US2935636A

US2935636A - Electron gun structure

Info

Publication number: US2935636A
Application number: US543680A
Authority: US
Inventors: Ronald C Knechtli
Original assignee: RCA Corp
Current assignee: RCA Corp
Priority date: 1955-10-31
Filing date: 1955-10-31
Publication date: 1960-05-03
Anticipated expiration: 1977-05-03

Description

- y 1969 R. c. KNECHTLI 2,935,636

ELECTRON cum STRUCTURE Filed Oct. 51, 1955 2 Sheets-Sheet 2 +zzv. 1 2w *410V. 752m HHHHH Hun IH/I/HHI/HHHHH1, HUM/,1, 1,11 H,

IN VEN TOR. RmNHu: [lKNEEHTLI IZTOR/VEY 2,935,636 ELECTRON STRUCTURE Ronald c. Knechtli, Cranbury, Nit, assignor ti) Radio Corporation of America, a corporation of Delaware s Claims. (Cl. 313-82) This invention relates to electron guns for cathode ray tubes, and it has for one of its objects the provision of a novel and improved high-voltage object defining apertureelectron gun for cathode ray tubes. 3

-Another object of .the invention is to minimize the beam spread between object and final lens of an electron gun h In applications where a small beam spot of constant size and of shape, possibly difierent from the round one, is required ,on the phosphor screen of a cathode ray tube or kinescope, the normal type of electron gun is not suit- .able. The beam spot obtained therefrom is round, of

variable size, depending on the current. Furthermore, the. spot edges are not sharp. In high-voltage object de- 7 .fining aperture guns, the spot is practically ofconstant {it is necessary to pass this current through the Object :fining aperture with the lowest possible value of the prodr uct P= /V .sin 0, where V =aperture voltage and c =angle of the outermost electron trajectory with the :axis of the gun at the object defining aperture. Hence, .the product x/V sin 0, for a given current can be conrsidered as a quality factor of the gun, which has to be minimized.

In one of the most efficient types of high-voltage object defining aperture electron guns heretofore proposed, the above mentioned product P equals 2.1 for a 1 ma. beam current passing through an object defining aperture of 4 x 15 mils. 'For currents of the order of 1 ma. or more, angular spread of the beam after passing the aperture is "or theorder of 3.5", at an aperture voltage of 1.5 kv.

The present invention provides an improved high-voltage object defining aperture gun in 'which the product P referred to above is reduced to P=1.6 to 1.3 or better at the same current of 1 ma., and in which the angle of beam spread is decreased to a minimum. This gun, hereinafter described in detail, comprises a crossover-forming system producing a true crossover in which the crosssection of the beam is decreased to the minimum possible dimensions; an electron optical lens system whose focus coincides or nearly coincides with said crossover; an object defining aperture flooded by the beam of electrons target surface of the screen. 7 c I The invention is described more in detail, in connection with the accompanying drawings, in which:

Fig. l is a longitudinal sectional view through a portion of a kinescope, illustrating diagrammatically the gen- United se s; p ht;

' immersion lens crossover forming system;

Fig. 3 is a similar view of a modified electron gun with l emerging from said lens system; and an electron optical" system imaging said object defining aperture onto the important.

eral structure and principles of operation of 'a higl'r-volt age object defining aperture electron gun embodying the I a laminar flow crossover system; and

Fig. 4 is a longitudinal sectional view of a test *tubej illustrating another embodiment of the invention.

The cathode ray tube or kinescope illustrated by way of example in Fig. 1 comprises an evacuated envelope 1 having a main chamber 2 in the form of a frustum which terminates at its large end in a window 4 which may constitute the transparent foundation plate of the conventional target or screen 12' of the kincscope conf taining a multiplicity of phosphor dots or other areas arranged in a manner well understood in the art.

The small end of the frustum 2 terminates in a tubular neck 3 which contains the electron gun forming thesubject matter of the present invention and comprising: a crossover-forming system A producing atrue crossover F in which the cross-section of the electron beam is decreased to the minimum possible dimensions; and electron optical lens system B whose focus coincides or nearly coincides with F; an object defining aperture C flooded by the beam of electrons emerging from the lens system B; and an electron optical system D imaging said object defining aperture onto the screen.

The crossover F formed by the crossover-forming system A is considered in first approximations as a point source of electrons for the lens systemB. In setting the focus of the lens system B on this point source, the electrons emerging from this lens system will flow. in first approximation in a parallel flow, as a consequence of the fundamental properties of lenses. The distance between the first principal plane of the lens system B and the crossover (equal to or nearly equal to the first focal length of the lens system B) is chosen such that the diameter of the beam emerging from this lens system is not appreciably larger than the largest dimension of the object defining aperture C following" it. Thus, the object defining aperture C is flooded by a parallei-flowing (essentially non-divergent) electron beam,la's illustrated in Fig. 1., For a round objectdefiningaperture, the transmission efliciency a (ratio of total beam current to transmitted current) will be close to Biro! a rectangular object defining aperture,,whereaperture'C 7 would have a width a and a length b, and with b a, one has in first approximation:

below a definite limit because of initialthermal electron- V velocities; and (2) aberrations in the lens system B.-

As the crossover voltage can be raised without dilli-v culty up to several kv. (this crossover can vwellbe-aFt a higher potential than the object defining aperture the first of the above-mentioned limitations is nottoo As for the second limitation,-byproper. design (e.g. several weak lenses in series) the aberrations in the lens system'B can be reducedto-a tolerable value. As a practical order of magnitude, it is possible for an witha n.

optimized design and with a rectangular object aperture of 4 x 15 mils at a voltage of 1.5 kv. (which may be lower than the crossover voltage) to obtain a current of 2 ma. through the object defining aperture C with an angle of beam spread 0,, 3. This corresponds to a product P= V sin 0,,,,' 2. Reduced to 1 ma. eflective beam current, this becomes P 1.3. This takes into account thermal limitations (finite size of crossover), beam spread due to space-charge, and lens aberrations. No secondary electron emission occurs in such electron gun to disturb the electron optical imaging of the aperture C onto the screen of the tube.

The crossover-forming system A producing the crossover F may be of the immersion lens type illustrated in Fig. 2. This immersion lens crossover-forming system'is designed according to standard practice; a typical design procedure for said system may be found in a paper by M. Polke, entitled Elementary Theory of Production of Electron Beams with Triode Systems, in Zeitschrift fiir Angewandte Physik, December 1951 and January 1952 (vols. 3 and 4). As the crossover voltage in the system of Fig. 2 normally does not exceed a few hundred volts, the electron optical system B can consist of one or a combination of bipotential lenses. Because of the short axial distances involved, aperture lenses are usually preferred. The cross-over point forming system of Fig. 2 comprises the cathode, a negative control grid 6; adjacent the gun cathode, and a high voltage screen grid G The lens system corresponding to B of Fig. 1 consists of the same high voltage screen G a low voltage electrode 6;, and a high voltage electrode G In the embodiment illustrated, the crossover-forming system A is designed to Work with the electrode G at a voltage equal to or of the order of magnitude of the voltage of the object defining aperture C. The system shown in Fig. 2 employs a screen grid G at the same highvolt-age as the object aperture C (V zl3 kv.).

Fig. 3 illustrates another embodiment of the invention "employing an'electron gun with a laminar flow crossover- .forming system. The said crossover-forming system A is a, structure of the type generally known as a Pierce gun (see Spangenberg, Vacuum Tubes, pp. 449-465).

This type of structure gives a good approximation to laminar spherical convergent flow, and it is particularly suited where high current, high current density and good cutoff characteristics are required. Because of the hlgh of Fig. 3 is preferably of the unipotential type. However, the potential of the electrode G through which the beam leavesthe lens system B may be lower than the potential of the electrode 6: through which the beam enters the lehs system. a

Fig. 4 illustrates a test tube containing an experimental electron gun which I have operated according to the principles of the invention described above in connection with Fig. 1. This tube comprises an evacuated envelope 5 having an internal silver coating 6, a window 7 constituting the transparent foundation plate of the aluminized phosphor screen 8. Within the envelope 5, there is a housing 9 enclosing a rectangular object defining aperture C, a conventional cathode and the electrodes G to G mounted on suitable "supporting rods 10. Electrode G supports a rectangular object defining aperture C measuring 4 x mils. The potentials applied to the electrodes for maximum current transmitted through the object defining aperture C were:

The control grid swing from 10 a. to 500 pa. screen current was 55 volts. It will be noted from the potential applied to the electrodes that this gun operated in the manner described above in connection with Fig. 1. The crossover is formed between electrodes G and G: by the fact that V 0 and V =1 kv. produce a strong immergent lens. Electrodes G --G G constitute a unipotential lens (the electron optical lens system B described above); G G is essentially an accelerating region without major significance; and G is the electrode in which the object defining aperture C is located. From the currents observed it will be appreciated that the secondary emission from the low voltage electrode G is negligible, being of the order of 50 a. for 2 ma. cathode current. The transmission efiicicncy of 25 to 30% through the rectangular aperture C corresponds to a round beam cross-section about equal to the length of the aperture, according to the equation for a rectangular object defining aperture of 4 x 15 mils given above in the description of Fig. 1.

The maximum beam spread in the system of Fig. 4 was observed in the direction of the long dimension of the rectangular aperture C, as is expected in a beam of rectangular cross-section spread by space-charge. The cross-section of the beam at a distance of 3" from the aperture was still nearly rectangular, the long dimension being about 5 mm. This corresponds to a maximum angle of beam spread 0 =2. With V,,,,=l.5 kv. this yields a product P= /V,, .sin 0,,,,=l.28. Extrapolating P=1 ma. according to the 3/2 power law, one obtains P=l.6. This represents an appreciable improvement over the best performing prior art high voltage limiting gun known to me, and, as stated above, values of the product P as low as 1.3 or better can be obtained by application of the principles of the invention. In the systern of Fig. 4, as in all others designed in accordance with the invention, secondary emission proved to be of negligible importance.

What is claimed is: V

1. An electron beam forming device comprising, in combination, cathode means providing a source of electrons, beam converging means disposed in proximity "to said cathode means for directing electrons from said source through a crossover point, lens means focused adjacent to said point for redirecting said electrons into a substantially parallel flow, electrode means having an aperture formed therein, said aperture being positioned transversely in the path of said parallel flow, the maximum dimension of said aperture being smaller than the corresponding minimum transverse dimension of said parallel flow of electrons, whereby a portion of the electrons of said parallel flow will pass through said 'aper ture and be sharply defined by the edges thereof, a screen and an electron-optical means imaging the portion of said substantially parallel flow emerging from said aperture onto said screen.

2. A device according to claim 1, wherein said parallel flow of electrons is of circular transverse cross-section and said aperture is of rectangular configuration.

3. A device according to claim 1, wherein said beam converging means comprises an immersion lens.

4. A device according to claim 1, in which said lens means comprises a combination of several bipotential lenses.

5. A high-voltage object defining aperture electron gun for cathode ray tubes, comprising an axially symmetrical electrode system including an electron-emissive cathode,

Electrode Cathode G G, G3 G G; G Screen Potential 7, (Volt), 0 v, -55 1000 1000 1500 V 1500 1600 Current (ma)- -2. 0 0 +0. 050 0. 050 0 0. 500 1. 000 0. 500

a crossover-forming means for reducing the cross-section of the electron beam from said cathode to a minimum, an

electron optical -lens whose focus coincides substantially with said minimum cross-section of said beam at said crossover-forming means, an electrode having an object defining aperture therein arranged to be flooded by the beam of electrons emerging-from said lens, and an elec- 2,172,739 Levin Sept. 12, 1939 Schlesinger 1m. e1. 1940;

Schlesinger July 8, 1941 Moss Oct. 11, 1949 McNaney Sept. 4, 1956 Broderick May 14, 1957 Dudley Mar. 4, 1958 7 OTHER REFERENCES 1 Malofl: Electron Optics in Television, McGraw-Hill,