US2903612A - Positive ion trap gun - Google Patents

Description

Sept 8, 1959 D. D. VAN oRMER POSITIVE ION TRAP GUN Filed Sept. 16, 1954 rally IN VEN TOR. /l V/a. MM 06M?? @l lva POSITIVE ION VATRAP GUN David 'D. Vanrmer, Lancaster,`Pa.,"assignor to'Radio This-invention is directed to a cathode ray tube and iparticularly to an electron gun structure providing a positive ion trap for use in cathode ray tubes such as television picture tubes.

Normally, a cathode ray tube for television or other ypurposes is one having 'an envelope with a large bulb portion and a tubular neck portion xed thereto.

Mounted Within the tubular neck portion are a plurality of electrodes for forming and focusing an electron beam along a path extending into the bulbportion. Magnetic or electrostatic field producing means 'are used forfmovingthe electron beamin any desiredlmanner over a wall portion -or face plate of theenvelope bulb portion. Theinner surface of the wall or face plate is coated With a iilm of phosphor material which luminesces With a visible light when struck by the electron beam. By modulating the current of the electron beam the scanned area of the phosphor screen can be Varied in a manner to produce a light path in accordance with modulating signals applied to the electron gun of the tube.

y Tubes of this type normally are highly evacuated. However, it has been found to be impossible to remove every trace of4 gaseous material from the tube envelope when it is sealed. Also, during the normal operating life of the cathode ray tube, certain occluded gases in the envelope wall or in other portions of the Ytube structure are caused to be released into lthe discharge space of the tube. The electron beam is normally accelerated at high energies between thevcathode gun and thephosphor screen of the tube. The electron energies are sutlicient to ionize gas particles which pass into the beam path during tube operation by knocking electrons out of the gas molecules forming positively charged -gas ions. The accelerating fields within the gun structure of the tube act on the positively charged ion particles and urge them substantially along the electron beam path toward the negative end of the electron gun. The positive ions being accelerated to high energies will strike the cathode surface with sufficient energy to destroy the electron emitting properties of the cathode material on the electrode. Positive ion bombardment of the cathode surfaces in tubes of this type causes an early breakdown of the thermionic emission from the cathode surface. This deleterious effect on the cathode electrode greatly shortens the life of the cathode ray tube.

It is therefore an object of this invention to provide an electron gun for a cathode ray tube which includes means for preventing positive ion bombardment of the cathode surface.

It is another object of the invention to provide a novel electron gun'means which includes structure'for trapping .positive ions formedin the electron beam path.

It is a further object of this invention to provide an electron gun for television Vpicture tubes which includes -means for preventing positive ions in the beam path from striking the cathode.

y The `invention is va novel electron -gun structure in 'which the beam-forming electrodes of the gun are offset United States Patent O ice seffgsg from the normal axis to the phosphor screen surface or are otset from the undeected beam path directed at the phosphor screen. The beam-forming electrodes of the gun establish an electron ybeam which is accelerated toward the screen axis into a non-symmetric electron lens which bends the beam to align it W-ith the screen axis normal to the screen at its center. The positive ions which in the absence of the invention would bombard the cathode surface are only those which are formed in the beam path outside of the field-free regions of the tube. These positive ions will normally be different in energy than the energy of the electrons and the negative ions of the beam and upon passing back down the beam path toward the cathode electrode, these positive ions lbecause of their difference in energy will be bent out of the beam pathvby the electrostatic bending field and will be collected by portions of the beam-forming electrodes of the gun.

Figure l'is a sectional view of the cathode ray tube having a gun structure in accordance with the invention.

Figure 2 is a schematic representation of the beam path of the gun of Fig. l.

Figure 3 is a cross-sectional View of another form of the invention.

The cathode ray tube envelop of Fig. 1 comprises` a large bulb portion 10 and a tubular neck portion 12 which is substantially a cylinder of glass formed withits axis 1'4 coincident with the axis of the bulb portion 10. The end of the bulb portion 10 opposite to the tubular neck 12 consists of a glass Wall portion or face plate `16 upon which is formed a lm or coating 18 of finely rdivided phosphor particles, which forms a phosphor screen.

Mounted Within the tubular envelope portion 12 is an electron'gun 20 consisting of a cathode electrode22 mounted Within an enclosing tubular control grid electrode 24. Closely spaced from the control grid 24-and axially aligned therewith is a short cup-shaped tubular accelerating electrode 26. The electrodes 22, 24 and 26 are aligned on a common axis which makes a small angle with the axis 14 of the tubular neck 12.

Mounted` coaxially Within the tubular neck 12 is a second accelerating electrode 28 consisting of a tubular metal member closed at one end by a circular plate structure 30 having a centrally located aperture 32 therein. Electrodes 24 and 26 are closed by centrally apertued plate structures 34 and 36 respectively. The apertures of these plates 34 and 36 are aligned and are located on a common axis of the electrodes 24 and 26. The adjacent ends of the electrodes 26 and 28 are cutjat an angle both to the axis i electrode 28 and the common axis of electrodes 22,24, and 26. The centrally located aperture of plate 34, of control grid 2.4 overlies one end ofthe cathode 22 which is closed by a solid Wall portion, the outer surface of which is coated with an electron emitting material.

The apertured plate 30 of the electrode 2S is connected by spring fingers 38 to a conductive coating {4i} formed on lthe inner surface of the tubular neck 12 and which extends from the neck 12 into the bulb portion 10 to a point adjacent the 'fluorescent screen 18. The electrodes of the electron gun 2l) are mounted in their spaced position and are insulated from each other by ceramic vor'vg'lass support rods, which 'are not shown.

Operative potentials are applied to the several electrodes shown, by connecting these Velectrodes with conductive leads '42, sealed through a glass Wall portion 44 closing the end of the 'tubular neck 12. Leads 42 are coated cathode surface. Voltages are applied to the sevleral electrodes having values of the order of those indicated in Fig. l. These potential values are not limiting but are illustrative of voltages which have been used in tubes of the type shown in Fig. l and for the purposes to be described.

In operation, electrons emitted from the coated cathode surface are urged through the apertured grid plate 34- by the accelerating field of electrode 26 and are formed into an electron beam 50 directed along the common axis of electrodes 24 and 26. Potentials on electrodes 26 and 28 form an electron lens field therebetween. Because the adjacent ends of electrodes 26 and 28 lie in parallel planes tilted at an angle to the common axis of electrodes 24 and 26 and also at an angle to the axis 14 of electrode 28, the lens field formed between electrodes 26 and 28 is non-symmetric with respect to these two axes. Therefore, the electron beam 58 passing along the axis of electrodes 24 and 26 passes into this lens iield at an angle thereto and is bent toward the axis of the lens ield which may be considered substantially normal to the parallel planes in which lie the adjacent ends of electrodes 26 and 28. The potential difference between electrodes 26 and 28 can be adjusted to control the amount of deflection or bending which the non-symmetric lens eld gives to electron beam 50. During tube operation the electron beam is bent to align the beam on axis 14 and directed substantially normal to the fluorescent screen 18 at its center. The angle which the adjacent ends of electrodes 26 and 28 make with axis 14 may also be varied to provide the desired amount of beam bending for the voltage ranges used on electrodes 26 and 28.

ln a successfully operated tube of the type described, the adjacent ends of electrodes 26 and 28 are formed at an angle `of substantially ten degrees to axis 14. The electron beam in passing through the non-symmetric lens between electrodes 26 and 28 is given its iinal and greatest acceleration so that in passing down the axis 14 of electrode 28 the electron beam is moving with high energy into a field-free space which extends from a point within tubular electrode 28 to the screen, since electrode 28 and the conductive coating 46 are maintained at a common potential.

The electron beam 58 passing into the non-symmetrical eld between electrodes 26 and 28 is a divergent beam. The non-symmetrical eld, provides only a small convergence to the beam so that the beam striking apertured plate 30 is still somewhat divergent. A magnetic eld provided by a coil winding 52 is used to focus the electron beam to a small spot on fluorescent screen 16. Coil 52 is enclosed in an iron armature casing 54 to provide a concentrated focusing eld on the tube axis 14. The electron beam 50 is scanned over the surface of target screen 18 by deflecting magnetic iields formed by pairs of deiiecting coils mounted in a neck yoke 56 on the tubular neck portion 12. These deecting coils consist of pairs of coils connected in series to sources of sawtooth currents for providing respectively line and frame scansion of the electron beam. Both the focusing means 52 and the deecting means 56 are conventional and do not constitute a part of this invention. Accordingly, they are not described in greater detail.

The scansion of beam 50 of screen 18 may be in any well known manner such as to provide for example a rectangular raster normally used in television picture tubes. The intensity of electron beam 50 is normally varied in accordance with signal voltages applied to the control grid 24. Thus the beam 58 can be modulated in accordance with video signals applied to screen 24 to provide a charge pattern of light on the uorescent screen 18, such as a television picture.

In tubes of the type described and shown in Fig. l, it is desirable that the tube envelope be completely evacuated for optimum operation. Traces of gas within the tube envelope provide a source of positive ions when struck by the high energy electrons and negative ions of beam 58. Since these ions are positively charged they pass down the electron beam path 58 and are accelerated by the electrostatic ields along the path to strike the cathode electron emitting surface at high energies. Positive ion bombardment of the cathode surface causes a deterioration of the emitting characteristics of the coating and tend to greatly shorten the life of the discharge tube.

Therefore, in accordance with the invention, the cathode 22 is oiset from the tube axis 14 and a non-symmetric electrostatic lens is formed along the beam path between the cathode 22 and screen 18 and at the portion of the beam path at which the electron beam has its greatest energy.

Fig. 2 is an enlarged schematic representation of the electron beam path 50 extending from cathode 22 into the tubular electrode 28. In describing the invention it is only necessary to consider the effect of those positive ions formed along the electron beam path by the direct collision of electrons and negative ions of the electron beam with gas molecules present in the beam path, since it is only those positive ions formed by direct collision which will pass back down the beam path to the cathode. Positive ions formed by other than direct collision with electrons or negative ions will be deflected away from the beam path and will be collected by electrode structures of the gun 20.

A direct collision between a gas molecule and a negative beam particle will give the positive ion formed an initial motion along the beam path in the direction of the screen 18. Considering the positive ions formed Within the electrode cylinder 28 between the bending lens ield and screen 18, if an ion is formed by direct collision within electrode 28, at point A (Fig. 2) it may receive suicient energy to force the positive ion toward screen 18 and into the field-free space extending from within electrode 28 to screen 18. This positive ion is thus carried out of any accelerating eld and will not be drawn back along the beam path 50 to the cathode 22.

A positive ion formed by direct collision with a negative particle at point B Within electrode 28 will be first forced by the action of the collision along the beam path toward screen 18, but because of the accelerating negative fields between point B and the cathode 22, the energy of collision will be insuflicient to carry the positive ion into the ield free space, but instead will cause the ion to reverse the direction of its motion and to be accelerated back along the beam path 50. As the positive ion passes the point of collision, B, it will have gained the energy of formation and will be further accelerated to enter the deecting field between electrodes 26 and 28 at a higher energy than the energy of the electron beam passing through the deiiecting field in the opposite direction. That is, the positive ion will enter the deflecting ield with substantially all of the energy imparted to it at collision in addition to the energy given to the positive ion by the high accelerating field between electrodes 26 and 28. Since the deflection of a moving charged particle by an electric field is inversely proportional to the kinetic energy of that particle, positive ions passing into the deflecting field between electrodes 26 and 28 will not follow the electron path to the cathode surface, since due to their inherently greater energy they will be deiiected less than the electrons of the beam following the path 50. Thus, these high energy positive ions will not pass through the aperture of plate 36 but will be collected thereby and thus be prevented from striking the coated surface of cathode 22.

Positive ions formed by direct collision in the deecting region, between electrodes 26 and 28, at points C and D for example, and at other points with sufficient energy to carry them into this field, will be deected by the non-symmetric field oppositely to the electron beam and will be randomly scattered. Positive ions, however, formed between the cathode and the deecting field at E, for example, with insucient energy of formation to drive them to the deflecting field will be returned to the cathode. However, positive ions formed within this region (at E) are formed at low energies and since they are not accelerated to high energies due to the low potential diierence between electrode 26 and cathode 22, the positive ions in this region will strike the cathode surface at relatively low energy. To provide complete trapping of the positive ions Within this region between the cathode and the deecting field, it would be necessary to maintain the positive potential of electrode 26 below the ionizing energy of the gas molecules present, and thus prevent the negative ions or electrons of the beam, accelerated by electrode 26 in this region, from having an ionizing effect on gas molecules in the beam path.

An alternative structure Which would provide trapping of positive ions in the low potential field adjacent to the cathode electrode is schematically shown in Figure 3. 'Ihis figure discloses a neck portion 7i) of a cathode ray tube andv corresponding in structure to the neck portion 12 of Figure 1. Mounted within the neck 70, by any appropriate means (not shown), is an electron gun structure 71. The gun structure 71 consists of a conventional cathode electrode 72 insulatingly mounted on a ceramic disc 74 Within a tubular control grid electrode 76. Control grid 76 includes an end wall 75 spaced from one end of cathode 72 which is closed and coated with electron emitting material. Cathode 72 and control grid 76 are coaxially arranged along a common axis '73 making substantially a 45 angle with the axis 78 of the tubular neck portion 70. An accelerating electrode 8G includes a plate portion 82 overlying and spaced parallel to the end wall portion 75 of control electrode 76. Spaced from electrode 80 is a second accelerating tubular electrode 84 coaxially mounted on the axis '78 of the tubular neck 70.

Control grid end wall 75 and accelerating electrode plate 82 each have apertures therethrough substantially at their centers and overlying the coated end of cathode 72. The apertures in plate portions 75 and `82, however, are ofset from each other or staggered, the aperture of plate 75 being on axis 73 and the aperture in plate 82 being slightly displaced from axis 73 in a direction away from the axis 78. Electrode Sti may have a portion 86 extending from plate 82 to a point spaced from the'adjacent end of tubular electrode 84, substantially as shown.

To operate the gun structure of Figure 3, appropriate voltages are applied in the order of the values disclosed for corresponding electrodes of Figure l. Cathode 72 is heated by a lament (not shown) to a thermionic temperature. Electrons, are emitted from the heated cathode electrode 72 and are eccelerated by electrode plate 32 through the offset apertures in adjacent plates 75 and 82. Because of the offset arrangement of these apertures, the electrostatic field between electrode portions 75 and 82 form an asymmetric electron lens which bends the electrons away from axis 73 and along a path which coincides with axis 78, as the beam enters the accelerating electrode 84. The electron beam is thus directed down the tube axis 78 by the accelerating lens field as described above for Fig. l. The beam then may be focused and scanned over a fluorescent screen in the manner described above for the structure of Fig. l.

The gun structure of Fig. 3 provides a bending field which extends through the aperture of grid plate 75 into the region adjacent to the cathode electrode 72. This bending field includes portions of the accelerating field of plate 82 which are below the ionization energies of gases. The negative particles of the beam passing through the negative grid aperture of plate 75 are eccelerated only by the asymmetric iield of the off-set apertures. Positive ions formed in the region between plates 75 and 82 will thus be in an accelerating bending` field. These positive ions will normally have a diferent energy than the negative particles in the electron beam so that when they are urged toward the cathode 72, they will be bent' differently by the asymmetric eld and will pass out' of the electron path to be captured by adjacent electrode portions. in the bending field between plate 82 and electrode 84,v will' also be deflected oli of the electron path as described' above for the structure of Fig. l. Thus, all points of the beam path between cathode 72 and the final accelerating electrode 84, and where the negative particles of the beam reach ionizing energies, will be` within the beam bending field. The structure then of Fig. 3 discloses a means for a complete trapping of all positive ions formedbetween the cathode and the phosphor screen of the cathode ray tube.

As pointedout above, the positive ions formed along, the electron beam path are trapped by the structure disclosed. The elimination of these positive ions eliminates the deleterious ion'bombardment of the cathode surface, land thus results in a longer cathode life to extend the operational life of the cathode ray tube. The trapping ofthe positive ions iseiectively done by a non-symmetric electrostatic beam bending field between the cathode electrode and the point of formation of the positive ions.

What is claimed is:

1. An electron gun for forming a beam of negative particles along a path, said electron gun including an' accelerating electrode aligned on an axis along said beam path, a' cathode electrode spaced from said accelerating electrode and offset in one direction from said axis for providingV an electron emission, and tubular electrode means along said beam path between said cathode and accelerating electrode for providing an electrostatic eld lens for bending said beam in said one direction to align said beam with said axis.

2. An electron gun for forming a beam of negative particles along a path, said electron gun including a first accelerating electrode aligned on an axis along said beam path, a cathode electrode spaced from said accelerating electrode and offsetV in one direction from said axis, and a tubular second accelerating electrode adjacent to said first accelerating electrode and between said first ac; celerating electrode and said cathode electrode, said secondaccelerating electrode being axially aligned with said cathode Aelectrode and'oiset in said one direction from said axis for providing an electrostatic field lens for bending said beam in said one direction to align said negative beam withk said axis. v

3. An electron gun for forming a beam of negative particles along'a path, said electron gun including a first accelerating electrode aligned on an axis along said beam path, a cathode electrode spaced from said accelerating electrode and offset in one direction from said axis, and a tubular second accelerating electrode mounted adjacent to said first accelerating electrode and between said first accelerating electrode and said cathode electrode, said tubular second accelerating electrode being axially aligned with said cathode electrode and offset in said one direction from said axis, the adjacent ends of said first and second accelerating electrodes lying in parallel planes disposedl at an angle to said axis for bending said beam in said one direction to align said beam along said axis.

4. An electron gun for forming a beam of negative particles along a path, said electron gun including an accelerating electrode aligned on an axis along said beam path, a cathode electrode spaced from said accelerating electrode and offset in one direction from said axis, and electrode means along said beam path between said cathode and accelerating electrodes for providing an electrostatic field for bending said beam in said one direction to align said negative beam with said axis, said electrode Also, positive ions formed means including a pair of parallel plates each having an aperture therethrough, said apertures being oifset from each other relative to said axis.

5. An electron discharge device comprising an electron gun for forming a beam of negative particles along a path, said electron gun including an accelerating electrode aligned on an axis along said beam path, a cathode electrode spaced from said accelerating electrode and offset in one direction from said axis, and electrode means along said beam path between said cathode and accelerating electrodes for providing an electrostatic eld aligning said negative beam with said axis, said elect-rode means including a pair of electrode members each having an aperture therethrough, said apertures being positioned in line with a portion of said cathode emitting surface and offset from each other relative to said axis for bending said negative beam in said one direction to align said beam along said axis.

6. A cathode ray tube comprising an envelope having a tubular portion, a fluorescent screen within said envelope, an electron gun structure spaced from said screen and including a cathode electrode within said tubular envelope portion for producing a beam of electrons along a path offset in one direction from the axis of the tubular envelope portion wherein positively charged ions may be present in said electron beam, means between said cathode electrode and said screen for providing an eletrostatic field for bending said electron beam in said one direction to align said electron beam with said tubular envelope axis.

7. A cathode ray tube comprising an envelope having a tubular portion, an electron gun structure including a cathode electrode within said tubular envelope portion for producing a beam of electrons along a path offset in one direction from the axis of the tubular envelope portion wherein positively charged ions may be present in said beam, said electron gun structure including as a part thereof means for forming a non-symmetric electrostatic field in the path of said beam for bending said electron beam in said one direction to align said beam with said tubular envelope axis.

8. A cathode ray tube comprising an envelope having a tubular portion, an electron gun structure including a cathode electrode within said tubular envelope portion and oifset in one direction from the axis thereof for producing a beam of electrons along a path wherein positively charged ions may be present in said beam, said electron gun structure including as a part thereof means for forming a non-symmetric electrostatic iield in the path of said beam for bending said beam in said one direction to align said beam with said tubular envelope axis, said means including a tubular electrode coaxially mounted Within said tubular envelope portion and a second electrode adjacent to said tubular electrode and mounted between said tubular electrode and said cathode electrode, adjacent portions of said tubular and second electrodes lying in parallel planes tilted in said one direction and at an angle to said axis, said electron gun structure including impermeable structure closely spaced from said beam path for collecting positive ions directed toward said cathode.

9. A cathode ray tube including an electron gun for forming a beam of negatively charged particles in which positive ions may be present, said electron gun including 'a cathode electrode having an electron emitting surface and a pair of electrodes each having an aperture overlying said cathode surface, an accelerating electrode having a centrally disposed aperture therethrough and mounted on an axis through said centrally disposed aperture, said pair of electrodes being iixed so that a straight line from said cathode emitting surface passes through the apertures of said pair of electrodes at an angle to said axis, the apertures of said pair of electrodes being otset from each other relative to said straight line.

l0. The method of separating positive ions from the negative particles of a cathode ray beam Within a cathode ray tube including a cathode electrode and a fluorescent screen, said method comprising the steps of forming the negative particle beam along a path offset from a second path directed at the phosphor screen, using an electrostatic lens field non-symmetrically disposed with said beam path to align said negative particle beam with said second beam path, collecting the positive ions which are separated from the negative particle beam by the nonsymmetrical electrostatic lens before the positive ions strike the cathode.

l1. The method of separating positive ions from the negative particles of a cathode ray beam within a cathode ray tube including a cathode electrode and a fluorescent screen, said method comprising the steps of forming electrous from said cathode into a beam which may contain other negative particles and which is displaced from the axis substantially normal to said fluorescent screen at its center, directing said negative beam along a path toward said axis through an electrostatic lens eld asymmetrieally disposed relative to said beam path and said axis, adjusting the strength of said asymmetric eld to align said negative beam with said axis.

12. The method of separating positive ions from the negative particles of a cathode ray beam within a cathode ray tube including a cathode electrode and a fluorescent screen, said method comprising the steps of, forming electrons from said cathode into a beam which may contain other negative particles and which is displaced from the axis substantially normal to said fluorescent screen at its center, directing said negative beam along a path toward said axis through an electrostatic lens field asymmetrically disposed relative to said beam path and said axis, adjusting the strength of said asymmetric iield to align said negative beam with said axis, collecting positive ions separated by the asymmetric lens from the negative particle beam before the positive ions strike the cathode electrode.

References Cited in the le of this patent UNITED STATES PATENTS 2,289,319 Strobel July 7, 1942 2,348,133 larns May 2, 1944 2,413,276 Wold Dec. 24, 1946 2,464,562 Diemer Mar. 15, 1949 2,562,242 Pohle July 31, 1951 2,562,243 Pohle et al. July 31, 1951 2,604,599 Breeden July 22, 1952 2,617,060 De Gier Nov.'4, 1952 2,658,160 Peterman Nov. 3, 1953 2,658,161 De Ano Nov. 3, 1953 2,680,204 Swedlund June 1, 1954 2,719,243 Hoagland Sept. 27, 1955 2,727,171 De Gier Dec. 13, 1955 2,732,511 Dichter Jan. 24, 1956

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