US3609439A

US3609439A - Anode having spaced cavities for suppression of secondary emission

Info

Publication number: US3609439A
Application number: US802797*A
Authority: US
Inventors: Barry M Singer
Original assignee: Machlett Laboratories Inc
Current assignee: Machlett Laboratories Inc
Priority date: 1969-02-27
Filing date: 1969-02-27
Publication date: 1971-09-28
Anticipated expiration: 1988-09-28

Abstract

A magnetically focused beam power tube having an anode provided with means for suppressing secondary emission, which means comprises Faraday cage-type cavity structures having magnetic shielding for providing substantially magnetic-free regions within the cavities.

Description

United States Patent Reverdin 3.13/ 106 X Lerbs et 3l3/106X Ferry et a1. 313/107 Doolittle 313/21 Prinz et a]... 3l5/39.75 X McArthur 315/39.75 X Gutton et al.... 315/39.75 Fiedor et al..... 315/8 X Schade 313/46 Primary Examiner-Robert Segal An0rneys-Harold A. Murphy and Joseph D. Pannone ABSTRACT: A magnetically focused beam power tube having an anode provided with means for suppressing secondary emission, which means comprises Faraday cage-type cavity structures having m agnetic shielding for providing substantiaily magnetic-free regions within the cavities.

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INVENTOR BARRY M. SINGER GEN ANODE HAVING SPACED CAVITIES FOR SUPPRESSION OF SECONDARY EMISSION BACKGROUND OF THE INVENTION This invention relates generally to magnetic beam tubes and is more particularly concerned with an anode structure which suppresses secondary emission from the internal surface thereof.

In the operation of a power triode tube, the control grid is driven positive in order to obtain the substantial flow of electrons necessary for power generation. When the control grid becomes positive, a portion of the electron flow is attracted to the grid. In tetrode power tube operations, the control grid is held at negative potentials, but the screen grid is maintained at relatively high positive potentials in order to accelerate electron flow from the cathode to the anode. As a result, a portion of the electron flow is attracted to the highly positive screen grid in the tetrode power tube. An excessive flow of electrons to the control grid or screen grid cannot be tolerated in power tubes because of the resulting high grid currents, grid heating and grid emission. Consequently, the anode of a conventional power tube is maintained at a sufficiently high voltage value to overcome the attractive force of the control grid or screen grid and draw most of the electrons to the anode. However, increasing the plate voltage of a power tube decreases the operating efficiency of the tube. Therefore, more efficient power tubes have been developed which do not use high plate voltages to solve the problems of minimizing grid interception of electrons.

U.S. Pat. No. 3,365,601 which has been granted to H. D. Doolittle and assigned to the assignee of this invention discloses a magnetic beam power tube that uses a magnetic field for controlling the direction of electron flow. Briefly, the tube described in the referenced patent comprises an array of parallel filament wires in spaced, parallel relationship with an array of control grid rods and further spaced, parallel relationship with an array of screen grid rods. This assembly is disposed in spaced, parallel relationship with the walls of an anode cavity that is located between the poles of a permanent magnet. Thus, a magnetic field is established across the interelectrode spaces between the parallel filament, control grid, screen grid and anode structures whereby the magnetic lines of flux are perpendicular to the elements of these structures. When an electron, emitted from a filament wire, travels directly toward the anode, it passes between adjacent rods of the intervening grid structures. In doing so, the electron moves along a path which is parallel to the magnetic lines of flux and, consequently, is unafiected by the magnetic field. However, when an emitted electron moves toward an adjacent grid rod, it crosses the magnetic lines of flux at an oblique angle. The velocity of this electron can be resolved into two right-angle components, one parallel and the other perpendicular to the magnetic flux lines. The component of velocity which is parallel to the magnetic lines of flux represents motion of the electron directly toward the anode and is unaffected by the magnetic field. The component of velocity which is perpendicular to the magnetic lines of flux interacts with the magnetic field and causes the electron to move in a circular path. The combination of motions due to both components of velocity results in the electron travelling in a helical path toward the anode. Thus, linear motion of an electron toward a grid rod is changed into spiralling motion around interacting magnetic lines of flux which are established between the grid rods. Consequently, the emitted electrons are focused into narrow beams which pass between adjacent grid rods and terminate on the internal surface of the anode. Thus, grid interception of electrons is minimized, and grid currents resulting therefrom are reduced considerably.

Since the magnetic field in a magnetic beam power tube prevents excessive grid currents, the plate voltage in this type of tube can be maintained at the minimum value required to draw electrons from the cathode to the anode. Furthermore, it has been found that the control grid of a triode or the screen grid of a tetrode can be raised to a higher positive value than the anode of a magnetic beam power tube without causing a substantial increase in grid current. With the anode maintained at the minimum required voltage level and a relatively higher positive voltage applied to the screen grid of a tetrode or the control grid of a triode, the operating efficiency of the tube can be very high. However, the problem of secondary emission from the anode surface becomes more complicated in magnetic beam power tubes than in conventional power tubes. With the anode at a lower positive voltage level than the screen grid or the control grid, an excessive number of secondary electrons escape from the'electrostatic field of the anode and are drawn toward the more positive grid. The resulting decrease in plate current produces distortion in the output of the tube. Some prior art tubes suppress secondary emission from the anode by providing recesses in the internal surface thereof. These recesses tend to trap secondary electrons because of the higher probability that a secondary electron will collide with the sides of a recessed cavity before emerging therefrom. However, there is less possibility of similar collisions occuring in magnetic beam power tubes. Since a transverse magnetic field causes the primary electrons to follow direct paths toward the anode, it also causes secondary electrons to follow direct paths away from the anode. Therefore, the problem is to suppress secondary emission from the anode surface in a magnetic beam power tube without increasing the plate voltage applied to the anode and thereby decreasing the efficiency of the tube.

SUMMARY OF THE INVENTION In accordance with this invention, a magnetic beam power tube is provided with an anode having a plurality of cavities recessed in the internal surfaces thereof. Each cavity comprises a Faraday cage which has an opening dimension that is no greater than one-third the depth dimension. Furthermore, each cavity is surrounded by a shield of permeable material, such as permalloy or soft iron, for example, which channels magnetic lines of flux around the cavity thereby providing a substantially magnetic free region within the cavity.

BRIEF DESCRIPTION OF THE DRAWINGS For a more complete understanding of this invention, reference is made to the drawing wherein:

FIG. 1 is an axial view, partly in section of a magnetic beam power tube which embodies this invention;

FIG. 2 is a fragmentary cross-sectional view taken along line 2-2 of FIG. 1 looking in the direction of the arrows; and

FIG. 3 is an enlarged, cross-sectional, diagrammatic representation of the electron flow in a tube embodying the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to the drawing wherein like characters of reference designate like parts throughout the several views, an electron discharge tube is shown in FIGS. 1 and 2 which comprises a gastight envelope closed at one end by an oval shaped, anode cup 10 made of highly conductive material, such as copper for example. Anode cup 10 has extended

flat sides

12 and 14 and encloses an oval-shaped cavity 16. The open end of anode cup 10 terminates in an outwardly extending flange 18 which has a circular perimeter and functions as the anode terminal of the tube.

A metallic sleeve 20, preferably kovar, is hermetically attached throughout one end to anode flange 18. The other end of sleeve 20 is peripherally sealed to one end of a dielectric cylinder 22, preferably ceramic, which is similarly sealed at of a metallic sleeve 28, preferably kovar. The other end of sleeve 28 is hermetically attached to one end of a hollow dielectric cylinder 29, preferably ceramic, which is similarly sealed at the opposite end to one end of another metallic sleeve 30, preferably kovar. The other end of sleeve 30 is peripherally attached to a flat side of a metallic ring 31, preferably copper, which functions as one of the cathode terminals of the tube. Cathode terminal ring 31 extends radially inward and is hermetically attached on the opposite side, adjacent the inner periphery thereof, to one end of a metallic sleeve 32, preferably kovar. The other end of sleeve 32 is peripherally sealed to one end of a hollow dielectric cylinder 34, preferably ceramic, which is similarly sealed at the opposite end to one end of a metallic sleeve 36, preferably kovar. The other end of sleeve 36 is peripherally attached to a flat side of a metallic ring 38, preferably copper, which serves as the other cathode terminal of the tube. Cathode terminal ring 38 extends radially inward and is hermetically attached on the opposite side, adjacent the inner periphery thereof, to one end of an exhaust tubulation 40, preferably copper, which is pinched OH to form a vacuum-tight seal after processing of the tube is completed.

Within the gastight envelope just described, a coaxial support cylinder 42, preferably copper, connects cathode terminal ring 38 to support deck 44. A concentric support cylinder 46, preferably copper, connects cathode terminal ring 31 to support deck 48. Support cylinder 46 is insulatingly spaced from support cylinder 42 and from the surrounding tube structure. For purposes of rigidity and alignment, support decks 44 and 48 are insulatingly attached to one another by conventional means, such as screws surrounded by dielectric bushings and washers (not shown) for example. Another support cylinder 50, preferably copper, is attached at one end, as by brazing, for example, to grid terminal ring 26 and extends longitudinally within the tube envelope in concentric, spaced relationship with cathode support cylinder 46. Grid support cylinder 50 terminates at the other end in opposing arcuate walls 52 and 54 which are attached, as by screws, for example, to

plates

56 and 58, respectively, adjacent the arcuate peripheries thereof.

Grid support plates

56 and 58 extend radially inward and terminate with respective straight sides spaced from one another to form a slot 60 therebetween. Parallel connecting rods 62 are attached at one end to support decks 44 and 48 by conventional means, such as brazing, for example, and extend longitudinally through slot 60, insulatingly spaced from the straight sides thereof. Alternate connecting rods are attached to cathode support deck 44 and pass insulatingly through aligned holes 63 in support deck 48 to extend between adjacent connecting rods attached to support deck 48. The distal ends of connecting rods 62 are attached, as by welding, for example, to respective ends of parallel, U- shaped filament wires 64. Each U-shaped filament wire 64 is attached, at one end, to a connecting rod 62 that extends from cathode support deck 44 and, at the other end, to an adjacent connecting rod 62 that extends from support deck 48. The resulting linear array of parallel, U-shaped filament wires 64 hangs longitudinally in a central axial plane of the anode cavity l6. Respective ends of grid rods 66 are attached, as by brazing, for example, to the straight sides of slot 60 at regular spaced intervals and extend longitudinally in planes parallel with the linear array of filament wire 62. As shown more clearly in FIG. 2, the grid rods 64 are disposed in transverse planes between the legs of the U-shaped filament wires 62. The respective opposite ends of grid rods 64 extend longitudinally beyond the respective closed ends of U-shaped filament wires 64 and are attached, as by brazing, for example, to opposing extended sides of an oblong plate 67. The plate 67 is insulatingly spaced from the closed ends of filament wires 64 and maintains the grid rods 66 in the desired spaced relationship with one another and with the linear array of filament wires 62.

The extended flat sides '12 and 14 of anode cup are provided with-a plurality of axially extending ,cavities 68 which are recessed in the internal surfaces thereof. Optimum design indicates that pairs of opposing cavities 68 should be disposed in transverse alignment with the respective filament wires 62 and located centrally between adjacent grid rods 66. Each cavity 68 comprises a Faraday cage wherein the lateral dimension of the cavity opening is no more than one-third the dimensional depth of the cavity. Each cavity 68 is provided with a lining 70 (FIG. 3) of permeable material, such as permalloy or soft iron, for example. A C-shaped, permanent magnet 72 having extended,

flat pole pieces

74 and 76 is located external of the tube envelope and folds around an arcuate side of the oval-shaped anode cup 10. The

respective pole pieces

74 and 76 of magnet 72 are disposed in respective planes that are parallel with the extended,

fiat sides

12 and 14 of anode cup 10, the linear array of filament wires 62 and the parallel arrays of grid rods 64. Thus, the magnetic lines of flux between the

pole pieces

74 and 76 are perpendicular to the planes of the filament, grid and anode electrodes and parallel with the intended direction of electron flow between the filament wires 62 and the

extended sides

12 and 14 of the anode cup 10.

During operation of the described tube, a strong heating current flows through the filament wires 64 and produces a heavy emission of electrons therefrom. A large signal voltage is impressed on the negatively biased grid rods 66 thereby driving them positive and accelerating a dense flow of electrons toward the

anode walls

12 and 14. Ordinarily, a significant portion of the electron flow would be diverted to the instantaneously positive grid rods 66 and result in a prohibitively high grid current. However, the magnetic field established by the magnet 72 interacts with electrons that move transversely through the field and causes them to spiral around magnetic lines of flux which extend between adjacent grid rods. As shown in FIG. 3, the electrons emitted from the filament wires 64 are focused into narrow beams which pass between adjacent grid rods 66 and terminate on the internal surface of the anode 10. The resulting low value of grid current permits operation of the tube with the anode l0 maintained at the minimum voltage level required to draw electrons from the filament wires 64 to the

anode walls

12 and 14. When this minimum value of plate voltage is applied to the anode 10, the grid rods 66 must be raised to a higher positive potential than in conventional tubes of this type, in order to achieve an electron flow of equivalent density and thereby obtain an output current of equal value. With these operating conditions the grid rods 66 may reach a higher positive voltage level than the anode 10 during intervals of maximum electron flow. However, the grid current does not increase accordingly, because the magnetic field minimizes grid interception of electrons. Thus, the described tube delivers substantially the same output current as similar conventional tubes but at a much lower plate voltage level, and therefore operates much more efficiently.

When the grid rods 66 reach a higher positive voltage level than the anode 10, the electrons emitted from the filament wires 64 produce an emission of secondary electrons from the respective internal surfaces of the

anode walls

12 and 14. If the secondary electrons are allowed to leave the anode l0 and travel to the grid rods 66, the resulting decrease in anode current would cause distortion in the output of the tube. Therefore, in order to suppress secondary emission from the anode 10, a plurality of longitudinal recesses 68 are provided in the respective internal surfaces of the

anode walls

12 and 14. Each recess 68 comprises a cavity having the dimensions of a Faraday cage wherein the width of the opening is no greater than one-third the depth of the cavity. In a Faraday cage having the specified dimensions, the probability is very high that a secondary electron produced therein will collide with the walls of the cavity and be absorbed before it can emerge therefrom.

FIG. 3 is an actual plot of computer-calculated electron trajectories in a typical magnetic beam power tube. As shown in FIG. 3, the narrow'beams of primary electrons resulting from the action of the magnetic field are compatible with the narrow, longitudinal openings of the respective cavities 68. Therefore, a well-designed power of this type requires only one pair of opposing cavities 68 in the anode wall 12 and 1 4 for each wire 64 of the filament array. The primary electrons readily enter the narrow openings of the Faraday-type cavities 68 and, upon striking the walls of the respective cavities, produce secondary electrons deep within the cavities. However, the secondary electrons leave the surface of the anode travelling at a much lower velocity than the incoming primary electrons. Consequently, the magnetic field will cause the secondary electrons to move in circles of smaller diameter than the primary electrons. Since the primary electrons readily enter the cavities 68 because they are focused into narrow beams by the magnetic field, the secondary electrons will emerge from the cavities 68 by being focused into similar narrow beams. Therefore, the probability of the secondary electrons colliding with the respective walls of the cavities 68 and being absorbed is reduced considerably.

In order to prevent the secondary electrons from spiralling around the interacting magnetic lines of flux and emerging from the respective cavities, each cavity 68 is surrounded by a shield 70 of highly permeable material, such as soft iron or permalloy, for example. Thus, the respective permeable linings 70 provide lower resistance magnetic paths and thereby channel the magnetic lines of flux around the respective cavities 68. Since the width of the cavity opening is no greater than one-third of the cavity depth, the magnetic lines of flux do not penetrate too far into the respective cavities 68 before diverting into the lower resistance magnetic paths provided by the respective permeable shields 70. With the effects of the magnetic field sharply reduced in the respective interiors of the cavities 68, there is higher probability that secondary electrons produced within the respective cavities will collide with the respective walls thereof and be absorbed. As a result, secondary emission from the anode is suppressed without increasing the plate voltage above the minimum required value and thereby decreasing the efficiency of the tube.

Thus, there has been disclosed herein a magnetic beam power tube having an anode electrode with a plurality of axially extending cavities recessed in the internal surfaces thereof. Although the magnetic shields 70 have been shown contiguous with the sides of the cavity, the permeable material may be embedded in the material of the anode and spaced from the cavity 68. As stated previously, since the openings of the respective longitudinal cavities 68 are less than one-third the depth thereof, the magnetic lines of flux do not penetrate very far into the interiors of the cavities. Consequently, the cavities need not be enclosed on three sides by the permeable shield, but would require permeable material only on the two opposing longitudinal sides of the respective cavities. The longitudinal cavities have been shown in transverse alignment with the respective filament wires and centrally spaced between adjacent grid rods because of the narrow beams of electrons produced by the magnetic field. However, a plurality of closely spaced, longitudinal cavities 68 could be disposed in the internal surface of the anode walls between each pair of adjacent grid rods to provide for any spreading of the respective electron beams. Although the respective longitudinal cavities 68 have been shown as continuous grooves, each groove may be replaced by a longitudinal series of individual cavities which may have various cross sections, such as elliptical or square, for examples. Furthermore, although the respective cavities are shown in FIG. 2 as having generally U- shaped cross sections, alternatively they could have been provided with V-shaped cross sections which would increase the possibility of secondary electrons colliding with the walls of the respective cavities.

Although the drawings and the foregoing description relate to and depict a magnetically focused beam power tube having so-called planar arrangements of electrodes therein, this invention is equally well adapted for use in such beam tubes as may have annular" electrode structures. Such an annular electrode arrangement is also disclosed in aforementioned U.S. Pat. No. 3,365,601. Furthermore, although the preferred embodiment has been shown and described herein as a triode tube, this invention may also be incorporated into other multiple electrode tubes, such as tetrode tubes, for example. These and other modifications of this type which may occur to those skilled in the art are deemed to be within the spirit and scope I of this invention, and, as such, are intended to be included in the claims appended hereto.

1 claim:

1. An anode for an electron tube comprising a hollow cylindrical copper body having flat sidewalls joined by circular end walls, the inner surfaces of side flat side walls having longitudinally, closely spaced cavities comprising Faraday cages for suppressing secondary electron emission therefrom when bombarded by electrons, said cavities being lined with magnetically permeable material and having openings there into having a lateral dimension no greater than one-third the depth of the cavities.

2. An anode as in claim 1 wherein said magnetically permeable material is one of permalloy and soft iron.