GB2227360A

GB2227360A - Cathode and heater assembly for electron beam devices

Info

Publication number: GB2227360A
Application number: GB8927439A
Authority: GB
Inventors: Nina Pavlovna Andreeva; Irina Yakovlevna Arstova; Igor Konstantinovich Batrak; Andrei Mikhailovich Bishaev; Alexandr Fedorovich Ezhov; Vyacheslav Anatolie Kalabukhov; Svetlana Vasilievna Shumskaya
Original assignee: Individual
Current assignee: Individual
Priority date: 1988-12-05
Filing date: 1989-12-05
Publication date: 1990-07-25
Also published as: JPH02250251A; FR2643505A1; DE3940215A1; GB8927439D0

Description

i 1 CATHODE AND HEATER ASSEMBLY FOR ELECTRON BEAM DEVICES This invention

relates to electron devices and more specifically to cathode and heater assemblies for elect5 ron beam devices.

This inventlon can be used in the electronics technology, in particular in devices for generating heavycurrent electron beams such as plasma injectors and particle accelerators.

Fixing the mask shaping the electron beam and the adjoining it emitter to the cathod/heater assembly is one of the most complicated tasks in cathode and heater asseirbly manufacture, especially so if the emitter can fall out under its weight, as in the case of horizontally mounter assemblies.

Available are cathode and heater assemblies, wherein reliable fixing of the emitter and mask is provided. but -their design allows emitter replacements (on exhaustion of its service life) only by extracting the cathode and heater assembly out of the electron device and its complete disassembly. Cathode and heater assemblies operate at high temperatures (about 1600OC), so that their fixing components may be deformed and broken during, disassembly. Reliable mask fixing, combined with quick and easy emitter replacement, would provide a greater 2 reliability of the entire cathode and heater assembly.

Known in the art is a cathode and heater assembly (SU9 Aq 376825), wherein a refractory metal carbide or graphite heater shaped as a cup with a conical side surface carrying a ring groove at Its open end to accomodate the emitter, Is mounted on the assembly body.

During assemblye the emitter Is firmly fitted into the heater. but due to the difference In thermal expansion coefficients of the emitter material (e.g. lanthanum hexaboride) and the heater material (graphite) a gap is formed and in case of a large gap the emitter may fall out of its mounting place. This makes such a cathode and heater assembly operable only with vertical mounting of the electron beam device and is the cause of its low perf ormance reliability. In case of failure, replacement of the emitter necessitates dismantling the entire cathode and heater assembly out of the electron beam device and this may lead to damaging its brittle graphite components, 20 Also known in the art is a cathode assembly (FR. A, 2395600) wherein the body houses a helical heater and vertically mounted cylindrical chamber with an emitter mounted inside it. The bottom part of this chamber is pro-4vided with a hole. the diameter whereof is smaller than that of the emitter, so that the latter rests on the chamber bottom and is pressed thereto by a ring spring, thus ensuring reliable emitter fixing. The hole in the chamber bottom also serves as the mask, i.e. shapes the 1 electron beam to the required cross-section.

This design configuration of the cathode and heater assembly ensures reliable emitter fixirg but replacement of the latter necessitates complete disassembling of the assembly.

The ring spring which fixes the emitter directly faces the heater and therefore is subjected to high tem peratures, at which its material crystallizes, thus losing its elastic properties and becoming prone to damage during disassembly. In other cases the ring spring may weld to the chamber walls.

Furtherr,-.ore, the rims at the chamber bottom from the emitter side make the interelectrode gap more prone to electric breakdown.

Also widely known In the art is a cathode and heater assembly for electron beam devices (V.V. Glebov, V.V. Gulanova et al. "Relaksatsiya i magnitnaya kompressiya puchkov v iii-nogolucheyom inzhektorell (Beam relaxation and compression in multibeam injectors) - Theses of Reports presented at the V-th All-Union Symposium on Heavy- Current Electronics, 1984, Tomsk.- In Russian), comprising a body housing a graphite heater, coaxially and sequentially positioned on the heater emitter and graphite mask in the form, of discs, and a means for fixing the mask and emitter to the body.

In this known in the art cathode and heater assembly design the means for fixing the mask and emitter to the body is a cylinder with a conical imer surface, 4 mounted coaxially relative to the emitter, with the mask constituting an integral part of the cylinder at its larger butt-end.

The body of this cathode and heater assembly is a cup with a flangeg the latter provided with conical ring bosses. The peripheral massive part of the cylinder wall with a maximal axial height is secured by pins to the cup flange, contacting its bosses only so as to reduce heat losses.

The heater is a graphite helix fitted at the non- -working side of the emitter.

As evident from the assembly designed described herein above, the means for fixing the mask and emitter is rather hard to -LEabricate and requires accurate match- ing of components fitted to the body. Daring actual operation at high temperatures the pins may be welded to the graphite cylinder walls. Daring disassembly, necessary to replace the emitter, the threading may be damaged. These factors lead to an inadequate reliability of this known in the art cathode and heater assembly.

It is an objective of this invent-fon to provide a cathode and heater assembly featuring a high reliability of mask and emmiter securing to the assembly body.

It is another objective of this invention to simpli- fy the procedure of emitter reDlacement in the cathode an and heater assembly.

The invention resides in that in a cathode and heater assembly for electron bean. devices the assembly t, body houses a graphite heater with coaxially and sequentially positioned thereon emitter and graphite mask shaped as discs; a means for securing the emitter and mask to the assembly body is designed as a split graphite ring with a conical outer side surface. positioned outside the mask and with a gap relatiTe to its side surface, with the larger base of the ring facing the emitter; the inner side surface of the split ring is provided with a boss for engagement to the mask; the heater is designed as a cup, with the inner side surface thereof proyided with a ring grooTe at the open end to accomodate the emitter and split ring; the side surface of the ring groove is provided with two sections congruent to the side surface of the emitter and to the conical outer side surface of the split ring.

respectively. It is suitable to integrate the split ring and mask into one unit, with equal axial thickness of these component parts. 20 It is reasonable that the boss on the inner surface of the split ring be positioned symmetrically relative to its free ends. It is expedient that the mask be provided with a conical side surface and face the emitter with its lar- ger base, and that the split ring have a conical side surface, wi-th substantially equal angles between their common axis and the generatrices of the inner conical 6 surface of the split ring and conical side surface of the mask.

It Is suitable that the angle between the generatrix of the conical outer surface of the split ring and its axis be within the limits from /9 to lL17.

It is also suitable that the gap between the conical side surface of the mask and the conical outer side surface of the split ring be defined according to the relation:

1 -5h tg -4 a 6.1v-3D 2 /(b + t tgy) where: a is the gap between the conical side surface of the mask and the conical outer surface of the split ring, is the angle between the generatrix of the conical outer side surface of the split ring and its axis, h is the thickness of the split ring, D is the outer diameter of the smaller base of the split ring.

b is the radial width of the smaller base of the split ring.

It is preferable that the angle between the generatrices of the conical side surface of the mask and of the conical Inner side surface of the split ring, and their 25 common axis be defined according to the relation.

arctg (a/h) r " arctg (4a/3h) 7 where is the angle between the generatrices of the conical side surface of the mask and conical inner side surface of the split ring, and their common axis.

The cathode and heater assembly for electron beam devices of this invention. due to the means for securing the mask and emitter to the assembly body being designed as a split ring fitted to the heater. ensures a high reliability of mask and emitter securing to the assembly body. Furthermore, such a design configuration provides easy replacement of the emitter without having to disassemble the entire cathode and heater assembly, this prolonging the assembly service life.

The cathode and heater assembly for electron beam device of this invention allows the use of different masks provided with split rings to shape electron beams of desired cross-sections, fitting a mask being a simple and reliable operation.

These and other objectives of the invention will become apparent from the following description of the invention and an embodiment thereof, and the accompanying drawL,igs, wherein:

Fig. 1 shows the cathode and heater assembly for electron beam devices according to the invention, longi- tudinal section; Fig. 2 shows the mask and s'Dlit ring, top view, according to the invention.

8 The cathode and heater assembly for electron beam devices comprises body 1 (Fig. 1) housing graphite heater 2 designed as a conical cup with slits. The inner side surface of this cup is provided with a ring groove 3 at the cup's open end. Emitter 4. for instance of lanthanum hexaboride, and graphite mask 5 are positioned coaxially and sequentially one after the other on the heater 2 and secured to it by a mask 5 and emitter 4 securing means designed as a split graphite ring 6 with conical side lo surfaces. Split ring 6 is fitted with a gap "all relatiTe to the side surflace of mask 5 and facing emitter 4 by its larger base.

Groove 3 housing emitter 4 and split ring 6 has two sections on its side surface. congruent to the side sur- face of emitter 4 and the conical side surface of split ring 6.

In this embodiment. split ring 6 and mask 5 are rated into a single unit and feature equal height "h" along the axis. This designed approach is the most adapted to streamlined production, because an adequate bending strength, tensile strength, and compressive strength of split ring 6 during its mounting along with mask 5 in groove 3 of heater 2.

Mask 5 is Provided with ring slit 7 to shape elect- rons into a diverging beam. Body 1 is of a refractive -ance tantalum, with screen 8 installed to metal, for inst reduce heat losses. Body 1 is mounted on base 9 of an 1 c 1 X 9 electrically Insulating material, for Instance boron carbonitride.

Angle,p at which the conical outer surface of split ring 6 is inclined was determined experimentally and should be within the limits from i(., /9 to,L/7. At angles less than the lower limit friction forces prove to be inadequate to hold split ring 6 and mask 5 inside groove 3 of heater 29 for instance at horizontal mounting of the cathode and heater assembly. At angles above 'fC/7 certain production difficulties are encountered in fabricating split ring 6.

Mask 5 has a conical side surface and angles at which the generatrices of the conical side surface of mask 5 and of the conical Inner side surface of sDlit ring 6 are inclined relative to their common axis are substan tially equal and are determined by the requirement of no direct line of sight through gap "all, so as to prevent electron transit via this gap during operation of the ca thode and heater assembly. The value of angle 4s de- fined according to the relation:

arctg (a/h) arctg (4a/3h) Lyleeting the lower limit ensures the rims of the larger base of mask 5 and smaller base of s'Olit ring 6 lying on a straight line parallel to emitter 4 axis, thus ensuring the absence of a direct line of sight, mentioned here- in ab ov e.

The upper level for angles r was determined experi- mentally to meet the condition of a guaranteed overlap- ping of the emitter working zone by at least one third of gap 11att Designing the geometry of mask 5 Is based on the dia- meter of the electron beam to be shaped by it and on its thickness "h", which is defined from the electron optics of the device, wherein the cathode and heater assembly is to be used.

The outer diameter, D of the smaller base of split ring 6, its radial width 11b11 in the plane of Its smaller base, gap "all the diameter of the smaller base of mask 5 are determined on the basic of providing an adequate mechanical strength of these component parts.

To facilitate mounting SDlit ring 6 and mask 5 in heater 2, the conical outer surface of split ring 6 is provided with recesses 10 (Fig. 2). Split ring 6 is comp ressed during mounting in heater 2 till the inner surface of split ring 6 contacts with the side surface of mask 5 in the vicinity of recesses 10, this necessitating that gap "all be larger than b. tg1P. Experiments indicate this providing that gap "all should exceed 1.5h tg ease of mounting and removing mask 5 with split ring 6.

The spacing between the free ends of split ring 6 should be at least 2a, with the upper limit of gap "all set by mechanical strength considerations, taking Into account the amount to which split ring 6 is bent at recesses 10 and the ratio of allowable tensile stress to the modulus t 1 11 0 of elasticity for graphite. Thus, the limits of gap "all variation are determined according to the relation:

1.5tg a 6.10-3D 2 / (b + h tg wherefrom, at a 0, it follows that 1.5 tg v = 1. 10-3D 2 /(b + h tg) and hence the upper limit for dimension 11b11 is b = (4.10-3D 2/h tg I, tg (2) (3) The lower limit for 11b11 is set by mechanical strength considerations. Calculations indicate that the lower limit for 11b11 is an order of magnitude lower than the upper limit and depends on emitter 4 weight.

In this eirbodiment base 11 (Fig. 2) on the inner side surface of split ring 6 is positioned symmetrically relative to the free ends of split ring 6 and its width "H" is determined from mechanical strength considerations.

Mask 5 is provided with ring slit 7 with three radial straps 12. In order embodiments the shape of slit 7 and the number of straps may be different, as determined by the required shape of the electron beam cross-section.

It should be noted. that the design of the means se curing mask 5 and emitter 4 to assembly body 1 of this in vention poses less stringent requirements to emitter 4 dimension accuracy, because split ring 6 ensures its re liable securing in heater 2, irrespective of the position ing of the cathode and heater assembly.

12 During emitter 4 and mask 5 mounting and securing by split ring 6 In the cathode and heater assembly, emitter 4 is positioned in groove 3 at the section. the side surface whereof Is congruent to the side surface of emitter 4.

Then. for instance using pincers, mask 5 with aplit ring 6 is compressed and fitted into the next section. The pincers are then released and split ring 6 sets to it initial positions reliably securing emitter 4 and mask 5 in heater 2.

In case of need, a failed emitter 4 may be quickly and easily replaced without dismantling the entire cathode and heater assembly, or mask 5 replaced for another one, providing a different specified shape of the electron beam cross-section.

The cathode and heater assembly functions as follows Applying a voltage to heater 2 current leads (not shown in the drawing) causes its heating to a working temperature of 19000 to 22000C and emitter 4 Is heated by its heat radiation.

When heated to its working temperature (14500to 1650OC), emitter 4 emits electrons from its working side faced toward mask 5, which acquire the required beam shape on passage through slit 7 in mask 5.

Screens 8 serve to reduce heat losses of the cathode and heater assembly. Simultaneously with emitter 4, mask and split ring are also heated, but to a temperature lower than that of emitter 4.

The thermal expansion coefltcients of lanthanum hexaboride and graphite, of which emitter 4 and mask 5 with split ring 6, respectively, are fabricated, are such, that mask 5 with split ring 6 expand more than emitter 4, 5 so that split ring 6 expands in ring groove 3 of heater 2 and therefore provides reliable securing of emitter 40 The following specific example serves to better explain the essence and point out the adTantages of this invention.

The mask Is of graphite, with a thickness of h= lmm.

The width of the smaller base of the split ring is b = 1.5 mm, at a diameter of this base D = 31 mm.

Angle r, - 500. The gap between the free ends of the split ring is 5 mm, angle 250, and the gap a Prom Eq. (2): 1.5 h tgYp= 0.;69 mm and 6.10-3D 2 Ab + h tg 2.4 mm; hence the selected gao (1 mm) is within the limits specified.

The upper and lower limi-.s for angle as set by Eq. (1), are arctg (4a/3h) = 540 and arctg (a/h)-450. respectively, therefore the Talue selected (500) is also within the limits specified.

The cathode and heater assembly was tested on an experimental setup by placing the assembly Into a vacuum chamber, wherein the pressure was maintained at (2 - 3).10-5 torr. The working brightness temperature of 14 a lanthanum hexaboride emitter was 145010C at a brightness temperature of the mask of 112000.

These tests proved that the design configuration of the means-securing the mask and emitter, according to this invention, allows multiple replacements of the emitter without dismantling the entire cathode and heater assembly.

1

Claims

What we claim is:

1. A cathode and heater assembly for electron beam devices, the body whereof houses a graphite heater with coaxially and sequentially mounted thereon emitter and mask designed as discs; a means for securing the mask and emitter to the body designed as a split graphite ring with a conical outer side surface, fitted outside the mask with a gap relative to its side surface and with larger base facing the emitter; the inner side surface of the split ring provided with a boss to engage the mask; the heater designed as a cup wital the inner side surface thereof, at the open side, provided with a ring groove to accomodate the emitter and split ring; the side surface of the ring groove having two sections congruent to the side surface of the emitter and to the conical outer side surface of the split ring respectively.

2. A cathode and heater assembly substantially as set forth in Claim 1. where4Ln the split ring and mask are integrated into a single unit with equal thickness in the axial direction.

3. A cathode and heater assembly substantiplly as set forth in Claims 1 or 2, wherein the boss on the inner side surface of the split ring Is positioned symmetrically relative to Its free ends.

4. A cathode and heater assembly substantially as set forth in any one of the preceding Claims 1 to 39 wherein the mask Is designed with a conical side surface 16 and faces the emitter with Its larger base and the split ring is designed with a conical inner side surface, with the angles at which the generatrices of the conical inner side surface of the split and of the conical side surface of the mask are inclined relative to their com mon axis are substant-fally equal. -

5. A cathode and heater assembly substantially as set f orth In any one of the preceeding Claims 1 to 4. wherein the angle between the generatrix of outer side surface of the split ring and its axis is within the 11mits f rom /9 to 7

6. A cathode and heater assembly substantially as set forth in any one of the preceeding Claims 2 to 5, wherein the gap between the conical side surface of the mask and the conical inner side surface of the split ring is defined according to the relation:

where a 1 -5h tg a 6.10-3D 2 /(b = h tg is the gap between the conical side surface of the mask and the conical outer side surface of the split-ring, is the angle between the generatrix of the conical outer side surface of the split ring and its axis, h is the thickness of the split ring and of the mask, D is the outer diameter of the smaller base of the split ring. is the radial width of the split ring 1 17 in the plane of Its smaller base.

7. A cathode and heater assembly substantiall as set forth in any one of the preceeding Claims 4 to 69 wherein the angle at which the generatrices of the conical side surface of the mask and of the conical inner side surface of the split ring are inclined relatiTe to their common axis Is defined according to the relation:

arctg (a/h) -<, arotg (4a/3h), where Is the angle of Inclination of the generatrices of the conical side surface of the mask and of the conical inner side surface of the split ring.

8. A cathode anz' heater assembly for electron beam devices substantially as set forth in any one of the preceeding Claims 1 to 7 and as described herein above with the reference to the accompanying drawings.

Published 1990 at, The PalentO:L,,ce. State House.66 71 High Holbc;.-n. LonderWC1R4TP-.larther copies maybe obtained fronnThe Patent Officc Sales Branch. St Ma.,:; Cray Orpingcr- Kent ERE 3RD- Printed by Multiplex techniclues ltd. St Ma-,y Cray. Kent Con 1 87