US3104345A

US3104345A - Plasma generator for a highly ionized electrical plasma

Info

Publication number: US3104345A
Application number: US157863A
Authority: US
Inventors: John M Wilcox; William R Baker
Original assignee: Individual
Current assignee: Individual
Priority date: 1961-12-07
Filing date: 1961-12-07
Publication date: 1963-09-17
Anticipated expiration: 1980-09-17
Also published as: NL285745A; GB959150A; DE1224415B

Description

p 1963 J. M. WILCOX ETAL 3,104,345 PLASMA GENERATOR FOR-A HIGHLY IONIZED ELECTRICAL PLASMA 2 Sheets -Sheet 1 Filed Dec. 7, 1961 momDOm w 0 mobwzmo M35 km\ 58;;

INVENTORS JOHN M. W/LCOX WILLIAM R. BAKER A TTOHNE Y Sept. 17, 1963 J. M. WILCOX ETAL PLASMA GENERATOR FOR A HIGHLY IONIZED ELECTRICAL PLASMA 2 Sheets Sheet 2 Filed Dec. '7, 1961 lgnifron 32 INVENTORS JOHN M. W/LCOX BY WILLIAM R. BAKER A TTORNE Y U i d States Patent 3,104,345 PLASMA GENERATOR FOR A HIGHLY IGNEZED I ELECTRHIAL PLASMA John M. Wilcox, Berkeley, and William R. Baker, Orinda, Calif., assignors to the United States of America as represented by the United States Atomic Energy Commission Filed Dec. 7, 1961, Ser. No. 157,863 Claims; (Cl. 315-111) Yreleasing contaminants which may prevent many desired nuclear interactions from occuring. To avoid this, in many devices the plasma is produced in one location and then moved to another location for heating so that no interfering electrodes will be present. However, the additional apparatus necessary for moving the plasma adds to the complexity of the device and most of the .,plasma may be lost in the process.

Thus, it would be highly desirable to. produce theplasma Within a region ,netic field within the chamber. An axial electrode encircled by a coaxial metallic sleeve projects for a short distance into one end of the chamber. A charge of deuterium is admitted to the chamber and a local radial electrical discharge is created from the central electrode to the sleeve by applying a high voltage pulse therebetween from a triggered power supply. The radial current, in

conjunction with the axial magnetic field, causes rotation of the plasma created by the discharge. A hydromagnetic ionization wave then proceeds from the electrode along the length of the chamber, following the magnetic field, leaving behind a gas that is practically fully ionized. The moving ionizing front, or transition region between the plasma and the neutral gas, is relatively thin and well defined.

Just before the hydromagnetic ionization front reaches the opposite end of the chamber, the inputpulse line is short circuited or crowbarred to halt the ionizing wave before it contacts the end wall so that impurities will not be released therefrom. The plasma rotation stops, m uch as-short-circuiting a freely spinning electric motor brakes it to a stop. The chamber is therefore left filled with a nearly fully ionized gas. Owing to the described novel structure this plasma has been created by remotely located electrodes and is therefore relatively free from contaminants.

The magnetic field intensity may then be increased to compress and heat theplasma and the configuration of the field may be altered to provide additional trapping eificiency. The heated plasma may be then utilized for various purposes, for instance, ions may be extracted, free neutrons may be produced, and fusion reactions may be initiated through processes well known in the art. Typical processes of this type are described, for example, in the text: Controlled Thermonuclear Reactions, by

,where it is surrounded by a containing magnetic field V 3,14,345 Patented Sept. 17, 1963 Glasstone and Lovberg, D. Van Nostrand and Co., Inc, 1960, pages 6-44.

It is accordingly'an object of the present invention to provide an improved means for creating and heating an electrical plasma.

It is a further object of the present invention to provide a means for readily obtaining a highly ionized plasma. 7

It is a further object of the present invention to provide a means for creating a plasma at a point remote from electrodes and other structure. a

It is still a further object ofv the present invention to provide a means for generating and heating a plasma with minimized introduction of imprities into the plasma.

It is yet another object of the present invention to provide a simple and eflicient means for producing a highly ionized plasma within a containing magnetic field by the initiation of an ionizing wave in a gas disposed within said fiield.

The invention together with further objects and advantages thereof will be better understood by reference to the accompanying drawing of which:

FIGURE 1 is a broken out longitudinal view of a plasma generating and containing device with certain associated components being shown schematically, and

FIGURE 2 is a longitudinal section view showing an element of the device of FIGURE 1 as modified for low gas pressure operation.

Referring now to the drawing and more particularly to FIGURE -1 thereof, there is shown an outer cylindrical shell 5 made of stainless steel or a similar conductive material and having in this instance a length considerably exceeding the diameter. First and secondflat circular end closures 7 and '8 are at opposite ends of the shell 6, forming the plasma chamber 9. Closure 7 is comprised of a conductive material and has a disc-like configuration with a central aperture 1 3. An electrode '11 of molybdenum or similar material is disposed axially within the aperture 13 and is encircled by a spaced apart coaxial cylindrical conductive sleeve 12, the electrode 11 being supported by an annular insulator I14 disposed between the sleeve and electrode. The sleeve 12 protrudes for a short distance into the chamber 9, extending well beyond the electrode I l. The outer end 15' of the electrode 11 is threaded and passes through a circular insulator :14 made of a material such as quartz or alumina ceramic. The electrode 11 is secured to the insulator 14 by a nut '16 engaged on the threaded stem portion 15 of the electrode.

The second end closure 8 of shell 6 may be made of either a conductive material or an insulative material, there being openings therein for connections to both a vacuum pump 17 which removes substantially all the atmosphere in the chamber 9 and a gas source 18 which supplies an ioniza-ble gas such as hydrogen, deuterium, or tritium to the chamber 9 through a control valve 20. Conventional O-ring vacuum seals 19 are disposed between the various elements Where necessary to maintain the vacuum.

To provide a containment field, an annular center magnet coil 21 is disposed coaxially around the outside of the shell 6. A first end coil 22 is disposed around the electrode 11 end of shell 6 and a second end coil 23 is disposed at the opposite side of the central magnet coil 21, the three coils being coaxial. Coils 22 and 23 are adapted to create a more intense magnetic field than the central coil 21, thereby forming the well known magnetic mirror type field. End coil 22 is made longer than coil 23 so that in the region within the sleeve 12 the magnetic field is substantially parallel to the axis of the apparatus. A power supply 24 is connected to the central coil 21 and to the magnetic mirror end coils 22 and 23 for supplying current thereto to establish the field within the chamber 9, the configuration of which field is indicated by dashed lines 25.

To generate the plasma it is required that a high energy pulse be applied to the electrode '11, creating a discharge from the central electrode 11 to the sleeve 12, which is at ground potential. Accordingly a high voltage power supply 26 is connected through a current limiting resistor 27 to a pulse line 28 comprised of a plurality of inductors 29 and capacitors 31 arranged as a low pass filter. The capacitors 31 are charged to the full potential of the power supply 26-, -a positive potential being shown in FIGURE 1, although a negative potential is also suitable. A firing ignitron 32 has an anode 33 connected to the pulse line 28. A trigger electrode 36 in the ignitron 32 is connected to the output of a trigger pulse generator 37 and receives turn-on pulses therefrom which initiates conduction between a mercury-pool cathode 38 and the anode 33. The cathode 38 is connected to the electrode 11. When the ignit-ron 32 is fired, a potential difference is created between the electrode 11 and the grounded sleeve 12 'and causes an ionizing wavefront to progress axially through the chamber 9. The pulse line 28 provides an approximately constant current over the time period it is utilized.

For reasons previously discussed, it is desirable to stop the wavefront before it strikes the second end closure 8 at the opposite end of the chamber 9. For this purpose a shorting or crowbar ignitron 39 has an anode 41 connected to the electrode 11 and has a mercury-pool cathode 42 connected to ground potential. The ionizing wavefront will reach the second end closure 8 at a definite interval after the firing ignitron 32 starts to conduct, such interval being a characteristic of the parameters of each particular embodiment and being readily determined empirically. Therefore a portion of the triggering pulse output of the trigger pulse generator 37 is passed through a delay circuit 43 for such interval. The output of such delay circuit is connected to a trigger electrode 44 in the shorting ignitron 39 and initiates conduction therethrough.

Considering now the operation of the invention, assume that the various operating potentials are applied, that the magnetic field coils 21, 2'2 and 23 are energized, and

that the vacuum system and gas sources are operative. The pulse line 28 is fully charged until the generation of a pulse from the trigger pulse generator 37, whereupon the firing ignitron 32 commences to conduct and the po- Itential from the pulse line 28 is applied between the electrode 11 and the sleeve 12. A local breakdown of the gas in the chamber 9 occurs, creating plasma, and a radial current flows from the sleeve 12 to the electrode 11. This radial current, together with the axial magnetic field 25, exerts an azimuthal force on the plasma which causes it to rotate and thus develop a hack electromotive force which tends to reduce the radial current flow. The ionization proceeds along the length of the chamber 9, following the field lines 25 so that the gas within a central axial volume of the chamber is ionized. The width of the transition region or wavefront between the warm highly ionized rotating plasma and the essentially neutral gas may be in the order of a few centimeters. The process is referred to as a switch-on ionizing wave, because as the wave front passes by, an azimuthal component of magnetic field is switched-on.

If the ionizing current continues to flow after the wavefront has reached the far end of the chamber 9, deleterious impurity elements from the second end plate 8 appear in the plasma. Therefore the potential applied to the electrode 11 is shorted out, or crowbarred, by the shorting ignitron 42 just as the ionizing wave reaches the end of the chamber 8. This stops the energy flow from the pulse line 28 to the chamber 9, and also abruptly stops the rotation of the plasma in the same way that a rotating motor or generator will abruptly stop if the armature is shorted out.

The gas in the central volume of the chamber 9 is nearly completely ionized by the above process, creating an essentially pure plasma which is entrapped within the magnetic mirror field 25. The plasma has been heated in the foregoing process of formation and further heating may be obtained if desired by increasing the intensity of the magnetic field or by other means well known in the art.

Under some conditions, especially with a low gas pressure in the chamber 9, breakdown of the gas is facilitated by providing a spark gap at the center of the electrode 11 as shown in FIGURE 2.

Referring now to FIGURE 2 in conjunction with FIG- URE 1, there is shown an enlarged modified view of the electrode 11' including the threaded stem 15', the modified electrode having an axial bore 51 therethrough. A single conductor 52 is disposed along the axis of the bore 51 and is electrically isolated from the electrode 11' by an insulator 53. A step 54 on the insulator 53 and a corresponding shoulder 55 in the bore 51 provide means for compressing an O-ring vacuum seal 19 between the insulator 53 and electrode 11'. The insulator 53 is secured in position by a nut 56 on the end of threaded portion 515'. At the chamber 9 side of the electrode 11' the conductor 52 extends beyond the insulator 53 so that a small gap separates the end of the conductor 52 from the electrode. A low capacity capacitor 57 is connected from the conductor 52 to ground.

In operation, when a high voltage is applied to the electrode 11' through the firing ignitron 32, for an instant the full pulse line 28 potential appears across the gap be tween the electrode 11 and the conductor 52. Breakdown occurs .across the gap, providing a copious quantity of electrons and ions which cause an immediate breakdown from the electrode 11 to the sleeve 12. The capacitor 57 quickly charges to the full potential of the pulse line 28 and the arc between the electrode 11 and conductor 52 is quickly extinguished. As a further modification, the energy for the spark may be supplied from a separate pulsed power supply which is synchronized with the functioning of the trigger pulse generator 37.

Ions may be extracted from the plasma and particle interactions in the heated plasma create neutrons which may be utilized for various purposes such as the irradiation of materials. As the degree of plasma heating is increased, fusion type reactions occur at an increasing rate through particle interactions which are also Well known within the art.

In one embodiment of the invention the chamber 9 is 86 centimeters long and 20 centimeters in diameter. Typical magnetic mirror field intensity is approximately 15 kilogauss requiring a total magnetic energy of 150 kilogauss. The pulse line 28 supplies an output potential of 10 kilovolts from ten 7.5 microfarad capacitors. The chamber 9 is filled with hydrogen gas at a pressure of 0.1 millimeter mercury and the degree of ionization exceeds 90 percent. The velocity of the ionizing wavefront is approximately 5 centimeters per microsecond, thus the shorting ignitron 39 is triggered approximately 15 microseconds after the firing ignitron 32 is triggered. The two ignitrons are both RCA type 5550.

The apparatus may be operated in various ways with regard to the manner in which the magnetic fields of coils 21, 2'2 and 23 are energized. The current from the magnet current power supply 24 may be adjusted to provide an axial magnetic field within the chamber 9 during the ionization process after which the current through coils 22 and 23 is increased to provide magnetic mirror fields at each end of the chamber 9 to suppress escape of the plasma.

It will therefore be apparent to those skilled in the art that many variations and modifications are possible without departing from the spirit and scope of the invention and thus it is not intended to limit the invention except as defined in the following claims.

What is claimed is:

1. In a magnetohydrodynamic device ionizing a gas by means of an ionizing wave, the combination comprising a cylindrical shell forming a vacuum chamber, means producing a longitudinally directed magnetic field in said chamber which field has increased intensity at each end of a plasma region within said chamber thereby forming a magnetic mirror field at each end of said region, a pair of spaced coaxial electrodes disposed at one end of said region within said mirror field thereat, a high volt-age power supply, a first switch coupling said power supply to a first of said electrodes, at second switch connected to provide a short circuit between said electrodes, control means closing said second switch an interval after said first switch closes, and a gas supply communicating with said chamber.

2. In a magnetohydrodynamic device as described in claim 1, the further combination comprising a spark initiating means disposed in the innermost of said electrodes.

3. A magnetohydrodynamic device as described in claim 1 further characterized by said control means having a timing element closing said second switch after an interval substantially equal to the transit time of the wavefront of said ionizing wave through said plasma region.

4. In a magnetohydrodynamic device, the combination comprising an annular magnetic field coil having spaced end sections each providing a strong mirror field and an intermediate section providing a field of less intensity which forms a plasma trapping region, a first electrode disposed within the field of a first of said end sections along .the axis thereof and being confined to a position outside said plasma trapping region, a second sleeve electrode disposed within the field of said first end section in coaxial relationship with said first electrode and radially spaced therefrom to form a discharge gap, a first switch, a high voltage power supply coupled to said first and second electrodes through said first switch for applying a potential difierence to said electrodes, a second switch connected between said first and second electrodes, control means closing said second switch an interval after said first switch closes, a vacuum vessel enclosing said plasma trapping region, and means supplying gas to said vacuum vessel.

5. In a device for generating a highly ionized plasma, the combination comprising a cylindrical shell defining a vacuum chamber, a first magnetic field coil disposed coaxially around :a central portion of said shell, a second and third magnetic field coil disposed coaxially around said shell one at each end of said first coil, said second and third coils providing more intense magnetic fields than said first coil whereby a plasma trapping region is established within said control portion of said shell, a first electrode disposed along the axis of said chamber and being limited to a longitudinal position therein corresponding to that of said second field coil, a second sleeve electrode disposed coaxially around said first electrode and being spaced radially from said first electrode and from said shell, a high voltage source, a first switch connected between said high voltage source and said first and second electrodes for applying a potential diiference therebetween, a second switch connected from said first elec trode to said second electrode, a switch control circuit connected to said first switch and said second switch, said control circuit closing said second switch a fixed interval after said first switch is closed, and a gas source communicated with said chamber. 7

6. A device as described in claim further charac- 6 terized by a spark initiating means disposed at said first electrode.

7. In a plasma generating and containment device, the combination comprising a long vacuum vessel, a magnetic field coil disposed around said vessel and adapted to provide a substantially longitudinal magnetic field therethrough which field includes a central plasma trapping region, an electrode disposed substantially at the axis of said vessel adjacent one end of said plasma trapping re gion, a sleeve coaxial with said electrode and extending beyond said electrode towards said plasma trapping region, a high voltage power supply, a pulse line connected to the output of said high voltage power supply, a first switch connected from said pulse line to said electrode, a second switch connected from said electrode to said sleeve, a switch control circuit connected to said first and second switches and closing said second switch an interval after the closure of said first switch, a vacuum pump coupled to said vessel, and a gas source coupled to said vessel.

8. A plasma generating and containment device as de scribed in claim 7, wherein said first and second switches are ignitrons, and a delay circuit connected from said control circuit to said second switch.

- 9. In a plasma heating and containment devicepthe combination comprising a. cylindrical vacuum enclosure, a magnet coil disposed coaxially anound said enclosure and having end sectors providing a magnetic field of higher intensity than a central sector, a magnet current power supply coupled to said magnet coil, an electrode disposed along the axis of said enclosure at a longitudinal position corresponding to that of a first of said end sectors of said coil, an annular sleeve disposed coaxially around said electrode and being radially spaced from said electrode and said enclosure, a high voltage power supply connected trom said electrode to said sleeve, a first ignitron connecting said high voltage power supply to said electrode, said first ignitron having a first trigger electrode, a pulse generator connected to said first trigger electrode, a second ignitron connected from said electrode to said sleeve and having a second trigger electrode, a delay circuit connected from said pulse generator to said second trigger electrode, a vacuum pump coupled to said enclosure, and a gas supply coupled to said enclosure.

10. In a gas ionizing device, the combination comprising a first cylindrical electrode, a second cylindrical electrode disposed coaxially around said first electrode and spaced apart therefrom, an electrical power supply connected across said first and said second electrodes, first switch means connected between said power supply and one of said electrodes, magnetic field producing means providing an axially directed magnetic field in the zone between said first and said second electrodes which field extends a substantial distance past said electrodes providing an unobstructed plasma containment region adjacent one end of said electrodes, a vacuum tank enclosing said first and said second electrodes and said containment region, a gas source communicable With'said tank, a second switch means connected between said first electrode and said second electrode, and a control circuit closing said second switch an interval after said first switch closes.

References Cited in the file of this patent UNITED STATES PATENTS 2,992,345 Hansen Julyrll, 1961 3,005,931 Dandl Oct. 24, 1961 3,048,736 Emmerich Aug. 7, 1962 3,064,178 Persson Nov. 13, 1962

Claims

1. IN A MAGNETOHYDRODYNAMIC DEVICE IONIZING A GAS BY MEANS OF AN IONIZING WAVE, THE COMBINATION COMPRISING A CYLINDRICAL SHELL FORMING A VACUUM CHAMBER, MEANS PRODUCING A LONGITUDINALLY DIRECTED MAGNETIC FIELD IN SAID CHAMBER WHICH FIELD HAS INCREASED INTENSITY AT EACH END OF A PLASMA REGION WITHIN SAID CHAMBER THEREBY FORMING A MAGNETIC MIRROR FIELD AT EACH END OF SAID REGION, A PAIR OF SPACED COAXIAL ELECTRODES DISPOSED AT ONE END OF SAID REGION WITHIN SAID MIRROR FIELD THEREAT, A HIGH VOLTAGE POWER SUPPLY, A FIRST SWITCH COUPLING SAID POWER SUPPLY TO A FIRST OF SAID ELECTRODES, A SECOND SWITCH CONNECTED TO PROVIDE A SHORT CIRCUIT BETWEEN SAID ELECTRODES, CONTROL MEANS CLOSING SAID SECOND SWITCH AN INTERVAL AFTER SAID FIRST SWITCH CLOSES, AND A GAS SUPPLY COMMUNICATING WITH SAID CHAMBER.