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

Charged particle gun with nonspherical emissive surface Download PDF

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Publication number
US3139552A
US3139552A US13333A US1333360A US3139552A US 3139552 A US3139552 A US 3139552A US 13333 A US13333 A US 13333A US 1333360 A US1333360 A US 1333360A US 3139552 A US3139552 A US 3139552A
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Prior art keywords
electron
axis
gun
cathode
electrons
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Expired - Lifetime
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US13333A
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English (en)
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George R Brewer
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Raytheon Co
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Hughes Aircraft Co
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Priority to US13333A priority Critical patent/US3139552A/en
Priority to GB4751/61A priority patent/GB908590A/en
Priority to DEH41705A priority patent/DE1162003B/de
Priority to FR854288A priority patent/FR1281804A/fr
Priority to BE600921A priority patent/BE600921A/fr
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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
    • H01J3/00—Details of electron-optical or ion-optical arrangements common to two or more basic types of discharge tubes or lamps
    • H01J3/02—Electron guns
    • H01J3/029—Schematic arrangements for beam forming

Definitions

  • 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.
  • microwave electron tubes such as traveling-wave tubes
  • 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.
  • perveance is defined as the ratio of total beam current to the three-halves power of the beam voltage.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • the stream maybe cylindrical, hollow cylindrical, planar, or the like.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • FIG. 11 is a schematic diagram of a convergent, hollow cylindrical charged particle gun in accordance with the present invention.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • they have too great a radially inward velocity.
  • 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.
  • 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.
  • 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.
  • 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.
  • a portion of a traveling-wave tube 66 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. 9 illustrates a curved, noncircular cathode of the present invention which is alternative in its manufacture to that shown in FIG. 5.
  • 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
  • FIG. 11 illustrates a convergent, hollow cylindrical charged particle gun of the present invention.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.

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  • Particle Accelerators (AREA)
US13333A 1960-03-07 1960-03-07 Charged particle gun with nonspherical emissive surface Expired - Lifetime US3139552A (en)

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Application Number Priority Date Filing Date Title
US13333A US3139552A (en) 1960-03-07 1960-03-07 Charged particle gun with nonspherical emissive surface
GB4751/61A GB908590A (en) 1960-03-07 1961-02-08 Charged particle gun
DEH41705A DE1162003B (de) 1960-03-07 1961-02-11 Einrichtung zur Erzeugung einer gebuendelten Stroemung von geladenen Teilchen
FR854288A FR1281804A (fr) 1960-03-07 1961-03-01 Canon à particules chargées
BE600921A BE600921A (fr) 1960-03-07 1961-03-06 Canon à particules chargées.

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Cited By (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3270511A (en) * 1963-10-10 1966-09-06 Intrusion Prepakt Inc Method of forming piles
US3558967A (en) * 1969-06-16 1971-01-26 Varian Associates Linear beam tube with plural cathode beamlets providing a convergent electron stream
US3594885A (en) * 1969-06-16 1971-07-27 Varian Associates Method for fabricating a dimpled concave dispenser cathode incorporating a grid
US3798499A (en) * 1971-11-02 1974-03-19 Siemens Ag Disc-sealed electron discharge tubes
US5552675A (en) * 1959-04-08 1996-09-03 Lemelson; Jerome H. High temperature reaction apparatus

Families Citing this family (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
GB2152741B (en) * 1980-04-28 1986-02-12 Emi Varian Ltd Producing an electron beam

Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2817040A (en) * 1955-05-31 1957-12-17 Joseph F Hull Broadband backward wave amplifier
US2840754A (en) * 1954-09-01 1958-06-24 Rca Corp Electron beam tube
US2921223A (en) * 1954-11-15 1960-01-12 Hughes Aircraft Co High-power traveling-wave tube

Family Cites Families (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2518472A (en) * 1949-02-03 1950-08-15 Heil Oskar Electron gun
US2812467A (en) * 1952-10-10 1957-11-05 Bell Telephone Labor Inc Electron beam system

Patent Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2840754A (en) * 1954-09-01 1958-06-24 Rca Corp Electron beam tube
US2921223A (en) * 1954-11-15 1960-01-12 Hughes Aircraft Co High-power traveling-wave tube
US2817040A (en) * 1955-05-31 1957-12-17 Joseph F Hull Broadband backward wave amplifier

Cited By (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US5552675A (en) * 1959-04-08 1996-09-03 Lemelson; Jerome H. High temperature reaction apparatus
US5628881A (en) * 1959-04-08 1997-05-13 Lemelson; Jerome H. High temperature reaction method
US3270511A (en) * 1963-10-10 1966-09-06 Intrusion Prepakt Inc Method of forming piles
US3558967A (en) * 1969-06-16 1971-01-26 Varian Associates Linear beam tube with plural cathode beamlets providing a convergent electron stream
US3594885A (en) * 1969-06-16 1971-07-27 Varian Associates Method for fabricating a dimpled concave dispenser cathode incorporating a grid
US3798499A (en) * 1971-11-02 1974-03-19 Siemens Ag Disc-sealed electron discharge tubes

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DE1162003B (de) 1964-01-30
GB908590A (en) 1962-10-17
BE600921A (fr) 1961-07-03

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