US2577314A - Electronic discharge device - Google Patents

Description

Dec. 4, 1951 E. EASTWOOD ETAL 2,577,314

ELECTRONIC DISCHARGE DEVICE 4 SheetsSheet 1 Filed Nov. 17, 1948 FIG. 1.

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I I I micro seconds FIG. 5.

Attorneys Dec. 4, 1951 4 Sheets-Sheet 5 Filed Nov. 17, 1948 Inventors: Eric Eastwood John M. Ferguson Kenneth W. Brown Attorneys D 1951 E. EASTWOOD ET AL ELECTRONIC DISCHARGE DEVICE 4 Sheets-Sheet 4 Filed Nov. 17, 1948 FIG. 7.

Inventors:

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B 6M ywwaw Attorneys Patented Dec. 4, 1951 v if '1'";

UNITED STATES PATENT OFFICES ELECTRONIC DISCHARGE DEVICE Eric Eastwood, Great Baddow, and John McIntyre Ferguson and Kenneth William Brown, Stafa ford, England, assignors to The English Electric Company Limited, London, England, a

British company Claims.

This invention relates to electron discharge devices in which a discharge is required to take place between cold electrodes in high vacuum and especially to starting the discharge in such I tion of the electrons will reach the main anode.

It is usual after a short period for the discharge to develop into a metallic arc of relatively low voltage.

In a discharge device according to the present invention, while similar conditions of discharge are ultimately achieved, the emission of electrons is initiated from the cathode by means of a high voltage pulse applied to a high resistance igniter element, which is in contact with the metal of the cathode. Due to the field conditions arising at the igniter-cathode interface and in the presence of a high anode accelerating potential, a very heavy discharge of electrons to the anode is initiated. With this type of cathode it is possible to reach very high currents above the range of a conventional hot cathode for a short period of the order of one microsecond. After this period the discharge passes into an arc with the normal low voltage characteristics.

With a suitable anode the device may be used as an X-ray tube and owing to the very high X-ray intensities obtainable it is possible to make radiographs with a single exposure of the order of one microsecond or less.

The invention is illustrated in the accompanying drawings in which:

Fig. 1 is a sectional elevation of an X-ray tube incorporating the invention;

Fig. 2 is an enlarged view of the cathode of the tube shown in Fig. 1;

Fig. 3 is an end view of the cathode of Fig. 2;

Fig. 4 is a typical oscillogram showing the voltage between anode and cathode in operation of the device;

Fig. 5 is an oscillogram showing the current;

Fig. 6 is a sectional elevation of a further form of X-ray tube and a schematic diagram of apparatus for supplying the operating voltage to the tube;

Fig. 'I is an enlarged view of the cathode of the tube shown in Fig. 6, and

Fig. 8 is an end view of the cathode of Fi '7.

Referring first to Figs. 1 to 3, a glass envelope l is provided at one end with a re-entrant foot tube 2 through which is sealed a lead-in conductor 3. Mounted on the lead-in conductor 3 within the envelope I is a cathode 4 consisting of a block of vacuum melted copper. The end surface 5 of the cathode 4 is concave and an axial bore 6 is first drilled and then reamed in the centre of the cathode 4 for the reception of one end of a cylindrical igniter element 1. The element l is composed of a mass of silicon carbide crystals having a'particle size of the order of 400 mesh and held together in a vitrified clay bond. The other end of the element 1 is held in the centre of a grid-like structure formed of wires 8 which in turn is carried on the end of a copper tube 9 which is fitted over the re-e'ntrant foot tube 2 and serves as a shield around the cathode 4. Electrical connection to the tube 9 and thence to the igniter element 1 is made by means of a leading in wire l0 which is sealed through the side wall of the envelope I at H. At the opposite end of the envelope l a further re-entrant foot tube I2 is provided through which is sealed a conductor l3 carrying an X- ray target [4. The target I l may be of tungsten or other suitable material depending upon the nature of the X-rays which it is desired to produce. A tubular metal shield I5 is welded to the conductor l3 and surrounds the inner end of the foot tube l2. In operation of the device a high voltage pulse of the order of 250 kv. is applied across a potential divider it the ends of which are connected respectively to the conductors 3 and I3. The potential divider is tapped at ll to produce a voltage of the order of 10 RV. between the igniter element 1 and the cathode 4.

Typical oscillograms of the voltage and current occurring in the operation of this tube are illustrated in Fig. 4 which shows the voltage between anode and cathode and Fig. 5 which shows the current in the device. At point B on the voltage oscillogram in Fig. l the metallic arc is starting to occur and the voltage begins to fall rapidly. At the same time the current correspondingly increases as is shown in Fig. 5. The current reaches a maximum when the voltage has fallen to a low value and by this time the X-radiation has practically ceased. It is beneficial that some resistance is inserted in series with the discharge tube to prevent oscillations occurring when the metallic arc has been set up.

A further form of X-ray tube according to the invention suitable for voltages of the order of 1,000,000 volts is illustrated in Figs. 6, 7 and 8. The device consists essentially of a cylindrical envelope composed of, a stack of; 48 porcelain rings each 1""thick. The rings 2llare metal.- lised on their end surfaces and soldered together with metal rings 2| between. A metal;

The end of the conductor forms the-catho'de of the device and carries a dish-shaped copper shield 21. in the end of the conductor 25 and is carried by agrid-like structure 29 which is connected eiectrically to the disc 22 This construction; is Shown more clearly-in Figs. '7 and 8. Phe shield 2l-is conveniently formed by stamping and has a, central aperture with curved shoulders toeffect a measure of focussing. A threaded boss isarranged (Jo-axial with the central aperture in the-shield 2-1 and this is screwed on to the threaded end of the conductor 25 until the end of; theconductor 25 is flush with the lower, surface of theshieldzl and is secured bymeansof a lock nut 3!. The electrode 28 fits closely into arecessdrilled and reamed for that purpose in tl gjendof the conductor 25.- The structure 29 is arranged to produceisymmetricalfieldcondi tions and is supported by four equallyspaced rods 32 extending throughapertures 3% in the shieldZl-and secured'to the-disefi. Thelower end of thetube is terminatedby. a flanged portion 34 which may. be provided with a pumping connection 35" or may be, sealedoif. T-he meme ber'3 4 carries an X.-ray targeted. At theoperating voltage of thedevice the -X-radiation ;pene-. trates the target. The metal rings 2t are each provided with a terminal lug connected to are;- sistance potential divider 38-having a total resistance of about 3000 ohms; and; serve to accelerate electrons fromthe cathode to the target 36; In practicetheterminallug on each successive ring is displaced 90 fromthe lug on the preceding ring and the potential dividen-SB is arranged helically aroundthe'outside of the envelope of the device sinceotherwise; the cur rent in the divider 33 will deflect the electrons within the envelope. For convenienceo f illustration, however, the potential divider 38 is shown as being arranged down one-side'of the envelope.

The supply and control circuit forthe-tube consists of a Marx generator comprising-a supply transformer 38 supplying rectifiersv 40 and 4| through a condenser 42; The rectifiers 40 and 4| are arranged to charge a numberof condensers 43 in parallel through -resistances i i. and spark gaps 45 are arranged; so that the;con,- denserstS may discharge inrseries'. Thecontrol ircuit includes: a; transfo mer 46' aving a switch in. its primary rcuit; itscs ndar erv o char e acondensertllsthroughareetn tier 9. and es a cefifly s arksan pt vided so that 1 the condenser dsmay; discharge through the primary of an aircored; step-up transforme econda y; e; trans? farmer; -2v nnec d; :between the grounded ends of the Mar eeenerator and an: 2'8le9t EQ 3-* 3 An-igniter electrode 28 is embedded" in the first spark gap 45. The first spark gap 45 may thus be induced to break downat a lower voltage than the remainder of the spark gaps in the Marx generator whereupon the voltage across the succeeding spark gaps will become such that they also will breakdown. The negative end of the generator is connected through aiurther spark gap 5% to the conductor 25 and to the cathode end of the potential divider 38. The positive 'end of the Marx generator is earthed and is also connected to the target 35.

V The discZZ and. consequently the igniter electrode 28is connected to an intermediate point onrthe; potential divider 33 such as to provide a voltagelpositive with respect to the cathode. A highgradient is. produced at the junction of the 'igniter electrode and the cathode suificient to give rise to emission of electrons. The optimum igniter'voltage varies from about 7 to 15 kv. according to the length of the igniter.

' It will be appreciated that the invention is not restricted to X-ray tubes and that in the case of the tube illustrated in Fig. 6 the target-36 may be omitted and the bottom of the member 34 may be in the form of a Duralumin Window, for example, 0.001 thick through which the electron beam may penetrate into the atmosphere below. Alternatively, the'device may be used asacontrol tube.

Whatwe claim as our invention and desire to secure by Letters Patentis:

1. A high. vacuum electron discharge device including an evacuated envelope, an anode and a cathode within said envelope, said cathode consisting of a body of metal in the solid state, and an igniter electrode of high resistance material, one portion of which is in intimate contact with said cathode, and another portion. of which is connected to a terminal .for a sourceof control impulses.

2.- A high vacuum electron discharge device including an, evacuated envelope, an anode, a cathode comprising a body of metal in the solid state, and. an igniter electrode of high resistance material one portion of which fits closely in. a .recess in the metal of said cathode, and another portion of which is connected to. a terminalfor a source of control impulses.

3. A high vacuum electron dischargedevice including. an evacuated envelope, an anode, a cathode comprising a body of metal in the solid state, and" an igniter electrode in the form of a rodzof silicon carbide crystals held together in a vitrified clay bond, an end of said rod fitting closely in a recess in the metal oifsaid cathode, the other. end connected to a terminal for a source of control impulses.

4. A high vacuum electric discharge device including an evacuated envelope, an anode, a cathode comprising a mass of metal, and an ig-niter electrode in the form of a rod of silicon carbide crystals of the order of 400 mesh held together in a vitrified clay bond, an end of said rod fitting closely in a recess in the metalof saidcathode.

5, A high vacuum electron discharge device including an evacuated envelope, an anode, a cathode consisting of a body of metal in the solid state, and a symmetrical grid-like structure supported on conductorsextending through apertures' in said cathode and carrying an igniter electrodeof high resistance material, a portion of which 1 electrode; is in Y intimatev contact with said cathode, the portion which is, in. contact with said grid-like structure connected thereby to a source of control impulses.

6. A high vacuum electron discharge device as claimed in claim 5 wherein said portion of said igniter electrode fits closely in a recess in said cathode.

7. A high vacuum electron discharge device including an evacuated envelope, an anode, a copper cathode and an igniter electrode composed of a rod of silicon carbide crystals held together in a vitrified clay bond, one end of said rod fitting closely in a recess in said cathode, the other end being connected to a terminal for a source of control impulses.

8. A high vacuum electron discharge device including an elongated evacuated envelope, an anode, a cathode consisting of abody of metal in the solid state, an igniter electrode composed of a rod of silicon carbide crystals held together in a vitrified clay bond one end of which fits closely in a recess in said cathode, the other end being connected to a terminal for a source of control impulses and a plurality of accelerating electrodes arranged between said cathode and said anode.

9. An X-ray generator, including an evacuated envelope, a cathode consisting of a body of metal in the solid state, an igniter electrode composed of a rod of silicon carbide crystals in a vitrified clay bond one end of which fits closely in a recess in said cathode,, and an X-ray producing target electrode, and another portion of which is connected to a terminal for a source of control impulses.

10. An X-ray generator including an elon- 6 gated evacuated envelope, a cathode consisting of a body of metal in the solid state, an igniter electrode comprising a rod composed of silicon carbide crystals held together in a vitrified clay bond one end of which rod fits closely in a recess in the metal of said cathode, the other end being connected to a terminal for a source of control impulses, an X-ray producing target electrode, and a plurality of accelerating electrodes arranged between said cathode and said target electrode.

ERIC EASTWOOD.

JOHN MCINTYRE FERGUSON.

KENNETH WILLIAM BROWN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,901,865 Bouwers Mar. 21, 1933 1,992,852 Bader Feb. 26, 1935 2,053,501 Spencer Sept. 8, 1936 2,144,518 Westendorp Jan. 17, 1939 2,145,727 Lloyd, Jr Jan. 31, 1939 2,261,569 Schutze Nov. 4, 1941 2,269,861 Rennie Jan. 13, 1942 2,270,324 Marshall Jan. 20, 1942 2,284,341 Pollock May 26, 1942 2,456,891 Rigrod Dec. 21, 1948 FOREIGN PATENTS Number Country Date 444,001 Great Britain Mar. 11, 1936

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