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Патент USA US2406824

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Sept. 3, 1946.
- 2,406,824
Filed June 25, 1941
Fig. 1;
' Attorney
Patented Sept. 3, 1946
John Heaver Fremlin, London, England, assignor
to International Stan dard Electric Corporation,
New York, N. Y.
Application June 25, 1941, Serial No. 399,598
In Great Britain June 21, 1940
6 Claims. (Cl. 250-463)
This invention relates to electron discharge ap
paratus incorporating focussing systems for elec
tron beams set up in the apparatus.
It is common practice to provide, as focussing
electrodes for an electron beam, tubular, aper- .
tured-disc and other-shaped metal bodies at dif
ferent points along the path of the beam such
that desired beam converging ?elds and beam
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
diverging ?elds may be set up mainly in the gaps
In the ?gure the potential of the space within
between the bodies by the application of different
metal tube E is made equal to that of the grid.
potentials to the bodies.
There is no reason, however, why the diameter
Proposals have also been made, particularly
and potential of DE should not have any desired
with a view to obtaining continuous potential
arbitrary value. In the same way the potential
gradients along substantial lengths of the beam
of the initial tubular portion B may be at any
path and thereby facilitating the concentration
arbitrary potential. In some cases it is conven
of beams of large intensity and density, to pro
ient to have the diameter and potential of B
vide interleaved conductors of different potential
or E intermediate between those of the grid and
in a common cylindrical surface, the conductors
of the continuous external conductor, the grid
at one- potential being tapered in one direction
wires being then supported at both ends for
and the conductors of the other potential being
example by thin sheets of mica.
tapered in the opposite direction along the axis.
From consideration of the conditions obtaining
The present invention provides alternative
structures providing greater freedom in design
and simplicity in manufacture.
in triodes it is known that when a grid at poten
tial 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 exist
ed over the whole surface de?ned by the grid,
The structure now proposed comprises a fo
raminate tube or tubular grid surrounded by a
co-axial tube capable of in?uencing the ?eld
where V depends both on Vg and Vp, on the
within the foraminate tube or tubular grid, one
mechanical structure of the grid, 1. e. on wire
or both of said tubular elements being of non 30 size and grid pitch and also on the distance
uniform diameter along the length of the system
apart of the grid and plate. The present in
Preferably we employ a tubular grid of uni
vention makes use of the property that the effec
form diameter and a ?ared outer tube. The grid
tive potential in a grid surface can be varied
may be a wire helix or it may be constructed of
continuously in any desired manner by suitably
parallel wires in a cylindrical boundary. The
shaping the outer relatively unbroken conductor
outer tube is preferably of sheet metal although 35 and by suitably arranging the characteristics of
it may be foraminate so long as the mesh is ?ne
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
the grid. Thus a convenient means is provided
for the construction of certain types of electron
Although in the figure the grid is shown as
of uniform diameter, the outer conductor being
?ared, similar e?ects can be secured by using
an outer tube of uniform diameter and a ?ared
grid. Alternatively both elements may be of
a sheet‘ metal tube G converging from a large 45
non-uniform diameter.
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
It will be clear that any desired potential dis
tribution along the electron path can be set up
by suitable grading of the tube diameters along
the path, choice of grid aperture and of poten
the mechanical support of the grid wires.
50 tials.
The tubular grid with its tubular extension E
In some cases it is desirable to arrange that
and the outer sheet metal tube G are adapted
the varying diameter changes in different senses
to be maintained at di?erent potentials.
at different points along the beam path. In
It will be seen from the ?gure that initially
cases where in the past three or more successive
electrons coming from below as indicated by ar 55 tubular or apertured electrodes each requiring a
rows H are moving in a space at a potential sen
different potential have been employed, equiva
lent ?eld 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 speci?ed intermediate potentials appear at
chosen points along the beam path.
What is claimed is:
1. Electron discharge apparatus comprising, as
a focussing system adapted When suitable poten
of varying diameter along said coextensive part
adapted when given a potential different from
that of the foraminate tubular grid to modify the
?eld 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
tials are applied to provide a desired potential 10
claim 2 wherein the foraminate tubular grid has
gradient along the path of an electron beam, a
a sheet metal tubular extension at a ?ared end
foraminate tubular electrode surrounded by a co
of the outer ‘tube.
axial tube capable of in?uencing the ?eld within
5. Electron discharge apparatus according to
the foraminate tubular electrode, said electrode
and tube being coextensive throughout at least 15 claim 2 wherein the outer tube converges upon
and extends beyond one end of the foraminate
part of their length and at least one of said tu
tubular grid.
bular elements being of non-uniform diameter
6. Electron discharge apparatus as claimed in
along the coextensive length of the system.
claim 2 comprising means for applying to the
2. Electron discharge apparatus comprising, as
inner and outer tubes of the focussing system
a focussing system adapted to provide a desired
potential gradient along the path of an electron 20 potentials differing from each other and from
that of a thermionic cathode disposed on the
beam, a foraminate tubular grid of substantially
axis of the focussing. system and co-operating
uniform diameter a coaxial tube surrounding said
with said system to form an electron gun.
foraminate tubular grid and extending at least
in part along the length of said grid and being
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