US1927475A - High vacuum discharge vessel in particular X-ray tubes - Google Patents

Description

leo b ld V'l/re/ y i M L. VULKEL Sept. 19, 1933.

HIGH VACUUM DISCHARGE VESSEL IN PARTICULAR X-RAY TUBES Filed Sept. 8, 1931 Patented Sept. 19, 1933 HIGH VACUUM DISCHARGE VESSEL IN PARTICULAR X-BAY TUBES Leopold Viilkel, Hamburg, Germany, assignor to G. H. F. Muller .Aktiengesellschaft, Hamburg,

Germany Application September 8, 1931,Serial No. 561,766, and in Germany Sep temberQ', 19 30 7 Claims (01. -0-35) This invention relates to discharge devices and more particularly to discharge devices which operate at high voltages and with pure electron discharge, and which are-provided with an envelope 0 of insulating material comprising a middle portion of conductive material, surrounding the discharge space. i

It is known that in such devices, the voltage drop along'the envelope is divided by the middle portion of conductive material. The divisionof voltage between the two insulating portions of the envelope depends on two factors.

The first factor is the symmetrical location of F the conductive portion between the two metal parts, for instance, between two electrodes, from 'wh ich the conductive portion is insulated by the insulating portions ofthe envelope and between which the voltage drop exists which the conductive portion is subdividing.

The second factor is the presence of free electrons in the discharge space. I

The first factor will tend to produce an equal division of the voltage between the 'two insulating portions; whensuch a discharge "tube isoperated on alternating current or on. a. rectified alternating current without using a special smoothing circuit,thus as long as thevoltage has a continuously varying value, such .equaldivi- .sion of voltage will be permanently maintained. However, in certain cases, as for instance, when working with pure direct current of very ihigh voltages, as used in therapeutical installations, the second factor, namely, the presence of free electrons, manifests itself and causes a space charge, which while partly counteracted 'by the discharge between the positive electrode and the conductive middle portion, .nevertheless will ,pre-

vent equal voltage division.

In the case just mentioned, the electrons .will cause the .conductive middle'portion ;to take up a negative charge, and .unless' special means are ,provided or the positive electrodeis closely spaced "to the middle portion, the middle portion willassume a potential which is nearer to that of .the

:the voltage is no more'equally divided, but the insulating portionbet'ween the'middle conductive portion and the anode has'to 'takecareof a much 4 larger portion of the total voltage, than the insulating portion between the conductive *middle portion and the cathode. j

It has already been proposedto retain the con- *ductive or 'metallic central part at ground potentialso as to apply between same and each of the two :electrodes half of the total voltage; This method, however, can only :be used with tubes having :a small output or with tubes operating at low voltages. With tubes operating at high voltages, the electrons impinging upon the metallic middle portion, due to the intense electric but having a large output, the current intensity in the common connection "to groundthrough the arrival of the correspondingly large number of electrons,becomes so high, that 'here again the *central' metallic part becomes very hot.

' Such heating up of the metallic part leads in bot oasesto the destruction-of the tube.

According to the present invention, it is possible to ground the central metallic part so as to obtain a symmetrical distribution of the voltage between the electrodes by disposing inside the discharge vessel a metal screen, which is insulated trom the electrodes and inside which the discharge occurs. The annular space between the outer wall of the tube and the just mentioned screen is-brought =toa high vacuum whereby this annular space forms a layer of high insulating capacity.. JConsequer-itly the danger of per-fora- -ticn of the tube is materially reduced. It has already been proposed to construct an X-ray tube made entirely of glass with a protective casing of metal or glass. This arrangement has the drawback inter aliaof reducing the insulatingca- .pacity .of the vacuum between the inner glass .walliand the protective casing, in case of dis- :turbances .eccurring in .its relation to the vacuum in thedischargespace. For this reason, the an- .nular space between the .outer wall and the inner screen,;according to the present invention is comzpletely sealed with reference to the inner space and evacuated independently therefrom. Thus the stray :electrons :from :the electrodes can no more can onthe :metallicwentral .part of the .out- :er wall and consequently the latter :may .be directly connected tmground :whereby athezdistribution of potential in the tube is perfectlysymmetrical. This provides the further advantage, .that it is possible to support the tubes directly through theircentral :metalpart even when op- V leratingathigh potenials. .cathode than that of the anode, consequen'tly,

A -further improvement of the above :described discharge vessel :is accomplished by using metal for. the central partof -thescreen-surrounding the discharge space aswell as for the central part 'lOf the outer-wall. This metal vpartsurrounds the electrodes and causes-theelectrons to fall on the central -metal' par't of the screen. 'Thereby' the load ,isxevenly distributed over :the :whole :length whereby constant conditions are Obtained during the dischargev and variations in the cathodic ray beam and thus in the size of the cathodic spot is prevented.

It is found, however, that not only the electrons falling on the wall bounding the discharge space have such a detrimental action on the discharge and on the constancy of its operating conditions, but also that the metal particles falling on the wall, and due to the atomizing and vaporizing of the electrode material, may also have such an action. If this wall is of glass, the electric con ditions vary during the life of the discharge vessel as the metal deposit on the glass wall increases constantly during the life of the tube. This drawback is removed by making the central part of the inner screen originally of metal, whereby the projection of metal particles on the wall surrounding the discharge space, while gradually forming an increasingly thick coating, do not influence the conditions of the discharge.

A further increase of safety as to voltage is obtained in such a discharge vessel by having the inner metal part surrounding the discharge space project on both ends by equal amounts beyond the edges of the opposing central metallic part of the outer wall. The size of these two central metallic parts is chosen such that the areas of the annular metal surfaces lying opposing each other are equal. Owing to the difierent diameters of the two metal rings the area of the inner ring would be smaller than that of the outer ring for equal widths of said rings, but this is compensated by making the inner ring larger. If this care is taken to give equal areas to the two rings they form a condenser, in which a homogeneous electric field prevails and points of greater field intensities which might cause perforations, are avoided.

In the accompanying figure is shown by way of example of a form of execution of the invention, a cylindrical Xeray tube having an outer glass wall 1 provided with a central metal part 3. To the outer wall is welded the hollow cylinder 2 the major part of which is also of glass and the central part 4 of which is or". metal in the form of execution illustrated. The size of the metal parts 3 and 4. is so selected that the opposing surfaces have equal areas. Between the glass parts 1 and 2 lies the annular space 5 closed with reference to the inner discharge space and brought to a high vacuum. For the mainoutlet of the X-rays, there are provided in the metal parts 3 and 4 outlet windows 7 and 8, for instance of glass.

I claim:

1. A high voltage electric discharge tube comprising a receptacle and at least two electrodes, said receptacle comprising a wall having a metallic portion and two substantially similar insulating portions on each side of said metallic portion, said electrodes being supported by said insulating portions, a partition dividing said receptacle into two sealed concentric chambers, the inside chamber containing the electrode members and the outer chamber forming a highly evacuated annular. jacket around the first chamber, said partition comprising a metallic portion surrounding the operating portionsv of the electrodes and being insulated from said electrodes and from the metallic portion of the outer wall.

2. An electric discharge tube comprising electrode members and a wall supporting same, which including substantially similar parts of insulating by an integral metallic portion is subdivided into two substantially similar insulating portions, a metal screen disposed within said metallic portion and concentric therewith, said screen surrounding those parts of the electrodes between which the discharge takes place and extending beyond the edges of said metallic portion, and substantially similar insulating members between said screen and each of said electrode members.

An electric discharge tube having electrode members and a wall supporting same, said wall comprising a metallic part and two substantially similar insulating portions, a metal screen arranged concentrically within the metallic part, said screen being insulated from the electrodes and said metallic part, and surrounding the electrodes at their portions where the discharge takes place, said screen having substantially the same area as the said metallic part.

4. A high voltage X-ray tube for direct current operation comprising a cylindrical envelope having a metallic ring and two substantially similar insulating portions, one on each side of said ring, electrodes in said envelope, a substantially cylindrical partition disposed within said envelope, interposed between said electrodes and said envelope, and hermetically sealed to said envelope, and comprising a metallic ring opposing said first ring and concentric therewith, said first andsecond rings surrounding the operating portions of the electrodes, and X-ray transmitting windows provided in each of said metallic rings topermit the X-rays to pass from said tube, the window of the first ring being slightly larger than the window of the second ring.

5. An electric discharge tube, electrodes therefor, an enveloping wall structure which mechanically connects said electrodes, said wall structure comprising a middle portion of conductive material and two substantially insulating portions on each side of said conductive material, each insulating portion supporting one electrode, a second envelope within said first envelope and surrounding said electrodes, said second envelope being welded'to the first envelope and forming a sealed chamber therein, the chamber formed between the first and second envelope forming a high vacuurn insulating jacket.

waist section of conductive material, a screen interposed between said wall and said electrode members and hermetically sealed to said wall to form a sealed high vacuum chamber between said wall and said screen.

'7. An electric discharge tube having electrode members between which during the operation of the tube a high potential difference occurs, said members being connected to each other by a wall material, said wall having at its middle portion a waist section of conductive material, a screen having a metallic waist section opposing the first waist section interposed between said wall and said electrodes and welded to said wall, to form a sealed highly evacuated chamber between said wall and said screen.

. LEOPOLD voLKEL.

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