US2406824A - Electron discharge apparatus - Google Patents

Description

Sept. 3, 1946.

'J. H. FREM LIN- ELECTRON DISCHARGE APPARATUS Filed June 25, 1941 Fig. 1;

Ina/enter Attorney Patented Sept. 3, 1946 ELECTRON DISCHARGE APPARATUS London, England, assignor John Heaver Fremlin,

to International Stan New York, N. Y.

dard Electric Corporation,

Application June 25, 1941, Serial No. 399,598

In Great 6 Claims.

This invention relates to electron discharge apparatus incorporating focussing systems for electron beams set up in the apparatus.

It is common practice to provide, as focussing electrodes for an electron tured-disc and other-shaped metal bodies at different points along the path of the beam such that desired beam converging fields and beam diverging fields may be set up mainly in the gaps between the bodie by the application of different potentials to the bodies.

Proposals have also been made, particularly with a view to obtaining continuous potential gradients along substantial lengths of the beam path and thereby facilitating the concentration of beams of large intensity and density, to provide interleaved conductors of different potential in a common cylindrical surface, the conductors at onepotential being tapered in one direction and the conductors of the other potential being tapered in the opposite direction along the axis.

The present invention provides alternative structures providing greater freedom in design and simplicity in manufacture.

The structure now proposed comprises a foraminate tube or tubular grid surrounded by a co-axial tube capable of influencing the field within the foraminate tube or tubular grid, one or both of said tubular elements being of nonuniform diameter along the length of the system Preferably we employ a tubular grid of uniform diameter and a flared outer tube. The grid may be a wire helix or it may be constructed of parallel wires in a cylindrical boundary. The outer tube is preferably of sheet metal although it may be foraminate so long as the mesh is fine compared with that of the tubular grid.

A focussing structure embodying the invention is shown in the accompanying drawing, Fig. 2 of which shows a section along the line A-A of Fig. 1. The structure shown comprises a tu bular grid of parallel wires F extending from a metal tube E. Surrounding the tubular grid is a sheet metal tube G converging from a large diameter end adjacent the tube E to a cylindrical portion 13 of diameter comparable with the grid diameter at the free end of the grid wires F. A transverse insulating annulus C is provided for the mechanical support of the grid wires.

The tubular grid with its tubular extension E and the outer sheet metal tube G are adapted to be maintained at difierent potentials.

It will be seen from the figure that initially electrons coming from below as indicated by arrows H are moving in a space at a potential senbeam, tubular, aper- Britain June 21, 1940 sibly equal to that of the conductor B. They then pass through the space within the grid wires in which the potential changes continuously. Finally they emerge into the equi-potential space within metal tube E. The potential gradient within the grid depends on the curvature of the wall of the outer conductor G and can be made of any desired form by suitably choosing this curvature.

In the figure the potential of the space within metal tube E is made equal to that of the grid. There is no reason, however, why the diameter and potential of DE should not have any desired arbitrary value. In the same way the potential of the initial tubular portion B may be at any arbitrary potential. In some cases it is convenient to have the diameter and potential of B or E intermediate between those of the grid and of the continuous external conductor, the grid wires being then supported at both ends for example by thin sheets of mica.

From consideration of the conditions obtaining in triodes it is known that when a grid at potential Vg is placed near to a plate at a different potential Vp then the grid will behave in some respects as though an effective potential V existed over the whole surface defined by the grid, where V depends both on V and Vp, on the mechanical structure of the grid, 1. e. on wire size and grid pitch and also on the distance apart of the grid and plate. The present invention makes use of the property that the effective potential in a grid surface can be varied continuously in any desired manner by suitably shaping the outer relatively unbroken conductor and by suitably arranging the characteristics of the grid. Thus a convenient means is provided for the construction of certain types of electron gun.

Although in the figure the grid is shown as of uniform diameter, the outer conductor being flared, similar efiects can be secured by using an outer tube of uniform diameter and a flared grid. Alternatively both elements may be of non-uniform diameter.

It will be clear that any desired potential distribution along the electron path can be set up by suitable grading of the tube diameters along the path, choice of grid aperture and of potentials.

In some cases it is desirable to arrange that the varying diameter changes in different senses at different points along the beam path. In cases where in the past three or more successive tubular or apertured electrodes each requiring a different potential have been employed, equivalent field conditions can be now set up by the use of but two electrodes of different potential. The shape of the electrodes can be arranged so that specified intermediate potentials appear at chosen points along the beam path.

What i claimed is:

1. Electron discharge apparatus comprising, as a focussing system adapted When suitable potentials are applied to provide a desired potential gradient along the path of an electron beam, a foraminate tubular electrode surrounded by a coaxial tube capable of influencing the field within the foraminate tubular electrode, said electrode and tube being coextensive throughout at least part of their length and at least one of said tubular elements being of non-uniform diameter along the coextensive length of the ystem.

2. Electron discharge apparatus comprising, as a focussing system adapted to provide a desired potential gradient along the path of an electron beam, a foraminate tubular grid of substantially uniform diameter a coaxial tube surrounding said foraminate tubular grid and extending at least in part along the length of said grid and being of varying diameter along said coextensive part adapted when given a potential different from that of the foraminate tubular grid to modify the field within said foraminate tube or tubular grid.

3. Electron discharge apparatus according to claim 2 wherein parallel wires in a cylindrical boundary serve as tubular grid and a flared sheet metal tube constitutes the outer co-axial tube.

4. Electron discharge apparatus according to claim 2 wherein the foraminate tubular grid has a sheet metal tubular extension at a flared end of the outer tube.

5. Electron discharge apparatus according to claim 2 wherein the outer tube converges upon and extends beyond one end of the foraminate tubular grid.

6. Electron discharge apparatus as claimed in claim 2 comprising mean for applying to the inner and outer tubes of the focussing system potentials differing from each other and from that of a thermionic cathode disposed on the axis of the focussing. system and co-operating with said system to form an electron gun.

JOHN HEAVER FREMLIN.

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