US3299308A

US3299308A - Electron beam traverse of narrow aperture in barrier separating regions of differentpressure

Info

Publication number: US3299308A
Application number: US296175A
Authority: US
Inventors: Charles W Hanks
Original assignee: Temescal Metallurgical Corp
Current assignee: Temescal Metallurgical Corp
Priority date: 1963-07-19
Filing date: 1963-07-19
Publication date: 1967-01-17
Anticipated expiration: 1984-01-17

Description

c. w. HANKS' 3,299,308

ERTURE IN BARRIER SEPARATING REGIONS OF DIFFERENT PRESSURE Jan; 17, 1967 ELECTRON BEAM TRAVERSE OF NARROW AP Filed July 19, 1963 fizz; 1

Char/es W Hanks United States Patent Office 3,299,308 ELECTRON BEAM TRAVERSE F NARROW APER- TURE IN BARRIER SEPATING REGIONS OF DIFFERENT PRESSURE Charles W. Hanks, Orinda, Calif., assignor to Temescal Metallurgical Corporation, Berkeley, Calif, 21 corporation of California Filed July 19, 1963, Ser. No. 296,175 Claims. (Cl. 313-160) The present invention relates generally to an electron beam apparatus and, more particularly, to an improved electron beam apparatus for treating materials.

Electron beam apparatus has been employed for various material treating processes, such as melting, annealing, cleaning, vapor-plating, etc. Normally, an electron beam apparatus includes a source of electrons and focusing means for forming the electrons into a beam and directing the same at the material to be treated, the material to be treated and the electron source being enclosed in an evacuated chamber. Either electrostatic fields or magnetic fields may be employed for focusing electrons into a beam. The beam electrons bombard the work piece and heat the same. Gases and vapors of various kinds are emitted by the heated material. These gases may cause arc discharges in the electrostatic fields if they are not immediately removed from the chamber.

In previously available electron beam apparatus, the gases have been removed by providing a vacuum pump system which had a sufficient capacity to handle the maximum gas emitted by the heated metal. Such pump systems are expensive and bulky.

Various sources of electrons have been provided in the electron beam apparatus. For high power applications, the so-called Pierce guns have been employed. A Pierce gun generally includes an emitting surface which may be an indirectly heated cathode or a directly heated cathode. The beam is produced by the cathode and is formed by a focus electrode and an anode into a desired shape, for example, round, rectangular etc. The shape of the beam is determined by the shape of apertures in the focus electrode and the anode. The focus electrode which is at the same potential as the cathode extends outwardly from the cathode at an angle of about 67 /2 degrees with respect to the axis of the desired beam. The anode of the gun is spaced from the cathode and extends outwardly from the edges of the desired beam at a predetermined angle. The placement of the focus electrode and the anode is normally critical.

If the anode is placed too close to the cathode, a divergent beam will be formed which beam is normally unusable. For more beam current, it is desirable to place the anode close to the cathode. It has been difficult to provide a gun having a higher perveance, which is equal to the DC. beam current divided by the three half power of the DC. beam voltage, than 0.5 1() and still produce a convergent or parallel beam.

An object of the present invention is the provision of an improved electron beam apparatus for treating metal. Another object of the invention is the provision of an electron beam apparatus which permits efficient use of high output electron guns. A further object is the provision of an electron beam apparatus in which relatively small vacuum pumps are employed. A still further object is the provision of an electron beam apparatus which is economical to manufacture and use and which is durable in operation.

Other objects and advantages of the present invention will become apparent with reference to the following description and accompanying drawings.

In the drawings:

FIGURE 1 is a schematic cross sectional view of one 3,299,308 Patented Jan. 17, 1967 embodiment of the electron beam apparatus showing various features of the present invention;

FIGURE 2 is an enlarged schematic cross section of an electron gun which may be employed in the electron beam apparatus shown in FIGURE 1; and

FIGURE 3 is a schematic cross sectional view of another embodiment of an electron beam apparatus showing various features of the present invention.

The electron beam apparatus shown in FIGURE 1 includes an air-tight enclosure 10 which is divided into three

chambers

12, 14 and 16 by a pair of spaced

barriers

18 and 20. An electron source or gun 22 is disposed in one end chamber (gun chamber 12) and a target 24 (the material to be treated) is disposed in the other end chamber (treating chamber 16). Each of the

barriers

18 and 20 has an

aperture

26 and 28, respectively, therein which is in alignment with the axis of a beam produced by the electron gun 22. Means 30 is provided for establishing a uniform magnetic field extending parallel to the axis of the beam of electrons whereby electrons from the gun 22 follow helical paths to the

apertures

26 and 28. The distance between the

barriers

18 and 20 and between the barrier 18 and gun 22 is made substantially equal to an integral number of revolutions of the helical path of the electrons. Pump means 32, 34 and 36 are connected to the

chambers

12, 14 and 16, respectively, for evacuating the same.

If an electron moves parallel to a uniform magnetic field, it is not influenced and travels along a straight line. If the electron moves in a direction perpendicular to the uniform field, its path is a circle. If an electron moves in a direction between that of parallel and perpendicular, its path is helical, the axis of the helix being parallel to the magnetic field. Electrons leaving a point on a magnetic field line at the same angle, although in different directions, will arrive back at the field line at the same time and at the same point which is farther along the field line by a distance equal to a revolution of the helix (the pitch). The pitch of the helix depends on the cosine of the angle of departure of electrons from the field line. For small angles the pitch does not change much with the change of angle. Hence, if a magnetic field is placed parallel to a beam of electrons all moving generally in the same direction, the electrons will all return to positions corresponding to their original position in a distance along the axis of the beam equal to the pitch of the helix. Thus, an image of an emitting surface is produced one or more revolutions or pitches of the helical path of the electrons along the field line. The length of a revolution or the pitch of the helical path is equal to Where V is the electron velocity in equivalent volts; B is the magnetic flux density in webers per square meter, and 0 is the angle of depature from the field line.

In the illustrated embodiment, this production of images of the emitting surface is utilized to reduce the amount of gas or vapors which reach the electron gun. More specifically, as shown in FIGURE 1, the electron beam apparatus includes the air-tight enclosure 10 which may be of any suitable shape such as rectangular, cylindrical, etc. The enclosure 10 is divided into three

chambers

12, 14 and 16 by the pair of spaced

parallel barriers

18 and 20. The barriers may be of any suitable material which will withstand the temperature and environmental conditions of the apparatus.

In FIGURE 1, the central part of each

barrier

18 and 20 is formed of a

circular plate

38 and 40 of ferromagnetic material, such as steel. The remainder of the barriers are made of a material, such as copper. The

plates

38 and 40 are provided with central and aligned

apertures

26 and 28, described more fully hereinafter. Besides serving as part of the barriers, the

plates

38 and 40 also serve as pole pieces for providing a uniform magnetic field in the intermediate or transition chamber 14 parallel to an axis extending between the

apertures

26 and 28. The magnetic field is generated by a plurality of iron core coils 42 extending between the

plates

38 and 40 and disposed adjacent the periphery thereof.

As shown in FIGURE 1, the electron source or gun 22 is disposed in the end or gun chamber 12 in a position such as to direct a beam of electrons along the axis of the

apertures

26 and 28. The electron gun 22 may be any suitable linear type gun but preferably, for high current output, is a Pierce type gun. The electron gun 22 is suitably supported in spaced relation to the barrier 18 and is disposed in a central aperture of a pole piece 44 which may be a plate of ferromagnetic material similar to the

plates

38 and 40.

A uniform field is generated parallel to an axis extending between the electron gun 22 and the aperture 26 by iron core coils 46 extending between the

plates

38 and 44 adjacent the periphery thereof. Thus the electrons, upon being emitted by the emitting surface of the electron gun 22, are immediately influenced by the uniform field. Consequently, the electrons follow a helical path upon their emission from the emitting surface and form images of the emitting surface at an integral number of revolutions or pitches of the helical path.

The spacing between the

pole pieces

38 and 40 and the spacing between the emitting surface of the electron gun 22 and the pole piece 38 are made such that an image of the emitting surface is produced at each of the

apertures

26 and 28. In this connection, the distance between the

apertures

26 and 28 and between the emitting surface and the aperture 26 is selected so as to be equal to an integral number of the revolutions or pitches of the helical path.

As previously indicated, the electron gun 22 is pre- \ferably a Pierce type gun. Such a gun provides a high current beam and thus provides a higher heating effect on the material 24 to be treated. As shown in FIGURE 2, the Pierce type gun of the illustrated embodiment includes an emitting surface or cathode 48 which is in the form of an elongated cylinder or filament of a material such as tungsten. The filament is heated by passing current through the same. An indirectly heated cathode may be employed in certain applications. A pair of

plates

50 and 52 which serve as the focus electrode extend from the cathode 48 at an angle of 67 /2 relative to the axis of the beam of electrons. The

plates

50 and 52 are made of a material which will withstand the temperature to which they are exposed. A suitable material is tungsten or tantalum. The focus electrodes may be formed of a single plate with a slot to receive the cathode. The cathode 48 and the pair of focusing

plates

50 and 52 are connected to a source of high negative potential 54.

In the illustrated embodiment, the anode of the electron gun is formed bya pair of metal rods 56 and 58 which extend parallel to the cathode and on either side of the beam of electrons. The rods, which may be loosely supported, are made of a refractory metal, such as tungsten, tantalum, etc. The rods 56 and 58, as well as the

pole pieces

44, 38, and 40 and the target 24 are grounded. As previously indicated, the electrons are constrained and directed by the magnetic field as soon as they are emitted from the cathode and, hence, the electrons are prevented from diverging. This permits the use of loosely held rods as anodes and the use of a relatively small distance between the anode and cathode and hence a higher beam current. Without the magnetic field, such a construction would produce a divergent beam which would normally be unusable. Also, the magnetic focusing reduces the amount of anode bombardment by the electron beam and hence, even at high outputs, anode cooling, other than radiation, is not required.

Since an image of the emitting surface is produced at the

apertures

26 and 28, the

apertures

26 and 28 are made of the same shape as the electron emitting surface and are aligned with the emitting surface. Thus, in the illustrated embodiment, the

apertures

26 and 28 are made rectangular since the cathode is rectangular. The edges of the

apertures

26 and 28 are beveled to prevent the electrons from striking the

pole pieces

38 and 48. However, the bevel is not critical.

Gas and vapor is evolved by the electron beam impinging on and heating the target 24. The

barriers

40 and 38 greatly reduce the amount of gas and vapor which enters the gun chamber 12. Gas and vapor can only enter the gun chamber 12 by passing through the

apertures

26 and 28. The

apertures

26 and 28 are made about the size and shape of the emitter and hence a relatively minor amount of leakage occurs between chambers. The leakage through the aperture 26 is especially minor since at the normal operating pressures of

chambers

12 and 14, so-called molecular flow occurs through the aperture 26 instead of the normal viscous flow. In this connection, the mass flow of gases and vapors through orifices drops significantly as soon as a pressure is reached where molecularfiow occurs instead of the viscous flow. This is defined as a pressure such that the distance between the sides of the orifice is equal to or less than the mean free path of the vapor or gas molecules. The molecular flow of gases through an orifice is about 43% less than that of viscous flow through the orifice. By making the aperture 26 a narrow rectangle, molecular flow occurs at a higher pressure than for a cylindrical aperture of the same area. This results in a considerable saving in the size of pumps required to maintain a given pressure drop across the aperture, or conversely, it permits an aperture with a larger cross sectional area for a given size pump. 1

The target 24 to be treated, which may be of metal or non-metal, is suitably mounted in the treating chamber 16. The beam emerging from the last aperture 28 is no longer under the influence of the magnetic field and hence, the beam tends to spread, due to space charge repulsion. The position of the target relative to the last aperture 28 depends on the operations to be performed on the target and the amount of power required. The closer the target is to an image point of the beam the more intense the beam will be. For example, to melt the metal, the beam is normally spread over its entire surface to reduce hot spots.

To minimize the amount of ions and metal vapor which enter the transition chamber and the gun chamber, the target may be positioned out of alignment with the

apertures

26 and 28. The electron beam is focused on the target by a traverse magnetic field provided by a suitable means (not shown) after the last aperture 28, the field bending the electron beam as it emerges from the last aperture 28. Thus, vapor and ions produced by electron bombardment of the target tend to strike the barrier 40 rather than passing through the aperture 28.

The

chambers

12, 14 and 16 are evacuated by

suitable pumps

32, 34 and 36, respectively. Preferably, for the most stability, the electron gun chamber 12 is evacuated to as high a vacuum as possible, i.e., higher than about 0.1 micron of Hg. Because of the pressure barrier system of the illustrated apparatus, very little vapor or gas from the treating chamber 16 enters the electron gun chamber 12. Thus a high speed diffusion pump can handle the low mass vapor flow involved and maintain the electron gun chamber 12 at a high vacuum.

The pressure at which the treating chamber 16 is operated depends upon the operation to be performed on the target 24. For normal operations, the treating chamher is preferably evacuated at a speed sufficient to quickly remove the vapor and gases evolved during the operation.

A relatively low vacuum may be tolerated in the treating chamber 16, since high voltage electrostatic fields are not present in the treating chamber. Thus a higher pressure pump, such as a mechanical vacuum pump, can be employed to evacuate the treating chamber 16.

The transition chamber 14 is evacuated to a vacuum intermediate that of the treating chamber :16 and the gun chamber 12. The allowable pressure differential between the chambers depends upon the amount of gases evolved. For lower amounts of gases, a pressure differential of up to about 100 to 1 can be tolerated. In certain applications, the transition chamber may be eliminated. For large amounts of evolved gases the pressure differential, preferably is not .greater than to 1. Additional transition chambers can be provided to provide the lower differential pressure across the barrier.

In one illustrated embodiment of the electron beam apparatus the emitter size is A: inch wide by A2 inch long. The apertures in the barriers are A; inch wide by 1 inch 'long. The pole pieces are made of steel and are spaced approximately 8 inches apart. The magnetic field intensity is 108 gausses for 10 kv. electrons and 130 gausses for kv. electrons. The gun chamber is evacuated to 0.6 micron of Hg, the transition chamber to 17 microns of Hg, and the treating chamber to 100 microns of Hg.

In the embodiment shown in FIGURE 3, wherein parts similar to those shown in FIGURE 1 are indicated with the same reference numeral and with the suffix a, the uniform magnetic field is generated by

Helmholtz coils

60, 62 and 64. Helmholtz coils are coaxial air core coils which are spaced so that the distance between the coil centers is equal to the average diameter of the coils. The

barriers

18a and 20a between the

chambers

12a, 14a, and 16a are made of non-magnetic material, such as watercooled copper.

The Helmholtz coil system of FIGURE 3 has two main advantages over the ferromagnetic system shown in FIG- URE 1. First, the magnetic lines have no curvatures at the apertures but continue in straight parallel lines, unaffected by the presence of the orifice or its thickness. Thus, in FIGURE 3, the electrons do not traverse a re gion where the magnetic lines of flux are not parallel but are divergent on one side of the aperture and convergent on the other side. The non-parallel lines of flux may unequally aifect the electrons in the beam, depending on the position of the electrons in the aperture. Secondly, the inductance of the air core coils is very much less than that of the iron core coils. Hence, the magnetic field intensity may be changed at a higher rate, as by means of a servo system, to follow any changes in high voltage in the gun cathode to keep the beam of electrons always focused on the aperture.

As can be seen from the above, an electron beam apparatus is provided which permits efficient use of high perveance electron guns which normally produce unusable divergent beams. Also, relatively inexpensive pumps may be employed in the apparatus to maintain the required vacuum in the system.

It should be realized that while one electron gun is shown in the drawings, more than one electron gun may be provided along with the associated apertures. Also, as previously indicate-d, in certain applications it may be desirable to only use one barrier, that is, a barrier to separate the enclosure into only two chambers, a gun chamber and a treating chamber. In other apparatus it may be desirable to provide more than three chambers. Various other changes and modifications may be made in the above described electron beam apparatus without deviating or departing from the spirit or scope of the present invention.

Various features of the present invention are set forth in the accompanying claims.

What is claimed is:

1. An electron beam apparatus for treating material comprising an air-tight enclosure, a barrier in said enclosure for dividing the same into two chambers, said barrier having an elongated aperture therein, an elongated electron emitting surface in one of said chambers, said surface being parallel to and aligned with said aperture, said aperture being of substantially the same size as said surface, means for directing said electrons at said aperture, means for establishing a uniform magnetic field having parallel lines of flux from the emitting surface to the aperture whereby electrons from said surface follow a helical path to said aperture, the distance between the aperture and the emitting surface being substantially equal to an integral number of revolutions of the helical path, and means connected to said chambers for evacuating the same to maintain a pressure differential on opposite sides of the barrier, the narrow dimension of the aperture being less than the mean free path of the gas molecules in the chambers.

2. An electron beam apparatus for treating material comprising an air-tight enclosure, a pair of spaced barriers in said enclosure for dividing the same into three chambers, each of said barriers having an elongated aperture therein, an elongated electron emitting surface in a first of said chambers, said surface and each of said apertures being parallel, of substantially the same size, and in line, means for directing said electrons at said aperture, means for establishing a uniform magnetic field having parallel lines of fiux from the emitting surface to the apertures whereby electrons from said emitting surface follow a helical path, the distance between the apertures and between the aperture and said surface being substantially equal to an integral number of revolutions of the helical path, and means connected to each of said chambers for evacuating the same to maintain a pressure differential on opposite sides of the barrier, the narrow dimension of the aperture being less than the mean free path of the gas molecules in the chambers.

3. An electron beam apparatus for treating material comprising an air-tight enclosure, a barrier in said en;

closure for dividing the same into two chambers, said barrier having an elongated aperture therein, an elongated electron emitting surface in one of said chambers, said surface being parallel to and aligned with said aperture, means for directing said electrons at said aperture, means for establishing a uniform magnetic field having parallel lines of flux from the emitting surface to the aperture whereby electrons from said surface follow a helical path to said aperture, the distance between the aperture and the emitting surface being substantially equal to an integral number of revolutions of the helical path, and means connected to said chambers for evacuating the same, the pressure difference on the opposite sides of the barrier being of the order of between 10 to 1 and to l.

4. An electron beam apparatus comprising an air-tight enclosure, a pair of spaced generally parallel barriers in said enclosure for dividing the enclosure into three chambers, means for evacuating each of said chambers, each of said barriers being formed at least in part by a plate of ferromagnetic material, a third plateof ferromagnetic material in a first of said compartments, said third plate being spaced from and generally parallel to said first mentioned plates, a Pierce type electron gun extending through an aperture in said third plate, said gun producing an elongated beam of electrons which beam is directed at said first mentioned plates, each of said first mentioned plates having an elongated aperture substantially the same size as the elongated beam of electrons, said elongated apertures being in alignment with and parallel to said elongated beam of electrons, the mean free path of gas molecules in said aperture closer to said gun being greater than the width of said aperture, and a plurality of cores extending between each pair of plates, each of said cores having a winding thereon, whereby when current is passed through said winding a uniform magnetic field is set up between said plates and electrons in said beam follow a helical path to said elongated apertures, said elongated apertures being spaced from each other and said gun being spaced from the adjacent elongate-d aperture by a distance equal to an integral number of revolutions of the helical path.

5. An electron beam apparatus for treating material comprising an vair-tight enclosure, three equal, fiat, circular air core coils disposed in spaced coaxial relationship in said enclosure, the spacing between said coils being equal to the diameter of one of the coils, said coils being connected in series so as to provide a uniform field within said coils, a barrier in said chamber at each of two of said coils for dividing the enclosure into three chambers, an elongated aperture in each of said barriers, a

Pierce type electron gun disposed within said remaining coil, said gun producing an elongated electron beam and directing the same toward said apertures, whereby electrons in said beam follow a helical path to said apertures, said apertures and said beam being substantially of the same size, aligned and parallel to each other, the spacing between the apertures and between the gun and 8 the adjacent aperture being equal to an integral number of revolutions of said helix, and means for exhausting each of said compartments, the width of the aperture closer to said gun being less than the mean free path of gas molecules in that aperture.

References Cited by the Examiner UNITED STATES PATENTS 1,941,157 12/1933 Smith 313-84 X 2,072,658 3/1937 Von Bronk 31386 X 2,234,281 3/1941 Ruska 313-84 X 2,266,218 12/1941 Krause 313-84 X 2,293,567 8/1942 Skellett 31386 X 2,369,782 2/1945 Hillier 31384 2,429,558 10/1947 Marton 31384 X 2,841,726 7/1958 Knechtli 3137 3,150,256 9/1964 Wilska 31384 X JAMES W. LAWRENCE, Primary Examiner.

R. SEGAL, Assistant Examiner.