US3139552A - Charged particle gun with nonspherical emissive surface - Google Patents

Description

June 30, 1964 BREWER 3,139,552

CHARGED PARTICLE GUN WITH NON-SPHERICAL EMISSIVE SURFACE 3 Sheets-Sheet 1 Filed March 7, 1960 5y i r W haw? Arm/#04 .N @NNN. g H\ N W June 30, 4 s. R. BREWER 3,139,552

CHARGED PARTICLE GUN WITH NON-SPHERICAL EMISSIVE SURFACE WT W June 30, 19 4 cs. R. BREWER 3,139,552

CHARGED PARTICLE GUN WITH NON-SPHERICAL EMISSIVE SURFACE.

Filed March 7, 1960 3 Sheets-Sheet 3 United States Patent 3,139,552 CHARGED PARTICLE GUN WITH NON- PHERICAL EMISSIVE SURFACE George R. Brewer, Los Angeles, Calif, assignor to Hughes Aircraft ompany, Culver City, Calif, a corporation of Delaware Filed Mar. 7, 1960, Ser. No. 13,333

3 Claims. (Cl. 313-83) This invention relates to devices for formating high density charged particle beams, and more specifically to an improved particle gun for producing a collimated beam having an improved uniformity of particle density across its cross section.

In recent years, a number of new types of microwave electron tubes, such as traveling-wave tubes, have been developed. This invention is not limited to electron devices per se but encompasses devices utilizing other charged particles as well; however, henceforth in this specification the term electron will be used and should be taken to include, where applicable, other charged particles.

Amplification in a traveling-wave tube involves an accumulative interaction between an electromagnetic wave and an electron beam moving in a predetermined relationship With respect to the wave. This specification is concerned in part with some of the details of the production of this electron beam and will have occasion to refer to beams in terms of perveance, which is defined as the ratio of total beam current to the three-halves power of the beam voltage. For useful performance and interaction between the electron beam and a traveling wave, a traveling-wave tube requires a minimum perveance of the order of one hundred times that in the conventional cathode ray tube beam such as, for example, in an oscilloscope tube or television picture reproducing tube. In higher power traveling-wave tubes the perveance required is up to one thousand times this minimum value that is 1 to x10- amperes/volt 3/2. Because of such a high value of perveance, which is a measure of the tendency for beam spreading due to space charge repulsion of the electrons, beams for use in traveling-wave tubes must be provided with some type of focusing or constraining means to counterbalance the space charge force of the electrons in order to obtain a reasonably smooth, constant diameter beam. This is necessary in part because it is desired for maximum interaction that the electron stream travel along the interaction or slow-wave structure of the traveling-wave tube as closely thereto as possible. Such a high power beam if not well focused would be intercepted by the structure and would immediately damage or destroy the interaction structure. A common way of accomplishing this focusingis by immersing all or a part of the electron stream in a uniform magnetic field.

The gradual development of traveling-wave tubes to higher output power capabilities has required the production of beams of higher current in which the beam current density frequently exceeds that available from the cathode. For example, cathodes may generally emit of the order of from 10 to 20 amperes per square centimeter of surface, while the required beam current density may be of the order of a few hundred amperes per square centimeter. A converging beam gun in which the cathode area is several times larger than the ultimate beam area has become a commonly accepted means for forming the initial beam. The electron gun is usually disposed outside of the focusing magnetic field so that the initial beam formation results principally from electric fields in the gun. It is desired to design the shape of the gun electrodes in such a way that the electric fields result in uniform emission and a well collimated beam resulting in a uniform current density over the cross section of the beam. Such a gun is essentially a diode with a large concave 3,139,552 Patented June 30, 1964 emissive cathode emitting electrons toward an anode having a central aperture for permitting the beam to pass therethrough toward the remainder of the traveling-wave tube and into the environment of an axial focusing magnetic field.

In the anode aperture transverse electric fields cause an outward deflection of the electrons. This effect is frequently treated by considering the anode aperture as an electrosatic lens with a certain focal length. This is a thin lens concept and as an approximation is useful in guns of low perveance, for example, less than 0.1 1()" amperes per volt 3/2. However, this approximation breaks down rapidly as the gun perveance is increased. This lens effect is important and theradius and slope of the beam emerging from the gun must be designed to be compatible with the beamperveance and the magnetic field system in order to obtain optimum beam focusing in the region beyond the gun. It may therefore be seen that the details of the electron gun design are of critical'im portance in order to obtain a satisfactory focused beam.

If the focal length of the anode aperture lens mentioned above could be made constantwith respect to electrons entering the lens at varying radial distances off axis, so that in the absence of any magnetic field all the electrons from the cathode would converge and focus to or toward one point, it would be comparatively straightforward to introduce the beam of electrons into an environment of an axial magnetic field and to constrain them to flow in a well collimated beam through that environment. The type of flow is Brillouin flow in which the centrifugal and space charge forces on the spiraling electrons are balanced by the radially directed magnetic forces as the beam moves through the structure. However, when a hole is cut in the anode to allow the beam to pass through, a number of problems arise. If a grid is placed over the anode aperture so as tomaintain a unipotential surface or an effectively continuous electrode, or if the diameter of this aperture is small compared with the anode-cathode distance, as in low perveance guns, the aperture may be ignored. However, in a high voltage gun, the power dissipated in a grid from electron interception would normally cause the grid to operate at a prohibitively high temperature; and as the perveance of a gridless gun is increased the anode aperture must be made larger compared with the cathode-anode distance and the distortion in the electrostatic field is increased. This field distortion is the major cause of the variation in'focal length of the electric lens as seen by electrons entering the lens at different radii and is analogous to spherical aberration in an optical lens. This concept and problem of field distortion will, in accordance with the terminology in the art, be hereinafter referred to as aberration or spherical aberration in the electron lens. This aberration in the electron lens gives rise to a highly nonuniform current density distribution across the cross section of the electron stream. Electrons in the regions of higher density tend to have a greater component of radially inward velocity as they emerge from the electron gun. They soon, after leaving the gun, approach the axis where they subsequently begin to diverge because of their space charge repulsion forces. After expanding to a larger radius they are again focused inwardly by the magnetic field and the processes repeats along the length of the traveling-wave tube causing the beam to be scalloped. The interaction structure of the tube must be placed at a radial distance from the axis represented by the maximum amplitude of radial excursion of the scalloping electrons in order to preclude melting or other damage to the helix or other structure.

In the past, there have been a number of attempts to correct or to compensate for this lens aberration. One approach has been to increase the strength of the magnetic field above that required for theoretical Brillouin flow.

This, however, is expensive as regards electromagnet power and weight requirements in mobile or airborne equipment. Further, the solution is not adequate because the stronger magnetic field does not cure the scalloping but, for the most part, merely changes its period and outer diameter. Another approach is described in United States Patent 2,811,667, issued to George R. Brewer, on October 29, 1957, entitled Electron Gun. The approach as described there is to utilize a second anode downstream from the primary anode maintained at a higher direct current potential and designed to project an electrostatic field effect into the cathode-anode region which compensates for the field distortion of the character above referred to. Another approach is to attempt to correct for the field distortion by utilizing additional electrodes in the cathode-anode region itself or to shape the anode in a manner to attempt to eliminate the field distortion. These and other approaches are inadequate or suifer limitations in high perveance guns such as to cause them to be less than a complete solution in high perveance systems where a high degree of collimation and focusing is essential.

It is therefore an object of the present invention to provide an electron gun which does not suffer the disadvan- ,tages of the prior art.

It is in particular an object to provide an electron gun to produce a stream of charged particles which is well collimated and may be focused over an extended path,

such as the length of a traveling-wave tube or the like.

The stream maybe cylindrical, hollow cylindrical, planar, or the like.

It is a further object to provide an electron gun which ,does not suifer a serious degree of lens aberration.

It is a further object to provide a converging beam electron gun for producing a high perveance or very high perveance electron stream which may be well focused and confined throughout an extended longitudinal beam path.

Briefly, in accordance with the present invention, these ,and other objects are achieved by a new charged particle gun having a source with a curved emissive surface which is shaped in a manner to provide a converging stream of particles which may be of extremely high density and perveance. In one example, in which a very high perveance cylindrical beam of electrons is produced, the cathode surface is a concave figure of revolution about the axis of a traveling-wave tube. The radius of curvature of the curved surface varies as a function of radial distance from the axis in a manner such that the curvature of the cathode surface decreases with increasing radial distance from the axis. Inthis manner electrons emitted from the regions toward the periphery of the concave cathode are focused, in the absence of a magnetic field, toward the same point on the axis as are electrons emitted from portions of the cathode near the axis. The increased curvature near the axis causes these latter electrons to be emitted with a greater inward velocity than would be the case with a conventional spherical cathode surface. This initial and extra radial velocity compensates for the effect of the more peripheral electrons being bent outwardly less in the lens, due to the spherical aberration suffered by conventional electron guns.

In a preferred embodiment of an electron gun of the present invention a focusing electrode is placed sym- "metrically about the converging electron stream between the curved cathode surface and the anode. The focusing electrode is asi'ngle, unipotential electrode which is shaped to create fields in the region external to the converging beam, satisfying proper boundary conditions at the beam edge so that the electrons in the beam behave as though the beam and its space charge continue to a far greater distance. The design of the shape of the focusing electrode, as well as that of the other electrodes in the gun, is carried out by means of an electrolytic tank analog of that part of the gun structure external to the beam in the cathode-anode region.

'trons.

In another example, a hollow cylindrical convergent beam of charged particles is provided about an elongated axis by a gun having a source of particles in a toroid-like figure of revolution about the elongated axis. The cross section of the emissive surface which is rotated around the axis in a curve symmetric about a line parallel to the axis. The curvature of this curve decreases as a function of distance from the line.

The novel features of this invention, as well as the in vention itself, both as to its organization and method of operation, will best be understood from the following description, taken in conjunction with the accompanying drawings, in which like reference numerals refer to like parts, and in which:

FIGURE 1 is a selected diagram of a traveling-wave tube of the prior art; FIG. 2 is a series of graphs of electron density taken across the scalloped electron beam of FIG. 1; each of the individual plots being directly below the point in the beam of FIG. 1 where it represents the beam cross section;

FIG. 3 is a diagram illustrating electron trajectories in the gun of the tube of FIG. 1;

FIG. 4 is a graph illustrating spherical aberration in electron guns;

FIG. 5 is a diagram illustrating the noncircular curvature of the cathode in a gun of the present invention;

FIG. 6 is a graph illustrating curvature of the cathode of FIG. 5 as a function of radial distance oif axis;

FIG. 7 is a sectioned diagram of a traveling-wave tube utilizing the electron gun of the present invention;

FIG. 8 is a series of graphs illustrating the electron density across the stream of the tube of FIG. 7; the individual graphs again being directly below the point of the beam where they represent the cross section of the beam;

FIG. 9 is a diagram illustrating another nonspherical cathode of the invention;

FIG. 10 is a graph illustrating curvature of the cathode of FIG. 9; and

FIG. 11 is a schematic diagram of a convergent, hollow cylindrical charged particle gun in accordance with the present invention.

Referring more particularly to FIG. 1, an electron gun 10 is shown as utilizing a thermally emissive cathode 12 which is heated by a filament heater 14. The heater 14 is energized by a voltage source, not shown. The cathode 12 has a circularly or spherically curved emissive surface 16 which emits an initially converging stream 18 of elec- A focusing electrode 20 aids in the initial beam formation by compensating along the boundaries of the beam in the cathode-anode region for space charge effects in the beam. An anode 22 is disposed adjacently to and downstream from the focusing electrode 20. The anode 20 has a central aperture 24 to permit passage of the electron stream 18 out of the electron gun. The electrodes of the gun 10 are connected to a source of potential 26 to provide them with appropriate operating potentials. The electron stream 18 after emerging from the anode aperture 24 enters the environment of an axial focusing magnetic field B. In this regard it may be noted that the anode 22 may be of a ferromagnetic material in order to shield the cathode-anode region from this magnetic field. The axial magnetic field is produced by a magnet 28 which may be an electromagnet or a permanent magnet.

The shading in the electron stream 18 of FIG. 1 makes apparent the scalloping of high density portions of the stream. The first graph 30 of FIG. 2 illustrates the variation of electron density across the stream as it emerges from the anode aperture 24. It may be seen that the high density peaks occur near the outer periphery of the beam. It may also be seen that the electrons in these high density portions of the beam have an inward component of velocity so that slightly further down the stream where its cross section is represented by the graph 32 the high density regions of the beam are closer to the axis. At the point represented by graph 34 the high density regions are yet closer to the axis and is the point in the first scallop where the high density regions are closest to the axis. Here the space charge repulsion forces associated with the electrons begin to take over and force the high density portions of the beam to expand radially outward, as illustrated by the graph 36. At the point in the stream corresponding to the graph 38 the high density electrons have reached their maximum excursion from the axis of the stream and again the magnetic field B begins to constrain them and force them toward the center. Graph 42 illustratesthe next in the scalloping amplitude of the high density portions of the beam while graph 44 represents the succeeding maximum. Graph 46 illustrates the next maximum and also shows the extent to which the beam itself, as well as the high density regions within the beam, has spread. This of course represents defocusing of the stream and points up the requirement that the interaction structure, represented by the dotted lines 47, must be placed at a wastefully great distance from the scalloped electron stream in order to prevent melting or other destruction due to electron interception. The dotted lines 49 illustrate the lower density portions of the stream comprising, for the most part, electrons emitted from the less peripheral portions of the cathode. It is to be noted that this portion of the beam is also scalloped and that the relative phase of the scalloping is such that the denser portion sometimes passes outside of the less dense portions. This is illustrated to some extent by the graph 38.

FIG. 3 illustrates a number of individual electron trajectories from the cathode of the gun of FIG. 1 to the point on the axis to which the electron would be focused in the absence of space charge effects and a constraining magnetic field. The individual electron trajectories are labeled i, ii, iii, iv, v, vi, vii and viii, with i representing an electron emitted from near the periphery of the circularly or spherically curved ernissive surface 16, While viii represents an electron emitted near the axis. The spherical aberration in the lens between the cathode 12 and the anode 22 mainfests itself in causing the outer electrons to be bent less outwardly than they would be if the lens was a pure spherical lens without aberration. This results in the electron trajectory i intercepting the axis at a far shorter focal length than the electron trajectory viii. A nonhomocentric beam is defined as having such properties. The fact that the electrons do not have a common focal length precludes the selection of a magnetic field strength B which will provide ideal Brillouin flow for the entire beam. It also causes the bunching of the electrons near the outer periphery of the beam in the region 48 near the anode aperture 24. Crossing-over of the electrons in this region gives rise to the sharp density peaks on the graph 30 of FIG. 2. It may also be seen in observing the group of trajectories in the region 43 that these trajectories have a considerably greater radial inward component of velocity than the inner portions of the beam and, as discussed above, this is one of the factors in the undesirable beam scalloping illustrated in FIGS. 1 and 2.

FIG. 4 is a graph which is useful in representing the degree of excess slope in the region 48. The observed slope of electrons emitted from a prior art electron gun at the anode aperture 24 is plotted in this graph. If the lens was purely without aberration, the slopes of the electrons would increase linearly with radial distance from the axis to the diameter of the anode aperture. In FIG. 4 this condition is represented by the straight line 50, while the curve line 52 represents the observed values. The region 48 of FIG. 3 is represented on FIG. 4 by the bracket near the upper end of the curve 52. It may also be seen in the region from .04 inch to .06 inch in the axis that there is excess slope to the trajectories, while below .03 inch the slope is somewhat less than ideal. To restate one of the objects of this invention, it is desired to provide an electron gun where the actual plot of tra jectory slope is a straight line as represented by the line 5% in FIG. 4.

As stated earlier, the scalloping and the uneven distribution of electrons across the beam is caused by the electrons emitted from the outer regions of the cathode being not bent suificiently outwardly by the cathode-anode lens. In other words, with respect to electrons emitted near the periphery, they have too great a radially inward velocity. In FIG. 5 it is illustrated how this excess inward velocity of the outer electrons is corrected in the electron gun of the present invention. An electron gun 54 having a cathode 56 is illustrated. The curved emissive surface 58 is not spherical or circular but has a curvature which increases near the axis or center line of the gun. The variation of the curved surface 58 from a circular surface is shown by comparing it with a circle shown by the dotted line 60. At the periphery of these two surfaces 58 and 60 their curvatures are equal but near the center line the curvature of surface 58 is greater than that of surface 60. Thus, electrons which are emitted near the periphery of the cathode 56 have approximately the same radial veloc ity as those emitted from a circular cathode and thus would focus like the electronindicated by the trajectory i of FIG. 3. However, electrons emitted near the axis have a greater inward radial velocity than those of the prior art guns and are thus caused to focus with a shorter focal length than if they were emitted from a spherical surface and thus the convergent beam may be a homocentric one.

FIG. 6 is a plot of the curvature of the emissive surface 58 as a function of radial distance from the axis. On the abscissa, R represents the outer edge of the cathode surface 58. The 'line 62 is straight and is parallel to the abscissa representing that in a circular or spherical surface the radius of curvature, or the curvature defined as the reciprocal of the radius of curvature, is constant across the spherical surface. Line 64 represents the curvature of the surface 53 and it is seen that the curvature is greater near the axis than at its periphery. It is in fact equalto the curvature of the spherical cathode at the periphery and significantly less than at the axis. I

Referring to FIG. 7, a portion of a traveling-wave tube 66 is shown which utilizes the electron gun 54 of FIG. 5 The cathode 56 has a curved emissive surface 58 which varies from the spherical, as may be seen by comparing it to the dotted circular line 60. A focusing electrode 68 is shown in place to aid in the initial beam formation, while a ferromagnetic anode 70 is illustrated with an anode aperture 72 through which an electron stream 74 emerges. After the stream emerges from the aperture 72 it enters the environment of an axial focusing magnetic field B which is produced by an electromagnet 76. Be cause of the good focusing and collimation of the electron gun 54 the stream may effectively be placed much closer to the interaction structure 78 which is represented by dotted lines.

FIG. 8 illustrates the improved electron density distributions across the stream 74 by a series of individual plots or profiles 80, 82, 84, 86, 88, 90, 92, 94 and 96 which may be compared to those of FIG. 2 for showing improved collimation and focusing of the electron stream 74 as compared to that of electron stream 18 of the prior art. In particular, it may be seen from the electron density curves of FIG. 8 that the severe high density peaks of the prior art as illustrated in FIG. 2 do not occur and the stream is greatly improved as regards its lack of scalloping and spreading. Obviously, with electron gun 54 higher beam currents and higher perveance may be achieved and utilized without excessive beam spreading and scalloping so that a higher energy beam may be projected closer to an interaction structure, such as a helix, to provide substantially increased interaction between traveling waves, for example, and the electron stream.

FIG. 9 illustrates a curved, noncircular cathode of the present invention which is alternative in its manufacture to that shown in FIG. 5. In the cathode 97 of FIG. 9 the curvature of the emissive surface 98 in its central portion near the axis is substantially circular or spherical as may be seen by comparing it to the circular dotted line 100. Near the periphery of the curved surface, however, the curvature is decreased and the emissive surface falls away from the circular line 100 which may be taken as representing a prior art cathode.

The graph of FIG. 10 relates to the structure of FIG. 9

and illustrates the curvature of surface 98 by line 102 which may be readily compared with the line 104 which shows the constant curvature of the circle 100. The curvatures are equal near the axis and diverge at radial distances toward the edge of the cathode represented by R on the abscissa. The result is substantially the same as with the nonspherical cathode of FIG. the outer electrons are given less radially inward velocity to make possible better collimation of the beam.

FIG. 11 illustrates a convergent, hollow cylindrical charged particle gun of the present invention. Here, the

emitter surface 106 has the same general properties as those of the previously described guns, for example, that of FIG. 5. That is, the emissive surface 106, or a cross section taken through it, is a curve symmetrically disposed about a line 108 which is substantially parallel to the center line of the gun. Here again, the curvature of the curve of the emissive surface 106 decreases as a function of distance from the line 108. The emissive surface 106 is a toroid-like figure of revolution about the center line or axis of the gun. A toroidal focusing electrode 110 is shown disposed adjacent the cathode to aid in forming a convergent annular beam of charged particles. An annular accelerating electrode 112 is positioned next downstream from and adjacently to the focusing electrode.

As set forth above the scope of the invention includes devices for forming beams of charged particles other than electrons, for example, ions or protons or the like. Further, the invention also includes guns for producing other than beams of cylindrical form, for example, FIG. 5 illustrates a gun which may provide a planar or sheet beam in which case the (12 reference is not a single line axis but is a longitudinal reference plane lying perpendicular to the plane of the drawing.

I claim:

1. A charged particle gun for producing a well-collimated stream of charged particles along a linear path comprising: a charged particle emitter having a concave equipotential charged particle emissive surface, said emissive surface being a non-spherical figure of revolution having an axis of revolution coincident with said path, the intersection of said surface with a plane containing said axis defining a curve extending continuously from one side of said axis to the other, the radius of curvature of said curve increasing as a function of transverse distance from said axis to said curve, and means for focusing the charged particles emitted from said surface into a wellcollimated stream along said linear path.

2. In an electron gun for emitting a Well-collirnated electron stream along a predetermined linear path, a concave equipotential electron emissive surface defining a non-spherical figure of revolution having an axis of revolution coincident with said path, the intersection of said surface with a plane containing said axis defining a curve extending continuously from one side of said axis to the other, and the curvature of said curve decreasing as a function of transverse distance from said curve.

3. A charged particle gun for emitting a Well-collimated, hollow, cylindrical stream of charged particles along a predetermined axis comprising: a curved equipotential charged particle emissive surface, said emissive surface being a toroid-like figure of revolution having an axis of revolution coincident with said predetermined axis, the intersection of said surface with a plane containing said axis defining a pair of curves, each curve of said pair being symmetrically disposed about a line parallel to said axis and extending continuously from one side of said line to the other, the curvature of each said curve decreasing as a function of transverse distance from its said line of symmetry, and means for focusing the charged particles emitted from said surface into a well-collimated, hollow, cylindrical stream.

References Cited in the file of this patent UNITED STATES PATENTS 2,817,040 Hull Dec. 17, 1957 2,840,754 Linder June 28, 1958 2,921,223 Birdsall Jan. 12, 1960

Claims (1)

1. 2. IN AN ELECTRON GUN FOR EMITTING A WELL-COLLIMATED ELECTRON STREAM ALONG A PREDETERMINED LINEAR PATH, A CONCAVE EQUIPOTENTIAL ELECTRON EMISSIVE SURFACE DEFINING A NON-SPHERICAL FIGURE OF REVOLUTION HAVING AN AXIS OF REVOLUTION COINCIDENT WITH SAID PATH, THE INTERSECTION OF SAID SURFACE WITH A PLANE CONTAINING SAID AXIS DEFINING A CURVE EXTENDING CONTINUOUSLY FROM ONE SIDE OF SAID AXIS TO THE OTHER, AND THE CURVATURE OF SAID CURVE DECREASING AS A FUNCTION OF TRANSVERSE DISTANCE FROM SAID CURVE.

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