US3383541A - Glow discharge cathode having a large electron beam emitting aperture - Google Patents

Description

3,383,541 LARGE ELECTRON 2 Sheets-Sheet l I N VEN TOR. FERNAND J. FERREIRA OLA-A4 ATTORNE F. J. FERREIRA RGB CATHODE HAVING A BEAM EMITTING APERTURE www3 May 14, 1968 GLOW DISCHA Filed Nov. 17, 1965 May 14, 1968 F. J. FERREHQA 3,383,541

GLOW DISCHARGE CATHODE HAVING A LARGE ELECTRON BEAM EMITTING APERTURE Filed Nov. 17, 1965 2 Sheets-Sheet 2 United States Patent 3,383,541 GLOW DISCHARGE CATl'llODE HAVING A LARGE ELECTRON BEAM EMl'l'llNG APERTURE Fernand I. Ferreira, Hazardville, Conn., assignor to United Aircraft Corporation, East Hartford, Conn a corporation of Delaware Filed Nov. 17, 1965, Ser. No. 508,302 12 Claims. (Cl. S13-207) ABSTRACT F THE DISCLOSURE A large apertured hollow cathode is described for producing a collimated beam of electrons and wherein the aperture is either equal or larger than the cathode fall or bears a predetermined relationship to the entire frontal beam emitting surface of the cathode.

Conventional means for producing electron beams involve the liberation of electrons from the surface of a heated cathode by thermionic emission. Recently, electron beams have been produced from a narrow apertured glow discharge hollow cathode as a result of the volume production of electrons within a hollow chamber enclosed by the cathode. In glow discharge devices various operating modes may be encountered if one, for instance, varies the potential difference between the cathode and the anode and th'e gas density of the environment. Onel of these modes produces a well-defined electron beam which may predictably and advantageously be used to work materials. Other modes such as the arc mode alsol isa good source of electrons but this mode exhibits erratic behavior and does not produce a high energy beam.

These narrow apertured hollowcathodes have hollow chambers fabricated from a wire mesh or solid metal with a single aperture in one end. When the cathode is subjected to a high negative potential with respect to an anode and with the proper cathode geometry and pressure level in the hollow chamber, a wel1-detned pencil beam of high current density, high energy electrons emanate from the aperture. An example of the versatility of the configurations possible with the apertured hollow cathode .may be found in my copending application Ser. No. 417,- 399, filed Dec. 10, 1964, en-titled Annular Hollow Cath ode Discharge Apparatus and assigned to the same as signee.

'The hollow cathode has a cathode fall region adjacent its external surface. The electrons for the beam are obB tained, however, from a plasma generated by an intense discharge within the chamber enclosed by the cathode. If the aperture through which the electron beam emerges is then so shaped to where at least one cross-sectional di mension is less than the size of the cathode fall region, an electrostatic focussing eifect o n lthe electrons is obn tained and a well-defined beam is produced.

,The range of gas densities and voltages in which this narrow' apertured hollow cathode operates with stability with little tendency to enter the arc mode is generally small. For instance, an apertured cathode operating in argon at 24 microns of mercury pressure must operate at 1200 volts potential and will enter the arc mode if the potential is raised to 2000 volts or the pressure is inn creased to 40 microns. At about 7 microns of pressure, a potential of 10,000 volts can be used but an increase .in the pressure to microns will 1result in an arc mode. Hence, with each particular gas density necessary for an electron beam mode, there is a maximum voltage at which a relatively small increase in gas density will shift the operation into the arc mode.

3,383,541 Patented May 14, 1968 A particular operating regime cannot be easily defined for the narrow apertured hollow cathode since this depends upon many factors such as type of gas used, the density of the gas, operating voltage, geometric shape of 5 the cathode, beam power requirements, stability, and

where the anode is close to the cathode -the cathode-to anode spacing, etc. Nevertheless, it cannbe stated with reasonable accuracy that its operatioifVT for most useful applications involves gas pressures of about 50 microns of mercury or less. For high voltage lapplications of greater than 20 kv., much lower pressure regimes of the order of 10 microns or' less are required and are subject to instability with relatively small excursions in the gas pressure due to outgassing or other factors. Narrow aper= tured hollow cathodes have been operated in the electron beam mode at high pressures but their dimensions are then so small as to seriously limit the power levels below practical utility for most electron beam applications.

The many useful functions to which electron beams may be put are illustrated for instance by the patents to Steigerwald 2,987,610 directed at deep penetration of materials and Patent No. 2,902,583 directed to pulsed electron beam working of materials. To achieve the'dee'p penetration, the power density of the beam at the work piece must be sufficiently high. In the event conventional welding, a surface electron beam working technique, is used, suliicient power is needed to heat and melt an area on the surface of the workpiece with subsequent fusion at deeper levels in the material occurring by the conduction of the heat from the surface. Y l.

As a result of working of materials with an electron beam, a substantial amount of outgassing and evaporation from the workpiece occurs. This causes excursionsY from` a mean pressure level of the gas and increases theprob= ability of arcing in the apertured hollow cathode. Further? more, temperature variations of the gas cause excursions in its density and further reduces the stability of the operation. In practical applications these factors have made high. voltage operation of about 15,000 volts highlynu'rigy stable with an increase in the pressure of several microns causing an arc mode and a comparable decrease causing extinguishment of the discharge. In fact, complicated and expensive closed loop gas pressure controls have been found necessary to maintain the electron beam from fan apertured hollow cathode. These disadvantages have been overcome with a newly developed glow discharge cathode. Accordingly, it is an object of this invention to provide a glow discharge device for producing an electron beam at high pressure levels. It is a further object of this invention to provide a gow discharge device operating in a gaseous envirtin'-v ment for producing an electron beam the stabilityffof which is insensitive to excursions inthe density of the gas. Across a region, called the cathode fall, and adjacent the cathode, most of the potential drop existing between the cathode and anode occurs. In the apertured hollow cathode the aperture has at least one cross-sectional-diu mension -which is less than the cathode fall distance D., and the beam produced by it is noticeably smaller' than the aperture. It has been found however that a stableelecd tron 'beam can also be produced if the aperture becomes equal or larger at long as the side walls still defnefa cavity within which the electron beam can be formed. The size of the aperture is characterized by the shape of the electron beam it produces. The shape of the beam having cross-sectional dimensions that are equal or larger than the corresponding aperture dimensions. Althouglithe relatively large aperture size of the cavity reduces. the focussing action, the electron beam that emergesfrom the cavity may be advantageously used for the working of materials. f

The large aperture has a dimensional characteristic which has been found to also have a critical relationship with a corresponding dimension of the cathode wall in which it appears. Since the electron bea-m emitted from such a large apertured device obtains most of its elec trons from the plasma in the cavity it is classiled as a large apertured hollow cathoden It is therefore an object of this invention to provide an electron beam from a cathode enclosing a cavity having an aperture where the electron beam has cross-sectional dimensions that are equal or larger than corre-= sponding aperture dimensions.

It is a further object of this invention to provide a device for producing a sta'ble electron beam from a hollow cathode having an aperture in a wall which bears a spe cic relationship with said wall. y

It is still another object of this invention to provide a device for producing an electron vbeam from a hollow cathode having an aperture of cross-sectional dimensions that are equal or larger than the cathode fall'.

These and other objects of this invention will become apparent upon a review of the following description and. the accompanying figures wherein:

FIGURE l'shows a cross-sectional view of a large apertured hollow annular cathode of this invention.

FIGURE 2 shows a prior art narrow apertured hollow cathode.

FIGURE 3 is a side view of a large apertured hollow cathode.

FIGURE 4 illustrates a wide apertured hollow cathode of the single bounding wall type.

FIGURE 5 is an end. view of the cathode of FIG URE 4.

FIGURE 6 illustrates an outwardly electron beam ra= diating hollow cathode of the wide apertured type.

lIn FIGURE l a hollow annular cathode 1 is located in a chamber 3 of which a portion of one of its walls 5 is shown. The chamber 3 is evacuated to the appropriate pressures necessary for establishing an electron beam from the hollow cathode. The cathode 1 has a peripheral rear wall 7 and extending inwardly therefrom two side walls 9 and 11. The side walls 9 and 11 intersect the extremitiesof rear wall 7 at right angles so that an annular cavity 14 is formed. The cathode 1 has an annular aperture 16 which is formed by the inward ends of walls 9 and 11..

The cathode 1 is annular but other closed circuitous arrangements of the cathode are possible, such as for the working of irregularly shapedv` workpieces.

Coaxial with the annular cathode is a workpiece 18 which serves an anode and is connected to the positive side of a high voltage source 20. In the event the work= piece 18 is a nonconductor, the positive side of the high voltage power supply may be connected to any large com ductive surface within the chamber 3. The negative side of the source 20 is connected to the cathode 1 via the high voltage cable 22 and conductor feedthrough assembly 24. The cathode 1 is supported at several places around its perimeter with nonconductive pin assembly 26. The feedn through assembly 24 and the pin support assembly 26 may be of the type described in the copending applica tion by Allan P. Walch, entitled, Mounting for a Glow Discharge Cathode, Ser. No, 508,201, filed Nov. 1.7, 1965, and assigned to the same assignee.

The cathode 1 is surrounded Iby a shield 28 to suppress the emission of electrons from the cathode surfaces oppo= site the shield. The gap between the shield and the cathode is adjusted according to well-known gas pressure scaling laws.

FIGURE 2 shows a prior art narrow apertured hollow cathode without a shield as described in my copending application Ser. No. 417,399. The cathode 30 is provided with an aperture 32 of height A and length L in one wall 34 which has a height H. The length L of the wall 34 and, of course, the whole cathode may vary and form a continuous annular configuration or a straight section= The aperture may be circular if its length L is small in 4 which case a pencil beam maybe produced from the deu vice. The aperture-to-wall ratio of A/H is less than .7 and the aperture has at least one cross-sectional dirnena sion such as A which is less than the cathode fall Dc.

The hollow cathode 1 when charged to the proper potential and exposed to the proper pressure level will produce a well-defined electron beam whose width is ape proximately that of the aperture 16. The aperture 16, however, must have a predetermined size to establish the sta-ble electron beam. Its dimensions may bev character= ized by the electron beam-toaperture size ratios. Since the narrow apertured hollow cathode produces an electron beam whose half power or half-power density profile is smaller than the aperture from which theI beam is pr0 duced, a substantial change in these profiles is observed with the larger apertures in the hollow cathodes of this invention. With large apertured hollow cathodes, the elec tron beam half power and half-power density profiles are substantially equal or larger than the aperture and the resultant -beam is unexpectedly stable.

The electrons for the beam are produced to a large extent as a result of the trapping effect in the cavity 14. The plasma in this cavity can be made more intense by adding extensions such as 36 and 38 in FIGURE 3 to narrow the aperture 16. There is a well-defined limit, however, to the size of these extensions and this may be determined bythe relationship 'between the A dmen sion of the aperture and H dimension of the wall in which it appears. Below approximately an A/I-I ratio of .7

the beam protruding from theqcathode is much smaller than the aperture and has the characteristic of prior art narrow apertured hollow cathode electron beams. Above this ratio of .7 the beam produced vby the cathode is substantially determined by the size of the aperture and exhibits great stability with varying gas pressures and anode-to-cathode potentials. Gas pressures in excess of 200 microns and at voltages of 10 kilovolts or more may be usedvto produce an electron beam whose stability is unaffected 'by voltage variations and gas density excur-s sions. 0f course, in cases where there is no aperture wall, the ratio approaches unity but this ratio still produces a stable beam. The aperture 16 is usually also provided with cross-sectional dimensions which are equal or larger than the cathode fall distance Dc. The cathode fall dis tance encompasses for most applications that distance from the cathode where most of the potential rise from cathode to anode occurs. These distances may be determined ex perimentally or may beobtained for instance of FIGURE 8.10, p. 229, from the book Gaseous Conductor by James Dillon Cabine, a 1958 Dover publication. The cathode fall distance for a cathode operating in helium at a pressure of 500 microns and at 2000 volts is approx= imately 1 cm.

Having described an annular arrangement, other con figurations are possible such as a cylindrical device as in FIGURE 3 where one wall of the cylinder is provided with an aperture whose cross-sectionaldimensions meet the criteria of the annular arrangement described above. The aperture-to-wall dimensions cannot fall 'below .7 if the large stable beam etect'is to be maintained.

Another circuitous configuration wherein the electron beam is projected outwardly from the circuitous cathode is possible with this invention and especially suited for electron beam working inside hollow chambersD such as pipes.

Other cathode shapes possible with this invention are described in my copending application Ser. No. 417,399.

It is to be understood that the invention is not limited to the specic embodiment herein illustrated and de= scribed but may be used in other ways Without departure from its spirit as defined by the following claims.

FIGURE 4 shows a cathode having a single backwall 7 and a single bounding wall 9C The end view of FIG z URE 4 is shown in FIGURE 5 and shows the cylindrical arrangement of the cathode structuretherein. A shield 28 is selectively spaced from the cathode structure to provide the suppression of the glow discharge from the wal.s enclosed. The aperture 16' of the cathode structure is greater than the cathode fall. The cavityf14' is suicient to produce the beam of electrons from the hot plasma present therein.

FIGURE 6 illustratesan outwardly radiating annular cathode structure having an inner wall7 'and two bound= ing walls 9 extending outwardly from? the inner wall 7 to define an annular cavity 14 having an a'perture 16. The aperture 16,#again, is greater than the "cathode fall to permit a wide beam of electrons to be emitted from the annular cavity 14.

I claim:

1. A` hollow cathode structure operating at a high neg= ative potential with respect to an anode in a gaseous chamber evacuated to a predetermined density range of the gas comprising:

means for establishing a glow discharge and a cathode fall for the production of an electron beam from th cathode structure,

said vcathode structure having a peripheral' wall,

bounding walls extending away from said peripheral wall to define a cavity of sufficient size to establish said glow discharge therein,4 v

said-,bounding walls further forming an aperture,

said aperture having all its cross-sectional dimensions substantially equal or greater than the cathode fall,

and f a shield` selectively spaced from and enclosing said peripheral and bounding walls, said shield inhibiting the generation of electrons from the enclosed walls.

2. A device as recited in claim 1 and further comprising:

an aperture wall extending from at least one of said bounding walls across the aperture to reduce atl least one( of the cross-sectional dimensions of said aperture and where the reduced aperture dimensions rbears a ratio with respect to the unreduced aperture diQ v mension of greater than .7.

3. A device as lrecited in claim 1 where said peripheral wall is circuitous and defines two circuitous edges .and

where the bounding wals extend from said edges.

4. A device as recited in claim 3 where'said bounding walls are substantially transverse to said peripheral walla 5. A hollow cathode structure operating at a high negative potential with respect to an'anode in a gaseous ated to a predetermined density range of cathode structure having a back wall,

a bounding wall extending away from said back wall to form a cavity of suicient size to establish a glow discharge therein for generation of a beam of elec .trons,

said bounding wall further forming..lan opening,

said opening having all cross-sectional dimensions equal or greater than the cathode fall, Iand a shield selectively spaced 'from and enclosing said peripheral and bounding walls, said shield inhibiting the ow of electrons from the enclosed walls and enhance the generation of a beam. l6. A device as recited in claim and further comprising: t

an aperture wall extending from said bounding wall across the opening to form an aperture having crosssectional dimensions smaller than the corresponding dimensions of the opening, and t where said corresponding cross-sectional dimensions of the aperture and the opening bear a respective ratio l greater than .7.

7. A hollow cathode structure as recited in claim 1 wherein said bounding walls extend outwardly from said peripheral -wall to define a cavity of suicient size to establish said glow discharge therein 8. A hollow cathode structure as recited in claim 1 wherein said the 6 bounding walls extend inwardly from said peripheral wall to deiine a cavity of sufficient size to establish said glow discharge thereine 9. A hollow cathode structure operating at a high nega 5 tive potential with respect to an anode in a gaseous chamber evacuated to a predetermined density range of the gas comprising:

means for establishing a glow discharge and a cathode fall for the production of an electron beam from the cathode structure,

said cathode structure having a peripheral wall,

bounding walls extending away from said peripheral wall to define a cavity of suliicient size to establish said glow discharge therein,

said bounding walls further forming an opening,

an aperture wall extending from at least one of said bounding walls across the opening to form an aperture having a cross-sectional dimension smaller than the corresponding dimension of the opening,

where said"y corresponding cross-sectional dimensions of the aperture and the opening bear a respective ratio greater than .7, and

a shield selectively spaced from and enclosing said peripheral and bounding walls, said shield inhibiting the4 generation of electrons from the enclosed walls.

10. A hollow cathode structure operating at a high negative potential with respect to an anode in a gaseous chamber evacuated to a predetermined density range of the gas comprising: t

said cathode structure having a back wall,

a bounding wall extending away from said back wall to forma cavity with said back wall and of sufficient size to establish a glow discharge therein,

said bounding wall further dening an opening,

an aperture wall extending from said bounding wall across the opening to form an aperture having cross`1 sectional dimensions smaller than the corresponding dimensions of the opening,

where said corresponding cross-sectional dimensions of the aperture and the opening bear a respective ratio greater than .7, and

a shield selectively spaced from and enclosing said peripheral and bounding walls, said shield inhibiting the generation of electrons from the enclosed walls.'l

11. A hollow cathode structure having an aperture-'and operating ata high negative potential with respect to an anode in a gaseous chamber evacuated to a predetermined density range' of the gas comprising:

means for establishing a glow discharge and a cathode fall for the production of an electron beam from the cathode structure,

said cathode structure having a peripheral wall,

bounding walls extending away from said peripheral wall to define a cavity of sufficient size yto establish said glow discharge therein,

said bounding walls further forming an opening opu posite said peripheral wall,

an aperture wall extending from at least one of said bounding walls across the opening to form an aperture having a width smaller than the corresponding dimension of the opening, f

where said corresponding width of the aperture and the opening bear a respective ratio greater than .7,

where the electron bea-m is characterized by having cross-sectional dimensions that are larger than the corresponding aperture dimensions, and

a shield selectively spaced from and enclosingsaid peripheral and bounding Walls, said shield inhibiting the generation of electrons from the enclosed walls.

12. A hollow cathode structure having an aperture and y operating at a high negative potential` with respect to .an

anode in a gaseous chamber evacuated to a predetermined density range of the gas comprising:

means for establishing a glow discharge and a cathode fall for the production of an electron beam from the cathode structure,

said cathode structure having a. back w-all,

a bounding wall extending away from said vback wall to form a cavity with said back wall and of suflicient size to establish a glow discharge therein,

said bounding wall further dening an opening opposite said peripheral wall,

an aperture wall extending from said bounding wall across the opening to form an aperture having a smaller dimension than the corresponding di-menn sions of the opening,

where said corresponding smaller dimensions of the aperture and the opening bear a respective ratio greater than :7,

the generation of electrons from the enclosed Walls..

References Cited UNITED STATES PATENTS 3,172,007 3/1965 Hanks 219-121 3,262,003 7/1966 Allen 219-121 JAMES W. LAWRENCE, Primary Examiner.

STANLEY D SCHLOSSER, Examiner.

R. JUDD, Assistant Examinen7

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