US3379910A - Plasma extraction guns and applications therefor - Google Patents

Description

April 23, 1968 P. CHORNEY ETAL 3,379,910

PLASMA EXTRACTION GUNS AND APPLICATIONS THEREFOR Filed July 9, 1965 4 Sheets-Sheet 1 INVENTORS. 5 P601, 6/weA/67 BY JEN/V V 74 N 2/17 E ha M Filed July 9, 1965 P. CHORNEY ETAL 3,379,910

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PLASMA EXTRACTION GUNS AND APPLICATIONS THEREFOR Filed July 9, 1965 4 Sheets-Sheet L INVENTORj. Paw, awe/v5 Ja/m/ mum United States Patent 3,379,910 PLASMA EXTRACTEUN GUNS AND APPLICATIONS THEREFOR Paul Chorney, Norwood, and John Valun, Framingham, Mass., assignors, by mesne assignments, to the United ISqtates of America as represented by the Secretary of the avy Filed July 9, 1965, Ser. No. 470,944 5 Ciaims. (Cl. 31363) ABSTRACT 0F THE DISCLOSURE A plasma is created and a high perveance beam is extracted from the plasma reliably and stably over long periods of time. Three stacked panallel flat electrodes supported in an atmosphere of 1-100 microns and normal to a unidirectional constant magnetic field generate the plasma. The two outer electrodes are connected in common and have one or more in-line perforation pairs. The intermediate electrode is at about fifteen volts positive relative to the other two electrodes. One of the other two electrodes is of nickel cermat thermionic emissive material and has a skirt directed away from the intermediate electrode and supports a heater therein. The intermediate electrode has a hole slightly larger than the outide dimension of the emitter electrode. At least one extraction anode is supported parallel with but beyond the repeller electrode and has a perforation in-line with perforations in the commonly-connected electrodes and at a high positive potential relative to the three plasma forming electrodes. A collector electrode is supported beyond the extraction anode or beyond the emitter electrode in the line of particles that issue from the perforation in the adjacent electrode.

This invention relates to improvements in technique for extracting a beam of charged particles from a plasma and to applications therefor.

The Philips Ion Gage or Penning Ion Gage (P.I.G.) type of device has been used for beam extraction from plasma. The subject is treated at length in a report entitled Extraction and Modulation of Electron Beam from Philips Ion Gage by R. E. Lundgren, Scientific Report No. 9 of Electronics Research Laboratory of University of California, Berkeley, Calif, 1960, Astia Number AD 247,432. The basic components of the P.I.G. include a cylindrical electrode whose axis is normal to two fiat electrodes, one beyond each end of the cylindrical electrode. A ring or perforated plate are alternatives to the cylindrical electrode. The electrodes are supported in a gaseous atmosphere at a pressure below l() torr (1 micron). A undirectional constant magnetic field is directed axially of the cylindrical electrode and a positive potential is applied thereto relative to the two flat electrodes sufficient to generate a plasma between the flat electrodes. The required voltage depends upon the fiat electrode material, the gas pressure, and the particular gas. A beam of electrons or positive ions has been extracted from this type of device by perforating one of the fiat electrodes and disposing another electrode beyond the perforated electrode to apply a suitable accelerating potential.

Ion bombardment limits the capability of these devices to provide high perveance beams, limits their useful life to an impractically short period, and limits their utility to extremely low pressure environment.

Perveance (K) of an electron gun is defined as where I is in amperes and V is in volts. Perveance is considered high when it is greater than 1 l0" amps/ volts The previously known P.I.G. type devices are not useful for high perveance beams.

In P.I.G. type devices electrons in the gaseous atmosphere accelerate toward the cylindrical electrode or anode from the end electrodes or cathodes but are confined to a spiral path by the axial magnetic field and on approaching either end electrode or cathode are repelled. The electrons oscillate axially through the anode until they collide with a gas atom or molecule and ionize it. The newly created slow electron oscillates in the same manner as the ionizing electron and the positive ion is accelerated directly into the nearer cathode. In this manner, a plasma is created which reaches an equilibrium state determined by the rate at which electrons reach the anode. Electrons reach the anode as a result of multiple collisions with gas particles.

The Voltage required for the P.I.G. is determined by the work function and temperature of the cathode, the type of gas, the pressure, the magnetic field, and the plasma density. This voltage ranges from several hundred to several thousand volts depending on the choice of panameters. Unless the gas pressure and plasma density are very low, the cathodes undergo severe ion bombardment; therefore these devices have been limited to gaseous pressures :below a micron and for short periods of time until the electrodes and supporting insulators have been rendered unsatisfactory for proper operation by ion bombardment and sputtering.

An object of this invention is to provide a plasma extraction gun for supplying a high perveance beam reliably over long periods of time.

A further object is to provide a beam plasma amplifier with a reliable electron beam source.

A further object is to provide an electron beam source of high perveance for electron beam welding and various scientific studies.

A further object is to provide an ion beam source for target bombardment and sputtering, for scientific studies, and for ion engine propulsion.

A further object is to provide a more reliable, more durable, more stable, and generally superior beam extraction gun.

Other objects and advantages will appear from the following description of an example of the invention, and the novel features will be particularly point out in the appended claims.

PEG. 1 shows an embodiment of an electron beam extraction gun, partly in section, partly in elevation, in simplified form, omitting parts not essential to illustrate the principles of this invention;

FIG. 2 is a semi-log plot of retarding potential characteristic of the embodiment in FIG. 1 wherein the negative retarding potential is applied to the first anode.

FIG. 3 shows a fragment of the embodiment of FIG. 1 modified for ion beam extraction,

FIG. 4 illustrates in block form a beam plasma amplifier embodying this invention,

FIG. 5 illustrates in block form an electron beam welding arrangement embodying this invention,

FIG. 6 illustrates in block form an ion beam sputtering or bombarding arrangement embodying this invention, and

FIG. 7 illustrates another structural arrangement embodying the same principles as the structure of FIG. 1.

This invention concerns the modification of a PlG. type of discharge device. In this invention one of the cathodes is a thermionic emitter and has an ion extraction hole in line with an electron extraction hole in the other cathode and is formed of nickel cermet, for example, which is exceptionally resistant to ion bombardment and an excellent emitter. The discharge is operated at a very low voltage, e.g., 15 volts minimizing ion bombardment. The gas pressure is substantially higher than in a P.I.G. making possible a high plasma density and a high perveance beam. The invention contemplates a cathode that has a plurality of extraction holes, the same number in both cathodes and wherein corresponding holes are colinear.

The embodiment shown in FIG. 1 includes an anode 10, and cathodes 12 and 14 corresponding to the electrodes of a PIG. The electrodes have radial symmetry and are affixed to three parallel insulating support rods 16, only two of which are shown. The anode is a washer shaped member. The cathode 12 is a cylindrical member whose outside diameter is smaller than the inside diameter of the anode and preferably is of nickel cermet described in US. patent application Ser. No. 375,986, filed June 17, 1964, now Pat. No. 3,303,378, for Cathode by Paul Chorney and John Valun and assigned to the US. Government. The end of the cathode 12 directed toward anode 10 is flat, normal to the axis of the electrodes, and is formed with an axial perforation 22. A heater coil 24 is supported in the cathode. The cathode i2 is mounted in a cathode support 26 and operates as a thermionic emitter. The other cathode 14 is a repeller and is formed with an axial perforation 28 which has a tapered opening narrowing down to a diameter at the smaller end equal to the diameter of the perforation 22 in the cathode. The anode 10 is intermediate the end face of cathode 12 and the repeller 14. Two electron beam extraction anodes 30 and 32 are supported beyond the repeller 14 and have axial perforations 34 and 36 respectively of essentially the same diameter as the perforation in the cathode. An ion collector 38 is supported beyond the cathode 12. A collector or target for the electron beam is not shown in FIG. 1. To inhibit diffusion of the plasma discharge outside of the bounds of electrodes 10, 12, and 14 and causing supurious discharges, a glass sleeve 40 is bonded at one end to the repeller and extends coaxially therefrom toward and beyond the emitter cathode 12 and serves as a diffusion shield. The gun is confined in a gastight glass envelope 42 in which the selected gas atmosphere at the selected pressure is established by conventional techniques. As an alternative to the diffusion shield, the glass envelope may be necked down around and against the repeller.

Molybdenum is a satisfactory material for the electrodes other than the emitter cathode.

One set of dimensions for the gun in the illustrated embodiment is as follows. The diameter of cathode 12 is 0.250 inch and the diameter of the perforation 22 is 0.040 inch. The anode 10 has an inside diameter of 0.300 inch and the perforation 28 is the repeller 14 narrows down through a 120 degree taper to 0.040 inch, the same diameter as the perforation in the cathode. The first anode 30 is spaced 0.060 inch from the 0.040 inch end of the perforation in the repeller and is operable to extract the electron beam current from the discharge. The second anode 32 post accelerates the extracted bean and determines the beam voltage.

The disclosed embodiment may be operated with a cathode temperature on the order of 800 C., an anode voltage which is a small fraction of 100 volts, e.g., as low as volts, and a second extraction anode voltage on the order of l kv. If the first extraction anode is now operated at a positive potential on the order of 200 volts, a very high perveance electron beam is extracted. A beam of micropervs was obtained with the embodiment described. To lower the perveance a lower or negative potential is applied to the first extraction anode; a negative potential turns back a percentage of the electrons tending to leave the discharge thereby lowering the perveance of the beam. If a semi logarithmic plot is made of the retarding potential characteristic, i.e., beam current in milliamps plotted logarithmically as the ordinate and retarding potential on the first extraction anode as the abscissa, the plot appears very nearly linear on these coordinates, as illustrated in FIG. 2, and has the same qualitative characteristic exhibited by thermionically emitting cathodes. The retarding potential characteristic of common cathodes is governed by the law where: I is the cathode current with zero potential applied to the anode,

e is the charge of an electron,

V is the retarding potential,

k is Boltzmanns constant,

T is the absolute temperature of the cathode.

Assuming that the virtual cathode of the plasma extraction gun is governed by the same law, an effective temperature of the virtual cathode may be inferred from the slope of the retarding potential characteristic. The resulting effective temperature with the structure described was found to be approximately 100,000 degrees Kelvin. I

Although there are high densities of ions formed in the plasma discharge, the damage to the emitter cathode and the repeller due to ion bombardment is minor because of the low discharge voltage (about 15 volts) in comparison with beam voltages (about 1 kv. and greater) and because of the use of nickel cermet for the emitter cathode. High energy ions formed in the electron beam which can bombard only a small portion of the cathode surface in line with the beam cause no damage because of the perforation in the cathode, directly opposite the hole in the repeller, that serves as a drain for the high energy ions coming from the electron beam region. High emission densities were obtained continuously and high perveances were obtained with extraction guns of the type described.

In the illustrated embodiment, there is only one perforation in the emitter cathode and in the repeller. For two or more beams the anode and the repeller are formed with a plurality of colinear perforations substantially as the one set shown in FIG. 1.

In FIG. 3, there is shown a fragment of the beam extraction gun of FIG. 1 modified to provide an ion beam. The ion collector 38 of FIG. 1 is perforated to provide an ion beam extractor 38a. A negative accelerating potential is applied to beam extractor 38a. The repeller 14 need not be perforated and the anodes 30 and 32 are omitted in an ion beam extraction gun. Also, an ion beam may be derived from the embodiment shown in FIG. 1 when large negative potentials are applied to the first extraction anode 30. The ion beam is post accelerated by also applying a large negative potential to the second extraction anode 32.

In a beam plasma amplifier a beam of electrons modulated with a microwave signal is directed through a plasma for amplification of the microwave signal. The plasma density required for amplification is high, requiring gas pressure on the order of 1 to microns. When a conventional gun is operated in a gaseous environment at these pressures, the gun is short lived. The gun undergoes severe ion bombardment Which sputters and poisons the emitting surfaces. In FIG. 4, there is shown a beam plasma amplifier embodying this invention. An electron beam extraction gun 50, a plasma interaction region 52 and an electron beam collector 54 are supported in line in a gas-tight envelope 56 in which the selected gas at the selected pressure is established by conventional techniques through a valve 58 which is closed when the desired environment is established in the envelope.

The beam extraction gun 50 takes full advantage of the gas atmosphere required for the plasma amplifier. The constricted end of the tapered hole in the repeller is, in effect, the virtual cathode for the electron beam. Compared with conventional microwave tubes, the gas pressure in a beam power amplifier is very high, on the order of l to 100 microns. The relatively high pressure is necessary for the production of the plasma density required for amplification. Of course it is obvious that the electron beam derived from the extraction gun can be used for interactions with media other than plasmas.

A high perveance electron beam is useful for specialized welding application. In FIG. 5, there is shown an electron beam extraction gun as in FIG. 1 wherein the envelope 58 has a separable part 66 to permit insertion and removal of a workpiece 62 which functions as the beam collector. The gun may be single beam or multibeam for this application.

In FIG. 6, there is shown an arrangement similar to that in FIG. 5 for an ion beam gun wherein a workpiece 62a is bombarded, sputtered, etc. by the ion beam.

In the electron beam extraction gun embodiment illustrated in FIG. 7, the electrodes and spacers form the container. This embodiment includes a plurality of circular electrodes corresponding to those in FIG. 1 in spaced coaxial relationship. The electrodes include an ion collector 70, a thermionic emitter cathode 72 welded into a conductive support 74, a discharge anode '76, a discharge repeller 78, a first beam extraction anode 80 and a second or post accelerator anode 82. Target is not shown.

The embodiment in FIG. 7 is more compact, simpler to operate, is generally superior for scientific studies of controlled variation of parameters on beam extraction. By perforating the plate 70 and applying a negative potential thereto, a structure as in FIG. 3 is operable as an ion beam source or an ion propulsion engine: alternatively, this mode of operation can be effected by applying a high negative potential to the anode 80.

It will be understood that various changes in the details, materials, and arrangements of parts (and steps), which have been herein described and illustrated in order to explain the nature of the invention, may be made by those skilled in the art within the principle and scope of the invention as expressed in the appended claims.

We claim:

1. An electron gun for use in a low pressure gaseous atmosphere and aligned with a unidirectional constant magnetic field comprising:

a flat electrode,

a thermionic emitter cathode having a heater therein and having a flat face directed toward and parallel to the flat electrode,

means connecting said flat electrode and said thermionic emitter in common,

an electrode supported between the fiat electrode and the thermionic emitter cathode and having a perforation therethrough in line with and larger than the flat face of the thermionic emitter cathode,

said thermionic emitter cathode and said fiat electrode having colinear perforations therethrough, and

an extraction anode supported beyond the fiat electrode and perforated colinearly with perforations in the emitter cathode and flat electrode.

2. In an electron gun as defined in claim 1, wherein said thermionic emitter cathode is of nickel cermet.

3. An electron gun for use in a low pressure gaseous atmosphere and aligned with a unidirectional magnetic field comprising:

a flat electrode,

a thermionic emitter cathode having a heater therein and having a flat face directed toward and parallel to the fiat electrode,

means connecting said fiat electrode and said thermionic emitter in common,

an electrode supported intermediate the flat electrode and the thermionic emitter cathode and having a perforation therethrough in line with and larger than the fiat face of the thermionic emitter cathode,

said thermionic emitter cathode and said flat electrode having colinear perforations,

an electrode supported on the side of said cathode opposite the intermediate electrode to intercept ions issuing from the perforated cathode,

electrode means for extracting an electron beam from the discharge supported on the side of the flat electrode opposite the intermediate fiat electrode.

4. An electron gun as define in claim 3, further comprising:

direct current means establishing a positive potential on the intermediate electrode that is about fifteen volts relative to both the thermionic emitter cathode and the fiat electrode.

5. An electron gun for use in a low pressure gaseous atmosphere and aligned with a unidirectional constant magnetic field comprising:

a cup-shaped electrode of thermionic emitter material supporting a heater therein and having a flat end with at least one perforation therethrough,

a fiat repeller electrode supported in parallel with and spaced from the flat end of the cup-shaped electrode and having a perforation in line with each perforation in the cup-shaped electrode,

means connecting in common the cup-shaped electrode and the repeller electrode,

a flat anode supported between and parallel to the flat end of the cup-shaped electrode and the repeller electrode and formed with a hole in line with and slightly larger than the outside dimension of the fiat end of the cup-shaped emitter,

means connected to the three electrodes to render the anode approximately fifteen volts positive relative to the cup-shaped electrode and repeller electrode,

a first extraction anode supported beyond the repeller electrode and having a perforation in line with perforations in the repeller electrode and cup-shaped electrode,

a second extraction anode supported between the repeller electrode and the first extraction anode for adjusting beam perveance,

means connected to the extraction anodes, flat anode, repeller electrode, and cup-shaped electrode to render the first extraction anode approximately onethousand volts positive and the second extraction anode approximately two hundred volts positive relative to the other electrodes, and

a collector electrode free of perforations supported beyond the cup-shaped electrode.

References Cited UNITED STATES PATENTS 2,137,198 11/1938 Smith 313-161 2,141,654 12/1938 Kott 3l3-250 X 12,212,643 8/1940 Koros et al. 313-250 X 2,376,439 5/1945 Machlett et al. 313346 2,486,134 10/1949 Elder 313195 3,303,378 2/1967 Chorney et al. 313346 FOREIGN PATENTS 205,632 11/1955 Australia. 766,171 5/ 1954 Germany.

JOHN W. H'UCKERT, Primary Examiner.

A. 1. JAMES, Assistant Examiner.

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