Electron beam device

Abstract

An electron beam device in which a hollow target is symmetrically irradiated by a high energy, pulsed electron beam about its periphery and wherein the outer portion of the target has a thickness slightly greater than required to absorb the electron beam pulse energy.

Description

BACKGROUND OF INVENTION

Many more techniques and devices are being developed in order to produce high energy plasmas and other materials which may result in fusion or the like, or to be used for material studies, testing of deposition of high energy in materials or for similar purposes. One such technique or device is high intensity electron beam generating machines. Such machines are being built or developed which are capable of producing electron beams in pulses having currents of from about 100,000 to greater than 1,000,000 amperes at voltages of from about 100,000 volts to greater than 10,000,000 volts in time periods of 100 nanoseconds or less.

In order to achieve the desired interactions and reactions of the materials to achieve these purposes, the electron beam device should desirably focus this beam energy to a very small point or space in a symmetrical or uniform manner. This is particularly true in plasma and fusion-type applications where an amount of material to be ionized or fused must be heated to very high temperatures in the neighborhood of from about 10.sup.5 to 10.sup.8 .degree.C and compressed to extremely high densities. It has been found to be difficult to achieve desired high energy electron beam or beams having suitable energy and pulse durations and then to focus the so-produced electron beams to a desired size which may be absorbed in a time period and configuration which would produce significantly high density and high temperature plasmas and/or fusion.

Attempts to achieve nuclear fusion with an electron beam are referred to by Rudakov and Samarsky in the Proceedings of the 6th European Conference on Controlled Fusion and Plasma Physics, July-August, 1973, at pages 487-490, wherein an electron beam would be directed against a target from one side with the target and beam parameters selected to have the beam penetrate target and enclosed fuel material to the targets' far side. Interior portions of the target would be vaporized and the vapor accelerated against the fuel material. The vapor would be to act as a relatively hot, low density tamping material which might not be effective in producing the levels of fuel compression desirable in many fusion and plasma applications.

SUMMARY OF INVENTION

In view of the above, it is an object of this invention to provide an electron beam device which includes a target capable of absorbing and utilizing a high energy electron beam pulse for symmetrical compression thereof.

It is a further object of this invention to provide an electron beam device which may deposit a high energy, electron beam pulse into a spherical target in nanosecond time periods at high energies.

It is a further object of this invention to provide a novel electron beam implosion target.

Various other objects and advantages will appear from the following description of the invention, and the most novel features will be particularly pointed out hereinafter in connection with the appended claims. It will be understood that various changes in the details, materials and arrangements of the parts, which are herein described and illustrated in order to explain the nature of the invention, may be made by those skilled in the art.

The invention comprises a target composed of an outer hollow shell of electron beam absorbing material having a thickness to diameter ratio of about 1 to 10 or 20 and a material enclosed within the shell which it is desired to implode, and means for producing a high energy electron beam pulse deposited in the outer surface of the target for vaporizing outer portions of the shell and for imploding inner portions of the shell against the enclosed material.

DESCRIPTION OF DRAWING

The invention is illustrated in the accompanying drawing wherein:

FIG. 1 is a partially cutaway view of a target including the features of this invention;

FIG. 2 is a cutaway and somewhat diagrammatic view of an electron beam device which may be utilized to irradiate the target illustrated in FIG. 1;

FIG. 3 is a graph illustrating the effects produced by electron beam irradiation of the target of FIG. 1; and

FIG. 4 is an elevation view of an alternate target and anode arrangement which may be utilized in the device of FIG. 2.

DETAILED DESCRIPTION

The target 10 which may be utilized in the electron beam device of this invention is illustrated in FIG. 1. Target 10 includes a hollow spherical shell 12 which is formed of an appropriate material of prescribed shell thickness and diameter. The shell material should be such that will efficiently absorb a beam of high energy electrons. For example, the shell material may be made of a high density material having a density of from about 15 to 20 grams per cubic centimeter, such as gold, tungsten and uranium or the like and alloys with these materials, or an appropriate electron absorbing lower density material, such as lead, iron or the like. The shell 12 must be formed with proper thickness and diameter to absorb substantially all of the electron beam energy before the electron beam pulse terminates and the shell is ablated and imploded. The shell 12 wall thickness should be great enough to prevent or block substantially all of the electrons from penetrating the shell into its interior. It has been found that the shell thickness and shell outer diameter should be at a ratio of from about 1 to 5 to about 1 to 50, preferably about 1 to 10 or 20, with the shell thickness being in the range of from about 0.1 to 1.0 millimeters and the outer shell diameter in a range of from about 1 to 10 millimeters. A shell 12 made of the above mateials, and particularly the high density materials, with these dimensions will efficiently absorb the electron beam energy in outer portions of the shell and act as a pusher by accelerating essentially cold, high density material inwardly to compress and heat the fuel and further function as a tamper during burning of the fuel.

The interior portion 14 of shell 12 may include hydrogen isotope fuel in an appropriate form which is substantially shielded from the electrons in the electron beam by shell 12. For example, the hydrogen isotope may be a gas, liquid, or solid or be in compound form with such as lithium or carbon. The hydrogen isotope may be either deuterium or tritium or a mixture of the two isotopes. As a mixture, the deuterium and tritium may be at about equal atomic concentrations (e.g., 40% deuterium and 60% tritium by weight). It is desirable, for effective utilization, that the hydrogen isotope by present in portion 14 of shell 12 with about 10 to 1,000 micrograms of isotope.

The shell 12 may be formed of a single material as shown, or it may be in discrete layers of different materials which will provide enhanced operation of the respective functions of the shell. For example, the shell 12 may include an outer layer which is highly efficient in absorption of electron beam energy with one or more inner layers which function better as pushers for compression of the fuel in portion 14 and as a tamper during fuel ignition and burning. For example, the shell segment 12a shown in FIG. 1a may include an outer layer 16 which is most effective as an electron beam absorber and an intermediate or inner layer 18 of gold or tungsten or the like which functions more effectively as a pusher and tamper material during fuel compression and burning.

In order to provide the desired electron beam absorption, pushing and tamping in target 10, the target 10 must be irradiated by a high energy and short duration electron beam pulse having voltage of from about 0.5 to 5 megavolts, energy of from about 0.1 to 10 megajoules, power of from about 10.sup.13 to 10.sup.15 watts, and duration of from about 5 to 20 nanoseconds. In addition, the electron beam pulse having these characteristics should be directed against the target 10 in a generally symmetrical, focused manner substantially encompassing and encircling the outer surface of the target 10. It has been found that such may be achieved by using a diode-type of discharge in an arrangement as illustrated in FIG. 2 with a diode electron beam generator having an impedance below or about 10 ohms and an inductance below or about 1.0 nanohenry. In the arrangement of FIG. 2 the target 10 may be positioned in an appropriate manner generally at the center of a circular, disc-shaped anode electrode 22 intermediate the cylindrical cathode electrodes 24a and 24b. Anode 22 may be in the form of a thin sheet or foil or be a vapor or plasma and may be provided with an aperture or other position for receiving or holding target 10. It has been found that the anode 22 and cathodes 24a and 24b should have a diameter to discharge gap width ratio of greater than 10 to 1 in order to provide the above recited impedance and inductance characteristics. In addition, a plasma discharge may be produced near the axis of the anode and cathodes and therebetween in an appropriate manner, such as by vaporizing a wire, plasma injection or the like, to provide space charge neutralization to enhance beam pinching and further reduce impedance levels. The anode 22 and cathodes 24a and 24b may be energized by an appropriate high energy and high switching speed power supply 26 by a suitable control device 28. The power supply 26 may include a capacitor energy storage bank with appropriate high speed switching mechanisms, explosive driven high energy electrical generator, or the like. Another diode-type electron beam discharge device which may be utilized is such as is described in U.S. Pat. No. 3,760,286 entitled "Electron Beam Generator" by John G. Kelly and issued Sept. 18, 1973. Other types of single or multiple electron beam generating apparatus may be utilized which will produce the desired irradiation of the target 10. If it is desired, a single cathode and anode may be employed for some operations where less uniform electron beam discharges may be utilized.

A combination of target parameters and electron beam characteristics to provide operation in accordance with this invention may include a target having an outside diameter of about 4 millimeters and a shell thickness of about 0.75 millimeters using a 3 megavolt electron beam pulse having a duration of about 15 nanoseconds.

When the energy stored in power supply 26 is appropriately coupled to the anode 22 and cathodes 24a and 24b across dischage regions or gaps 30a and 30b, the electrons may be symmetrically emitted from the discharge surfaces of cathodes 24a and 24b towards anode 22 along the paths indicated by lines 32a and 32b. Because of the arrangement of the electrodes 22, 234a and 24b, the electrons will travel along paths 32a and 32b radially inward towards the center or axis of anode 22 and cathodes 24a and 24b from all directions and portions of the discharge surfaces of cathodes 24a and 24b, principally from the outer perimeter of cathodes 24a and 24b, as shown, and will be pinched and focused by forces produced in the beams so as to impinge against and encircle target 10 in a relatively uniform manner about its outer surface. The self-pinching of the electron beam results in a large spread in angles of the beam electrons incident on the target 10 which causes them to behave like a high temperature electron gas. The outer surface and portions of pellet 12 will ablate and vaporize under the influence of the electron beam and drive inner portions of the shell 12 inward to implode the same against the fuel incorporated in portion 14. The implosion may heat and compress the fuel and cause ingnition thereof. During the burning of the fuel, the remaining portions of the shell 12 will serve as a tamper to hold the fuel compressed for a period of time to insure consumption of a substantial fraction of the fuel.

FIG. 3 illustrates these effects graphically on a target which is irradiated with an about 0.75 megavolt electron beam having a power of about 1.5 .times. 10.sup.14 watts and about 0.9 megajoules of energy. The target is a gold shell 1.8 mm in diameter and 0.15 mm thick and encloses about 14.5 micrograms of a 50-50 deuterium-tritium mixture. Outer portions of the shell will be vaporized and expand as indicated by curve 40 when the target is irradiated beginning at time zero. Portions of the shell generally to the inner limit of energy deposition, as indicated by the curve 42, will be vaporized and will then produce a shock wave, depicted by curve 44, and drive the remaining unvaporized inner portions of the shell inward against the fuel mixture. The inner surface of the shell, e.g., the shell-fuel mixture interface, will follow the curve 46. The high pressure region produced in the shell from electron beam absorption, together with the shock wave, pushes the inner portion of the shell inward to compress and heat the fuel mixture. The maximum compression may occur at near 8.7 nanoseconds after which a neutron pulse, such as pulse 48, may be produced having an amplitude related to the compression and temperature achieved in the fuel mixture.

If it is desired to provide repetitive operation of the device shown in FIG. 2, after ignition and burning of the target 10, the control device 28 may release or otherwise inject additional targets, such as additional targets 10' and 10" in tube 34, in an appropriate manner into the discharge regions 30a and 30b between anode 22 and 24 at the center of anode 22. For example, the targets 10' and 10" may be held magnetically within tube 34 and be released one at a time upon receipt of suitable control signals from the control device 28 and dropped by gravity through an appropriate passageway 36 in cathode 24a into the discharge regions. When the so released target reaches the desired central position on anode 22, the control device 28 may then initiate power supply 26 to provide another electron beam pulse discharge against the new target. For applications using a solid anode 22, the anode can be formed as a rotating disc 22' or as a sliding or otherwise moveable elongated sheet (now shown) with multiple targets 10, 10' and 10" positioned therein, as shown in FIG. 4. The targets may then be sequentially rotated about shaft 38 into the position shown in FIG. 2. Any number of targets may be supported on disc 22' within the limits of the discharge dimensions between cathodes 24a and 24b and the diameter of anode 22'.

It will be understood that the preferred spherical shaped target 10 may be varied to other configurations or the thickness of shell 12 varied to tailor the target to provide a uniform compression of the fuel material. For instance, the target 10 may have an ovate or elliptical shape which may be positioned in a suitable manner into a desired orientation with respect to the electron beam for some electron beam generators having non-symmetrical beam shapes.

Claims

What is claimed is:

1. An electron beam device comprising a target consisting essentially of a hollow shell of an electron absorbing material, said shell having a thickness to diameter ratio of from about 1 to 5 to about 1 to 50, a thickness of from about 0.1 to 1.0 mm sufficient to absorb in outer portions of said shell substantially all electrons impinging on the shell, and hydrogen isotope disposed in the interior thereof; and means adjacent and generally encircling said shell for producing a high energy pulse beam of electrons circumferentially encompassing and focused on and substantially encircling said target for accelerating inner portions of said shell inwardly against said hydrogen isotope.

2. The device of claim 1 wherein said electron beam producing means includes a diode discharge means for generating a pulsed electron beam having voltage of from about 0.5 to 5 megavolts of from about 5 to 20 nanoseconds in duration.

3. The device of claim 1 wherein said hydrogen isotope is selected from the group consisting of deuterium and tritium and mixtures thereof with from about 10 to 1,000 micrograms of isotope.

4. The device of claim 3 wherein said deuterium and tritium are present in about equal atomic concentrations.

5. The device of claim 1 wherein said shell is of sufficient thickness to block substantially all of said electrons from said hydrogen isotope.

6. The device of claim 5 wherein said shell includes an outer layer for absorption of said electron beam and an inner layer for acceleration against said hydrogen isotope.

7. The device of claim 1 wherein said target is spherical in shape.

8. A target for use in an electron beam device consisting essentially of a hollow spherical shell of an electron absorbing material, said shell having a thickness to diameter ratio of from about 1 to 5 to about 1 to 50, a thickness of from about 0.1 to 1.0 mm sufficient to absorb substantially all electrons impinging on the shell from said device, and hydrogen isotope disposed in the interior thereof.