US2509763A - Electric discharge tube with directional electron beam - Google Patents

Description

May 30, 1950 2,509,763

J. DE GIER ELECTRIC DISCHARGE TUBE WITH DIRECTIONAL ELECTRON BEAM Filed March 16, 1948 Patented May 30, 1950 ELECTRIC DISCHARGE TUBE WITH DERECTIONAL ELECTRON BEAM Johannes dc Gier, Eindhoven, Netherlands, as.-

signor to Hartford National Bank and Trust Company, Hartford, Conn, as trustee Application March 16, 1948, Serial No. 15,141 In the Netherlands April 9, 1947' 4 Claims. 1

The invention relates to electric discharge tubes comprising an electrode system for producing a directional electron beam and in particular to tubes for reproduction of television images.

Electric charges produced on the wall of insulating material of a discharge tube comprising electrodes between which a great voltage difference at least 20,000 volts, is maintained during operation of the tube, lead to uneven voltage distributions along the wall, which are liable to provoke jump-over spark discharges and breakdowns of the wall. If these phenomena appear, the serviceability of the tube is materially reduced.

The space between the electrodes has in some cases been surrounded by a metal screen. This screen constitutes in certain cases part of the Wall and is connected to a point of voltage source located between the high-voltage terminals. In a further known construction the screen is enclosed between the wall and the electrodes and is connected to the anode. Due to the charge of these screens being conducted away, there is a considerable loss of electrons which, at least in part, are supplied by the cathode of the tube, so that the latter is more heavily charged.

With these known constructions the tube diameter must be considerably larger than the diameter of the electrodes, since due to the great voltage difierence cold emission may arise at the edges of the electrodes. In order to obviate the occurrence of emission phenomena, efiorts are made to work up the electrode surface with sufiicient accuracy to prevent unevennesses thereon, while constituting it generally by a metal of high Work function.

The invention has for its object to obviate with greater certainty undue occurrence of emission phenomena at electrodes so as to enable the tube diameter to be reduced. It is particularly useful for electron-ray tubes with directional electron beam, a magnet coil being arranged to surround the tube so as to concentrate the electron rays to form a beam of small sectional area, as is desired in the case of television tubes. In addition, magnet coils for producing deflecting fields may be arranged outside the tube. The smaller the tube diameter, the smaller is the energy required for the production of the fields by the magnet coils and the smaller may be the dimensions of the supply devices.

The invention relates toelectric discharge tubes with a directional electron beam in which the space intermediate two field electrodes insulated from one another for high voltage is surrounded by a wall of insulating substance, from which the field electrode adjacent the cathode is spaced by a short distance (not exceeding half the distance by which the field electrodes are spaced) The term field electrodes is to be understood to mean here those electrodes which circumscribe the electric field which the electrons have to traverse. According to the invention, the wall and at least the field electrode adjacent the cathode are separated by a metal screen which is insulated from the electrodes and extends from the wall into the space intermediate the field electrodes and terminates therein and which. screen intersects electron paths which would otherwise connect the wall to the field electrode.

The screen is preferably designed such that its section through a plane passing through the axis of the electron beam is curved, its convex side facing the wall and exhibits a curvature which increases the greater the distance away from the wall.

In order that the invention may be more clearly understood and readily carried into effect, it will now be described more fully with reference to the accompanying drawing, in which Figs. 1 and 2 show, by way of example, forms of discharge tubes comprising an electrode system to which the invention is applied.

The figures show the cylindrical part of an electron-ray tube for reproduction or transmission of television images, its glass wall I surrounding an electron-emitting cathode 2 and an accelerating anode 3. The cathode 2 is secured in the tube by means of an insulating ring 4, made, for example, of ceramic material, whereas the anode 3 is gripped in the tube by means of spring members 5. A voltage difference of from 20 to 30,000 v. is applied between cathode and anode.

The insulating ring s also serves for the arrangement of a controhelectrode 5, which is supplied with a voltage which may vary between cathode potential and a negative value of some hundreds of volts.

The. space between the control-electrode 6 and the wall is minimized so as to limit the tube diameter. Set up along the wall of the tube is a substantially uniform distribution of the anode voltage, which, while the tube is in use, is maintained between a conductive coating 1, which is applied to the wall and on which springs 5 for gripping the anode. 3 bear and the area at which the insulating ring engages the wall I. At this voltage distribution there prevails between the wall of the tube and the control-plate B at the between the field electrodes.

area at which it bends inwards a voltage difference which, owing to the small spacing, is found to be suificient for the dislodgement of electrons from the surface of the control-electrode. The electron emission is effected by the increased field strength at the edge of the control-cap 6 and is assisted in that small particles become detached from the cathode and the image screen and deposit on the electrodes. Under the action of the potential difference the electrons move towards the tube Wall and on collision with the wall more secondary electrons are generally dislodged than there are electrons attracted so that the wall becomes higher positive. The potential difference relative to the control-electrode is thus increased, which results in an increase in electron emission and a further increase in wall potential. This shift of potential along the tube wall due to the emission of secondary electrons is found to give rise to the occurrence of serious disturbances in the tube, such as break-down of the wall and internal back-firing, with the result that the tube becomes unserviceable.

The screen 8, provided in accordance with the invention; prevents electrons emitted from the surface of the control-electrode from reaching the glass wall I. It is, however, not sufficient for the electrons to reach the screen instead of impinging on the tube wall, since any production of secondary emission at the point of impact 'on the screen arranged to be insulated would also give rise to undue potential shift. The screen extends from the wall of the tube into the space between the field electrodes 3 and 6 and its shape is such that the secondary electrons cannot move directly according to the electric lines of force towards the other field electrode 3.

The potential of the screen 8 may be prevented from exceeding a definite value by forming the screen in the shape shown in Fig. 2, a small aperture only being left between the controlelectrode 6 and the screen 8. In addition, the tube shown in Fig. 2 comprises a second screen 9 which is inserted between the tube wall I and the field electrode 3 and extends into the space This screen retains secondary electrons which are dislodged from the wall of the tube if this wall is struck by electrons of difierent origin, in which case the drainage of these secondary electrons towards the anode would nevertheless result in potential shifts.

What I claim is:

1. An electron discharge tube comprising a tubular envelope portion of electrical insulating material, a cathode source of a beam of electrons within said portion, a first field electrode arranged in the path of the beam and spaced from said cathode source, a second field electrode interposed between and spaced from said cathode source and said first field electrode and from said tubular envelope portion, the spacing between said second field electrode and said envelope portion being smaller than one-half the spacing between said second field electrode and said first field electrode, and a metal screening member interposed between said second field electrode and said tubular portion and insulated from said field electrodes and said tubular portion, said metal screening member having electron opaque portions shielding said tubular portion from electrons from said cathode source and from said second field electrode.

-2. An electron discharge tube comprising a tubular envelope portion of electrical insulating material, a cathode source of a beam of electrons within said portion, a first field electrode arranged in the path of the beam and spaced from said cathode source, a cup shaped field electrode interposed between and spaced from said cathode source and said first field electrode and from said tubular envelope portion, the spacing between said cup shaped field electrode and said envelope portion being smaller than one-half the spacing between said cup shaped field electrode and said first field electrode, and a cup shaped metal screening member interposed between said cup shaped field electrode and said tubular portion and insulated from said field electrodes and said tubular portion, said cup shaped metal screening member having electron opaque portions shielding said tubular portion from electrons from said cathode source and from said cup shaped field electrode.

3. An electron discharge tube comprising a tubular envelope portion of electrical insulating material, a cathode source of a beam of electrons within said portion, a first field electrode arranged in the path of the beam and spaced from said cathode source, a cup shaped field electrode interposed between and spaced from said cathode source and said first field electrode and from said tubular envelope portion, the spacing between said cup shaped field electrode and said envelope portion being smaller than one-half the spacing between said cup shaped field electrode and said first field electrode, an insulating disc supporting said cathode and said cup shaped field electrode and insulating each from the other, and a cup shaped metal screening member interposed between said cup shaped field electrode and said tubular portion and supported on said insulating disc and insulated from said field electrodes and said tubular portion, said cup shaped metal screening member having electron opaque portions shielding said tubular portion from electrons from said cathode source and from said second field electrode.

l. An electron discharge tube comprising a tubular envelope portion of electrical insulating material, a cathode source of a beam. of electrons within said portion, a first field electrode arranged in the path of the beam and spaced from said cathode source, a cup shaped field electrode positioned between and spaced from said cathode source and said first field electrode and from said tubular envelope portion, the spacing between said cup shaped field electrode and said envelope portion being smaller than one-half the spacing between said cup shaped field electrode and said first field electrode, an insulating disc supporting said cathode and said cup shaped field electrode and insulating each from the other, a first cup shaped metal screening member positioned between said cup shaped field electrode and said tubular portion and supported on said insulating disc and insulated from said field electrodes and said tubular portion, said first cup shaped metal screening member having electron. opaque portions shielding said tubular portion from electrons from said cathode source and from said cup shaped field electrode, and a second metal screening member interposed between and spaced from said first metal screening member and said first field electrode, said second metal screening member having electron opaque portions shielding said first field electrode from electrons from said tubular portion.

JOHANNES DE GIER).

(References on following page) REFERENCES CITED Number The following references are of record in the 2'174-853 file of this \patent: UNITED STATES PATENTS 5 213551795 Number Name Date 1,927,807 Sabbath Sept. 19, 1933 1,979,992 Lubcke Nov. 6, 1934 Number 2,079,163 Gardner et a1 May 4, 1937 500,017

Name Date Bowie Oct. 3, 1939 Baier Apr. 2, 1940 Laico Nov. 24, 1942 Glass Aug. 15, 1944 FOREIGN PATENTS Country Date Great Britain Feb. 1, 1939

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