US2892114A

US2892114A - Continuous plasma generator

Info

Publication number: US2892114A
Application number: US734078A
Authority: US
Inventors: Wallace D Kilpatrick
Original assignee: Individual
Current assignee: Individual
Priority date: 1958-05-06
Filing date: 1958-05-06
Publication date: 1959-06-23
Anticipated expiration: 1976-06-23
Also published as: GB862900A; CH354177A; FR1223198A; DE1153463B

Description

June 23, 1959 w. D. KILPATRICK conwmuous PLASMA GENERATOR Filed May 6, 1958 l j w ,7, km M F 8\ 3 6 3 T 4 9 2 2 Y L x W I U 3 1 S 4 R I E H j P s- 3 T 4 E 6 N I m w m J M 2 &%u 4 b 2 f m a 4 I 2 4 6 22m z z z.

FIG. 2.

DC HIGH VOLTAGE YINVENTOR. WALLACE D. KILPATRICK 52 MAGNET POWER SUPPLY ATTORNEY.

United States Patent CONTINUOUS PLASMA GENERATOR Wallace D. Kilpatrick, Livermore, Califi, assignor to the United States of America as represented by the United States Atomic Energy Commission Application May 6, 1958, Serial No. 734,078 7 Claims. (Cl. 313-63) The present invention relates to the production of a plasma in a continuous fashion to the end of emitting a steady flow of highly ionized and space-charged neutralized gas.

The art of producing ions, that is the stripping of one or more orbital electrons from an atom or molecule of elemental matter, has been relatively well investigated and innumerable ion sources or generators are known in the art. Widespread applicability of ion sources has resulted from the development of charged particle accelerators, mass spectrometers, leak detectors, pressure gauges and the various other electrical devices wherein magnetic and electric fields are employed to operate upon charged particles such as ions to the end of observing the action of the charged particle under varying conditions of interest or producing particular end results with the charged particles, Although many well-known ion sources employ arc plasmas as an ion reservoir from which ions are extracted recent developments have identified applicability of plasmas for purposes other than in the production of ions. The present invention operates to produce a continuous beam of plasma emerging from the generator proper and useful in a Wide variety of application including that of providing a source of ions and electrons as well as those applications wherein the plasma itself is employed as an entity, such as for example in a device for heating and compressing gases.

In order to clarify the description of the present invention it is of interest to note a definition of plasma particularly as here employed. Plasma is taken to mean a highly ionized gas composed of atomic or molecular particles having one or more orbital electrons removed therefrom and thus constituting ions together with a sufficient number of relatively free electrons to counterbalance the electrical charge of the ions whereby the resultant plasma is substantial electrically neutral, or in other words is space-charge neutralized. Plasmas may be operated upon by electric and magnetic fields inasmuch as the plasma is composed of positively and negatively charged particles as described above, and one applicability of plasmas lies in the field establishing nuclear reactions for it is possible to compress a plasma, as for example by a magnetic field and to thereby accelerate individual particles therein, and to thermalize the plasma thereby so that the average energy of the particles is increased and to consequently produce interactions between ions within the plasma wherein nuclear transformations result. One material advantage of such nuclear reactions is noted to be in the production of neutrons which are in themselves applicable for the bombardment of certain materials to produce transformations therein. Under certain known circumstances the foregoing plasma manipulation may be accomplished to produce thermonuclear reactions wherein the average ion energy within the plasma is sufiicient to carry out a continuing nuclear reaction therebetween to the end of transforming the original nuclei and at the same time liberating quantities of energy far in excess of that employed in the generation of the plasma or in the subsequent manipulation thereof. Inasmuch as the applications of the present invention form no part thereof and are in fact known to various people skilled in the art no further discussion of the same is herein included; however, reference is made to various scientific publications relating to the conditions for furthering thermonuclear reactions.

Whatever the ultimate application of the present inven tion certain substantial advantages are to be found in connection therewith and resulting from the advantageous plasma production. Contrary to conventional devices the present invention is capable of producing a continuous beam of plasma as distinguished from a pulsed beam. Additionally, the present invention is particularly directed to the production of a plasma having a relatively low percentage of tin-ionized atoms or molecules therein, and further the invention is highly advantageous in that there is ejected therefrom a very high percentage of the plasma formed therein. Also contrary to prior art devices the present invention is readily controllable as to the density of the output plasma therefrom.

it is an object of the present invention to provide means for producing continuous beams of plasma.

It is another object of the present invention to provide a plasma generator employing electron refluxing for maximum ionization and including entirely unobstructed plasma ejection means.

It is a further object of the present invention to provide a plasma generator utilizing a magnetic field gradient therein to control the ejection of plasma therefrom.

It is yet another object of the present invention to provide a plasma generator wherein ionization is mainly accomplished at an arc end to minimize contamination of the resultant plasma with un-ionized particles.

It is still another object of the present invention to pr0- vide a plasma generator including means establishing a magnetic field gradient longitudinally of a generated plasma for urging the plasma from the generator in the form of a continuous beam.

Yet another object of the present invention is to pro vide a plasma generator including an axial magnetic field with a maximum intensity at a controllable point longitudinal of the plasma within the generator for controlling ejection of plasma from the generator.

Various Other possible objects of the present invention together with numerous advantages of the invention will become apparent to those skilled in the art from the following disclosure taken together with the accompanying drawings wherein:

Figure l is a longitudinal sectional View taken on the center line of a preferred embodiment of the invention and showing certain associated electrical circuitry.

Figure 2 is a schematic representation of the embodiment of the invention illustrated in Figure l and including electrical circuits connected to the separate parts thereof; and

Figure 3 is a graph of magnetic field intensity along the axial length of the generator.

Considering now the present invention in some detail and referring first to Figure 1 of the drawings, there will be seen to be included therein a central anode cylinder 11 having at the rear end thereof a flange 12 and at the forward end thereof a flange 13. These

anode cylinder flanges

12 and 13 extend exteriorly from the open ends of the anode cylinder and taper toward each other so as to individually define a general conical surface at each end of the anode cylinder. Across the rear end of the anode cylinder there is disposed an end plate 14 having a generally conical depression in the surface thereof facing the anode cylinder and secured to the anode cylinder by bolts 16 disposed through the end plate 14 and flange 12 about the peripheries thereof. The

. 3 endplate 14 is adapted for insulated spaced relationship to the anode cylinder 11 and end flange 12 thereof and to this end there is provided an annular insulator 17 disposed between the end plate 14 and flange 12 preferably seated in annular grooves provided in each. Inasmuch as the connection of the end plate and anode cylinder must be vacuum tight there may be provided sealing means such as O-rings 18 on each side of the insulator 17, as indicated in Figure 1. As regards the connecting bolts 16, electrical connection of the anode cylinder and end plate are prevented by the expedient of inserting insulating bushings 19 in the anode cylinder flange 12 with the bolts 16 extending through such cylinders and thereby remaining out of electrical contact with the anode cylinder flange.

Within the anode 11 there is thereby defined a plasma chamber 21 adapted to be maintained at a reduced pres-- sure and in facing relation to the ends of this chamber at the end plate 14 there is disposed a cathode 22 which, however, is to be distinguished from conventional cathodes in that same is not herein necessarily intended to perform as an electron emitter. The term cathode is herein utilized to identify the particular element as regards the relative potential thereof compared to the anode. This cathode 22 in the preferred embodiment illustrated in Figure 1 includes a disc 23 connected to the inner surface of the end plate 14 directly adjacent to the end of the plasma chamber 21 within an indentation formed in the plate 14. The disc 23 is adapted to provide at the center thereof gaseous atoms or molecules for ionization within the chamber 21 to form the desired plasma therein. Various means may be provided for introducing gas into the plasma chamber at the disc 23 and in the illustrated embodiment there is employed a pipe 24 extending through the end plate 14 to the back side of the disc 23 and communicating with one or more minute apertures through the center of the disc. The pipe 24 is adapted for connection to a source of gas to be ionized within the plasma chamber and by the aforementioned minute aperture 26 through the disc 23. This gas is fed into the chamber axially thereof at the axis for reasons set forth with more detail below.

At the opposite end of the anode cylinder 11 there is provided an outlet electrode 27 having the form of a hollow cylinder with the internal bore 28 therethrough having substantially the same diameter as the plasma chamber 21 defined within the anode cylinder 11. This output electrode includes a flange 29 about one end thereof with the flange being dished outwardly to conform to the configuration of the anode flange 12'. Connection is made between the output electrode 27 and the anode 11 by means of a plurality of bolts 31 extending through both flanges about the peripheries thereof with insulating bushings 32 separating the bolts from electrical contact with the anode flange 13. The output electrode 27 and anode cylinder 11 are disposed in axial alignment in close proximity and Without electrical contact therebetween. This relative disposition is accomplished by the provision of an annular electrical insulator 33 disposed between the anode flange 13 and electrode flange 29,

preferably within annular facing indentations therein, and suitable vacuum sealing means such as tJ-rings 34 may be provided within these indentations to assure a vacuum-tight connection of electrode 27 and anode 11. Also provided as a portion of the output electrode 27 is an external flange about the output end thereof provided for the purpose of connecting to the output electrode a tube 37 having a flanged end 38 joined to the output cylinder by bolts 39 through the flanges of these elements. This tube 37 is adapted for extension into apparatus for utilizing a plasma produced within the present invention and is disposed in axial alignment with both the anode cylinder 11 and output electrode 27 while having an internal bore of substantially the same diameter as the aforementioned elements. Appropriate evacuation of the plasma chamber 21 may be accomplished from the apparatus to which the invention is connected via the tube 37 and output electrode 27 inasmuch as the plasma beamed from the invention would normally be employed in evacuated apparatus. However, separate evacuation means may be connected to the invention as required or desired.

As a further portion of the present invention, there are provided a plurality of magnet windings disposed about the above-described elements. Included in this magnet array are a number of separate axially aligned and identical solenoid windings 41 to 44 disposed about the anode cylinder and the end plate 14 to thereby encompass the plasma chamber 21. These solenoids are mounted coaxially with the plasma chamber by suitable means not herein illustrated. In addition to the noted plurality of solenoids each having a limited axial length there is further provided in alignment therewith an additional magnetic field winding or solenoid 46 extending from the end of the plasma chamber 21 about the output electrode 27 and at least a portion of tube 37.

Considering now the electrical connections of the elements of the present invention described above and referring to Figure 2, there will be seen to be provided a source of direct current voltage 51 having a positive terminal connected to the anode 11. The cathode 14 and output electrode 27 are maintained at the same electrical potential which is negative in relation to the potential of the anode 11 and this may be readily accomplished by electrically grounding both the cathode and output electrode. A return circuit for the electrical path is provided by grounding the negative terminal of the direct current voltage supply 51.

An additional direct current power supply 52 is provided for the purpose of energizing the above-described magnet windings. Separate electrical connections are made from the direct current power supply or magnet power supply 52 to each of the solenoids 41 to 44 and 46 to the end of individually energizing each of the solenoids whereby the current through each solenoid is individually adjustable. Suitable means are provided internally of the magnet power supply 52 for accomplishing the control of current flow to each of the magnet windings or solenoids, however, it is possible to in certain circumstances energize the output solenoid 46 about the output electrode 27 with a constant current inasmuch as for many applications it is not necessary to control or adjust the magnetic field produced by this output solenoid 46.

Considering now the operation of the present invention, it is first noted that with the magnet power supply 52 operating to pass current through each of the solenoids 41 to 44 and 46 about the anode and output cylinder there is produced by these energized solenoids a magnetic field axial through the plasma chamber 21 and the bore of the output cylinder 27 as well as through the tube 37 leading into apparatus for utilizing the plasma to be formed and ejected by the present invention. This magnetic field is at all times oriented to dispose the flux lines thereof axially through the plasma chamber and communicating out the end of the invention with these flux lines thus emerging normal to the cathode disc 23 into the plasma chamber. Variation of the current flow to the solenoids individually or collectively serves only to vary the intensity of the magnetic field within the plasma chamber 21 and in no way afiects the direction of the flux lines therein. Energization of the electrode elements of the invention by the direct current high voltage source 51 operates to produce a large voltage difierential between the anode and the cathode at one end and output electrode at the other. This large potential diifercnce thereby establishes a strong electrostatic field which operates on such free charged particles within the plasma chamber as may be present. Even though this plasma chamber be highly evacuated it is well established that there are always to be found some few charged particles naturally occurring in the atmosphere however much depleted. The aforementioned strong electric field operates upon these charged particles to accelerate same and as gas is admitted to the plasma chamber through the center of the disc 23 these accelerated charged particles within the chamber will encounter ionizing collisions with the un-ionized particles and a cascading effect will result until there is produced a large quantity of ionized gas within the chamber 21 including both positively charged ions and electrically negative electrons. The axial magnetic field through the plasma chamber 21 constrains charged particles therein to travel generally axially of the chamber. The relatively negative potential of the output electrode 27 and cathode 23 initially tends to attract ions from the anode and to repel electrons into the anode so that same are thereby electrostatically accelerated whereupon they suffer further ionizing collisions with entering gas and there then results a substantially totally ionized plasma within the chamber 21. Charged particles travelling, for example, with a radial component of velocity within the chamber 21 will be acted upon by a force produced at right angles to the direction of motion and to the direction of the magnetic field owing to the well-known interaction of electrical charges and magnetic fields whereby such particles are urged to travel about the chamber 21 and are in fact restrained from reaching the walls of the anode 11.

There is consequently formed Within the chamber 21 a very dense highly ionized space-charge neutralized plasma extending substantially to the cathode disc 23 and having but relatively small charged particle lossage to the walls of the anode 11. Additional gas is fed into this plasma through the center of the cathode 23 whereupon such gas is ionized and the probability of any un-ionized particle moving generally at random within the anode 11 of reaching the output end thereof or entering the output electrode 27 without first having passed through intense plasma and therein being ionized is extremely remote. It will be further appreciated as regards the ionizing action within the present invention that the anode 11 and

end electrodes

14 and 27 together comprise substantially a Phillips ion gauge configuration wherein relatively negative potentials are maintained at the end of a hollow central anode so that electrons are repelled to a certain extent from the ends of the anode to thereby oscillate within the anode and attain very long paths such that the probability of an ionizing collision by the electron prior to capture of same at an electrode surface is quite high. While this action is substantial in the start-up period of the present invention wherein the initial ionization is occurring the subsequent production of a very intense plasma within the chamber 21 modifies this condition. Of the charged particles that may overcome the lateral magnetic restraint those in the vicinity of the anode ends are prevented from damaging the insulators by the slanted flange configuration for the particles are quite unlikely to negotiate the path necessary to reach the insulators before first striking the flanges.

It will be appreciated from the known theory of arcs and plasmas that an are or plasma tends to take on the potential of the most positive electrode serving to establish same and further that there is only a very small potential drop from one end of an arc to another. Such is herein the case following initiation of the plasma and build up of same to a substantial density, and following the initial stage of ionization the plasma then extends from the vicinity of the disc 23 through the chamber 21 and consequently into and through the output bore 28. Under these circumstances the relatively negative potential of the output electrode 27 is ineffectual to penetrate the arc plasma or plasma boundaries. Consequently, the relatively negative potential of the output electrode 27 does not materially affect the plasma and this plasma passes unimpeded into the electrode 27 without separation of the thereof.

In addition to the above-noted action of the magnetic field in constraining charged particles from travelling laterally outwardly of the plasma to impinge upon the walls of the anode, the magnetic field is further employed herein as a means for expelling plasma from the source. As may be noted from Figure 3, the magnetic field intensity along the axis of the present invention is adjusted by the magnet power supply 52 to produce a maximum field intensity within the anode cylinder 11 with a decreasing magnet field strength on opposite sides of this point. Thus at some predetermined point within the anode 11 there is found to be a decreasing magnetic field axially therefrom in either direction. This magnetic field gradient operates to repel charged particles. More specifically, the increasing field gradient applies to charged particles a repulsive force tending to urge same toward the vicinity of a lesser magnetic field. This fact, which is well known in the field of physics, is herein employed to provide a force urging the plasma axially out of the plasma chamber 21 and through the output electrode 27' and attached tube 37. It will be appreciated that the magnitude of this force applied by the magnetic field gradient is proportional to the magnitude of the gradient and thus if this gradient is made sulficiently large almost no charged particles initially disposed adjacent the cathode will be able to pass longitudinally down the anode and through the maximum field region, however, is not here intended to employ magnetic field gradients of this magnitude. Rather there is employed a magnetic field gradient sufficient to apply force to the charged particles but not to prevent the passage of any therethrough in opposition to the force. It will be appreciated that in a plasma such as is herein formed, charged particles tend to diffuse from the ends thereof and in the present invention the established magnetic field gradient is utilized as an additive to this diffusion process whereby large quantities of plasma travel out of the plasma chamber through the output electrode 2'7. As noted above, the magnet power supply provides adjustment for the amount of current that is passed through the individual solenoids 41 to 44 and there is thus provided a means of varying the magnetic field gradient within the plasma chamber 21. An increase in this gradient results in a greater force being applied to charged particles within the chamber and consequently increases the plasma current or density leaving the anode per unit time. Furthermore the location of the point of maximum field intensity longitudinally of the anode 11 may be also varied to the end of controlling the amount of plasma expelled from the generator.

A further important contribution of the present invention provided by the peaked magnetic field intensity indicated as a maximum value at 53 of Figure 3 of the rawings is that of trapping a certain portion of the plasma adjacent the cathode. The increasing magnetic field axially of the anode away from the cathode tends to urge charged particles toward the cathode and thereby to prevent undue depletion of the plasma in this area. These charged particles, both ions and electrons, urged toward the cathode by the magnetic field gradient strike the cathode thereby producing secondary electrons which serve further to ionize gas introduced into the anode through the central cathode aperture. Contrary to certain other known plasma generators the present invention thus guards against the sweeping of the plasma from the generator such that the arc therein would be extinguished. In the present invention the magnetic field operates to ensure the continuing existence of a plasma within the chamber 21 so long as the elements of the invention are energized and gas is fed into the plasma chamber. Consequently, the present invention is able positive and negative charges to produce a continuous plasma beam which is accelerated from the generator by the action of the mag netic field. Note further that in the plasma generator of this invention no magnetic field variations are necessary to produce plasma acceleration and for any particular operating conditions fixed electrical potentials and solenoid energization are employed.

A further important part of the present invention is the structure of the output electrode 27 for same will be seen to materially ditter from similar electrodes sometimes employed in ion sources. It has been found, insofar as ion source work is concerned, that output density of ejected ion beam does not increase proportionally to the increase in size of the output aperture. While this condition is undoubtedly true for ion sources same has been found not to constitute a limitation upon plasma generators or at least upon the present plasma generator. Herein, acceleration of the plasma is in no way dependent upon electric fields but instead magnetic fields are employed to produce the accelerating force upon the emerging plasma and consequently the greater the size of the output aperture the greater the number of particles that may be ejected therethrough. In this invention the output electrode 27 has the bore 28 therethrough formed of a size equal to that of the plasma chamber 21 insofar as cross-sectional area is concerned and, consequently no lilnitation whatsoever is imposed upon the departure of the plasma from the plasma chamber by the output electrode 27.

The above-described magnetic field configuration within the plasma chamber 21. of this invention is sometimes termed a mirror magnetic field or a mirror field inasmuch as same tends to reflect charged particles approaching same without regard to the polarity of such particles. As noted above, there is employed herein only a relatively imperfect mirror field configuration inasmuch as it is not desired to reflect the maximum number of charged particles therefrom but merely to control the number of charged particles passing therethrough to the end of sharing maintenance of the ionizing plasma within the chamber 21 and also the continuous acceleration of plasma from the chamber.

What I claim is:

1. A plasma generator for producing a continuous plasma beam comprising an elongated operated anode, a first electrode disposed in closing relation to one end of said anode, a second electrode disposed adjacent the opposite end of said anode and having an opening therethrough substantially the same size as the anode aperture, means communicating with said anode aperture for introducing gas therein to be ionized, means maintaining said anode at a substantial positive potential with respect to said first and second electrodes for establishing a high intensity are discharge within said anode aperture, and means establishing a magnetic field having lines of force extend-- ing axially through said anode aperture with a peak intensity within said anode aperture whereby said magnetic field restrains charged particles from impinging upon said anode and additionally urges charged particles from said anode through the aperture in said second electrode for the continuous ejection of plasma therefrom.

2. A plasma generator as claimed in claim 1 further defined by said means for introducing a gas within said anode aperture composing means defining at least one aperture in said first electrode adjacent the axis of said anode aperture whereby gas to be ionized enters said anode aperture substantially at the end of the arc plasma established therein for maximum ionization of entering gas.

3. A plasma generator as claimed in claim 1 further defined by the means establishing said magnetic field comprising a plurality of solenoids disposed about said anode, and power supply means separately connected to each of said solenoids with means for varying the current through each of the solenoids for establishing within said anode aperture a magnetic mirror field of variable intensity.

4-. A plasma generator comprising means defining an elongated plasma chamber, electrode means at opposite ends of said plasma chamber with one of said means having an opening theretlrough of substantially the same lateral dimensions as said plasma chamber, direct current power supply means maintaining said electrode means at a negative potential with respect to the means defining said plasma chamber for establishing an electrical discharge through said chamber, means supplying a gas to be ionized to said plasma chamber through one of said electrode means substantially upon an extension of the axis of said plasma chamber, and means establishing a magnetic field axially through said plasma chamber and electrode means with a peak magnetic field intensity within said plasma chamber and a magnetic field gradient extending therefrom toward said electrode means whereby plasma tormed within said plasma chamber is in part magnetically urged from said chamber through said apertured electrode means and is in part retained within said plasma chamber for insuring a continuous plasma generation therein.

5. A plasma generator as claimed in claim 4 further defined by the means establishing said magnetic field including a direct current power supply separately energizing individual magnet field windings with a controllable current for varying the magnetic gradient within said plasma chamber to control the density of the plasma ejected from the chamber.

6. A plasma generator comprising a hollow cylindrical anode defining therein a plasma chamber, a cathode disposed in closing relation to one end of said anode and having gas inlet means extending therethrough in communication with said plasma chamber axially thereof, an output electrode disposed in close proximity with the opposite end of said anode and having an opening therethrough of substantially the same lateral dimensions as the plasma chamber, direct current power supply means maintaining said anode at a positive potential with respect to said cathode and output electrode for establishing an electrical discharge within said anode whereby gas introduced therein through said cathode is ionized to produce a plasma column within said anode, and means establishing a magnetic field having line of force extending axially through said anode with a magnetic field intensity decreasing toward said cathode and toward said output electrode for maintaining continuous the plasma column within said anode and for ejecting a continuous plasma beam through said output electrode.

7. A plasma. generator as claimed in claim 6 further defined by said magnetic field means comprising a plurality of aligned solenoids disposed coaxially about said anode and said cathode and output electrode, magnet power supply means separately energizing each of said solenoids, and means for varying the current to each of the said solenoids for controlling the density of the plasma beam ejected from the generator.

References Cited in the file of this patent UNITED STATES PATENTS 2,553,944 Schlesman May 22, 1951 2,764,707 Crawford et a1. Sept. 25, 1956 2,798,181 Poster, July 2, 1957 2,817,032 Batteau Dec. 17, 1957 2,826,708 Foster Mar. 11, 1958 2,826,709 Von Ardenne Mar. 11, 1958 2,831,996 Martina Apr. 22, 1958