US2134348A - High tension discharge tube - Google Patents

Description

Oct. 25, 1938. J. H. VAN DER TUUK 2,134,343

HIGH TENSION DISCHARGE TUBE Filed March 5, 1936 has l b .1. "u

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AUaR/YEY all) Patented Oct. 25, 1938 UNITED STATES HIGH TENSION DISCHARGE TUBE Jacob Harmannus van der Tuuk, Eindhoven,

Netherlands, assignor to N. V. Philips Gloeilampenfabrieken, Eindhoven, Netherlands Application March 5,

1936, Serial No. 67,381

In Germany March 21, 1935 2 Claims.

My invention relates to high-tension discharge tubes, and more particularly to the degasifying of such tubes by means of gas-binding agents.

Gas-binding agents, commonly known as getters, have been used for some time to absorb the harmful residual gases from various types of discharge tubes. For this purpose the getter-consisting of a substance which readily absorbs gasesis placed in -a compact form within the tube, and 'is then volatilized or disintegrated whereby it is deposited in the form of a very thin layer upon portions of the tube. The getter *therebycleans up theresidual gases remaining in the-tube after'its exhaust by a pump, and usually'a'lsoremains in the tube to absorb gases which are liberated during the operation of the-tube.

Whi-le the getters are generally used for lowvoltage discharge tubes, and also for voltages -up to about a few thousands volts, so far all attempts to successfully use them in discharge tubes for very high voltages as far as I know 'haveibeen unsuccessful; this because of several difficulties arising when getters are introduced into discharge tubes for very high voltages, such as X-ray tubes or transmitting or rectifying tubes of voltagesabove 30,000 kv. or, especially in the case of X-ray tubes of voltages up to some hundreds of kilovolts.

Such difiiculties are, for example, that the getterwhich is usually conductive substance, for example, magnesiumdeposits on undesired portions of the tube. For example, when depositing on the insulating material separating two electrodes between which a potential difference exists it decreases the insulating resistance and breakdown voltage between such electrodes. Again, the getter material may cause the'emission of secondary electrons or so-called coldemisSion, which makes the operation of the tube irregular if not entirely impossible. Especially serious is the deposit of getter on electrodes which should not emit.

While various means have been tried to avoid 'these difficulties, none of these has been suc- =cessful.

And it-has been generally assumed that getters could not be used in very high-voltage tubes, for example X-ray tubes, by introducing same into :the discharge tube proper, and that if getters were :to be used at all for such tubes, a separate lappending auxiliary vessel connected to the dis- .charge tube -to be evacuated was necessary to house the getter. However, such an auxiliary --vesse'l could not remain permanently attached to the discharge tube, especially to the present type X-ray tubes, as its attachment, besides increasing the size and fragility .of the tube, changes the shape of :the tube to an extent as to make its use diflicult or impossible. with existing fixtures.

On the other hand, the mere sealing off of the auxiliary vessel again liberates harmful gases, and furthermore in such cases the getter action is only utilized for removing the residual gases after :thedischarge tube has been exhaustedyand not to remove gases which are liberated in the 'tube during its operation. This latter function of the getter, however, "is also very important as it prolongsthe lifeof the tube. Therefore to obtain the full advantage of getters they should remain in the vacuum space of the tube during the life of the tube.

Altogether, the use of getters in high-tension discharge tubes such as iX-ray tubes,-even if they were used in an auxiliary chamber which was sealed off from the tube or allowed to remain connected thereto, have been unsuccessful and up till now the degasifying .of such tubes has been-carried out by pumping and heating of the :wall and the metal parts of the tube. A high voltage is usually applied between theelectrodes .of the tube and also high frequent :eddy .currents may be used :for heating the metal parts. The time required to exhaust these :tubes by means of a pump'withoutrthe :useof a getter'is very long. For the-usual types of 'X-ray tubes for instance, the pumping "time ranges generally from 5 to 15 hours depending upon the type of the-tube, the pumping equipment and the heating arrangement. This makes the degasifying of such expensive both because .of the expense of the heating equipmenhand the slowness of the operation. Moreover the high voltage applied during the degasifying process causes considerable complications.

The object of my invention is to overcome all the above-mentioned difficulties by providing a getter in a high-tension discharge tube in such a manner that the deposits of the getter material will have no deleterious effect upon the operation ofthe tube, while at the same time the getter action will be present throughout the life :of the tube.

further object of my invention is .to decrease the cost of degasifying such tubes by decreasing the pumping time and by making use .of'2high tension in the process of degasifying unnecessary. For instancesbyusing a getter in an X-ray tube in accordance with my invention, the pumping .time undersuitable conditions maybe reduced tola fraction of the time required heretofore, for example to 80 minutes or even to 20 minutes depending upon the tubes being degasified and the pumping equipment used. Moreover the use of high tension is unnecessary. I

In accordance with the invention, I provide a suitable getter, preferably an alkaline earth metal such as magnesium, within the vacuum space of the tube, but in such a part of the tube as to be fully protected from the impact of moving particles, such as electrons, ions, gas atoms or molecules, etc.

Whether or not the success of this arrangement is primarily due to the fact that the affecting of the getter deposits by impact of the electrons, ions, or the like is avoided, and thus for instance gases previously absorbed by the getter material are not reliberated, or that the undesired discharges caused by secondary emission are eliminated, I am unprepared to state. Whatever the reason may be, the tubes provided with a getter .in accordance with the invention utilize all the advantages of the presence of the getter, without its disadvantages.

If the discharge tubes, for example X-ray tubes, are of such construction, as is often the case, that'they have one or more internal chambers which are separated from the discharge space by partitions, such chambers may be used to house the getter provided they are connected to the discharge space only by means of one or more apertures through which the carriers of charge cannot readily pass. In such a case I dispose the getter in one of such chambers, so that it forms a deposit on the wall thereof. On the other hand, if the tube construction does not provide such a chamber, Iprovide a suitable chamber for this purpose.

If an electric field is present in such a chamher, the chamber should be narrow, so that movable particles, accelerated by the electric forces cannot assume a velocity by which they exercise a destructive action upon the getter substance. As a rule a width of 3 cm. or less as measured in the direction of the lines of force is suflicient for this purpose.

Preferably the getter is disposed at a place where it is in contact with a leading in wire, so that a fixed potential is applied to the getter film. For this purpose a special conductor may be sealed in the tube wall if there is no conductive contact between the getter substance and one of the electrodes of the tube.

To prevent emission from the getter deposit and the resulting disturbances caused thereby, the getter material is preferably deposited on such portion of the chamber wall as to extend within a portion of the tube which is free from any electric field or to form part of the boundary surface of a fieldless portion of the evacuated space.

In order that my invention may be clearly understood and readily carried into effect, it will be more fully described with reference to the accompanying drawing, in which:

Figure l is a sectionized side view of an X-ray tube according to the invention;

Fig. 2 is a sectionized View of an X-ray tube according to another embodiment of my invention;

Fig. 3 is a front view of the volatilizer shown in Fig. 1;

Fig. 4 is a front view of the volatilizer shown in Fig. 2.

.While I am illustrating my invention as applied to an X-ray tube and show two embodiments thereof, it should be understood that the invention is applicable to high-tension discharge tubes in general and that various other embodiments will suggest themselves to those skilled in the art without departing from the spirit of the invention.

The envelope of the X-ray tube shown in Fig. 1 comprises a central'metal portion 3 to the ends of which are sealed two glass portions l and. 2 respectively, having reentrant parts 31 and 38 carrying an anode 4 and a cathode structure 50 respectively. The anode'4 is provided with a disc-shaped target I!) of suitable refractory material, for example tungsten.

The cathode structure comprises a cathode cap or focusing device 6 provided with a central slot 39. Secured to the cap 6 is a tungsten plate 9.v Disposed within the slot 39 is an incandescible cathode filament 5 having one end connected to a lead 40, through a support-lead 1 passing through the plate 9 and insulated therefrom. As shown in the drawing the lead 1 passes through an aperture in the plate 9 and thus is insulatingly spaced therefrom. In this former case a baffle plate 4i is secured to the lead i for a purpose later to be described. However, the lead 1 may be insulated from the plate 9 by a suitable insulating bushing. The other end of the filament 5 is connected to the plate 9 and through a lead 8 to supply lead 42.

During operation, the electrons emitted by the cathode filament 5 impinge upon the target ID to produce X-rays which leave the tube through a glass window I! sealed into the central portion 3.

It will be noted that a chamber [3 is formed by the plate 9, the lower portion of cap 6, end portion 20 of the reentrant portion 38 and pinch l2. The chamber I3 forms part of the vacuum space of the tube, but is almost completely separated from the discharge space l4. To interconnect the main discharge space l4 with the chamber l3, the plates is provided with one or more small bores, for instance as shown at [5. Thereby the chamber I3 is connected to the space M through the bore l5 and the slot 39 and the chamber I3 forms part of the vacuum space of the tube, but is connected thereto only through a very constrictedpath.

Disposed within the chamber I3 is a so-called getter-volatilizer l1. As shown more clearly in Fig. 3, the getter-volatilizer I1 comprises a getter receptacle IS in the form of a small tube of insulating material, and a heating wire 43 helically wound around the receptacle l6. Disposed within the receptacle I6 is a small rod 44 of an electro-positive metal for example magnesium or barium enclosed e. g. in a' copper sheath. The heating wire I I has one end connected to a lead I9 whereas its other end is connected at l8 to the lead 42.

The X-ray tube is exhausted by means of a vacuum pump or by other suitable means to a high degree of vacuum for instance 0.01-0.001 micron of mercury. Subsequently, and preferably after the tube has been sealed off and separated from the pump, the heating wire I! is heated to incandescence by applying a suitable potential between leads I9 and 42. Heating of the wire I! causes the electropositive material enclosed in the copper sheath of the rod 44 to volatilize. The volatilized material condenses .on the inner surface of the wall of chamber l3 to form a very thin layer as indicatedat 45. As

cylindrical vitreous the bore IS in the plate 9 keeps the'pressures in chamber l3 the same as that in the main discharge space M, the residual gases present in both spaces are absorbed by the getter layer 45.

It should be noted that the layer 45 of getter material remains in place during the entire life of the tube and thus, in addition to absorbing the residual gases remaining after the pumping of the tube, will also absorb any gases liberated in the tube during its operation.

As the chamber I3 is closed from the main discharge path except for the small aperture l5 and the space between the lead 1 and plate 9, the entrance of charged particles such as ions, electrons of high velocity is practically prevented. In this connection it should be noted that the baffle 4| serves to restrict the passage of such moving particles of high velocity through the space between the lead 7 and the plate 9 and also to intercept X-rays passing through this space.

Furthermore, except for the negligible electric field created by the low potential difference existing between the cap 6 and the wire I, there is practically no electric field set up within the chamber l3. Thus, the layer 45 formed on the inner surface of cap 6 and also on part of the inner surface of the glass portion 20, surrounds a space which is practically free from any electric field so that the presence of the getter material does not cause any disturbance in charge of the tube.

If desired the getter I! may be disposed at other points within the chamber l3, for example adjacent to the pinch l2, in which case most of the getter material will be deposited on the surface of the glass portion 20 instead of on the metal cap 6. In such a case, it is preferable to electrically connect the layer of getter material to the cathode structure in order to prevent the establishing of a potential difference therebedrawing this is accomplished by a spring 46 mechanically secured to the lead 8 and pressing against the inner surface of portion 20 to thereby electrically connect the layer 45 to the lead 8.

The X-ray tube shown in Fig. 2 comprises a envelope 24 provided with a concentric vitreouspartition 25 surrounding an anode 2| and a cathode structure 21. The partition 25 is sealed to tends close to the cathode end of the tube to form a chamber 23 connected to the main discharge space 26 of the tube by means of a narrow ring-shaped opening 35.

The cathode structure 21 comprises a focusing device 52 supported by a plurality of supports 28 from a metal sleeve secured to a reentrant portion. Within the focusing device 52 is a cathode filament 41 having its ends connected to supply leads 48 and 49 through leads 32 and 33 sealed in a pinch 3|.

It will be noted that in this construction, an arrangement of the getter in a position similar to that of Fig. 1 would not give the advantages of the invention as the getter material would be deposited upon the walls of the envelope in the main discharge space. On the other hand the ring-shaped space or chamber23 is very well suitthe main disthe envelope 24 at 26 and ex- 7 space, a layer of getter getter as the charge-carriers or moving particles are practically ed to house the other harmful excluded therefrom.

of pressing the wire into a recess 54 of the metal plate 53. In Fig. 2 the protruding rib 55 is the backside of this recess.

To volatilize the getter material 36, the plate 53 is heatedpreferably after the tube is sealed off the pumpby eddy currents induced therein by means of an external source of high-frequency current. After being volatilized the getter materialcondenses upon the inner surfaces of the chamber 23 in the form of a thin layer as indicated at 49 and absorbs the residual gases remaining in the vacuum space after exhausting and also serves to absorb any gases which are liberated within the tube during the operation. The rod 34 may serve to apply a fixed potential to the getter film 49.

As has been stated, the degasifying of hightension discharge tubes by using getters disposed in the tube in the manner of the present invention, has the advantage that the pumping time can be considerably reduced and the use of highfrequency heating current in the degasifying can be dispensed with. Furthermore, as the getter remains in the tube throughout its life it protects the tube from puncturing due to disruption of gas.

While I have described my invention in connection with specific examples and applications I do not wish to be limited thereto but desire the appended claims to be construed as broadly as permissible in view of the prior art.

What I claim is:

1. A high-tension discharge tube comprising an evacuated envelope, a cathode structure therein comprising a filament an electrode spaced from to form a discharge space,

comprising partitioning means dividing the vacuum space of the tube to form a chamber communicating with the discharge space through a constricted opening, and a getter within the chamber and forming a coating on the walls thereof.

2. A high-tension discharge tube comprising an evacuated envelope, a cathode structure comprising a filament and a focusing device cooperating therewith, an electrode spaced from the cathode structure to form a discharge space, said focusing device comprising partitioning means dividing the vacuum space of the tubeto form a chamber in constricted communication with the discharge surface of the wall of said chamber, and a member electrically connecting said layer to the cathode.

JACOB HARMANNUS VAN DER TU'UK.

and a focusing. devicematerial upon the inner

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