This document describes improvements to ion traps in cathode-ray tubes. Specifically, it describes a cathode-ray tube design where the axis of the tubular anode adjacent to the cathode is not coaxial with but intersects the common axis of preceding electrodes. This allows the electrode system and overall tube length to be shorter compared to traditional designs where the anode has a bent coaxial section, reducing material needs and providing more space for getters. The design achieves satisfactory ion trapping through deflecting electrons and ions with a magnetic field at the point where the anode axis intersects the common axis of preceding electrodes.

1. * GB780040 (A)
Description: GB780040 (A) ? 1957-07-31
Improvements in or relating to ion traps in cathode-ray tubes
Description of GB780040 (A)
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PATENT SPECIFICATION
Date of Application and filing Complete Specification: Dec. 1, 1954.
780,040 No. 34811/54.
)k If t &BApplication made in Netherlands on Dec. 3, 1953.
Complete Specification Published: July 31, 1957.
Index at acceptance:-Class 39(1), 04(A4:A7:E1:E3B2:E8:J:K4).
International Classification:-H01j.
COMPLETE SPECIFICATION
Improvements in or relating to Ion Traps in Cathode-Ray Tubes We,
PHILIPS ELECTRICAL INDUSTRIES LIMITED, of Spencer House, South Place,
Finsbury, London, E.C.2, a British Company, do hereby declare the
invention, for which we pray that a patent may be granted to us, and
the method by which it is to be performed, to be particularly
described in and by the following statement:-
The invention relates to cathode-ray tubes, having an electron-gun
construction so that the electrons can be separated from the negative

2. ions.
It is known that in cathode-ray tubes (e.g., those used for
television, oscillography or radar purposes) negative ions have a
troublesome effect, since they travel in the same direction as the
electrons and strike the screen. If use is made of electromagnetic
deflection means the deflecting fields thus
20produced do not deflect these ions, which, after some time, produce
a dark spot in the centre of the screen. Many solutions have been
suggested to obviate this disadvantage.
Use is frequently made of an electron gun of a construction such that
the negative ions are collected on part of one of the electrodes,
generally the anode. In order to prevent the electrons from striking
the same area of this electrode, the paths of the electrons are
deflected by a magnetic field in a manner such that the electrons do
not strike this electrode, but approximately the centre of the screen.
The negative ions are substantially not deflected by this magnetic
transverse field and thus find their way to the collecting electrode.
In order to obtain the aforesaid ion trap effect, the anode is
frequently constructed in the form of a bent tube; Then the part of
the anode adjacent the cathode is coaxial to the electrodes preceding
the anode. The axis of the part after the bend is directed to the
centre of the screen. The direction of the electrons and the ions
before entering the anode intersects the wall of the anode. With the
aid of a magnetic transverse field approx[Price 3/61 imately at the
area of the bend in the anode the electrons are deflected and
conducted substantially into the direction of the axis of the anode
part after the bend. The negative ions travel substantially straight
on and.strike 50 the wall of the anode.
It has now been found that a satisfactory operation of this ion trap
does not require that the anode should have a part which is coaxial to
the electrodes preceding the anode 55 which previously thought
requirement had disadvantages.
An object of the invention is to provide a cathode-ray tube of which
the construction of the electron gun is such that these disadvantages
are substantially avoided, whilst satisfactory operation of the ion
trap is maintained.
According to the invention, a cathode-ray tube including an ion-trap
gun having a 65 cathode, a control electrode, and a tubular anode
having its end surface parallel to an adjacent end surface of at least
one electrode between the anode and the control-electrode, is
characterized in that the axis of the part of 70 the tubular anode
adjacent the cathode is not coaxial with but intersects the common
axis of the electrodes preceding the anode.
Between the anode and the control-electrode provision may be made of

3. more than 75 one electrode. A very common construction of the electron
gun comprises an acceleration anode preceding the anode. The following
description is based on such a construction of an electron gun,
although the invention is 80 not confined thereto. As an alternative
the anode and the control-electrode may have an electrode between them
which does not operate as an acceleration anode. Moreover, in a system
comprising an acceleration anode a 85 further electrode may be
provided between the acceleration anode and the control-electrode, so
that a pentode construction of the gun is obtained.
An embodiment of the invention will now 901 780,040 be described with
reference, by way of example, to Figs. 1 and 2 of the accompanying
drawing.
Fig. 1 shows diagrammatically a cross sectional view of a cathode-ray
tube according to the invention; Fig. 2 shows, on an enlarged scale,
the electron gun of the tube shown in Fig. 1; and Fig. 3 shows a gun
of known construction for comparison.
Referring to Fig. 1, the cathode-ray tube comprises a neck 1 and an
adjacent conical part 2, which is closed by a window 3, provided with
a screen 4 of luminescent substance. In the neck i of the tube
provision is made of an electron gun comprising a cathode 5, a
control-electrode 6, an acceleration anode 7 and an anode 8. The neck
of the tube is furthermore surrounded by a focusing coil 9. For the
sake of simplicity the deflectors for the electron beam are not shown.
Owing to the co-operation of the cathode 5, the control-electrode 6
and the accelerating anode 7, a concentrated beam 10 of electrons and
negative ions is produced. The direction of this beam is such that it
strikes the wall of the anode 8 at 11. A miagnetic field, produced by
a magnet 12-12 produces approximately at point 13 a separation between
the electrons and the negative ions.
The direction of the negative ions remains substantially the same,
whereas the electrons are deflected substantially into the direction
of the axis of the anode 8. Due to the fact that the centre line of
the magnetic field does not intersect the axis of the anode 8 in point
13 the electrons are not accurately deflected into the axis of the
anode 8. This is not a serious objection, since the axis of the anode
408 may be arranged slightly slanting relative to the axis going
through the centre of the screen 4, so that this small divergence may
be compensated. If necessary, with the aid of an additional magnetic
field the electrons may in a manner known per se be deflected
accurately into the axis of the anode, so that the axis is directed to
the centre of the screen.
From the figure it is evident that the axis of the part of the tubular
anode facing the accelerating anode is at an angle to the common axis
of the cathode 5, the control-electrode 6 and the accelerating anode

4. 7. The further shape of the anode 8 is not important.
It may for example have a non-circular sectional area or be slightly
conical.
The advantages obtained by this construction are best explained with
reference to a comparison of a known electron gun (shown in Fig. 3)
with an electron gun according to the invention, used in the
cathode-ray tube of Fig. 1 and shown in Fig. 2 on the same scale as
the corresponding known system of Fig. 3.
Referring to Fig. 2, the electrodes are designated by the same
reference numerals as those shown in Fig. 1. Referring to Fig. 3, the
electrode system comprises a cathode 14, a control-electrode 15. an
accelerating anode 16 and a bent anode 17. In the two figures the line
a-a indicates the axis common to 70 the cathode. the control-electrode
and the accelerating anode. in Fig. 3 the anode 17 has a piece 1t in
li _ with the accelerating anode 16 and thus hlts the axis a-a. In
this figure the axis of the second part of the anode 75 17 is
indicated bx the line c-c. In Fig. 2, the axes a-a and b-b intersect
one another at point 13 and approximately at this point the electrons
are serpirated from negative ions. The ions continue their way along
the 80 axis a-a and the electrons are deflected approximately in the
line b-b. which is directed substantially to the centre of the screen
by the magnetic transverse field. The distance of point 13 from the
end of the 85 anode is designated vby I and the angle between the
lines a-a and b-b by Y. Of course, I tg -. is a measure of the effect
of the ion trap.
In Fig. 3 the axis of the anode 17 is designated by c-c. It intersects
the axis a-a at point 19, approximately at this point the negative
ions are separated from the electrons under the action of a magnetic
transverse field. The distance of point 19 from the end 95 of the
anode is designated by in and the angle between the axes a and c-c by
/. A measure of the effect of the ion trap is thus in tg /. If the
effect of the ion traps is the same, i.e., if 1 tg = tg;i and if I =
in, 100 the length of the electrode system according to the invention.
shown in Fig. 2, is smaller than the length of the known system shown
in Fig. 3.
The effect of the ion traps may, of course, 105 be increased by
increasing the angles X and A3. However, this is soon limited, since
in the case of large angles a stronger magnetic transverse field is
required: moreover, in the case of larger angles, image defects may
occur. 110 The shorter length of the electrode system of a tube in
accordance with the invention may be utilized in various ways. In the
first place, the anode construction requires a smaller quantity of
material, the shape is 115 simpler and the mounting is easier, which
is of great improtance in manufacture. Moreover, more space is left

5. for the getter; this getter is always arranged slightly beyond the end
of the anode remote from the cathode. 120 Since the total length of
the tube, i.e., the distance of the screen from the cathode is the
same, space is saved for the arrangement of the getter.
More important than the aforesaid advantages is the fact that the
reference plane of the main lens produced for example with the aid of
a coil as shown in Fig. I and designated by 9 may be nearer the
crossover 20, located in the proximity of the aperture of 130 780,040
the control-electrode. In Figs. 2 and 3 the reference plane of the
main lens is designated by pq and the distance of this plane from the
crossover 20 is designated by d and D respectively. As is evident from
the figures, the distance d is smaller than the distance D. This is of
great importance, since the so-called deflection defocusing becomes
smaller as the distance of the reference plane of the main lens from
the crossover is smaller, if the distance of the screen from the
cathode and the distance of the deflection centre from the cathode are
kept constant.
Particularly in the case of electrostatic main lenses the advantage of
a construction in accordance with the invention is quite obvious.
It is known that deflection defocusing is understood to mean the
phenomenon that the diameter and the shape of the spot on the screen
vary with the angle of deflection.
The periphery of the aperture of the anode on the cathode side in a
circular-cylindrical anode is, of course, elliptical. If desired, the
slight field distortion thus produced may be compensated for example
by closing this aperture by means of a diaphragm having a circular
aperture.
The centre of this aperture may be so located outside the axis of the
electrodes preceding the anode but nearer that side of the anode which
is intersected by this axis, that after the deflection of the electron
beam by the magnetic transverse field the beam follows the axis of the
anode part remote from the cathode.
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