Ion trap type electron gun

Jan. 15, 1952

C. S. SZEGHO

ION TRAP TYPE ELECTRON GUN

Filed Sept. 29, 1950

2,582,402

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Patented Jan. 15, 1952

2^582,492

UNITED STATES PATENT OFFICE

2,582,402

ION TRAP TYPE ELECTRON GUN

Constantin S. Szegho, Chicago, Ill., assignor to The Rauland Corporation, a corporation of Illinois

Application September 29,1950, Serial No. 187,432

Claims.

This invention relates to cathode-ray tubes suitable for use in television receivers and pertains, more particularly, to an improved electrongun structure for such tubes.

In cathode-ray tubes utilized in present-day television receivers, an electron beam is developed by an electron gun mounted at one end of the tube, and is directed thereby to a fluorescent screen disposed at the opposite end. In many instances the electron beam in such tubes is deflected over the screen area by means of an electro-magnetic yoke which responds to periodic line- and field-scanning signals. When magnetic deflection is utilized a dark spot sometimes appears on the screen after the tube has been in operation for an appreciable time. This spot is caused by the bombardment of negative ions which appear to originate in the vicinity of the cathode of the electron gun and which have a mass that is materially greater than that of the electrons.

The accelerating fields of the cathode-ray tube accelerate the negative ions along with the electrons constituting the cathode-ray beam although the negative ions travel at a slower rate than the electrons due to their greater mass. It is well-known that the force exerted on a charged particle by a magnetic field is a function of velocity as well as the mass of this particle. For this reason, the electrons of the cathode-ray beam are readily deflected over the entire screen area of the cathode-ray tube by the magnetic field set up therein by the deflection yoke, but the negative ions are not affected by this field to any appreciable extent. Therefore, the ions impinge upon a very limited portion of the screen area, usually in the immediate vicinity of the undeflected beam, and after the tube has been in use for some time, this impingement causes contamination of the screen and gives rise to a socalled ion spot” at the point of impingement.

Several arrangements have been proposed for preventing the harmful ions from reaching the screen of the cathode-ray tube. These arrangements usually utilize the combined effects of electro-static and magnetic fields to trap the ions. An electro-static field is used to deflect the mixed beam of electrons and ions away from the fluorescent screen and towards a trapping surface, while a magnetic field is used to return solely the electrons of the beam to a path directed to the fluorescent screen.

Many ion-trap arrangements presently known to the art require the assembling of parts that are asymmetrically shaped and of cumbersome configuration. This adds materially to the difficulty and cost of construction, and renders it difficult to attain the required high degree of alignment of these components. Other arrange(Cl. 313—85) ments require off set apertures in the various components of the electron gun, but the necessary working tolerances made the commercial use of such arrangements infeasible. Yet other arS rangements require peculiar configurations of certain components of the gun which renders the construction and assemblage thereof unduly costly and complicated.

It is an object of the present invention to pro10 vide an improved ion-trap of electron gun structure which utilizes standard symmetrical component parts well-known to the art, which overcomes the aforementioned disadvantages of the prior art, and which may be constructed conven15 iently and economically.

The features of this invention which are believed to be new are set forth with particularity in the appended claims. The invention itself, however, together with further objects and ad20 vantages thereof may best be understood by reference to the following description when taken in conjunction with the accompanying drawing, in which:

Figures 1-3 show various embodiments of the 25 invention.

Referring now to Figure 1, a section of the neck portion 10 of a cathode-ray tflbe is illustrated in which, the electron gun of the present invention may be mounted. Neck to may have 30 the customary base 11 affixed thereto to accommodate a plurality of pins 12 extending therethrough and electrically connected to various elements of the electron gun by way of terminal connections 12a to establish these elements at ₃₅ desired operating potentials. The electron gun includes a tubular control electrode or first grid 14 supported from tube base 11 in any convenient manner and carrying radially extending lugs 15 which are suitably secured to a plurality of lon4Q gitudinally disposed support rods 13. The control electrode has a centrally located aperture 16 in its upper surface through which the cathoderay beam passes. A usual cathode structure Π is mounted within electrode 14 with its upper ac45 tivated surface aligned with aperture 16 and spaced therefrom by a fixed axial distance determined in any well-known maimer.

A tubular electrode or second grid 18 is secured to and supported by rods 13 by means of 50 lugs 19 and is positioned in coaxial alignment with the first grid 14. Electrode 18 has a central aperture 20 in its lower transverse surface which is aligned with aperture 16. A tubular anode electrode 21 is supported in coaxial align55 ment with electrode 18 by means of lugs 22 which likewise are attached to rods 13. Electrode 21 has a transverse wall 23 closing its upper end and having a central aperture 24 formed therein. The electrode 21 may be established at a desired op2,683,403 erating potential by means of a terminal connection 25 which leads, in well-known manner, to the internal conductive surface of the tube (not shown!.

In accordance with one embodiment of the invention a wire or conductive strip 26 is supported by and electrically connected to the inner peripheral surface of electrode 18 and projects toward anode electrode 21 so as to be exposed to at least a portion of the latter electrode across the axis of the cathode-ray tube. A potential difference is established between electrodes 18 and 2i through the terminal connections and sets up an eleptro-static accelerating field therebetween. The presence of conductive strip 26 established a£ the potential of electrode 18 distorts this electro-static field and establishes a component thereof between the strip and anode electrode 21 which is transverse to the common axis of the electrodes. A tubular member 27 is mounted on the external surface of neck (0 and supports a pair of spaced permanent magnetic rings 28, 29. This member may be moved longitudinally or rotated on the neck surface for reasons to be described.

In utilizing the electron gun of the present invention, anode 21 is established at a fixed positive potential that is large relative to the potential of electrode 18 to establish an electro-static accelerating field therebetween. Cathode ί 7 emits electrons which may be formed into a mixed beam of ions and electrons by suitable external magnetic focusing means (not shown), the mixed beam passing along the common axis of electrodes 14, 18 through aperture 20 in tubular electrode IB. The transverse component of the electro-static field between electrodes 18,21 produced by strip 26 tends to cause the mixed beam to enter the anode electrode along a path 30 directed away from aperture 24 in the transverse wall 23 and towards the internal peripheral surface of the anode. Under these conditions, neither the electrons nor ions are directed to aperture 24 in transverse wall 23.

The permanent magnet ring 28 is oriented to produce a magnetic field within neck i 0 to direct solely the electrons of the mixed beam to the common axis of the electrodes and through aperture 24 to the fluorescent screen (not shown) along a path indicated by broken construction line 31. It is preferable in this type of construction to include a second magnetic ring 29 oriented to produce a magnetic field in the opposite sense to the field of magnet 28 to compensate for any over shooting effect of the field of magnet 27 and to insure that the electron beam emerging from aperture 24 is directed along the common axis of the electrodes. Since tubular member 27 may be rotated about and moved longitudinally along neck 10, the fields of magnets 28 and 29 may be adjusted until the desired deflection of the electron beam through aperture 24 is obtained.

It is apparent that similar results may be obtained when the conductive strip 26 is supported by and electrically connected to anode electrode 21 instead of electrode 18 as in the illustrated embodiment in Figure 1.

The embodiment of Figure 2 differs from the embodiment of Figure 1 only in that a pair of conductive strips 26a and 26b are utilized to establish the transverse component of the electro-static field, these strips being connected to and supported by anode electrode 21 and electrode 18 respectively.

In accordance with the preferred embodiment of the invention, illustrated in Figure 3, a relatively small deflection electrode 32 is secured to the inside surface of the end-wall portion of electrode 18 and exposed to a portion of electrode 21 across the common axis of the electrodes. Electrode 32 is in the form of an L-shaped plate and is mounted in close proximity to aperture

20. It has been found that the transverse component of the electro-static field between electrode IB and electrode 21 set up by plate 32 has sufficient strength to deflect away from aperture 24, the mixed beam of ions and electrons emerging from aperture 20. Moreover, the strength of this transverse component of the electro-static field is not excessive and allows for convenient deflection by magnets 28, 29 of the electron beam, so that it is directed along path 3i towards aperture 24.

The invention provides, therefore, an improved ion-trap type of electron-gun structure for use in cathode-ray tubes, which operates to prevent ions from reaching and contaminating the fluorescent screen thereof. As shown in the drawing, the improved gun is composed of standard symmetrical components that may be conveniently and economically assembled by any well-known fabrication process; ion trapping being accomplished by the simple expedient of securing a small conductive strip to at least one of the electrodes. There are no requirements in the gun structure of the present invention for components of irregular shapes and sizes, or for eccentrically positioned apertures.

While particular embodiments of the invention have been shown and described, modifications may be made and it is intended in the appended claims to cover all such modifications as may fall within the true spirit and scope of the invention.