field similar to a Z pinch. Taking into accountincreased internal pressure resulting from the nonohmicprocess, an equilibrium pinch configuration is formedwith currents in the muhi-megampere range, with sig-nificant reduction in heat conduction losses. Relative tothe typical classical Z-pinch, the generation of theanomalous pinch is considerably faster.

For a solid relativistic electron beam schematicallyillustrated in FIG. 11, the anomalously generated azi-muthal magnetic field 150 and heated plasma column148 is illustrated in FIG. 23. The axial nonuniformity inthe azimuthal magnetic field strength of azimuthal mag-netic field 150 is similiar to the energy deposition illus-trated in FIG. 6. Primary energy loss from the anoma-lous pinch is indicated by arrows. The presence of an

4.263.095.,—-19

external axial magnetic field and proximity of the radialwall, together provide stable operation.

FIG. 24 is a schematic illustration of the arrangementschematically disclosed in FIG. 11 and disclosed in theabove referenced copending application for producingan anomalous pinch. As shown, relativistic electronbeam generator 152 produces a solid relativistic beam154 which propagates through the vacuum tube andadiabatic compressor 156 and adjacent modulator 158.The relativistic electron beam 154 then penetrates foil160, passes through the low-density plasma channel 162,penetrates foil 164, and anomalously transfers a fractionof its energy and momentum to the target plasma 170 togenerate the anomalous pinch illustrated in FIG. 23.Windows 172 and 174 allow the laser ionization beam178 to penetrate the target plasma container 168 andlow-density gas chamber 166. A salt or sapphire win-dow is used for C02 or HF lasers, respectively. Anionization beam intensity of 109 to 1010 watts/cm2 issufficient to fully ionize the plasma.

A fully-ionized plasma with sufficient axial unifor-mity can be formed using the configuration shown inFIG. 24. The laser energy is transferred to the targetplasma through inverse Bremsstrahlung. Consequently,the target plasma exhibits a slightly decreasing gradientalong the direction of propagation of the relativisticelectron beam 154. Such a decreasing gradient tends toincrease the strength of the deposition, since its effecton the nonlinear dynamics is similar to premodulation.In fact, the ability of the streaming instabilities to coun-teract self-consistent plasma hydrodynamic gradients isrelated to this dynamic effect.

FIG. 25 is an alternative arrangement in which twolasers 208 and 210 apply ionization beams 212 and 214transverse to the axis of the relativistic electron beam182. Windows 204 and 206 in the low-density gas cham-ber 194 and windows 200 and 202 in the target plasmacontainer 196 allow passage of the ionization beams tothe target plasma 198. FIG. 26 is a schematic end viewof an additional alternative arrangement utilizing threelasers 234,236, and 238 which produce ionization beams240, 242 and 244. Windows 228, 230 and 232 in thelow-density gas chamber 216 and windows 222,224 and226 in the target plasma container 218 provide ioniza-tion beams 240, 242 and 244 access to the target plasma220. The advantage of the arrangement shown in FIG.26 is that lasers 234, 236 and 238 are arranged in anoff-axis position such that laser beams 240, 242 and 244are not directed at other lasers.

Although the single laser beam configuration shownin FIG. 24 produces the desired target plasma, addi-tional magnetic field energy is required to deflect theresidual relativistic electron beam so that the electronbeam does not impinge upon laser 176. Also, the costand technology associated with a single large laser isgreater than with a number of smaller lasers with thesame combined energy. Thus, the multiple laser config-urations shown in FIGS. 25 and 26 are considered thepreferred method of operation.

The preceding laser configurations m-e also appropri-ate for systems which use the high-energy densityplasma to drive a fast liner to implode a structuredmicrosphere according to the present invention, sche-matically shown in FIG, 12. Since the laser intensity isquite low, the hot electron spectrum generated by sucha beam interacting directly with a power multiplicationdevice is negligible. The components of FIG. 11 andtheir operation are identical to the components of FIG.

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2012 with the exception of the target plasma container 66and relativistic electron beam 34. Similarly, the deposi-tion of electron beam energy in the target plasma 112occurs in the same manner described with respect toFIGS. 5, 6, 11 and 22 to 26. Therefore, the remainingdisclosure of FIGS. 27 through 38 relates only to themanner in which a hot annular plasma 112 drives a fastliner according to the invention disclosed in copendingapplication Ser. No. 9,703 entitled “Device and Methodfor Relativistic Electron Beam Heating of a High-Den-sity Plasma to Drive Fast Liners” filed Feb. 5, 1979 byLester E. Thode, and a fast liner to implode a structuredmicrosphere according to the present invention.

Historically, high explosives or magnetically driventhin, cylindrical metallic shells have been referred to asliners. These hybrid devices incorporate concepts com-mon to both magnetic and inertial confinement ofplasma. Liners have been used to compress magneticfields, compress and heat magnetically confined plas-mas, and generate radiation. According to the presentinvention, this type of power multiplication device canbe generalized to include spherical and ellipsoidalshapes. Since the liners are multilayer in design, they aremuch like laser fusion pellets.

A configuration suitable for driving fast spherical orcylindrical liners, such as disclosed in the above refer-enced copending application Ser. No. 9,703 entitled“Device and Method for Relativistic Electron BeamHeating of a High-Density Plasma to Drive Fast Lin-ers” filed Feb. 5, 1979 by Lester E. Thode, is shown inFIGS. 27 and 28, respectively. As shown in FIGS. 27and 28, a single laser ionization beam 252 enteringthrough window 254 is used. Multiple laser ionizationconfigurations, as shown in FIGS.” 25 and 26, may beemployed to obtain full ionization. The use of lasers forpreionization lowers the relativistic electron beam tech-nology requirements as disclosed above. Thus, the laserionization sources should be considered as optional.

Referring again to FIGS. 27 and 28, an annular rela-tivistic electron beam 260 which corresponds to beam76 produced by the device of FIG. 12, penetrates theinitiation foil 246, which also acts as an end plug tocontain the low-temperature plasma or gas. As the volt-age and current density rise, the anomalous couplingcoefficient increases to its optimal value, and the beamtransfers a large fraction of its energy and momentum tothe annular plasma region 258. The beam driven azi-muthal magnetic field 256, in turn, directs the annularplasma thermal energy to the fast spherical liner 250 orfast cylindrical liner 262. Since the source of the azi-muthal magnetic field 256 is the result of an axial cur-rent flow in the annular plasma 258, magnetic field 256is not present in the vicinity of the fast spherical liner250 or fast cylindrical liner 262, The presence of an axialexternal magnetic field generated by external magneticfield source 110 shown in FIG. 12, can be used to in-crease the anomalous coupling coefficient. However,since the annular plasma column 258 is very high beta,the external magnetic field produced by source 110 isexcluded during operation.

The radial wall of the plasma target container 248 issufficiently thick to ensure magnetic flux containmentand sufficiently massive to provide radial inertial con-finement (tamper) on the relativistic electron beam timescale, i.e. S IMl ns. Thus, radial energy loss to the con-tainer wall is limited by both the azimuthal magneticfield 256 and excluded external magnetic field producedby source 110. Heat conduction is limited axially on the

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beam time scale by the lower axial temperature gradi-ent, azimuthal magnetic field 256, and self-mirroring ofthe external magnetic field 110. Thus, the geometrytakes advantage of anomalous coupling and classicalheat conduction to rapidly and efficiently remove en-ergy from the relativistic electron beam 260 and trans-port it to the fast liner.

A cross-sectional view of the basic configurationwith dual laser ionization beams 268 and 270, 282 and284 for driving fast liners as disclosed in the abovereferenced copending application is shown in FIGS. 29and 30, respectively. In this configuration, windows 264and 266, 278 and 280 are placed in the radial wall of theplasma container. The heated annular plasmas 274 and286 drive the spherical and cylindrical fast liner to im-plosion by explosive or ablative means as disclosedpreviously.

Detail of the fast spherical liner 250 and fast cylindri-cal liner 262 as disclosed in the above referenced appli-cation is shown in FIGS. 31 and 32, respectively. Eachof the fast liners consists of ablators 292 and 298, push-ers 294 and 300 and solid buffers 296 and 302. The abla-tor is boiled off through heat conduction, to propel thepusher and solid buffer to high-implosion velocity.Since the thermal conductivity of a plasma is a strongfunction of temperature, the rate at which energy istransported to the liner increases in time throughout thebeam pulse. Thus, some natural shaping of the plasmadriving source is obtained. Such shaping leads to stron-ger compression and heating of the gas fuel 272 and 288,as disclosed by R. J. Mason et al., Phys. Fluids 18, 814(1975) and S. D. Bertke et al., Nucl. Fusion 18, 509(1978).

Structured spherical pellets similar to the liner 250have been studied extensively with respect to laser im-plosion. The ablators 292 and 298 of both the sphericaland cylindrical liners are a low Z, low mass densitymaterial such as LiDT, Be, ND3BT3, boron hydride, orCDT. Pushers 294 and 300 are typically a higher Z andmass density material such as glass, aluminum, gold, ornickel. Plastic embedded with high Z atoms is also used.Solid DT or LiDT can be used for the solid buffers 296and 302. Depending upon the desired implosion veloc-ity and various stability considerations, the total mass offast liner 250 and 262 varies from 1 to 100 milligrams.

In the case of the cylindrical liner 262, shaping mini-mizes loss of the enclosed fuel 288 out the ends, asshown in FIG. 32. Alternatively, the ends can beplugged if the annular plasma 286 and fuel 288 are dif-ferent. For example, gas fuel 272 and 288 for both spher-ical and enclosed cylindrical liner may comprise DT,DD, DHeJ, HLib, or HB II whereas the target plasma274 and 286 may comprise H2, He, DT, DD, or otherlow Z gas. An ellipsoidal shaped liner can also be used.

FIG. 33 illustrates another embodiment utilizing afast liner. According to FIG. 33, a solid beam penetratesfoil 320 to form an anomalous pinch 318 within thecylindrical liner 262, which is, in turn, driven by anannular beam entering through foil 304. Deflector 306provides initial ionization in the anomalous pinch region318. The cylindrical liner 262 implodes upon the plasma318 to enhance compression and burn.

Target geometry, utilizing two annular relativisticelectron beams to drive a spherical liner 250 is schemati-cally illustrated in FIG. 34. In operation, beam deflec-tion is minimized as beams 326 and 328 pass through thebeam driven azimuthal magnetic field regions, as indi-cated in FIG. 14.

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22According to the present invention, the approach of

obtaining an intense radiation, neutron and/or alphaparticle source is to use a high-density, multi-kilovoltrelativistic electron beam produced plasma to drive afast liner, which then produces an intense, shaped en-ergy pulse suitable for imploding a structured micro-sphere such as disclosed R. J. Mason, Phys. Fluids 18,814 (1975) and Los Alamos Scientific Report LA-5898-MS (October 1975), S. D. Bertke et al., Nucl. Fusion 18,509 (1978), and G. S. Fraley et al., Phys. Fluids 17, 474(1974). Such a microsphere can be filled with either DT,DD, DHeJHLiG, or HBI 1, for example.

The basic geometry of the approach of the presentinvention is illustrated in FIGS. 35 and 36, for a singlelaser ionization beam 340 entering through window 342.The multiple laser ionization configurations as shown inFIGS. 25 and 26 can also be used. As pointed out above,the use of lasers for preionization, lowers the relativisitcelectron beam technology requirements. Therefore, thelaser ionization sources should be considered as op-tional.

An annular relativistic electron beam 344 penetratesthe initiation foil 346, which also acts as an end plug tocontain the low-temperature plasma. As the voltage andcurrent density rise, the anomalous coupling coefficientincreases to its optimal value, and the beam transfers alarge fraction of its energy and momentum to the annu-lar plasma region 348. The beam driven azimuthal mag-netic field 350, in turn, directs annular plasma 348 ther-mal energy to the fast spherical liner 352 of FIG. 35, orfast cylindrical liner 354 of FIG. 36. Since the source ofthe azimuthal magnetic field 350 is the result of an axialcurrent flow in the annular plasma 348, resulting fromconservation of momentum, the azimuthal magneticfield 350 is not present in the vicinity of the fast spheri-cal liner 352 or fast cylindrical liner 354, The presenceof an axial external magnetic field generated by source110 of FIG. 12 can be used to increase the anomalouscoupling coeftlcient. However, since the annularplasma column 348 and plasma engulfing the liner arevery h