US3614510A

US3614510A - Nonthermionic cathode discharge devices

Info

Publication number: US3614510A
Application number: US829544A
Authority: US
Inventors: Clifford William Alfre Maskell
Original assignee: UK Atomic Energy Authority
Current assignee: UK Atomic Energy Authority
Priority date: 1968-06-04
Filing date: 1969-06-02
Publication date: 1971-10-19
Anticipated expiration: 1988-10-19
Also published as: JPS493119B1; GB1275261A; DE1927979A1; NL6908499A; FR2013329A1

Abstract

A nonthermionic cathode glow discharge device comprising an enclosure and means to maintain gas in the enclosure at a predetermined pressure. An anode is mounted in the enclosure or forms a part of a wall of the enclosure. A cathode having a surface directed at a treatment zone is suspended in the enclosure and has an aperture therein in which is positioned a control electrode. The said surface of the cathode may extend longitudinally or may be shaped as a segment of a sphere. By varying the potential applied to the control electrode with respect to the cathode, the focal point of the electron or ion beam and the current of the output beam can be controlled. A third electrode may be provided to surround part of the cathode to act as a screen to minimize unwanted electron emission from the cathode.

Description

l0-l9-7l XR 396149510 1 O Umted States Patent [111 ,6 14,510

[72] lnventor Clifford William Alfred Maskell 3,383,541 5/1968 Ferreira 313/207 Abingdon, England 3,414,702 12/1968 Stauffer.... 219/121 [21] Appl. No 829,544 3,430,091 2/1969 Davis 313/210 [22] Filed June 2,1969 3,466,487 9/1969 Davis et a1. 313/210 X [45] Paten 061-1 197 3,482,133 12/1969 Dugdale et a1. 313/210 [73] Assignee United Kingdom Atomic Energy Authority FOREIGN PATENTS London, England [32] Priority June 4,1968 1,506,933 11/1967 France 313/231 [33] Great Britain Primary Examiner-Roy Lake {31 26600/68 Assistant Examiner-Palmer C. Demeo Attorney-Larson, Taylor and Hinds [54] SS JEEE CATHODE DISCHARGE ABSTRACT: A nonthermionic cathode glow discharge device 13 Claims, 4 Drawing Figs. comprising an enclosure and means to mamtain gas in the enclosure at a predetermined pressure. An anode 15 mounted in US. Cl the enclosure or forms a pan of a wall of [he enclosure, A 219/121 313/231, 315/111 cathode having a surface directed at a treatment zone is [51] lnt.C| ..H01j 17/04, uspe ded in the enclosure and has an aperture therein in 1 17/26 which is positioned a control electrode. The said surface of the [50] Field of Search 313/207, cathode may extend longitudinally or may be shaped as a 2101 231i 315/] l ling/121 ment of a sphere. By varying the potential applied to the control electrode with respect to the cathode, the focal point of [56] Reierences cued the electron or ion beam and the current of the output beam UNITED STATES PATENTS can be controlled. A third electrode may be provided to sur- 3,262,003 7/1966 Allen et al. 313/207 round part of the cathode to act as a screen to minimize un- 3,381,157 4/1968 Ferreira 313/210 X wanted electron emission fromthc cathode.

NONTHERMIONIC CATHODE DISCHARGE DEVICES This invention relates to nonthermionic cathode glow discharge devices which form electron beam or ion beam generating devices.

According to the present invention a nonthermionic cathode glow discharge device comprises an enclosure, means to maintain gas in the enclosure at a predetermined pressure, an anode mounted within or forming part of the enclosure, a cathode having a surface directed toward a treatment zone and provided with an aperture therein, a second electrode positioned in or adjacent said aperture and means to apply to the anode. cathode and second electrode suitable operating potentials.

Preferably the surface is a curved surface and may be part spherical if desired. The cathode may be in the form of a segment of a hollow sphere and the treatment zone will then be at the center of the segment of the sphere but if desired the outline of the cathode may be of any shape such as for example an elongated arcuate plate or an elliptical curved, or annular shape when viewed in plan. Such an arrangement is suitable for producing a sheetlike beam of electrons or ions which may extend linearly in a straight or curved line or in a ring.

If desired a third electrode may surround part of the cathode and to enable a focused electron beam to be produced the third electrode is maintained positive with respect to the cathode and serves to minimize unwanted electron emission from the cathode. The cathode and the electrodes may be made from sheet metal which may or may not be perforated or constructed from a wire mesh. Alternatively, to enable a focused beam of ions to be produced the third electrode is made from sheet material and the cathode is made from perforated sheet metal or from a wire mesh. Some of the positive ions bombarding the cathode during operation of the device pass through the perforations or mesh and are focused by the potentials applied to the anode the cathode and said electrodes and by the pressure of the gas in the enclosure.

The operating potential applied to the second electrode may be the same as, or positive or negative relative to, that applied to the cathode. The second electrode may be solid or a hollow, and if desired may be moveable or alternatively fixed relative to the cathode. The second electrode may be positioned in the aperture or to one side of the plane of the aperture. Preferably it is positioned on the side of the aperture adjacent a concave side of the curved surface of the cathode. The cathode may be made from any suitable metal such as for example, tantalum or aluminum.

The present invention will now be described by way of an example only, with reference to the accompanying drawings in which;

FIG. 1 is a diagrammatic section of one form of cold cathode device according to the present invention.

FIG. 2 illustrates diagrammatically a modification to the electrode assembly of the device of FIG. 1.

FIG. 3 illustrates diagrammatically a further modification to the electrode assembly of FIG. 2.

FIG. 4 illustrates diagrammatically the electrode assembly of FIG. 3 modified to enable tubes or rods to be heated at a zone intermediate the ends of the rod or tube.

Referring to FIG. 1 of the drawings there is shown a cold cathode discharge device comprising tube 1 of heat-resisting glass and end plates 2 and 3. The end plates 2 and 3 are metal and are earthed so as to form anodes in operation of the device. O-rings (not shown) are provided between the mating surfaces of the plates 2 and 3 and the glass tube 1 to ensure a gastight joint and the end plates 2 and 3 are suitably secured to the ends of the tube 1. A tantalum or aluminum cathode 4, in the shape of a segment of a hollow 6 -inch diameter sphere, is suspended within the enclosure by a connecting tube 5 attached to a supply terminal 6 and an insulator 29. The supply terminal 6 has a bore 7 for the passage of coolant, and is secured to a removable portion 8 of the plate 2 by an insulating cylinder 9. A central aperture communicating with the bore ofthe tube 5 is provided in the cathode 4. A second electrode 10 which may be in the form of a plate as shown in FIG. 1 is supported adjacent the aperture in the cathode 4 by a connecting rod 11 which is secured to an insulating cylinder 12 attached to the supply terminal 6. A third electrode 13 forms a screen which surrounds the outer curved surface of the cathode 4 and is supported in the enclosure by a tube 14 which extends coaxial with, and encircles the tube 5. The tube 14 is attached to the removable portion 8 of the plate 2 and together with the screen 13 forms an anode in operation of the device. The spaces between the

tubes

5 and 14 and between the screen 13 and cathode 4 is kept small to minimize spurious electron emission therebetween.

The cathode 4 is of a refractory sheet metal and if desired may be perforated. Alternatively the cathode may be constructed from refractory wire mesh, such as, for example tantalum.

A crucible 16 is mounted on a shaft 17 secured to a removable portion 18 of the plate 3. In operation of the device the crucible 16 may form an anode if desired.

The enclosure is provided with an outlet opening 19 through which the interior of the enclosure is partially evacuated by a pump 20, and an inlet opening 21 through which gas from a gas supply 22 via valve means 23 is admitted to the interior of the enclosure. Small holes 28 ensure fluid communication between the interior of the enclosure 1 and the space between the insulator 9 and the tube 14.

In operation of the device, power is derived from a highvoltage direct current source 24. A suitable negative potential is applied to the terminal 6 via the fixed resistor 25 and the variable resistor 26 which are in series. The junction of the

resistors

25 and 26 is connected to the further electrode 10 by way of the rod 11. A suitable potential is applied to the plates 2 and 3 and the crucible 16. A voltmeter and ammeter (not shown) are included in the power supply circuit.

One way in which the device may be operated will now be described.

The enclosure is evacuated to a pressure approximately SXIOtorr and then filled with helium to a pressure of 4.5Xl0torr. The resistor 23 is set so that an equal negative potential of approximately 2.75 kilovolts is applied to the cathode and further electrode 10. When outgassing of the cathode has occurred the helium pressure is allowed to rise slowly, this is done by adjustment of the valve 23. A glow discharge develops, positive ions bombard the cathode and electrons are emitted from the cathode and directed by the cathode and the potential field lines in the vicinity of the cathode to form an inverted conical beam. The shape of the cathode 4 focuses the beam on the axis of the cathode 4 approximately at the center of the sphere of which the curved surface of the cathode 4' forms a part. The degree of focus is dependent on the pressure of the helium and the potential of the second electrode. With the cathode 4 at 2.75 kilovolts the beam 27 is focused to a small point at about l5 10 torr, but if the power of the beam 27 is increased by making the potential of the cathode more negative, the beam 27 is focused to a point at a lower helium pressure. With the cathode at 4.0 kilovolts the beam is focused to a point at a lower helium pressure. With the cathode at 4.0 kilovolts the beam is focused to a point at about 8X 1 0 torr.

The second electrode may be fixed relative to the cathode or if desired may be moveable in a direction parallel to the longitudinal axis of the rod 11. This has the effect of altering the position of the focus of the beam 27.

If desired, the second electrode 10 may be connected to a power supply which is separate to that for the cathode 4 so as to enable a positive or negative potential relative to the cathode to be applied to the second electrode 10.

The application of a positive potential to the second electrode 10 with respect to the cathode tends to move the point of focus of the electron beam 28 towards the cathode 4 whereas a negative potential with respect to the cathode tends to move the focus away from the cathode 4.

The application of a positive potential to the second electrode It) also has the effect of increasing the power of the electron beam by increasing the current of the beam. For example with helium in the enclosure at a pressure of 2X10" torr with Zero bias applied to the second electrode 10 (i.e. the second electrode I is at the same potential as the cathode) the beam current was found to be about 80 milliamperes when a potential difference of kv. was maintained between the anodes 2 and 3 and cathode 4. However when a positive potential of 800 volts with respect to the cathode was applied to the second electrode with the potential applied between the cathode and anodes 2 and 3 and the pressure of the helium maintained the same as before, the current of the beam increased to about 140 milliamperes. In other words, the power of the beam was increased from 400 watts to 700 watts.

To use the device, a specimen to be heated is arranged within the enclosure in the crucible 16 at the focal point of the beam. If desired, the crucible 16 may be moveable so as to enable the specimen to be moved into or out of the electron beam 27. It is to be understood that the crucible 16 may be dispensed with in some uses where it is not required to support the material to be heated in a crucible.

The device may be operated with other gases for example oxygen or argon and may be used for various purposes, such as, for example, heat treatment of materials, welding, melting, and zone refinement of, for example, rods or tubes. If oxygen is used it is desirable to make the cathode 4 of an oxidation resistant material.

1 Although the enclosure is described above as being formed in part by a cylindrical tube of heat-resisting glass, it is to be understood that the tube may be made from any suitable material. If desired the tube may be metal in'which case the tube together with the end plates 2 and 3 forms an anode in operation of the device. It is to be further understood that the enclosure may be of any desired shape.

Referring to FIG. 2 it will be seen that the third electrode or screen 13 has been extended so as to curve around the outer edge of the cathode 4. By extending the screen 13 around to cover a part of the concave surface of the cathode 4 the screen serves the function of defining the outer extent of the beam 27 by cutting off the edge of the beam. Thus the screen 13 can be used to minimize the area of electron emission of the cathode as desired. The inturned portion of the screen 13 may also serve the function of forming a lense by altering the electric field lines in the vicinity of the cathode.

Referring to FIG. 3 the electrode assembly of the device as illustrated in FIG. 2 may be further modified by making the rod 11 hollow and by flairing the lower end of the tube thus formed outwardly, in which case the plate 10 shown in FIGS. 1 and 2 may be dispensed with or may be in the form of an annular plate (not shown) fastened to the end of the hollow rod 11 of FIG. 3. By making the rod 11 hollow as shown in FIG. 3 it is possible to heat long objects such as rods or tubes by passing the objects up the bore of the hollow rod 11 and heating a ring shaped zone on the object. Such an arrangement is useful for welding rods or tubes together. The curvature of the cathode shown in FIGS. 1, 2 and 3, may be arranged such that in operation the resulting electron beam is brought to either a single spot or point focus by the potentials applied to the electrodes and the pressure of the gas or alternatively to a line focus, for example a ring focus.

Referring to FIG. 4 there is illustrated diagrammatically the electrode assembly of FIG. 3 modified particularly to enable rods or tubes 36 and 37 to be butt welded. Component parts of FIG. 4 corresponding to similar components illustrated in FIG. 3 have the same reference numeral ascribed to them as is ascribed to the similar component parts of FIG. 3. The second electrode 10 is secured by means of a flange 30 and insulator 31 bolted to the flange 32 of an enclosure or work chamber 33 by bolts (not shown). The bottom wall of the chamber 33 is metallic and is provided with an aperture 35 large enough to permit the work to be heated, for example tubes 36 and 37 to pass through the wall of the chamber. The third electrode 13 is secured by means of a flange 39 and insulator 40 to flange 41 of a lower work chamber 42 by means of bolts (not shown). The second electrode 10 and electrode 48 are maintained in spaced relationship by an insulator 43 by means of nuts and bolts 44. The inturned lip electrode 48 performs a similar function to the inturned lip of electrode 10 of FIG. 2.

The cathode 4 is supported in the insulator 43 by radially inward projecting insulators (not shown) arranged around the periphery of the cathode 4. The third electrode 13 is similarly supported from the insulator 43. Power is fed to the cathode 4 by means of a rod 51 mounted in an insulator 52 which is mounted in the bore of the tube 14. The tube 14 has a flange 53 for fixing the tube 14 to the insulator 43 and thus ensures that the cathode 4 and screen electrode 13 are firmly supported from the insulator 43. A gas inlet connector 45 provided in the wall of chamber 33 to enable gas to be admitted to the chamber 33 and an outlet connector (not shown) communicates with the interior of chamber 42 via which gas admitted to chamber 33 passing through the

apertures

35 and 46 in the

walls

34 and 47 of the

chambers

33 and 42 respectively may be pumped out by a pump (not shown). An outlet port may be provided in the chamber 33 via which the chamber 33 may be pumped down independently of the chamber 42 by a pump not shown. Any suitable pump such as for example a roughing pump or a diffusion pump may be employed depending on the degree of evacuation required.

Valves (not shown) may be employed to effectively blank off the

apertures

35 and 46 to enable the electrode assembly to be removed as a whole from the

chambers

33 and 42. In this case the

flanges

30 and 39 are unbolted from the flanges 32 and 41. It is preferable to maintain the

chambers

33 and 42 in spaced relationship, and this may be achieved by using a temporary jig (not shown) or by using permanent adjustable members (not shown) which function as struts during removal of the electrode assembly.

The cathode 4 and screen 13 is accessible with or without removing the whole electrode assembly by removing the insulator 43. The cathode 4 screen 13 and insulator 43 may, if

desired, be a split assembly which permits the sections to be removed without disturbing the

electrodes

10 and 13.

If desired the

chambers

33 and 42 may be dispensed with and the inlet and outlet ports referred to above may communicate with the

spaces

49 and 50 fon'ned between the electrodes l0 and 48 and walls equivalent to the

walls

34 and 47 with their

respective apertures

35 and 46 blanked off. In this instance the whole of the device may be constructed as a split assembly to enable the device to be assembled around the object to be heated in a gastight manner and then subsequently disassembled. However, if desired, the

apertures

35 and 46 need not be blanked off in which case a seal (not shown) which permits relative sliding movement between the object to be heated and the device while maintaining a substantially gastight seal between the device and the object to be heated is inserted in the

apertures

35 and 46.

In operation of the device of FIG. 4 gas, for example, helium is maintained at a predetermined pressure in the

chambers

33 and 42 and/or in the

spaces

49 and 50, and operating potentials are applied to the cathode 4, the screen 13, the

electrodes

10 and 48 and

walls

34 and 47. A negative potential is applied to the cathode 4 and the screen electrode 13 is maintained positive with respect to the cathode 4. The

electrodes

10 and 48 are connected to a power supply such that they may be maintained at the same potential as the cathode. The electrodes l0 and 48 may be independently maintained at a positive or negative potential with respect to the cathode 4. By varying the potential applied to the

electrodes

10 and 48 the beam 53 may be further focused. If desired the tubes 36 and 37 may be connected to form an anode in operation of the device.

It is to be understood that modifications may be made to the devices described above without departing from the scope of the present invention, for example if desired a jig may be provided to hold the tubes 36 and 37 (FIG. 4) together and this jig may carry further focusing electrodes ,to suit the particular operation required.

The accompanying drawings illustrate electrodes which are in effect segments of spheres but it is to be understood that the electrodes 4, l0, l3 and 48 may be'longitudinally extending plate type electrodes and the structure surrounding the electrodes, i.e., the enclosures or chambers may be modified to accommodate the elongated electrodes. In this respect FIGS. 1 to 3 of accompanying drawings may be regarded as illustrating cross-sectional views of elongated electrodes.

The invention is not restricted to the details of the foregoing examples.

lclaim:

l. A nonthermionic cathode glow discharge device for producing an electron beam, the device being of the type wherein the electron beam is produced by the emission of secondary electrons from the surface of a cathode and comprising: an enclosure; means for admitting gas to the interior of the enclosure, said gas, during the operation of the device, being maintained therein at a predetermined pressure; a cathode having an electron emissive surface facing a treatment zone, said cathode having at least one edge for defining an aperture; an electrode positioned in the vicinity of said aperture; an anode within said enclosure and spaced from said cathode; means for applying operating potentials to said anode, said cathode and said electrode such that, in operation, the gas in the said enclosure becomes ionized and positive ions bombard the cathode causing secondary electron emission from said electron emissive surface of the cathode which electrons are directed towards the treatment zone along paths which extend generally at right angles to the electron emissive surface of said cathode, said means including means for selectively applying a potential to said electrode in a range between predetermined positive and negative values with respect to the potential applied to said cathode; and means for positioning said electrode with respect to said cathode such that when, in operation, said potential is selectively applied to said electrode the electron beam is selectively deflected. I

2. A device according to claim 1 wherein said surface is curved.

3. A device according to claim 1 wherein the cathode is a segment of a hollow sphere.

4. A device according to claim I wherein the cathode is an arcuate plate.

5. A device according to claim 1 wherein a third electrode surrounds part of the cathode.

6. A device according to claim 5 wherein the third electrode has a portion which extends around an edge of the cathode and surrounds a part of the concave surface of the cathode.

7. A device according to claim 5 wherein the third electrode is made of solid sheet metal.

8. A device according to claim 5 wherein the third electrode is made of a wire mesh or perforated sheet metal.

9. A device according to claim 1 wherein the second electrode is a hollow tube.

10 A device according to claim 7 wherein the tube is outwardly flaired at its end adjacent the said surface of the cathode.

11. A device according to claim 1 wherein means are provided to vary the potential applied to said second electrode, with respect to the cathode.

12. A device according to claim I wherein the device is a split assembly which may be assembled around an object to be heated and subsequently disassembled.

13. A device according to claim 1 wherein the cathode is made of perforated sheet metal or wire mesh.

Claims

1. A nonthermionic cathode glow discharge device for producing an electron beam, the device being of the type wherein the electron beam is produced by the emission of secondary electrons from the surface of a cathode and comprising: an enclosure; means for admitting gas to the interior of the enclosure, said gas, during the operation of the device, being maintained therein at a predetermined pressure; a cathode having an electron emissive surface facing a treatment zone, said cathode having at least one edge for defining an aperture; an electrode positioned in the vicinity of said aperture; an anode within said enclosure and spaced from said cathode; means for applying operating potentials to said anode, said cathode and said electrode such that, in Operation, the gas in the said enclosure becomes ionized and positive ions bombard the cathode causing secondary electron emission from said electron emissive surface of the cathode which electrons are directed towards the treatment zone along paths which extend generally at right angles to the electron emissive surface of said cathode, said means including means for selectively applying a potential to said electrode in a range between predetermined positive and negative values with respect to the potential applied to said cathode; and means for positioning said electrode with respect to said cathode such that when, in operation, said potential is selectively applied to said electrode the electron beam is selectively deflected.

2. A device according to claim 1 wherein said surface is curved.

4. A device according to claim 1 wherein the cathode is an arcuate plate.

9. A device according to claim 1 wherein the second electrode is a hollow tube. 10 A device according to claim 7 wherein the tube is outwardly flaired at its end adjacent the said surface of the cathode.

12. A device according to claim 1 wherein the device is a split assembly which may be assembled around an object to be heated and subsequently disassembled.