US2810091A - Ion trap - Google Patents

Description

M. D. HARSH Oct. 15, 1957 10N TRAP Filed' March s1, 1954 N am wm V L ww NN NN QN Mm.

TTORNEY United States Patent O ION TRAP Maurice D. Harsh, Nefsville, Pa., assignor to-Radio Corporation of America, a corporation of Deiaware Application March 31, 1954, Serial No. 419,965

6 Claims. (Cl. 315-8) This invention relates to a device for re-centering the electron beam in a cathode-ray tube after the entire beam has been 'deflected to trap the negative ions in the beam.

In cathode-ray tubes using electromagnetic deflection, the negative ions present in the beam are essentially unaffected by the magnetic deflection field. These negative ions maintain an unwavering bombardment of the fluorescent screen in one small area, thus causing a permanent darkening or discoloration on the screen called an ion spot. These ion spots may be eliminated by an ion trap which consists of an electrostatic lens field tilted to the longitudinal axis of the electron gun, and two magnetic fields that are transverse to the same longitudinal axis. Th-e electrostatic lens field deflects the entire beam from the axis of the electron gun. One magnetic field defiects the electron portion of the beam back toward the axis of the cathode-ray tube. The second magnetic field, opposite in direction to the first magnetic field, bends the electron beam into approximate alignment with the axis of the cathode-ray tube. But since the negative ions are essentially unaffected by the magnetic fields, they continue their travel on the ofi-axis deflection path, and eventually strike an electrode that collects such ions.

Despite the two magnetic fields provided in the ion trap, it is still difficult to re-center accurately the deflected electron beam on the axis of the electron gun. This difficulty remains because it has not been possible to produce magnetic fields that are narrow and sharp enough to compensate exactly for the deflection of the electrons introduced by the tilted electrostatic lens. The two magnetic ion trap fields also tend to distort the electron beam.

An object of this invention is to improve the centering of the electrostatically deflected electron beam in a cathode-ray tube containing an ion trap.

Another object of this invention is to reduce the magnetic distortion of the electron beam in the region between the cathode and the tilted electrostatic lens in the cathode ray tube.

Another object of this invention is to provide narrow magnetic ion tr-ap fields that produce more accurate compensation for the Ideflection of the electron beam caused by the tilted electrostatic lens in a cathode-ray tube ion trap arrangement.

Anther object of this invention is to provide accurate centering of the electrostatically deflected electron beam in a cathode-ray tube 'containing an ion trap, by means that are inexpensive and simple.

To provide a more accurate and sharply defined magnetic ion trap field in the cathode-ray tube ion trap, this invention shields the control grid of' the electron gun from the magnetic ion trap field. This shielding may be accomplished by placing a ferro-magnetic material, such as iron, near the control grid, and between the control grid and the magnetic field. This shielding isolates a portion of the beam path enclosed by the control grid r 2,810,091 Patented Oct. 15, 1957 ICC from the magnetic ion trap field, thus effectively narrowing the magnetic field as shown in the following drawings, in which:

Fig. l is a longitudinal sectional view of a cathode ray tube showing the beam paths in an ion trap not utilizing this invention. v

Fig. 2 is a longitudinal sectional view of a portion of a cathode-ray tube showing the beam paths in an ion trap utilizing one embodiment of this invention.

Figs. 3, 4, and 5 are longitudinal sectional views of other embodiments of the magnetic shield.

Referring first to Fig. l, a typical cathode-ray tube is indicated by 10. The tube 10 is evacuated and has a glass neck 11 closed at one end, and a diverging glass or metal bulb 12 attached to the opposite end of the neck 11. The bulb 12 is closed by a glass face plate 13 that is coated on its inner surface with a fluorescent screen 15. Theneck 11 contains the electrongun structure which, in its usual form, comprises an electron emitting cathode 16, a heater filament 18 contained within said cathode, a cylindrical control grid 2f) having a centrally apertured disk 21 at its end toward the glass plate 13, a cylindrical first anode 22 having a centrally apertured disk 23 at its end next to the control grid 2f), a cylindrical second anode 24 having a centrally apertured disk 25 at its end toward the glass plate 13, and a third anode 26 formed by coating the inner surface of a portion of the tube neck 11 and bulb 12 with an electrically conducting material such as aquadag. This third anode 26 may or may not be electrically connected to the second anode 24, depending on the exact purpose and use of the cathoderay tube. The control grid 2f) and disk 21, the first anode 22 and disk 23, and the second anode 24 and disk 25 are all made of a nonmagnetic, metallic material such as stainless steel, and are physically spaced so that the longitudinal axis of each coincides with the longitudinal axis 40 of the electron gun. An electromagnetic deflecting yoke 28 encircles the neck 11. ln practice, this yoke 28 includes a pair of horizontal deflecting coils and a pair of vertical deflecting coils. However, since these coils form no essential part of this invention per se and yoke 23 is merely indicated.

The cathode-ray tube, as shown in Fig. l and las generally used, has the usual potentials applied to the various components of the electron-gun structure so as to project the electrons emitted from the cathode toward the screen Y15. The electrostatic lens 36, formedl between the first anode 22 and the second anode 24k has its axis of symmetry tilted about the axis 4f) of the cathode-ray tube gun. The two magnetic ion trap fields, which are in the region surrounding the electrostatic lens 3i), and which transversely deflect the beam of electrons in opposite directions are not shown, but their resultant lines of force `are represented by the lines 32 and 34 which are terminated by arrows indicating the directions of the forces.

In Fig. l, the beam 50 formed by the electron gun contains both electrons and negative ions. The electrons of the composite beam are deflected in the direction indicated by the line of for-ce 32 of one magnetic ion trap field so that they follow path S2. The ions continue along their original path 50 until they enter the region of the electrostatic lens 3f). This lens deflects the negative ions so that they follow path 54 and strike the second anode 24 which collects them. This lens 3f) also deflects the beam of electrons so that it follows path 56 back toward the electron-gun axis 40. Under the resultant influence of the electrostatic lens 30 and the forces 32 and 34 of the magnetic ion trap fields, the electrons of the beam finally proceed along path 58.

It is desirable that the last deflected path SS of the electron beam coincides as nearly as possible with the axis 40 of the cathode-ray tube. However, because of the unrestricted magnetic fields previously employed to redeliect the electron beam, it has beendiicult to recenter accurately the electron beam so that it coincides a nonmagnetic Ametallic cylinder and, an apertured disk 21 in accordance with this invention lmade of a ferromagnetic material such as iron.` Because of its high permeability, the ferromagnetic disk '217 shields the space within control grid`20 lfrom magnetic fields, thus narrowing and sharply defining the magnetic ion trap fields 32 and 34., As a result of this narrowing action on the magnetic fields, lthe electron beam isnot deflected by the magnetic fields 32 until itreaches a point 51 outside the shielding effect of the apertureddisk V21. Under the resultant inuenceof Athe electrostatic lens 30 and the forces 32 and 34 ofthe magnetic ion trap fields, the electron beam follows paths S2', 5'6 and finally 58. As before, the ions follow path 54 and are collected bythe second anode 24. Not only does the ferromagnetic disk 21' cause path 58 to coincide more closely with the cathode-'ray tube axis 40, but it'rednces the distortion of the electron beam. vThese advantages are accomplished because the shielding action of the disk 21" restricts the magnetic fields, thus allowing them to compensate more precisely for the electrostatic deflection without distorting the electron beam any more than necessary.

Another embodiment of 4this invention is shown in Fig. 3, wherein only the control grid portion of the electron-gun structure is shown. Instead of the control grid 20 and disk 21 both being made of a nonmagnetic metallic material, the cylinder 20 is made of a ferromagnetic material such as iron. This magnetic material shields the control grid in the same manner as did the magnetic apertured disk 21 in Fig.'2, thereby re-centering the electron beam more accurately and introducing less distortion to it.

Figs. 4A and 4B show `-diierent constructions of another embodiment 'of this invention. Instead of replacing the usual nonmagnetic, metallic, apcrtured disk 2'1 in Fig. 4A, or external of the control grid, as shown in r Fig. 4B. In either case, it shields the control grid in the same `way as did the ferromagnetic `apertured disk 21' shown in Fig. 2,`to give the same advantages.

Figs. 5A and 5B show different forms of another embodiment of the invention. Instead of replacing the usual nonmagnctic metallic cylinder 20 with a ferromagnetic cylinder 20 as shown in Fig. 3, a ferromagnetic collar 20 is positioned adjacent to the cylindrical portion 2G of the control grid either outside the cylindrical portion, as shown in Fig. 5A,"or within the cylindrical portion, as shown in Fig. 5B. The ferromagnetic collar 20 also shields the control grid 20 from the magnetic fields as did the ferromagnetic `cylindrical grid as shown in Fig. 3, thus restricting the magnetic field to give more accurate beam re-centering with less beam distortion.

What is claimed is:

1. An electron gun for a cathode ray tube adapted for use with an external magnetic field, said gun comprising an electron source and a plurality of electrodes spaced therefrom for forming an electron beam along a path, one of said electrodes including a wall portion for shielding said beam path from said magnetic field.

2. An electron gun for a cathode ,ray tube adapted for use with an external magnetic field, said gun comprising an electron source, a control electrode insulatingly spaced from said electron source, and an additional electrode insulatingly spaced from said control electrode to form electrons from said source into -a beam, said control electrode including a ferromagnetic wall portion for shielding said electron source from said magnetic field.

3. The Vinvention of claim 2, wherein said ferromagnetic wall portion includes an apertured member adjacent to said Velectron source.

4. The invention .of claim 2, wherein said ferromag netic wall portion includes a tubular wall portion enclosing said electron source.

5. `An electron Vgun for-a cathoderay tube adapted for use with an Aexternal 4magnetic field, said Ygun comprising an electron source, a control electrode insulatingly spaced from said electron source, and an additional electrode insulatingly spaced from said .control electrode to form electrons from said source into a beam, and a ferro magnetic wall kportion surrounding said electron source for magnetically shielding said source from said magnetic field.

6. An electron gun for a cathode ray tube adapted for use with an external magnetic field, said gun comprising an electronsource, a control electrode insulatingly spaced from said electron source, and an additional electrode insulatingly spaced from said control electrode to form electrons from said source into a beam along a path, and an apertured ferromagnetic wall portion spaced from said electron source along said beam path for shielding said source from said magnetic field.

References Citedin the le of this patent UNITED 'STATES PATENTS 2,522,872. Heppner Sept. 19, 1950

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