US3448315A - Ion gun improvements for operation in the micron pressure range and utilizing a diffuse discharge - Google Patents

Description

June 1969 R. L. HIRSCH ET AL 3,448,315

ION GUN IMPROVEMENTS FOR OPERATION IN THE MICRON PRESSURE RANGE AND UTILIZING A DIFFUSE DISCHARGE Oct. 11, Sheet of 4 i w I I E :E 0/1; 7 Em I J O /:J S g? P 1 2k: g 5 I I LL] I q 0A 1 -HI'L|% I g g 5% %8/ x 5 l Q U I %8 +LLO Il E8 7 Ring; E55? A J 2. 2 m g 8 H INVENTOR.

m ROBERT L. HiRSCH LT; BY GE A. MEEKS mm Z/OUCO, 414141) ATTO RNEYS June 3, 1969 Hmsc ET AL 3,448,315

IoN GUN IMPROVEMENTS FOR OPERATION IN THE MICRON PRESSURE RANGE AND UTILIZING A DIFFUSE DISCHARGE I Sheet 2 014 Filed Oct. 11. 1966 INVENTOR. ROBERT L. HIRSCH BY GEN A.

Z mcQ M ATTORNEYS June 3, 1969 R. HIRSCH ET AL ION GUN IMPROVEMENTS FOR OPERATION IN THE MICRON PRESSURE RANGE AND UTILIZING A DIFFUSE DISCHARGE Sheet 3 014 Filed Oct. 11, 1966 ION PATH ATTORNEYS June 3, 1969 R. L. HIRSCH ET AL 3,448,315

ION GUN IMPROVEMENTS FOR OPERATION IN THE MICRON PRESSURE RANGE AND UTILIZING A DIFFUSE DISCHARGE Filed Oct. 11, 1966 Sheet 4 of 4 ROBERT lf il l fi CH EN A. M EKS amp 5d-A)\+ ATTORNEYS United States Patent US. Cl. 313-63 18 Claims ABSTRACT OF THE DISCLOSURE This invention relates to an ion gun which includes a plasma chamber having a cathode disposed therein. An anode defines the chamber and contains an ion-transmitting orifice in one end thereof. An ion-extracting electrode is disposed adjacent to this orifice on the side opposite the chamber. The ion-extracting electrode has a beam-defining aperture oppoiste the orifice, which, in conjunction therewith, provides an electrostatic field for forming and directing a beam of ions from the chamber through both said orifice and aperture. Between the anode and ionextracting electrode is a region-enclosing wall having openings at its opposite ends, these openings being the orifice and aperture, respectively. This region thus defined inhibits the escape of neutral gas so as to provide an increase in operating efliciency. Electrons issuing from the cathode ionize neutral gas Within the chamber to form a plasma. By reason of the optics formed by the orifice and aperture, ions are extracted from the plasma, formed into a beam and discharged through the aperture in this form for subsequent utilization.

The present invention relates to an ion gun improvement, and more particularly to ion guns operated at relatively low gas pressures and utilizing electron discharges for the creation of ions. One of the most popular devices for the development of ion beams includes the duoplasmatron which accomplishes the two basic requirements of forming a plasma and extracting ions from the plasma. In the duoplasmatron, the plasma is produced in a few hundred microns pressure by a concentrated arc discharge immediately adjacent to an exit aperture. On the high vacuum side of this aperture, an extractor accelerates ions from the plasma and concentrates them into a beam, Which is then formed and accelerated for the particular application. A high degree of ionization is achieved in the concentrated are which makes it possible to obtain high gas efiiciencies. However, the high energy densities in the chamber where ionization occurs results in intense wall heating of the vessel and bombardment with associated impurity liberation. As a consequence, performance characteristics are necessarily limited.

This invention makes use of a diffuse, filament-excited discharge operating in the micron pressure range. This leads to great reduction of the energy density at the walls surrounding the discharge, thereby minimizing heating. In this arrangement, a relatively large exit aperture is used to extract a large number of ions from the plasma. In addition to this, this aperture can be enlarged effectively by the use of an extraction electrode which contributes to the development of an electric field which reaches through the aperture into a plasma chamber. By providing appropriate electrostatic focusing, the ions removed from the plasma are drawn into a fine beam which exits through a small aperture in the extraction electrode for subsequent usage. By incorporating the extraction electrode as a part of the discharge chamber, the small aperture therein becomes the only path for ion or neutral gas flow from the chamber. In this manner, low neutral gas flow, i.e., a relatively high gas efiiciency, is thus achieved.

It is therefore an object of this invention to provide an ion gun which uses relatively low gas pressures in connection with the development of a plasma from which one or more ion beams may be formed.

It is yet another object of this invention to provide an ion gun in which an extraction electrode with a small exit aperture is incorporated as a part of the ion production chamber in order to achieve a high ratio of ion output to neutral gas leakage.

It is yet another object of this invention to provide an ion gun wherein electron flow between cathode and anode electrodes follows relatively long paths through a plasma chamber such that the probability of ionization of neutral gas within this chamber is high. As a corollary of this object, there is provided means for lengthening normal straight line flow within such a chamber so as to increase significantly the probability of ionization.

It is still another object of this invention to provide an ion gun which is simple in design, inexpensive to build and maintain, and reliable in operation:

In the accomplishment of the aforesaid objectives, there is provided an ion gun comprising a plasma chamber having an electron-emitting electrode, or cathode, therein. An electron-collecting electrode, or anode, is incorporated as the chamber wall and is spaced from the cathode. An ion-transmitting orifice is provided in the vessel which encloses the chamber, and an ion-extracting electrode is disposed adjacent to this orifice on the side thereof opposite the chamber. The ion-extracting electrode has a beam-defining aperture opposite said orifice which, in conjunction with the orifice, provides ion-optical means for forming and directing a beam of ions from said chamber through both said orifice and aperture. Means is provided for introducing into the chamber a quantity of gas in the micron pressure regions, and an enclosed region between the orifice and aperture aforementioned is provided which inhibits the escape of neutral gas therefrom so as to provide an increase in gas efiiciency. Electrons issuing from the cathode ionize neutral gas within the chamber to form a plasma. By reason of the optics formed between the orifice and aperture, ions are extracted from the plasma, formed into a beam, and discharged through the aperture in this form for subsequent utilization.

The above-mentioned and other features and objects of this invention and the manner of attaining them will become more apparent and the invention itself will be best understood by reference to the following description of an embodiment of the invention taken in conjunction with the accompanying drawings, wherein:

FIG. 1 is a axial sectional view, partly diagrammed, of one embodiment of this invention;

FIG. 2 is an end view thereof;

FIG. 3 is an axial section of the orifice and aperture portions of the embodiment of FIG. 1 used in explaining the ion optics involved;

FIG. 4 is an enlarged fragmentary axial section of the arrangement of FIG. 1 used in explaining the magnetic influence on electrons flowing from the cathode to the anode;

FIG. 5 is an end view of another embodiment of this invention;

FIG. 6 is a cross-sectional view taken substantially along section line 6-6 of FIG. 5;

FIG. 7 is a longitudinal view of a portion of another embodiment of this invention;

FIG. 8 is an end view of still a further embodiment of this invention; and

FIG. 9 is a cross-sectional view taken substantially along section line 99 of FIG. 8.

Referring to the drawings, a hermetically sealed envelope is provided in the form of tubular anode having opposite end closures 12 and 14 as shown. The end closure 12 consists of a suitable strong metal plate 16 sealed to the left-hand end of the anode 10 and provided with feed-through insulators 18 and 20 which mount two conductive supporting bars 22 and 24 as shown. The mounting of these supporting bars 22 and 24 within the insulators 18 and 20 is conventional and is suitably rigid for supporting a thermionic, tungsten filament 26 on the inner ends thereof. Conductively connected to the end of the bar 22 and rigidly supported thereby is a metal cup 28 having a flat bottom and a tubular wall. The filament 26 is fitted within the cup 28 with a portion thereof projecting therebeyond as shown. The cup 28 is coaxially mounted with respect to the anode 10 and is suitable radially spaced therefrom as shown. Its purpose is to inhibit electron motion to the rear of the chamber and to provide a degree of electrostatic focusing of the discharge.

Centrally of the end plate 12 is an opening into which is fitted a gas-inlet tube 30 leading from a suitable sup ply 32 of gas. A valve 34 is provided for controlling manually the amount of gas fiowing from source 32 through tube 30 and into the interior of anode 10.

End closure 14 is in the form of a metal plate sealed at its perimeter to and conductively connected to anode 10 and provided with an enlarged orifice. Also, it will be noted that this aperture 36 is axially opposite filament 26. The shape of the rim of the orifice 36 may be smoothly rounded as shown but is determined by the optics of ion extraction.

Clamped onto the right-hand end of anode 10 and hermetically sealed thereto is an extractor assembly indicated generally by numeral 38. This assembly includes two feed-through insulators 40 and 42 having rigid barlike conductors 44 and 46 which extend radially inwardly toward the axis of anode 10. These rods 44 and 46 serve as voltage supply conductors as well as rigid mounts for an extractor 48 which is conically shaped in this embodiment and which is coaxially positioned as shown with respect to the orifice 36. The apex portion of the extractor 48 is provided with an aperture 50 which is much smaller than the orifice 36 but is positioned ade jacent thereto. The base of extractor 48 is remote from the orifice 36. Suitable screws 52 are used to fasten the extractor 48 to the innermost ends of the supporting bars 44 and 46, respectively. All of these parts are made of such material and to such size as to secure them rigidly in position.

A sleeve 54 of ceramic or other suitable insulating material is coaxially disposed between the end plate 14 and the extractor 48 as shown so as to have the ends thereof closed by these members. Thus, the sleeve 54 in conjunction with the radially inner portions of end plate 14 and extractor 48 provides a gas-tight enclosure having two openings, one being the orifice 36 and the other the aperture 50. The diameter of the sleeve 54 is intermediate that of the anode 10 and the orifice 36, closely approximating the latter. Extractor 48 is formed preferably of metal which produces minimum sputtering or liberation of impurities under conditions of operation of this invention.

A magnetic device 56 is mounted on the outside of anode tube 10. While different magnetic devices may be used without departing from the spirit and scope of this invention, a preferred arrangement and one which will not consume any power is a longtudinally split cylindrical permanent magnet. In the alternative, electrical focusing coils of the type conventionally used in the television are may be used. In any event, the arrangement of the device 56 is to have the axis of polarization thereof coincident with the axis of the anode tube 10. A magnet capable of providing a field strength within the anode tube 10 of the order of a few hundred Gauss is all that is required. As will appear from the description that follows, the present invention will work without the magnetic device 56; however, elficiency is generally improved by the use of magnets.

In dashed lines, and indicated by the numeral 58, is a typical evacuated chamber hermetically sealed to the right-hand end of the gun assembly described thus far, this chamber opening into and being in communication with extractor 48. A typical vacuum pump 60 may be connecetd to chamber 58 so as to reduce the pressure therein.

A transformer 62 is connected to the filament terminals as shown, the secondary winding of this transformer being center-tapped and connected to the negative terminal of a battery 64 having the positive side grounded. Anode 10 is also grounded as shown. The filament cup 28 is at the same potential as one side of the filament 26 by reason of the conductive connection to the bar 22.

The extractor 48 is insulated from all of the other parts of the gun, including the anode 10 and filament 26, and has the negative terminal of a battery 66 connected thereto, the positive terminal of this battery being grounded.

In operation, potentials as shown are applied to the various electrodes, the filament 26 being heated to the point at which it emits electrons. Anode 10 being positive with respect to filament 26 collects electrons emitted by the latter. As viewed in FIG. 1, the electrons will be projected toward the right by reason of the attractive force of the field set up between the filament cup 28 and the anode 10. Inasmuch as cup 28 is at the same potential as filament 26, it will be obvious that the forces acting on the electrons emitted by the filament will cause rightward movement.

-Assuming that the space within anode 10 has been evacuated to a pressure of, for example, 5 X10 mm. of mercury, while the valve 34 has been turned Off, the latter valve may be opened slightly to permit gas from the source 32 to flow into the interior of anode 10. A pressure of greater than about l-micron build-up within anode 10 constitutes one suitable operating parameter. Electrons flowing from filament 26 to anode 10 along various paths such as the path indicated by the dashed line 68 will collide with the neutral gas particles and cause ionization thereof. This results in the formation of a plasma within the space of anode 10 between the filament 26 and orifice 36, this space being characterized as a plasma chamber.

The extractor 48 being highly negative with respect to anode 10 and end plate 14 will provide a field having a a pattern something like the configuration shown in FIG. 3. This field reaches into the plasma chamber and withdraws ions therefrom and concentrates them through the orifice 36 and also through aperture 50. The shape of of this field is such that the ions are formed into a pencillike beam having a size and shape corresponding to that of aperture 50. Thus, the end plate 14 with the orifice 36 therein as well as the extractor 48 and its aperture 50 may be regarded as an ion lens which withdraws from the plasma chamber and forms them into a beam which is accelerated rightwardly into the evacuated chamber 58. The path followed by the ion beam is indicated by the dashed line arrow 70 in FIG. 1.

The operation thus far has ignored the presence of the magnet 56. It may be assumed that the magnet 56 was not being used. However, by applying magnet 56 in the position shown and described previously, flux lines are established something along the pattern shown in FIG. 4 diagrammatically. The result of these fiux lines is to impart a helical or corkscrew motion to the electrons in their transit from filament 26 to the anode 10 such that the path length becomes considerably greater than the straight-line path indicated by numeral 68. By thus imparting a helical motion, the probability of ionization becomes much greater. Thus, efficiency of ionization for a given filament power becomes greater.

For the embodiment illustrated, formation of an ion beam can be achieved through the use of a filament supply of 6-volt, l2-ampere capability. The voltage of the battery 64 is made variable between 50 and 500 volts, with 50-volts being typical for operation. Battery 66 similarly is preferably variable, a typical setting being 10-kilovolts.

Because the plasma from which the ions are extracted is dilfuse in this invention, the plasma chamber can be constructed in various sizes. For this reason, a variety of different extraction electrode geometries can be utilized to produce other than pencil-like ion beams. As an example, hollow ion beams have been produced utilizing orifices and extractors with circular slot geometry. A straight slot arrangement will provide a sheet-like beam. A number of pencil beams can be produced from an arrangement utilizing a number of similar extraction chambers disposed on the anode of the plasma chamber. Examples of these will be described hereinafter.

In the operating embodiment described herein, deuterium gas was utilized to provide deuterium ions. A variety of other gases such as oxygen, nitrogen, helium, argon, krypton, etc. can also be utilized to provide beams of each of their ions.

Suitable dimensions and materials for an operating embodiment of this invention are given in the following, it being understood that these dimensions are given by way of example only and not by way of limitation:

Inner diameter of anode 10 1% inches. Material of anode 10 304 stainless steel. Orifice 36 /2 inch diameter. Aperture 50 .070 inch diameter.

Approximate length of plasma chamber between filament 26 and end sten wire 0.012 inch dia. by 21 inch long.

Operation of the invention may be further considered with respect to two different modes of operation, one being the ignited and the other the unignited. Initiation of the ignited mode occurs above about l-micron of deuterium and 50 volts on the filament assembly. In the operating region, the current drawn by the discharge is solely determined by the filament emission. In the unignited mode, currents are limited to the order of milliamperes.

Based on the data given hereinabove, a l-milliampere ion output can be generated and extracted at minus 50 (-50) volts on the filament assembly with 400-milliamperes current. 12-kilovolts is used on the extractor 48. These conditions require of the order of -volts to the filament 26, the total power required for this output being of the order of 100-watts. This compares favorably with the order of 1,000-watts required for the same current from a duoplasmatron. Since the parts of this invention do not require extensive machining, the device is relatively simple and comparatively inexpensive to construct. Maintenance is confined to filament replacement, and at operations below l-ampere filament replacement is indeed rare. The exit aperture of .070 inch used in this invention represents 100 times the area of the .007 inch diameter apertures typical of the duoplasmatron. However, the fact of the present invention operating at a much reduced pressure over that of the duoplasmatron results in substantially decreased, neutral gas leakage.

Gas efficiencies defined as the ion beam current divided by the number of particles (both neutrals and ions) passing through the aperture 50 are high in this invention, a part of this greater efiiciency being attributable to the fact that the insulating sleeve 54 prevents escape of neutral molecules around the sides of the extractor 48 and into the evacuated chamber 58. Thus, neutral gas is restricted in the escape paths thereof which necessarily results in their being retained in or near the space of the plasma chamber thereby undergoing the greater probability of being ionized. Such paths are limited transversely by the sleeve 54 which is of a diameter intermediate the diameters of the anode 10 and the orifice 36, thereby maintaining the neutral gas in a space close to the orifice 10, so that it may penetrate into the chamber inside the anode 10, and also close to the ion beam which penetrates aperture 50. All of these factors contribute in this invention in the creation of higher output currents as well as greater efii-ciencies.

Referring to FIGS. 5 and 6, a different embodiment will be described in which a plurality of ion beams may be formed from a single gun structure. In these figures, similar parts are indicated by like numerals with the sufiix a added. The cathode cup 28a is annular in this embodiment, rather than circular as the cup 28 shown in FIG. 1. The filament 26a disposed therein is in the form of two concentric rings 70 and 72, respectively. These two rings are conductively connected together by means of a radially extending supporting bar 74 or the like (FIG. 5) and to the cup 28a such that filament voltage applied to the ring 70 and to the cup 28a may also be conducted to the ring 72.

A secure connection of the ring 70 to the supporting bar 24a is made by some suitable means such as spot welding. The bar 24a passes through a clearance opening in the cup 28a. The other supporting bar 22a is pressfitted or otherwise securely conductively connected to the cup 28a. By this means, the cup 28a, which is generally flat, as well as the filament structure 26a, in positioned substantially parallel to the end plate 16a and furthermore is disposed substantially coaxially inside the anode 10a.

The end closure 14a has a multiplicity of orifices 36a spaced apart in a circular pattern as shown. There are two circular series of these orifices 36a concentrically arranged and positioned substantially opposite the two filament rings 70 and 70a.

Mounted in spaced relation with respect to the end closure 14a is a metal plate or carrier 76 having a multiplicity of ion-extracting electrodes 480 formed therein. Each of the electrodes 48a has an aperture 50a in registry with respective orifice 36a. Sufiice it to say, any single combination of an orifice 36a and extracting electrode 48a is the same as the orifice and extracting electrode already described in connection with the arrangement of FIG. 1.

The plate 76 is positioned in parallelism with the end closure 14a on one side by the tubular member 54a of insulating material which is closed at its right-hand end by the plate 76 and its left-hand end by the end closure 14a. At diametrically opposite points on the outer peripheral portion thereof, the plate 76 is engaged by two brackets 78 of insulating material which are fixedly attached to the end assembly 38a on the anode 10a. Thus, plate 76 is rigidly held in position and, furthermore, it is insulated from anode 10a. A conventional feed-through insulator 80 is hermetically sealed to the end closure 16a and axially penetrates the entire assembly to be connected to the plate 76. This feed-through insulator 80 includes a sleeve 82 of insulating material and a central conductor 84 conductively connected at the right-hand end thereof to the plate 76. By means of this feed-through insulator 80, the necessary supply voltage may be coupled to the plate 76. Gas is admitted to the device by means of the inlet line 30a afiixed to the end 16a.

Operation of this device is essentially the same as described previously in connection with the embodiment of FIG. 1, suitable exciting voltage being applied to filament 26a which in turn emits electrons eventually collected by anode 10a or the end closure 14a. Collision of these electrons with neutral atoms and molecules of gas within the chamber formed between filament 26a and closure 14a results in formation of the plasma from which beams are extracted by means of orifice 36a and related extractors. Instead of forming a single beam, as in the case of the embodiment of FIG. 1, this arrangement is capable of forming and emitting a multiplicity of beams. Magnets, such as magnets 56 of FIG. 1, may or may not be used, as desired, with this arrangement of FIGS. and 6. Voltage is applied to extractors 48a by means of the plate 76.

Referring next to FIG. 7, a still further embodiment of this invention is illustrated. In this figure, like numerals indicate like parts with certain structure which is merely duplicative of that shown in FIG. 1 being omitted for clarity. The primary difference between the designs of FIGS. 7 and l resides in the configuration of the ion optics located at the right-hand end of the anode tube 10. As shown, anode tube has an end closure 1417 provided with an orifice 36b of annular shape. Radial bar-like support members (not shown) extending across orifice 36b locate and hold the central portion 86 of end closure 14]; in position. These radial supports are of minimal size so as to provide maximum openness of the orifice 36b.

The ion-extracting electrode 48b is also of annular shape and in radial cross-section appears to be generally triangular as shown. In the apex of the triangle is provided the aperture 50b which for the total ion-extracting elec trode 48b is annular. Aperture 50b is located axially opposite the annular orifice 36b. Insulating sleeve 54b is located between end plate 14b and extractor 48b and furthermore is situated peripherally outside the orifice and aperture. In operation, instead of a beam of solid, pencil-like cross-section, a hollow beam is formed. The principles and theories already explained in connection with FIG. 1 are equally applicable here in forming the beam. Close examination of orifice 36b will reveal that its cross-sectional shape is somewhat different than that of orifice 36 in FIG. 1 and 36a in FIG. 6. Suffice it to say, the shapes used will be those required in forming the necessary ion optical effect which will form the beam shape and size desired. More specifically, the orifice 36b is formed with a taper as shown which opens toward the triangular portion of the extractor 48b.

Another embodiment for forming an annular beam is shown in FIGS. 8 and 9, wherein the plasma chamber is end plate 160 closes the left-hand end of the chamber while another annular end plate 140 closes the right-hand end. The plate 14c is provided with an annular orifice 360 which is coaxial with respect to all of the other structure in the assembly.

Another closure plate 88 covers the right-hand end of the anode sleeve 10d and is secured in such a manner as to provide an hermetic seal. The two plates 14c and 88 may be regarded as a single disc when considering the operation of this embodiment.

A ring filament 26c is disposed coaxially within the chamber between the two anode sleeves 10c and 10d and furthermore is axially opposite the annular orifice 36c.

Coaxially mounted also in the assembly is a cathode cup 280 of annular shape within which the ring filament 26c is positioned. The supporting bar 24c of the feed-through insulator 200 is directly secured conductively to ring filament 26c as shown and passes through an enlarged hole within the cup 280 as shown to provide insulation. The other supporting bar 22c is secured directly to the bottom of cup 280 as shown and the filament 260 in turn is secured to a small post upstanding from the bottom of cup 28c for holding the filament in position and providing a conductive connection thereto.

A flat, metal plate 92 has the extractor 48b formed therein, the latter being of annular shape and positioned in registry with respect to the orifice 360. In radial crosssection, the extractor 48b is triangular with an annular orifice 50c being provided in the apex thereof. The orifice 36c and aperture 500 are in axial registry the same as is true of the preceding embodiments.

The sleeve 54c is interposed between and closed by end plate and the outer peripheral portion of plate 92 as shown. For securing plate 92 in position, two insulating bars 94 secured to the anode flange part 38c and clamped against plate 92 are used as shown.

For providing an electrical connection to plate 92, a centrally disposed feed-through insulator 96 is used, this insulator 96 being attached to the supporting plate 88 with the center conductor 98 thereof being connected at its right-hand end to plate 92. The gas inlet tube 300 is secured to plate and serves in feeding neutral gas to the plasma chamber.

In operation, potentials are applied in the same manner as previously described, setting up an ion optical effect between orifice 36c and aperture 500 whereby ions in the plasma between filament 26c and orifice 36c are extracted therefrom and formed into an annular beam. A magnet assembly 56c and 56d mounted on the outside of sleeve 10c and on the inside of sleeve 10d, respectively, is so polarized as to have an axis of polarization coaxially of the plasma chamber and coincident with a line drawn between filament 26c and aperture 500. The field produced by these magnets lengthens the transit of electrons from filament 26c to anode 10c, 10d, 14c such that the probability of ionization is greater than would be true without the magnets being present. However, as stated previously, the device will function as an ion gun without the magnets, but efficiency is markedly increased by having the magnets in the assembly.

While there have been described above the principles of this invention in connection with specific apparatus, it is to be clearly understood that this description is made only by way of example and not as a limitation to the scope of the invention.

What is claimed is:

'1. An ion gun comprising first means providing a plasma chamber, an electron-emitting electrode in said chamher, said first means including an electron collecting electrode spaced from said electron-emitting electrode in said chamber, an ion-transmitting orifice in said first means communicating with said chamber, second means including said electrodes for directing electrons through the space of said chamber, means for introducing neutral gas into said chamber, an ion-extracting electrode disposed adjacent said orifice on the side thereof opposite said chamber, said ion-extracting electrode having a beam-forming aperture opposite said orifice, ion-optical means including said orifice and said ion-extracting electrode for forming and directing a beam of ions from said chamber through both said orifice and aperture, and means for defining an enclosed region between said orifice and said aperture from which escape of neutral gas is inhibited, said enclosed region being peripherally defined by a wall disposed immediately adjacent to the outermost edge of said orifice, the transverse dimension of said region being smaller than said chamber and larger than said orifice.

2. The ion gun of claim 1 wherein said chamber is hermetically sealed for the maintenance therein of gas at sub-atmospheric pressure, said electron-emitting electrode being a filament, said chamber enclosing said filament and including said orifice, and means at a more negative potential than said electron-collecting electrode for directing flow of electrons from said filament in a direction into said chamber.

3. The ion gun of claim 1 wherein said chamber is hermetically sealed for the maintenance therein of gas at sub-atmospheric presure, said electron-emitting electrode being a filament, said chamber enclosing said filament and including said orifice, said orifice being larger than said aperture, and means for applying a unidirectional potential to said ion-extracting electrode which is negative with respect to said electron-collecting electrode.

4. The ion gun of claim 3 wherein said electron-collecting electrode includes a tubular anode having end closures, said anode surrounding said chamber, said orifice being in one end closure, said wall being of -a diameter intermediate the diameters of said anode and said orifice, said orifice being in one end closure, said one end closure being conductive, and said ion-extracting electrode being insulated from said one end closure.

5. The ion gun of claim 4 in which said ion-extracting electrode is a cone shaped conductive member coaxially disposed with respect to said orifice and with the apex portion thereof next adjacent to said orifice, said aperture being in the apex portion of said member, said wall including a tubular member of insulating material coaxially disposed between and being closed at its opposite ends by said cone shaped member and said one end closure, respectively. 1

6. The device of claim 5 wherein said filament is positioned in the end portion of said tubular anode opposite from said orifice, and a magnet device is disposed to provide a magnetic field for imparting a spiraling motion to electrons in transit from said filament to said anode.

7. The device of claim 6 in which said magnet device is disposed on "the outside of said anode, said magnet device having opposite ends polarized oppositely, said polarized ends being spaced apart in a direction parallel to the axis of said anode, and said orifice and aperture being coaxially located with respect to said anode.

8. The device of claim 7 including an element for establishing an electric field on the side of said filament opposite from said orifice such that electrons emitted by said filament will flow through said chamber and towards said anode.

9. The device of claim 8 in which said electric-field element includes a plate-like member disposed transversely of said anode on the side of said filament opposite said orifice, said plate-like member being conductively connected to said filament.

10. The device of claim 1 including means for maintaining the gas within said chamber at sub-atmospheric pressure in the range of about 10 to 10- millimeters of mercury.

11. The ion gun of claim 4 in which said ion-extracting electrode is a cone shaped conductive member coaxially disposed with respect to said orifice and with the apex portion thereof next adjacent to said orifice, said aperture being in the apex portion of said member, said memher being mounted on a plate-like carrier which extends radially from the base portion thereof, said region-defining means being a tubular wall of insulating material disposed between and being closed at its opposite ends by said carrier and said one end closure, respectively.

12. The device of claim 11 in which said one end closure has a plurality of orifices therethrough, said carrier being a conductive plate having a plurality of ionextracting electrodes thereon in registry with all of said orifices, respectively, said carrier having a peripheral portion which engages and closes one end of said insulating member, said carrier being insulated from said anode and said one end closure.

'13. The device of claim 12 in which said orifices and extracting electrodes are arranged in a pat-tern of two concentric circles, said filament being generally circular and disposed axially opposite said orifices.

14. The device of claim 4 in which both said orifice and said ion-extracting electrode are annular shaped and c0- axially positioned with respect to each other, said ionextracting electrode in radial section being generally triangular with its apex portion next adjacent to said orifice, said aperture being annular and in the apex portion of said ion-extracting electrode, said region-refining means being a tubular member of insulating material coaxially disposed between and being closed at its opposite ends by said ion-extracting electrode and said one end closure, respectively.

15. The device of claim 14 in which said chamber is or annular shape bounded on the inner and outer sides by said tubular anode, said anode including two coaxially disposed cylindrical sleeves.

16. The device of claim 15 in which said ion-extracting electrode is a part of a disc of conductive material, and a feedthrough insulator secured to said one end closure and having a centrally disposed conductive supporting bar secured at one end to said disc and extending axially through and being insulated from said one end closure.

17. The device of claim 16 in which a magnet assembly is disposed on the inner and outer peripheral sides of said inner and outer sleeves, said magnet assembly having an axis of polarization extending axially with respect to said chamber and a field which penetrates said chamber and controls the paths followed by the electrons in the transit thereof from the filament to said anode.

18. The device of claim 16 in which said filament is of annular shape disposed coaxially within said chamber axially opposite said annular orifice, and a cathode cup of annular shape within said chamber and opening toward said annular orifice, said annular filament being coaxially disposed within and conductively connected to said cup, and means fixedly supporting said cup in position in insulated relation with respect to said anode whereby an electric field may be established which directs electrons emitted by said filament away from said cup and toward said anode.

References Cited UNITED STATES PATENTS 2,975,277 3/1961 Von Ardenne 313-187 X 3,287,598 11/1966 Brooks 313-230 X 3,363,124 1/1968 Bensussan et al. 313230 X 3,367,114 2/1968 Webb 313-63 X JAMES W. LAWRENCE, Primary Examiner.

E. R. LA ROCHE, Assistant Examiner.

US. Cl. X.R.

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