US2172530A - Cathode bay tube - Google Patents

Description

Sept. 12, 1939. F, BRETT' 2,172,530

CATHODE RAY TUBE Filed Jan. l5, 1937 INVENTOR GEORGE FAIRBURN BRETT ATTORN EY Patented Sept. 12, 1939 UNITED STATES PATENT OFFICE GATHODE RAY TUBE George Fairburn Brett,

Landon, England, assignor to Radio Corporation of America, a corporation of Delaware 7 Claims.

This invention relates to cathode ray tubes and has forits main object to provide a cheap, simple and small gas focused cathode ray tube capable of operating with relatively low voltages and yet having relatively high electron beam current. Such tubes would be of considerable advantage for numerous purposes where the high cost and necessity for high operating voltages renders the normal type of cathode ray tube oscillograph impractical; e. g., tubes in accordance with this invention may be used in substitution for voltage or current indicating instruments and galvanometers generally, for example, in radio direction finding equipment, since they may be made cheaply enough to compare in cost with ordinary galvanometer and similar instruments, and since, moreover, they may be made to operate with voltages as low as the ordinarily available mains supply voltages.

According to this invention a gas filled cathode ray tube comprises an electron gun system consisting of a cathode whose point or area of emission is in the plane of or close to the plane of a transverse apertured electrode which surrounds said point or area of emission; a second transverse apertured electrode positioned a short distance in front of the cathode and having a small aperture in line with the cathode along the gun axis; and a third transverse apertured electrode which is in or near the plane of the second transverse electrode, and has an aperture which is much larger than the other apertures in the system; the tube comprising in addition to the electrodes already mentioned and constituting the electron gun, a target for the electrons from the gun. The gas filling may be argon or other rare gas at a suitable low pressure. The target will in most cases consist or a fluorescent screen preferably in the form of a metal plate prepared with fluorescent material, or in the form of fluorescent material deposited on a semi-transparent metal layer formed on the end wall of the tube. In the latter case contact is made to the metal by means of a connection 'passing through the envelope wall.

The invention is illustrated in the accomanying drawing which shows schematically two embodiments thereof.

Referring to Fig. l in the form of tube therein shown, a gas-filled approximately cylindrical envelope I, la is employed, this envelope being reduced in dimension towards the end la and being provided at that end with the usual reentrant footstep or pinch 2 for carrying the electron gun system. This system consists of an indirectly heated cathode 3 with a localized electron emissive source 3a said cathode being heated by heater wires 4 which pass into the cathode sleeve or tube. The said source 3a is positioned in an aperture 5a in a disc-like electrode 5 which surrounds the said source and is at right angles to the gun axis, said disc-like electrode 5 being in or nearly in the plane of the emissive source (in practice it is usually a little away from this plane in the direction away from the anode electrodes) and being connected (preferably as shown within the tube envelope) either to the cathode, or, where an indirectly heated cathode is employed (as is the case illustrated) to one leg of the cathode heater, in which case the said leg is preferably also connected to the cathode. A short distance in front of the cathode emission point-say from 1 to 1.5 mm. in front-is positioned a second disc-like electrode 6 which constitutes a first anode and has a small central aperture 60, preferably of about 0.6 mm. diameter. This aperture is in line with the emissive point or area 3a of the cathode along the gun axis. Just in front of the disclike electrode 6-about 0.5 mm. or 1essis a third disc-like electrode 1 constituting a second anode and of (say) about 20 mm. diameter with a wide aperture la of diameter about 5 mm. (5 to 10 mm. is a practical range). The external diameters of the electrodes 5, 6 and 1 may be, in each case, about 15 to 20 mm. At the other end of the tube is positioned a fluorescent screen 8 consisting of a metal plate prepared with a deposit of fluorescent material on the side facing the gun. The surface of this screen may be as shown at right angles to the gun axis. The third electrode 'l is connected either inside or outside the tube envelope to the metal back plate of the screen 8. As shown there is an external connection 9 to the plate of the screen 8. A typical value of the separation between the gun electrode 1 and the screen 8 is 12 cms. Instead of using a screen at right angles to the gun axis as shown, the said screen may, if desired, be tilted with respect to the gun axis for more convenient viewing of the fluorescent trace thereon the screen being viewed through the envelope walls. Deflection of the ray is efiected in the tube illustrated, magnetically, the N and S representing, diagrammatically, the pole pieces associated with one pair of magnetic deflecting coils. In use focusing control is obtained by varying the potential upon the second electrode and voltages of between about 10 and 30 volts positive (relative to the cathode) may be applied to this electrode. A positive potential (relative to the cathode) of the order of to 300 volts may be applied to the metal back plate of the screen and to the third electrode. Deflection of the ray as required is preferably accomplished electro-magnetically as indicated by means of external deflecting windings.

It has been found in experimental practice that a tube as described above With about 10 to 20 volts positive on the second disc-like electrode and about 200 to 240 volts positive on the screen and on the third electrode, will give an electron beam current as high as 50 micro-amperes. It Will be immediately apparent that this is a relatively high current having regard to the operating voltages employed, and in comparison with a present day commercially available gas focused cathode ray tube, for, a typical good quality commercially available gas focused tube will give only about 5 microamperes in the beam for 500 volts and only 15 microamperes for 800 volts, while at voltages as low as 250 the current in the beam is too small to be useful.

The principal advantage of tubes in accordance with this invention lies in the fact that relatively large beam currents can be obtained with relatively small operating voltages and this advantage derives from the nature of the electron gun arrangement. The possibility of using small operating voltages involves that the over-all length of the beam for a given value of sensitivity may be reduced, and this in turn, of course, cheapens the tube and makes it more compact. A further advantage is that, having regard to the low operating voltages used, a considerably increased sensitivity is obtained relative to that obtainable with existing commercially available tubes, while, since in use the third electrode and the screen are at the same potential, the conditions in the at present standard type of gas focused tube are virtually maintained there being no appreciable electron acceleration after deflection.

Although, as above stated, electromagnetic deflection is preferably employed, the invention is not limited to this form of deflection for electrostatic deflection may be used. The use of electrostatic deflection involves, however, some liability of distortion occurring due to inter-action between the deflector plates and the second (low voltage) anode, and in order to prevent this it is preferred, when using electrostatic deflection, to provide an additional (fourth) electrode consisting of an apertured disc of similar diameter to that of the third electrode about 10 mm. or so in front of the said third electrode. The aperture in this fourth electrode may be from 1 to 2 mm. or thereabouts, in diameter and should be in line with the other apertures. A cylindrical sheath of diameter substantially equal to the third and fourth electrode diameters may, with advantage, be placed round the said third and fourth electrodes so that the said electrodes, with said cylinder constitute a closed cylinder with unequal apertures in its end faces. The deflecting plates may be situated outside the cylinder on the screen side thereof so that the ray passes between the plates after leaving the fourth electrode. A complete system as just described but without deflector plates, may be used with electromagnetic deflection.

The accompanying Fig. 2 shows a tube including mutually perpendicular pairs II), II, of deflector plates. The principal difference between the electron gun system of the tube of Fig. 2

and that of the tube of Fig. 1 is that a cylindrical extension lb is provided for the electrode 1, the extension terminating in a disc 1c with an aperture 1d. The apertured discs 1 and 1c and the extension lb may be formed in one piece. I2 is a mica spacer between the first anode 6 and the second anode disc I. The structure 1, lb, 1c enisures minimum spot distortion due to the deflector plate fields. The envelope I in Fig. 2 is of the usual flared type and the screen 8 is somewhat differently arranged as compared to Fig. 1. In Fig. 2 the distance between the disc 1c and the plane of the nearest deflector plate edges may be about 5-10 mm.; the aperture 6a may be about 0.6 mm. diameter; the aperture 1a may be about 5 mm. diameter; the aperture 1d may be about 1.5 mm. diameter; the distance 3a to 6d may be about 1.0 to 1.5 mm.; the distance 6a to la. may be 0.5 mm. or less; the distance la to 1d may be about 12.5 mm.; and the diameters of the parts 5, 6, l and 10 may be about 15 to 20 mm..

Obviously the gun system shown in Fig. 2 might be employed in a tube as shown in Fig. 1 and vice versa and any suitable form of screen could be used in either tube; e. g., a semi-transparent metallized fluorescent screen on a mica backing, or (if voltages higher than about 250 are applied to the second anode) an ordinary unmetallized fluorescent screen. shown) to the deflector plates may be passed through the footstep to pins on a suitable multiple-pin base (not shown) in which case the leads are preferably provided with glass encasing sleeves to prevent their collecting any stray electrons. Preferably also a cylindrical metal sheath (not shown) surrounds the deflecting plate system.

In both the tubes illustrated the majority of the dimensions are by no means critical but for best results, the axial distance (for a tube of the general dimensions given) between 3a and 6a should not be appreciably less than 1 mm. nor more than 1.75 mm.; the aperture 6a should not exceed .75 mm. in diameter; and the aperture la should not be materially less than 4 mm. The size of the aperture Id in the construction of Fig. 2 is not critical and depends partly on the length of lb; for a length of lb of 12.5 mm. the aperture Id may have a diameter of about 1.5 mm. and for shorter or longer lengths this aperture diameter may be decreased or increased approximately in proportion.

In both figures indirectly heated cathodes are shown but filamentary cathodes can be used. The emitting cathode tip (3a in Figures 1 and 2) should be of approximately disc form and about .5 mm. in diameter.

Having now described my invention, what 1 claim is:

1. A cathode ray tube comprising an envelope, a cathode having a planar electron emissive surface positioned within the envelope, a fluorescent screen supported on a metallic layer in register with but remote from the cathode Within the envelope, a first disk} electrode surrounding the emissive surface of the cathode and lying in the same plane as said surface, a second apertured disk electrode closely adjacent to and in register with the cathode, a third apertured electrode in register with and closely adjacent to the second electrode, said third electrode having an aperture greater in diameter than the diameter of the aperture in the second electrode, and a press In Fig. 2 the leads (not member for supporting the cathode and the three electrodes.

2. A cathode ray tube comprising an envelope, a cathode having a planar electron emissive surface positioned within the envelope, a fluorescent screen supported on a metallic layer in register with but remote from the cathode within the envelope, a first disk electrode surrounding the emissive surface of the cathode and lying in the same plane as said surface, a second apertured disk electrode closely adjacent to and in register with the cathode, a third apertured disk electrode in register with and closely adjacent to the second electrode, said third electrode having an aperture greater in diameter than the diameter of the aperture in the second electrode, and a press member for supporting the cathode and the three electrodes.

3. A cathode ray tube comprising an envelope, a cathode having a planar electron emissive surface positioned within the envelope, a fluorescent screen supported on a metallic layer in register with but remote from the cathode within the envelope, a first disk electrode surrounding the emissive surface of the cathode and lying in the same plane as said surface, a second apertured disk electrode closely adjacent to and in register with the cathode, and an annular electrode having closed apertured ends, said electrode being positioned closely adjacent to the second electrode and in register therewith and having the diameter of the aperture of the end nearest the second electrode larger than the aperture of the second electrode and the aperture in the closed end more remote from the second electrode.

4. A cathode ray tube comprising an envelope, a cathode positioned within the envelope, a fluorescent screen supported on a metallic layer in register with but remote from the cathode within the envelope, a first d sk electrode surrounding a portion of the cathode nearest the screen, a second apertured disk electrode closely adjacent to and in register with the cathode, an annular electrode h'aving closed apertured ends, said electrode being positioned closely adjacent to the second electrode and in register therewith and having the diameter of the aperture of the end nearest the second electrode larger than the aperture of the second electrode and the aperture in the closed end more remote from the second electrode, and an insulating dis-k positioned between the second and the annular electrode.

5. A cathode ray tube comprising an envelope, a cathode positioned within the envelope, a fluorescent screen supported on a metallic layer in register with but remote from the cathode within the envelope, a first disk electrode surrounding a portion of the cathode nearest the screen, a second apertured disk electrode closely adjacent to and in register with the cathode, an annular electrode having closed apertured ends,

said electrode being positioned closely adjacent.

to the second electrode and in register therewith and having the diameter of the aperture of the end nearest the second electrode larger than the aperture of the second electrode and the aperture in the closed end more remote from the second electrode, an insulating disk positioned between the second and the annular electrode.

6. A gas filled cathode ray tube having an electron gun system including a cathode whose area of emission is in the plane of the plane of a transverse apertured disk electrode which surrounds said area of emission, a second transverse apertured disk electrode positioned a short distance in front of the cathode and having a small aperture in line with the cathode along the gun axis, and a third transverse apertured disk electrode in register with and adjacent to the second apertured disk electrode and having a larger aperture than said second electrode, and a metallized fluorescent screen positioned beyond and in register with said third electrode.

7. A cathode ray tube as claimed in claim 6 wherein the axial distance from the area of emis sion to the second apertured electrode is between 1 mm. and 1.75 mm., the diameter of the aperture in the said second apertured electrode does not exceed .75 mm., and the diameter of the aperture in the third apertured electrode is not less than about 4 mm.

GEORGE FAIRBURN BRETT.

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