US3610986A - Electron beam source including a pilot nonthermionic, electron source - Google Patents

Abstract

A device for generating an electron beam including a pilot gun which generates a pilot electron beam by nonthermionic means. The pilot electron beam is used to bombard a main cathode from which a main electron beam is generated.

Description

United States Patent 1 3,610,986

[72] Inventor James R. King [56] References Cited {21] APPL ggfgg UNITED STATES PATENTS [22] Filed May 1, 1970 3,393,339 7/1968 Hill et al. 250/419 SB X [45] Patented O t, 5, 1971 3,482,096 12/1969 Lewis et al. 313/74 X [73] Assignee Union Ca bid Corporation 3,517,240 6/1970 Dickinson 313/63 New York, N.Y. Primary ExaminerRoy Lake Assistant Examiner-Palmer C. Demeo Atto e Paul A. Rose, Harrie M. Humphreys and Dominic s4 ELECTRON BEAM SOURCE INCLUDING A PILOT M W NONTHERMIONIC, ELECTRON SOURCE J. Termmeuo 7 Claims, 7 Drawing Figs.

[52] US. Cl 313/63,

313/74, 313/231, 313/318 ABSTRACT: A device for generating an electron beam in- [51] Int. Cl H0lj 5/50, eluding a pilot gun which generates a pilot electron beam by H01 j 17/26 nonthermionic means. The pilot electron beam is used to [50] Field of Search 313/63, 74, bombard a main cathode from which a main electron beam is 230, 231, 318; 250/419 SB generated.

PATENTEU um 5197] 1 3510.99

' sum 1 0F 5 H.V. '10 KV INVENTOR. JAMES R. KING ATTORNEY PATENTEU um 5m CATHODE VOLTAGE SHEET 2 [IF 5 POTENTIAL DISTRIBUTION (PARALLEL ELECTRODES) 7 move I oaop CATHODE DROP Lu 0 O 2 2 b U m g g g F a. 2 B 5 :5 3 E LIGHT DISTRIBUTION 0 :1 Lu Q X m Q O x l I! z O a: g o E g 3 s w z 2 z o g a POSITIVE COLUMN 2 E I E 1 m {E 42 U u INVI'JNIHH. JAMES R. KING N N LUNINUL o RING VOLTAGE 8 CURRENT m PATENTEB 0m 5 |97| SHEET 5 OF 5 CONTROL RING VOLTAGE CURRENT 5KV ACCELERATING VOLTAGE 3" CATHODE TO TARGET DISTANCE INVFINIUR. JAMES R. KING ELECTRON BEAM SOURCE INCLUDING A PILOT NONTHERMIONIC ELECTRON SOURCE This invention relates to an improved electron beam gun and more particularly to such a gun which eliminates electromagnetic fields around the main cathode of the gun and vastly increases the operating life of the heated emitter of the main gun.

Apparatus for generating electron beams by utilizing a heat source which effects emission of electrons from a cathode by thermionic emission are well known. Also well known are the gaseous type electron generating guns frequently referred to as the plasma or cold cathode-type gun. It is also generally acknowledged that the power density of a hot cathode gun is greater than that achieved by a cold cathode gun. Up until now, thermionic emission was achieved in several ways, the most common being resistance heating of the filament. This, of course, depends upon body resistance of the filament material to the passage of high electrical currents.

Resistance heating has several accumulative drawbacks, not the least of which is related to the fact that emission is an area dependent function, and therefore, the greater the emission required from the gun, the larger the emitting area must be. The larger the area, the lower the resistance of the filament as resistance is inversely proportional to the cross sectional area of the conductor. As a consequence of H=FR the heating current must be greater to obtain greater emission which in turn creates a greater magnetic field around the filament, and of course, requires cables of larger cross section to carry the required current.

The first of the problems is to maintain a uniform heating current through the cathode which is changing resistance with temperature. Further, there is a decided change in contact resistance at the point where the cathode is clamped to the electrical heating supply. These clamps must function both mechanically and electrically to hold the cathode mechanically firm while it is being heated from ambient temperature to approximately 2300" K and to hold the cathode with unvarying electrical contact to conduct the required high current throughout the temperature range and while held for prolonged periods at high temperature.

A resistance-heated cathode has a very sensitive geometry, or distribution of resistance, such that it is difficult to restrict emission to the desired area. The cathode must be designed so that the area of highest resistance, and, therefore, the greatest heat, is concentrated at the center of the filament. Under ideal conditions this area, and only this area, is brought to emission temperature. ln addition to the geometry of the cathode itself, the attachments to the filament must be designed to function as heat sinks which will prevent extraneous heating.

The magnetic field surrounding the filament, as a result of the heating current through the filament, enters the interelectrode space between the anode and cathode .of the electron beam gun and causes an unwanted diversion of electrons and thereby, a bending of the electron beam. It can be shown that the initial angle of deflection exerted by the filament field depends upon the heating current which is varied for many reasons to satisfy the emission requirements. Further, the initial angle of deflection depends upon the velocity of the electrons as they enter the interelectrode space as determined by the square root of the accelerating voltage, thus creating a complex interdependency.

A second method of heating the emitter for the welding gun is less common due to technical difficulties involved in the solution, but nevertheless, exists in practice. This system is often called an indirectly heated or bombarded cathode, and it consists of a small electron beam gun behind the main gun. The sole purpose of the first gun is to produce an electron beam that is accelerated at low DC voltage to bombard the emitter of the main gun.-ln this case, advantage is taken of the kinetic energy conversion to heat energy at the point of impact of the accelerated'electrons, e=%mv*. The bombarding gun consists of a resistance-heated filament which is placed at some negative potential, say 1000 volts negative. The emitter of the welding gun is the target for the bombarding gun and is made positive with respect to the filament of the bombarding gun. Basically, the emitter is serving as the target anode of a crude work focus gun. It should be noted that approximately the same power is required to heat the given mass of the emitter, regardless of the method of heating used. In this case, however, lower currents are used in the milliampere range in conjunction with a higher voltage, than is used for resistance heating.

This technique is extremely effective and produces a very narrow and straight beam due to the absence of a magnetic field around the emitter of the main gun. The other relative disadvantages of the magnetically induced mechanical forces are also known eliminated. The system, however, has the in herent weakness that the bombarding gun is making use of a resistance-heated filament which will also eventually fail due to the geometry, thinness, and magnetically induced mechanical stresses.

Accordingly, it is the main object of this invention to provide a novel electron beam generating gun which eliminates unwanted electromagnetic deflection of the beam, decreases thermal aberration, and has vastly improved emitter life.

Another object is to provide a gun that can be used with electrical supply cables which are small in size and are sufficiently flexible to permit convenient coiling thereof in the gun chamber.

A further object is to provide a gun wherein the main cathode temperature is varied in response to changes in the main beam current in order to maintain constant current or to vary current according to a predetermined program such as a slope or taper program.

Still another object is to provide a gun provided with means for controlling backfiring through the gas due to the negative potential of the main cathode with respect to the chamber walls.

These and other objects will either be pointed out or become apparent from the following description and drawings wherein;

FIG. 1 is a front elevation view partially in cross section of an embodiment of the invention.

FIG. 2 is a graphic comparison of voltage distribution and light distribution in a normal glow discharge.

FIG. 3 is a sketch showing the important regions of the glow discharge in relation to the cold cathode gun.

FIG. 4 is a front elevation view partially in cross section of a second embodiment of the invention.

FIG. 5 is a front elevation view partially in cross section of a third embodiment of the invention.

FIG. 6 is a graph of a typical curve showing variation of beam current and control ring voltage as a function of pressure.

Briefly stated, the objects of the invention are accomplished by an electron beam gun which includes a pilot gun comprising an enclosure provided with a relatively low-pressure ionizable gaseous medium which serves as the source of electrons for the gun. Positioned in one wall of the enclosure is the pilot cathode. The pilot cathode has a concave surface with a radius of curvature approximately twice the cathode radius to give a convergent beam of electrons having a focal point beyond the Faraday dark space. Positioned in the same wall as the pilot cathode concentric with and spaced from the pilot cathode is a ring-shaped pilot anode. An accelerating voltage for the electrons is applied between the pilot cathode and pilot anode.

A main cathode emitter is positioned in the wall of the enclosure opposite the pilot cathode and is maintained at the same potential as the pilot anode. The distance of the main cathode-emitter from the pilot cathode is always beyond the Faraday dark space when operated within the desired pressure range-The main cathode-emitter is bombarded on its upper the emitter 360 but exposes its top surface to the bombarding beam and exposes a circular disc of a diameter appropriate to the desired emission area on its lower surface. The structure surrounding the emitter on the top side is a part of its electrical circuit of the cold cathode pilot gun. The structure in the vicinity of the emitter may be flat, concave, or convex. The convex curvature is preferred as it may aid in forming the equipotential surfaces between the pilot cathode and the emitter.

The lower side of the structure is electrically a part of the main gun and its shape primarily determines the shape of the equipotential surfaces between the main cathode electrode and the anode electrode. It is taught by the science of electron optics that the geometry of the equipotential surfaces between the cathode electrode and the anode electrode determines the electrostatic lens effect on the passage of electrons.

It can be seen that the upper and lower parts of the structure supporting the cathode emitter function simultaneously in two different circuits. The upper portion functions as a part of the anode electrode of the pilot gun and the lower portion functions as the cathode electrode of the main gun.

Spaced from the main cathode electrode is a main anode. Impressed between the main cathode electrode and main anode is an accelerating voltage to accelerate the electrons emitted from the main cathode emitter toward a workpiece. The accelerating voltage between the main cathode and anode is usually high relative to the pilot accelerating voltage but is not dependent upon any particular ratio or relationship.

The pilot gun of this invention is a plasma or cold cathode gun containing a low-pressure ionizable gas usually maintained at a pressure of from about to 30 microns, although the pilot gun can be operated at higher or lower pressures depending on the gas, voltage and electrode spacing. The gases can be ambient gases or inert gases, although inert gases are preferred for protection of the main cathode emitter. The pilot gun can be operated at various voltages and power levels; however, in this invention the pilot gun is preferably operated at a voltage up to about kilovolts (kv.) and a maximum current of 50 ma.

The main gun is usually operated at a pressure of approximately 0.] micron or less, at a voltage of about 60 kilovolts (kv.) and a current of 500 ma. which is wholly dependent upon the area and temperature of the emitter to produce an electron beam of 30 kilowatts. The work is usually placed in a chamber maintained at a pressure in the area of about 5 microns, although the pressure range might be at any value from infinitely low to l or above 1 atmosphere when the gun is used for high vacuum, intermediate vacuum, or in atmosphere. However, utilizing the concept of the invention, electron beams of different powers can be obtained by changing pressures, voltages, electrode spacing and kinds of electrodes and gases within the pilot gun such that emitters of greater or lesser area are heated to the appropriate emitting temperature.

Referring now to the drawing in FIG. 1, the electron beam gun is shown generally at G. The pilot gun portion of gun G is shown at P. Such pilot gun has a pilot cathode member 1 positioned in one wall 3 of an insulator enclosure 5 of pilot gun P. Outside of the pilot cathode l and insulated therefrom by insulator material 2 is a pilot anode 7 having a flange plate 9 at its top portion. Outside the pilot anode 7 is a control ring 11. Positioned in the wall 13 opposite the pilot cathode l is the main cathode emitter l5. Main cathode 16 is electrically connected to pilot anode 7 by conductor 17. The main function of conductor 17 is to carry electrons back from cathode emitter through pilot anode 7 to pilot cathode l. The main cathode emitter 15 is preferably a tungsten disc, although any good electron emitting material may be used such as tantalum or coatings having a low work function. Supporting main cathode emitter 15 is main cathode electrode 16. Spaced from said main cathode l6 and downstream therefrom is the main anode l9. Downstream from the main anode 19 is a main beam focusing coil 21.

The pilot cathode may be made of any suitable pure, alloyed or coated material such as molybdenum, copper, steel, and etc., although aluminum is preferred. it is well known that plasma cathode guns, having a flat cathode orifice plate produce a divergent electron beam. The power density of such beams is usually low so that some focusing means must be employed to form a convergent beam having higher power density which can be used as a heat source. In this invention the pilot cathode has a shallow concavity which serves as the plasma cavity in which the plasma that is the source of electrons is generated. This cavity has the characteristics of an electron lens. With this type of lens, when electrons approach the lens from the side of lower field strength, in this case the plasma cavity in the pilot cathode, the lens action is convergent. This convergence is obtained by the concave curvature of the cavity which results in equipotcntial surfaces which are convex and, therefore, exert a convergent action on the electrons.

It was discovered that a successful curvature of the pilot cathode approximates a shallow curve having a true radius, although Pierce shapes or parabolic shapes will function to a lesser degree. it was found that the radius of the curve is related to the CD. of the cathode such that the radius is approximately equal to twice the cathode radius. The choice of radius determines the depth of the curve; however, curves of smaller radii will function, provided that the depth does not exceed 0.130 to 0.180 inches. However, the most stable operation is achieved with larger radii.

The vertical position of the focal point was inversely related, to the depth of curvature; i.e., the deeper curvature, the nearer the focal point to the face of the cathode. On one test unit the focal point of a curvature of 0.750 radius and a depth of 0.250 was actually up inside the cavity, whereas a depth of 0.080" projected the focal point several inches from the cathode, and a depth of 0.130 projected the focal point approximately 2.5 inches from the cathode. As the focal point is projected farther away, the diameter of the focal region is also increased, thereby reducing the power density.

The plasma cathode gun involves the phenomenon of normal glow discharge. A normal glow discharge is divided up into areas of light and dark spaces of varying intensity. The bright areas are either areas of high ionization or areas of recombination of electrons and positive ions. The dark areas are conversely those of low ionization and recombination. It is well known that the space nearest the cathode is called the Aston dark space. The cathode glow space is the area in which the electrons have reached the critical minimum velocity for the given gas pressure. Next is the cathode dark space. In the cathode dark space, the electrons are above critical velocity and leave behind a large quantity of positive ions that were originated in the cathode glow space. The negative glow region is one of maximum potential drop and is extremely bright (the brightest of the entire column) and is an area of maximum ionization and recombination due to the presence of large numbers of ions, electrons, and gas molecules. The dark space beyond the negative glow region is the Faraday dark space and is one of potential minimum, with respect to cathode potential, due to the large number of electrons leaving the negative glow region. The electrons are slowed below critical ionizing velocity in this region and negative charges predominate. The so-called positive column is a plasma of equal charges of negative and positive ions. Following these are the anode glow and anode dark spaces which are of little consequence.

A graphic comparison of voltage distribution and light distribution between parallel electrodes is shown in FIG. 2. The main regions of discharge in relationship to the pilot gun elements are as shown in FIG. 3, wherein smaller parts bear similar reference characters.

It was found that a work surface (ground or positive potential) may be positioned at any point along the vertical height of the positive column with little change in beam current as long as it does not come into physical contact with the Faraday dark space 35 (see FIG. 3). Upon contacting the dark space with the work surface, the beam is suddenly reduced in intensity from its maximum value to some extremely low value which may be only one-tenth of the maximum. If the work is further raised until it contacts the now faint negative glow region 34, the beam current will decrease even more, or will extinguish entirely. Due to the drastic change as these areas are approached, we say that the discharge quenches when contact with the Faraday dark space is made.

Accordingly, while it is desirable from a beam power intensity point of view to have a focal point as close as possible to the cathode to form a small focal circle having a high-power density, it is not possible to utilize such a short focal distance due to the position of the Faraday dark space.

The distance of the Faraday dark space from the face of the cathode is determined primarily by the operating pressure, and secondarily, by the gun design and the applied voltage. Therefore, for a given set of design parameters the dark space must be tolerated at whatever distance it is positioned. It is known, as discussed hereinabove, that any surface that penetrates the dark space will diminish (quench) the beam current, and thus it is imperative that the focal point distance from the pilot cathode be chosen to be somewhat greater than the pilot cathode to the lower edge of the Faraday dark space distance.

ln order to accomplish a focal distance greater than the dark space distance, some power density must be sacrificed due to a larger beam cross section at the longer distance.

ln operation, in the embodiment shown in FIG. 1, the pressure in the pilot gun P is maintained between and 20 microns and is controlled by flowing gas from a source through an inlet 23 into a bubbler unit 25 filled with high dielectric fluid compatible with a vacuum environment. The purpose of the bubbler will be described hereinafter. From the bubbler unit 25 the gas flows into chamber C formed by enclosure 5. The top of the pilot gun P has a hole or holes 27 which communicates directly with the vacuum chamber VC so that the pilot gun chamber C is continually pumped by the vacuum system connected to the chamber VC at port 29. The gas might also be ducted out of the gun by a tube and independent pumping system. The pressure in the chamber C is maintained by the flowing gas e.g. 100 cc./min. and such flow is small enough not to affect the actual pressure of the vacuum chamber VC. The number and size of the gas bleed holes is related to the desired pressure within the enclosure and the desired flow rate. The source of the gas might also be a selfcontained source such as a self-enclosed gas bottle or an appropriate quantity of a porous material such as activated alumina, activated charcoal, or zeolite which will serve as a virtual leak in the enclosure. A material having a vapor pressure in the region of interest could also be contained within the cavity as a source of gas or vapor.

The pilot cathode 1 requires only about 0.025 amperes to provide enough power (about 250 watts) at 10 kilovolts to heat the main cathode to emission temperature. Up until now, the emitters in electron beam guns were customarily re- Sistance-heated by passing an electric current therethrough. Since large beam currents require a large heating current on the order of 6S amperes, a strong electromagnetic field is created which surrounds the emitter. The field electromagnetically bends the electron beam away from the axis of the gun and therefore, is a source of beam distortion. The present invention utilizes a power electron beam e.g. 250 watts or less to heat the emitter or main cathode for the high-powered '30,000-watt electron gun. This system has the advantage of eliminating the electromagnetic field surrounding the emitter. The pilot cathode itself is virtually indestructible and will not require frequent replacement. The main emitter may now fail only due to erosion or sputtering which is an uncontrollable process of vaporization of extremely hot metals under vacuum conditions.

The present electron gun utilizes a temperature controlled emitter or main cathode for purposes of varying beam current.

In practice, the magnitude of the main beam current is monitored by an appropriate circuit which sends a signal to a comparator circuit. A reference signal is also sent to the comparator circuit and the difference in signals is then sent to the power supply of the pilot gun to raise or lower the power delivered thereto to increase or decrease the pilot electron beam power. This, of course, raises or lowers the temperature of the emitter material, thus producing only the required quantity of electrons, thereby increasing the average life of the emitter or main cathode.

Another advantage of the gun of this invention is that because the cold cathode technique requires only about 0.025 amperes, compared to the 65 amperes used in prior guns, cables 31 and 33, which supply the pilot gun P, can be small and flexible so as to permit such cables to be conveniently coiled in and out of the chamber VC. For example, it is possible for a 0.250-inch diameter cable to replace a 2.5-inch diameter cable used with prior art guns, because that part of the cable bulk due to the cross section required to carry high current and the heating effect of the high current is eliminated.

The gun G includes a pilot gun P operating at l0 kv. and a main gun M operating at about 0.1-micron pressure at 60 kv. Electrically, the two guns are independent, but they are connected through conductor 17, described above, to a common cable 31. Accordingly, cable 31 carries an overall maximum potential of kv. The main cathode emitter 15 carries 60 kv. and contacts the gas chamber C of pilot gun P and therefore, there is a 601w. potential that might be applied through the gas to the walls of the vacuum chamber VC. Thus, the gun could fire in the reverse direction by the 60 kv. if the 60 kv. can see a positive surface through the gas outlet or inlet holes.

The gun G is provided with several features for preventing the above-described backfiring.

The first feature to eliminate backfiring through the gas input line 23 is the aforementioned bubbler unit 25. This unit partially filled with dielectric fluid provides physical discontinuity between electrical ground and the piping attached to the gun. This combination retards any backfiring tendency through the inlet assage.

A second method of eliminating backfiring through the inlet line is to feed the gas into the chamber through an insulated tube connected to a gas bottle which is insulated from ground. Of course the two techniques may be employed simultaneously.

Another feature incorporated in the gun to minimize backfiring involves the use of the flange 9 on pilot anode 7. The flange 9 is at the same potential as the pilot anode 7 and 60 kv. main cathode electrode 16. However, flange 9 is on the outside of the insulator material 2, putting it closer to the chamber walls. This effectively moves the field stress outside the gas region.

A third means of minimizing the tendency to backfire is to interpose a staggered hole bafi'le between the gas bleed holes and the gas plasma in the enclosure such that there is no electrical line of sight" for the voltage to see positive ground through the bleed holes. For example, three bleed holes might be provided in the top of the gun spaced apart such that the bleed holes appear at 0, 120, and 240. A ring baffle mounted below the area carrying the bleed holes would carry appropriately sized holes but spaced 60, 180, and 300 or any other position just so that the gas can find a path between the staggered holes. Due to the fact that they never are in exact alignment electrical lines of stress cannot see through them.

The lower baffle ring may either be an insulator or a conductor. When it is made of a conductive material a fourth means of backfiring suppression is provided.

A fourth feature used to minimize backfiring involves incorporating a control ring outside the pilot anode through which the gas inlet and outlet passages penetrate. The outer control ring contacts the Faraday dark space, and thus carries a high negative potential, which is related to pressure and applied voltage asshown in FIG. 6. As electrons approach the high negative pbtential in the control ring, they are repelled while positive ions are neutralized. The control ring may or may not be connected to the pilot anode through a high value of resistance.

The negative potential on the control ring is generated by taking advantage of the various regions of electrical potential created in a cold cathode discharge. It was found that the thickness of these regions and the outside diameter of the Faraday dark space is inversely influenced by ambient pressure. Thus, as pressure increases, the diameter of the region shrinks and thereby will move across a control ring positioned as shown. The ring acts like a slider of a potentiometer, and picks up a varying voltage across the dark space. The varying voltage picked up by the control ring is a function of voltage, pressure, current, cathode to ground surface distance, and position or geometry of the dark space.

Because of these relationships it can be seen that variety of control functions are possible both directly related to the invention of the pilot gun as well as other devices.

As shown in FIG. 6, there is a definite relationship between control ring voltage and beam current where pressure, electrode spacing and accelerating voltage are held constant.

By applying a supplementary voltage to this ring the dark space dimensions may also be varied to cause the beam current to vary. When the negative voltage picked up by this ring is decreased by applying a positive potential or by grounding the rings self-potential through a potentiometer of approximately I megohm, the current will be reduced as the resistance is reduced. A greater reduction in current may be obtained by choosing the parameters of pressure and spacing such that a reduction in the negative potential on the control ring will cause the dark space to increase in depth until its lower edge makes contact with a grounded ring or surface. In this manner the control ring may be used to modulate current flowing in the beam.

As the control ring potential also varies with changing cathode to ground surface distance when voltage and pressure are maintained constant, the ring may be used as a transducer which produces an output voltage variation that is the analog of changing cathode to surface distance as might be required as a control signal for a surface contour follower or a frictionless electronic cam follower.

Both the beam current and control ring voltage vary inversely as pressure varies when accelerating voltage and cathode to surface distance are held constant. Either or both parameters may be used to measure pressure over a range of at least 3 to more than 50 microns with a very high rate of response.

In the subject invention the negative potential picked up by the control ring with the discharge region is used as a self-biasing potential to help reduce baekfiring through the gas. This self-biasing could also be aided by the application of other voltage sources to the ring.

A further use of the self-biasing biasing potential picked up by the control ring which again be supplemented by other voltage sources is shown in FIG. which is another embodiment of the invention.

FIG. 5 is similar to FIG. 4 in most respects but has another element 226 added which functions to stabilize the focus of the cold cathode gun. Parts in FIG. 4 similar to the parts in FIG. I bear the same reference number, with the addition of one hundred thereto. Parts in FIG. 5 which are similar to those in FIG. 4 bear the same reference number, increased by one hundred.

A second embodiment of the gun is pictured in FIG. 4 generally shown at G. The pilot portion of gun G is shown at P. Such pilot gun has a pilot cathode member positioned in an insulator ring 102 which insulates the pilot cathode from the pilot anode I07. Attached to the pilot anode 107 is a metal enclosure I04 which is at the same potential as the pilot anode 107. Between the pilot anode 107 and the enclosure 104 is a control ring 111 supported by and spaced from the anode by insulator ring 106 which also contains gas inlet hole [24 connected to gas inlet tube 123. Positioned in the wall of the metal enclosure 104 opposite and in line with pilot cathode 101 is the main cathode emitter 115. The main cathode emitter is preferably a tungsten disc although any good electron emitting material such as tantalum or other metals coated with materials having a low work function could be used. Supporting the main cathode emitter is main cathode electrode I16. Opposite and in line with main cathode electrode 116 is main anode 119 followed in line by main beam focusing coil 12]. The entire assembly of the pilot gun is supported by a main gun insulator 114 which in turn is housed and supported by shielding cylinder 120 which is at ground or positive potential.

The two embodiments differ in that: FIG. I the pilot gun is assembled into and electrically insulative housing with wire 17 acting to close the electrical circuit between main cathode emitter I5 and pilot anode 7. In FIG. 4 the pilot gun is assembled into an electrically conductive housing in which the metal enclosure 104 is common to the main cathode emitter IIS and pilot anode electrode 107.

Both systems function similarly although the electrical field distribution and shape of equipotential surfaces differ somewhat. An insulating sleeve may be inserted into the enclosure between the walls of the metal enclosure and the gas space which would produce a field distribution in the FIG. 4 gun which would be more similar to the field distribution in the FIG. ll gun if desired.

It is possible to utilize a variable negative bias control in place of or in conjunction with temperature variation of the main cathode emitter for current control for the main gun. In such case, the main cathode emitter 15 is insulated from the main cathode electrode and electrically connected to a variable bias supply which makes the emitter negative with respect to the main cathode electrode.

A third embodiment FIG. 5 utilized a wire mesh cup or solid cup 226 attached to the control ring 211, spaced with a gap or by an insulating cylinder or insulating cone to prevent contact with metal enclosure 204. The cup at negative potential tends to remove positive ions from the beam region while at the same time creating a pinching or focusing action on the beam of electrons.

It is known that the focal length of the beam and its power density at the focal point is related to the lens effect of the virtual anode represented by the equipotential surface at the lower extremity of the Faraday dark space. The area of the pilot cathode involved in the creation of the beam determines the amount of current delivered by the gun. A low-current beam would make use of only the center portion of the pilot cathode, say an included angle of 20 as more current is drawn from the gun more of the cathode area glows and a greater included angle of, say 30 is involved. If boundary rays (straight lines) are drawn from the edges of the glowing area to intersect the axis of the gun the point of intersection represents the focal point along the axis. It can, therefore, be seen that the focal point moves up as more current is drawn and down as less current is drawn. When the angle is small, the focal region is collimated to some degree and the beam is in approximate focus over some distance, say 0.250" or greater. At larger angles, the focal region approximates a point.

The shape of the lower edge of the dark space and the dimensions of the dark space are also a function of the changing involvement of the area of the pilot cathode.

Without further refinements it can be said that the shape of cathode determines the range of focus for the range of current and pressure anticipated. Focus may further be accomplished by the use of permanent magnet fields or by appropriately shaped electromagnetic fields as taught by the science of electron optics.

It can be seen, through anexperirnental glass enclosure that the dark space grows thicker and comes closer to the cathode emitter 15 when the pressure approaches 5 microns as the reduced beam current tends to collimate the beam or to move the focal point beyond the emitter surface. Under these conditions the beam current tends to its minimum and the voltage on the control electrode tends toward its maximum which could approximate 250 to 2,000 volts under certain conditions. On the other hand, when the pressure rises to approach microns, the dark space thins and moves closer to the pilot cathode surface and a greater portion of the pilot cathode is involved in the glow, thus, increasing the beam current.

Under these conditions the voltage on the control ring may drop to or 50 volts as the pressure approaches 20 microns and beam current approaches maximum. The focal point moves higher and the beam at the emitter surface is divergent.

By connecting the cup 226 to control ring 2H1 the negative potential from the control ring is applied to the sides and bottom of the cup. The bottom of the cup is fitted with an aperture, the lips of which may either be the thickness of the cup material or increased by inserting a tube in the aperture to increase its electrically effective length. This aperture is large enough, say 0.250" or larger, to pass the main body of the beam when the focal point is at the midpoint of the aperture but small enough to intercept a small portion of the fringe electrons from the beam. The charge on the cup will be the sum of two electrostatic charges. If the focal point of the beam then moves higher the portion of the charge obtained from the control ring will decrease but at the same time, the portion of the charge coming from the interception of the fringe electrons of a divergent beam will increase. The net increase in the voltage on the cup and control ring will force the dark space to thicken (or move downward), thus forcing the focal point downward until the original equilibrium of the focal point in the aperture is reinstated.

This explanation could be made in terms of the fact that an increased negative potential on the control cup would attract and neutralize more positive ions, reduce the bombarding effect and secondary emission from the pilot cathode, and change the angle of area involvement on the face of the pilot cathode but the sense of explanation would remain the same.

The above explanation describes the stabilization of the focus of the beam to compensate for increases in pressure within the enclosure. A similar action takes place that (although less critical due to collimating effects at lower pressures) stabilizes the focus when pressure decreases.

As the pressure in the enclosure decreases the control ring potential will rise which would tend to decrease beam current and to decrease the angle of the pilot cathode area involved also the focal point would tend to move downward. Under these conditions the beam is of smaller diameter and does not tend to impinge fringe electrons upon the aperture edges to the same degree that high-pressure defocalization does. However, the relatively great increase of voltage on the control cup will tend to remove more positive ions from the beam which will neutralize a portion of the negative potential on the control ring which will allow the dark space to rise, increase the area of cathode involvement, increase beam current and to move the focal point upward to reinstate focal equilibrium in the aperture.

At all times the electrical servosystem will drive the accelerating potential in the proper direction to keep the emission from the cathode emitterat the preset value. The above stabilization is only an aid and the working of the system is not wholly dependent upon it.

Having described the invention with reference to certain preferred embodiments, it should be understood that certain modifications may be made to parts of the invention with respect to the arrangement thereof without departing from the spirit and scope of the invention.

What is claimed is: 1. An electron beam gun comprising: (A) a pilot gun having (i) An enclosure provided with a relatively low-pressure ionizable gaseous medium providing the source of electrons from said pilot gun; (ii) A pilot cathode positioned in one wall of said enclosure, said pilot cathode having a concave surface to give a convergent beam of electrons having a focal point beyond the Faraday dark space; (iii) A pilot anode positioned in said wall around and spaced from said pilot cathode the potential difference between said pilot cathode and pilot anode providing the electron accelerating voltage for the electrons generated from said gaseous medium.

(18) A main cathode at the same potential as said pilot anode positioned in the wall opposite said pilot cathode and at a distance from said pilot cathode beyond the Faraday dark space such that said main cathode is bombarded by electrons from said pilot gun and thereby itself emits electrons.

(C) A main anode spaced from said main cathode and having a high electron accelerating voltage therebetween to accelerate the electrons emitted from said main cathode toward a workpiece.

2. An electron beam gun according to claim 1 wherein said pilot cathode has a concave surface whose radius of curvature is approximately twice the cathode radius.

3. An electron beam gun according to claim 1 including means for flowing gas from a source thereof into said enclosure to provide and maintain said low-pressure ionizable gaseous medium.

4. An electron beam gun according to claim 1 including a self-contained source of gas in said enclosure.

5. An electron beam gun according to claim 3 and including a control ring positioned in said wall around said means for flowing gas into said enclosure and in contact with the Faraday dark space when the gun is in operation to thereby carry a high negative potential which repels electrons.

6. An electron beam gun according to claim 4 having a control cup fitted with an aperture at the bottom, in proximity to and in line with the cathode emitter for the purpose of aiding in the stabilization of the focal region of the beam, is positioned in said enclosure.

7. An electron beam gun according to claim 1 wherein power is supplied to said pilot gun through power cables having an outside diameter of about 0.250 and a conductor of as small as 400 circular mils cross section.

Claims (7)

1. An electron beam gun comprising: (A) a pilot gun having (i) An enclosure provided with a relatively low-pressure ionizable gaseous medium providing the source of electrons from said pilot gun; (ii) A pilot cathode positioned in one wall of said enclosure, said pilot cathode having a concave surface to give a convergent beam of electrons having a focal point beyond the Faraday dark space; (iii) A pilot anode positioned in said wall around and spaced from said pilot cathode the potential difference between said pilot cathode and pilot anode providing the electron accelerating voltage for the electrons generated from said gaseous medium. (B) A main cathode at the same potential as said pilot anode positioned in the wall opposite said pilot cathode and at a distance from said pilot cathode beyond the Faraday dark space such that said main cathode is bombarded by electrons from said pilot gun and thereby itself emits electrons. (C) A main anode spaced from said main cathode and having a high electron accelerating voltage therebetween to accelerate the electrons emitted from said main cathode toward a workpiece.

2. An Electron beam gun according to claim 1 wherein said pilot cathode has a concave surface whose radius of curvature is approximately twice the cathode radius.

3. An electron beam gun according to claim 1 including means for flowing gas from a source thereof into said enclosure to provide and maintain said low-pressure ionizable gaseous medium.

4. An electron beam gun according to claim 1 including a self-contained source of gas in said enclosure.

5. An electron beam gun according to claim 3 and including a control ring positioned in said wall around said means for flowing gas into said enclosure and in contact with the Faraday dark space when the gun is in operation to thereby carry a high negative potential which repels electrons.

6. An electron beam gun according to claim 4 having a control cup fitted with an aperture at the bottom, in proximity to and in line with the cathode emitter for the purpose of aiding in the stabilization of the focal region of the beam, is positioned in said enclosure.

7. An electron beam gun according to claim 1 wherein power is supplied to said pilot gun through power cables having an outside diameter of about 0.250 and a conductor of as small as 400 circular mils cross section.

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