US3329856A

US3329856A - Image dissector tube field mesh

Info

Publication number: US3329856A
Application number: US398891A
Authority: US
Inventors: Richard H Foote
Original assignee: Deutsche ITT Industries GmbH
Current assignee: TDK Micronas GmbH; ITT Inc
Priority date: 1964-09-24
Filing date: 1964-09-24
Publication date: 1967-07-04
Anticipated expiration: 1984-07-04
Also published as: GB1115509A

Description

July 4, 1967 R. H. FOOTE 3,329,856

IMAGE DISSECTOR TUBE FIELD MESH I Q 7 Filed Sept. 24, 1964 ELLE-Emmi 32 3; 36 9 l s. 1* JLIS BY I ATTORNEYS United States Patent M 3,329,856 IMAGE DISSECTOR TUBE FIELD MESH Richard H. Foote, Fort Wayne, Ind., assignor to International Telephone and Telegraph Corporation, Nutley, N.J., a corporation of Maryland Filed Sept. 24, 1964, Ser. No. 398,891 7 Claims. (Cl. 315-11) ABSTRACT OF THE DISCLOSURE A curved field mesh accelerator in an image dissector tube provides focusing for electrons from all areas of a planar photocathode directed toward a scanning aperture.

This invention relates generally to image dissector tubes, and more particularly to an image dissector tube in which the need for dynamic focusing is eliminated.

A common type of image dissector tube comprises an extended area, planar photocathode which provides a low velocity flood beam of electrons modulated in accordance with excitation of the photocathode by an optical image. The modulated flood beam is scanned over a small defining aperture so that the electrons which pass through the aperture at any instant emanate from only a single incremental area of the photocathode. The electrons which pass through the aperture are conventionally multiplied by a secondary electron multiplier to provide a time-based video output signal corresponding to the electron image which has been scanned over the aperture.

Since the transit time of electrons from the axial center of the photocathode to the defining aperture is normally less than the transit time of electrons emanating from the outer edge of the photocathode, conventional image dissector tubes have employed various dynamic focusing systems in an effort to achieve optimum focusing of electrons emanating from the entire area of the photocathode and thus to provide optimum resolution. Such dynamic focusing systems, however, require external circuitry for generating and applying the dynamic focusing waveforms thus increasing the complexity and in turn, the cost of the system in which the tube is employed.

It is accordingly an object of the invention to provide an image dissector tube in which optimum focusing of the electron beam is provided without the use of a dynamic focusing system.

The invention in its broader aspects provides an image dissector tube having a source of an extended area electron beam with first electrode means spaced axially from the beam source and having a defining aperture therein and with secondary electron multiplier means being provided for receiving and multiplying the electrons of the beam which pass through the aperture. A screen electrode is provided disposed between the beam source and the first electrode and means are provided acting upon the beam between the screen electrode and the first electrode for scanning the beam over the aperture. The beam is focused by means of an axial magnetic field extending through the tube and a suitable potential is applied to the screen electrode so that essentially all of the acceleration of the beam electrons occurs between the beam source and the electrode. In order to insure that the transit times of the electrons of the beam from all points on the beam source to the aperture are substantially equal to the times required for the electron to make an integral number of complete revolutions in the magnetic field, the screen electrode is curved toward the first electrode.

The above mentioned and other features and objects of this invention and the manner of attaining them will 3,329,856 Patented July 4, 1967 become more apparent and the invention itself will be best understood by reference to the following description of an embodiment of the invention taken in conjunction with the accompanying drawings, wherein:

FIG. 1 is a schematic cross-sectional view showing the improved image dissector tube of the invention; and

FIG. 2 is a diagram useful in explaining the invention.

Referring now to FIG. 1, the improved image dissector tube of the invention, generally indicated at 10, comprises a conventional elongated evacuated envelope 11 having a longitudinal axis 12, opposite ends 13, 14 and a side wall 15. A conventional photocathode 16 is deposited or otherwise positioned on the inner surface of the end wall 13 which is transparent to the wave length of the optical image 17. The optical image 17 is thus impressed upon the photocathode 16 which thus emits a low velocity flood beam of electrons area-modulated in accordance with the optical image 17; photocathode 16 thus emits an electron image corresponding to the optical image 17. Photocathode 16 is coupled to terminal 18 adapted to be connected to a suitable source of potential.

Electrode 19 is axially spaced from the photocathode 16 and has a defining aperture 20 formed therein concentric with the axis 12. Electrode 19 is connected to terminal 22 adapted to be connected to a suitable source of potential. A conventional secondary electron multiplier assembly 23 is disposed between electrode 19 and end wall 14 of the envelope 11 for receiving and multiplymg the electrons of the beam which pass through the defining aperture 20*. Electron multiplier 23 comprises a plurality of dynode stages 24-31 respectively connected to terminals 32-39 adapted to be connected to sources of progressively higher potentials. A final target electrode 40 is provided connected to terminals 42 which is adapted to be connected to a suitable source of potential and an output electrode 43 is positioned between target electrode 40 and the final dynode stage 31. Target electrode 43 is coupled by load resistor 44 to terminal 45 adapted to be connected to a suitable source of potential and is also connected to output terminal 46 by coupling capacitor 47.

In order to provide for initial acceleration of the beam electrons and also to provide an essentially field-free space within the envelope for deflection of the electron beam, a field screen electrode or mesh 48 is provided extending across envelope 11 between photocathode 16 and electrode 19 and closely spaced from the photocathode 16, as shown. Field mesh 48 is connected to terminal 49 adapted to be connected to a suitable source of accelerating potential. A tubular electrode 50 is provided extending axially substantially the entire distance between the field mesh electrode 48 and electrode 19. Electrode 50 may be formed as a conventional conductive coating on the interior-surface of side wall 15 of the envelope 11 and may be connected to the field mesh electrode 48- and the electrode 19, or may be electrically isolated therefrom, as shown. Tubular electrode 50 is formed of nonmagnetic material in order to permit magnetic deflection of the electron beam. Tubular electrode 50 is connected to terminal 52 adapted to be connected to a suitable source of potential. Terminals 49, '52 and 22 may be connected to the same potential, or

terminals

52 and 22 may be connected to a source of potential a few volts higher than that connected to terminal 49, either connection establishing an essentially field-free space within envelope 11 between the screen electrode 48 and the electrode 19 to permit magnetic deflection of the electron beam.

Conventional vertical and horizontal

magnetic deflection yokes

53, 54 are provided on the exterior of the side wall 15 of envelope 11 and thus act upon the electron beam within the tubular electrode 50 to scan the same over the defining aperture 20 in the electrode 19.

In order to focus the electrons of the beam onto the plane of the defining aperture 20, a solenoid coil 55 is provided coaxial with axis 12 and surrounding side wall 15 of the envelope 11 and the

deflection coils

53, 54. Solenoid coil 55, when suitably energized provides a solenoidal magnetic field extending axially through the envelope 11 parallel with the axis 12.

Proper focusing of the electron beam onto the plane of the aperture 20 is obtained when the photocathode 16 is spaced from the plane of the aperture 20 by an integral number of full loops of focus, i.e., when the transit time of the electrons from the photocathode 16 to the plane of the aperture 20 is equal to the time required for an electron to make an integral number of complete revolu tions in the magnetic field provided by the solenoid focusing coil 55.

Referring now additionally to FIG. 2, L is the distance between the photocathode 16 and the plane of the aperture 20, S is the distance along the axis 12 between photocathode 16 and

screen electrode

48, 56 is a point on the screen electrode 48 radially spaced from axis 12, b is the distance between point 56 and aperture 20, i.e., the line of travel of a beam electron from point 56 to aperture 20, a is the distance from the point 56 on screen electrode 48 to the photocathode 16 formed as an extension of the line of travel of the electron, and S is the distance from photocathode 16 to point 56 on screen electrode 48 parallel with the axis 12. Proper focus is obtained when the transit time I of an electron from the photocathode 16 to the aperture 20 is equal to the time required for the electron to make an integral number of complete revolutions in the magnetic field provided by the solenoid coil 55. Thus:

In the case of an electron emanating from point 58 on the photocathode 16 and thus traveling along axis 12:

so ;9 ifi w V/2 V V 2) It will be seen, however, that electrons emanating from point 59 on photocathode 16 or from any other point thereon radially spaced from axis 12, when deflected to the aperture 20 by the scanning field provided by

deflection coils

53, 54 move with a higher angular velocity. Thus:

Therefore, in order to provide the proper transit time for all electrons deflected to the aperture 20 without regard to their point of origin on the photocathode 16, the transit time of the off-axis electrons must be decreased so that:

From the geometry shown in FIG. 2, it will be seen that:

COS or Solving (7) and (8) for a,

a=(L+S cos a- COS a (L+S cos aL cos a (9) Thus, it is seen that the separation S between the photocathode 16 and the field screen electrode 48 at any point radially spaced from the axis 12 is:

S=(L+S Cos aL (10) In a specific example of a tube incorporating the invention in which the distance L is 6.25 inches and the distance S is 0.25 inch, the following values for S defining the curvature of the screen electrode 48 toward the aperture 20 are calculated in accordance with the Equation 10 above:

at S 1 0.2474 2 0.2422 3 0.2318 4 0.2188 5 0.2006

The foregoing specific example in which a focusing field of 40 gauss is provided, the following potentials may be applied:

Volts While I have described above the principles of my invention in connection with specific apparatus, it is to be clearly understood that this description is made only by way of example and not as a limitation to the scope of my invention.

What is claimed is:

1. An image dissector tube comprising: a source of an extended area electron beam; first electrode means spaced axially from said beam source and having a defining aperture therein; secondary electron multiplier means for receiving and multiplying the electrons of said beam which pass through said aperture; a screen electrode between said beam source and first electrode; means acting upon said beam between said screen electrode and first electrode for scanning said beam over said aperture; and means for providing a magnetic field extending axially through said tube for focusing said beam onto the plane of said aperture; said screen electrode being curved toward said first electrode so that the transit times of the electrons of said beam from all points on said beam source to said aperture are substantially equal to the times required for said electrons to make an integral number of complete revolutions in said magnetic field.

2. An image dissector tube comprising: an essentially planar source of an extended area electron beam, said beam source being normal to an axis; first electrode means spaced axially from said beam source and having a defining aperture therein on said axis; secondary electron multiplier means for receiving and multiplying the electrons of said beam which pass through said aperture; an extended area screen electrode extending across said axis between and facing said beam source and first electrode; means acting upon said beam betweeen said screen electrode and said first electrode for scanning said beam over said aperture; and means for providing a magnetic field extending parallel with said axis through said tube for focusing said beam onto the plane of said aperture; said screen electrode being smoothly curved toward said first electrode with its point on said axis being closer to said first electrode than points radially spaced from said axis so that the transit times of electrons of said beam from all points on said beam sources to said aperture are substantially equal to the times required for said electrons to make an integral number of complete revolutions in said magnetic field.

3. The tube of claim 2 wherein the curvature of said screen electrode responds to the equation:

S: (L-l-S cos OLL where:

L=the distance along said axis between said beam source and said aperture,

S =the distance along said axis from said beam source to said screen electrode,

Ot=th6 angle defined by said axis with the line of travel of a beam electron between one point on said screen radially spaced from said axis and said aperture,

S=the distance along a line parallel with said axis between said beam source and said one point on said screen electrode.

4. An image dissector tube comprising: an essentially planar source of an extended area electron beam, said source being normal to and concentric with an axis, first electrode means spaced axially from said source and having a defining aperture therein on said axis; secondary electron multiplier means for receiving said muliplying the electrons of said beam which pass through said aperture; an extended area screen electrode extending across and concentric with said axis between and facing said beam source and said first electrode; a tubular electrode concentric with said axis between said screen electrode and said first electrode; means for applying potentials to said screen, tubular and first electrodes thereby to accelerate the electrons of said beam between said beam source and said screen electrode and to provide an essentially field-free space between said screen electrode and said first electrode; magnetic deflection means exterior to said tubular electrode and acting upon said beam therein for scanning said beam over said aperture; and means for providnig a solenoidal magnetic field extending parallel with said axis through said tube for focusing said beam onto the plane of said aperture; said screen electrode being smoothly curved toward said first electrode with its point on said axis being closer to said first electrode than points radially spaced from said axis so that the transit times of electrons of said beam from all points on said beam source to said aperture are substantially equal to the times required for said electrons to make an integral number of complete revolutions in said magnetic field.

5. The tube of claim 4 wherein said beam source is axially spaced from the plane of said aperture by an integral number of full loops of focus of the electrons of said beam.

6. An image dissector tube comprising: an evacuated envelope having a longitudinal axis; an essentially planar source of an extended area electron beam in said envelope, said source being normal to and concentric with said axis; first electrode means in said envelope spaced axially from said source and having a defining aperture therein concentric with said axis; secondary electron multiplier means in said envelope for receiving and multiplying the electrons of said beam which pass through said aperture; an extended area screen electrode in said envelope extending across said axis and concentric therewith, said screen electrode being intermediate said source and said first electrode and closely spaced from said source; a tubular electrode in said envelope concentric with said axis and extending substantially between said screen electrode and said first electrode; means for applying potentials to said screen, tubular and first electrodes thereby to accelerate said beam between said source and said screen electrode and to provide an essentially field-free space between said screen electrode and said first electrode; magnetic deflection means exterior to said envelope and acting upon said beam within said tubular electrode for scanning said beam over said aperture; and a solenoid coil concentric with said axis and surrounding said envelope and said deflection means for providing a solenoidal magnetic field extending axially through said envelope for focusing said beam onto the plane of said aperture; said screen electrode being convexly curved toward said first electrode with its point on said axis being axially closer to said first electrode than points spaced radially from said axis so that the transit times of electrons of said beam from all points on said beam source to said aperture are substantially equal to the times required for said electrons to make an integral number of complete revolutions in said magnetic field; said beam source being axially spaced from the plane of said aperture by an integral number of full loops of focus of the electrons of said beam.

7. The tube of claim 6 wherein the curvature of said screen electrode responds to the equation:

S=(L+S C082 ocL where:

References Cited UNITED STATES PATENTS 3,207,997 3,286,114 11/1966 Schlesinger 3l383 JOHN W. CALDWELL, Primary Examiner.

T. A. GALLAGHER, Assistant Examiner.

9/1965 Eberhardt 315l1 X

Claims

1. AN IMAGE DISSECTOR TUBE COMPRISING: A SOURCE OF AN EXTENDED AREA ELECTRON BEAM; FIRST ELECTRODE MEANS SPACED AXIALLY FROM SAID BEAM SOURCE AND HAVING A DEFINING APERTURE THEREIN; SECONDARY ELECTRON MULTIPLIER MEANS FOR RECEIVING AND MULTIPLYING THE ELECTRONS OF SAID BEAM WHICH PASS THROUGH SAID APERTURE; A SCREEN ELECTRODE BETWEEN SAID BEAM SOURCE AND FIRST ELECTRODE; MEANS ACTING UPON SAID BEAM BETWEEN SAID SCREEN ELECTRODE AND FIRST ELECTRODE FOR SCANNING SAID BEAM OVER SAID APERTURE; AND MEANS FOR PROVIDING A MAGNETIC FIELD EXTENDING AXIALLY THROUGH SAID TUBE FOR FOCUSING SAID BEAM ONTO THE PLANE OF SAID APERTURE; SAID SCREEN ELECTRODE BEING CURVED TOWARD SAID FIRST ELECTRODE SO THAT THE TRANSIT TIMES OF THE ELECTRONS OF SAID BEAM FROM ALL POINTS ON SAID B EAM SOURCE TO SAID APERTURE ARE SUBSTANTIALLY EQUAL TO THE TIMES REQUIRED FOR SAID ELECTRONS TO MAKE AN INTEGRAL NUMBER OF COMPLETE REVOLUTIONS IN SAID MAGNETIC FIELD.