US1899568A - Cathode structure for vacuum tubes - Google Patents

Description

W. HOFMANN Feb. 28, 1933.

CATHODE STRUCTURE FOR VACUUM TUBES Filed Aug. 1927 OXIDE COATING Inventor": Walter- Hofmann,

Hts Attorney Patented Feb. 28, 1933 UNITED STATES WALTER HOFMANN, F RUDOLSTADT IN THURINGIA, GERMANY, ASSIGNORTO GENERAL ELECTRIC COMPANY, A CQRPORATION NEW YORK:v I

GATHODE STRUCTURE FOR VACUUM TUBES I Application filed August 1, 1927, Serial No 209,858, and in Germany August 7, 1926.

The present invention relates to vacuum tubes and more particularly to a cathode structure adapted to be employed in connection with X-ray apparatus.

When vacuum tubes and particularly tubes of the X-ray type are operated at high voltages, electronic discharges sometimes occur from portions of the apparatus other than the cathode electrode, due to existing edges within the tube or to surfaces having relatively small radii of curvature. Such discharges are harmful and are specially noticeable when the electrodes are spaced a short distance apart, such arrangement of elec- 5 trodes being generally quite advantageous.

To overcome this difliculty, it has been heretofore proposed that within the area of the strong electric field, the portion of the cathode structure, other than the part designed to emit electrons, be made everywhere slightly curved and that the surfaces of slight curvature be smoothened. 7

During the manufacture of vacuum tubes and especially high vacuum tubes, it is necessary during the pumping operation to bring all metal parts within the tube to as high a temperature as possible. At such a temperature it is usually found that metal points and sharpened edges which have been depressed in the polishing process become straightened out and later in the operation of the vacuum tube send out electronic discharges.

The capacity for emitting electrons varies with different materials and is inferior for all other materials than metal. In accordance with the present invention such parts of the cathode system in vacuum tubes which are not intended to send out any electronic discharge are formed of other materials than metal, or if formed of metal, refractory nonmetal coatings are applied tothe metal. Coatings particularly well adapted for use under such conditions are oxides, the structures of which are substantially constant at the high temperatures involved in the operation of vacuum tubes and which do not vary under ion or electron bombardment. I have found that chromium oxide is particularly well adapted for this purpose.

The novel features which I believe to be characteristic of my invention are set forth with particularity in the appended claims.

My invention itself, however, will best be understood from reference to the following specification when considered in connection with the accompanying drawing in which the single figure represents a sectional view partly broken away of an X-ray tube embodying the features of my invention.

Referring to thedrawing, I have indicated at 1, an evacuated receptacle having a reentrant stem 2 on which an elongated metallic cylindrical member 3 comprising a portion of the cathode structure is mounted. Memher 8 at its outer end is slightly depressed,'as

indicated at 4, and within the depressed portion a cathode or electron-emitting electrode 5 is mounted. The member 3 closely surrounds the cathode element 5. A pair of leading-in wires 6 and 7 are connected to the member 3 and electrode 5, respectively, so

that a potential may be supplied thereto in the normal operation of the tube. A cooperating anode or target 8 is mounted 0pposite the electrode 5. To prevent electron Eerature conditions and electron and ionbomardment and restricts the electron emission to the electrode 5 alone.

What'I claim as new and desire to secure by Letters Patent of the United States, is

1. An electric discharge device comprising an evacuated receptacle which terminates in a reentrant stem, a cathode and a cooperating .Such a coating re mains substantially constant under high temanode contained in the receptacle, a support for the cathode comprising a metal tube secured at one end to said stem and supporting the cathode atthe other end, said tube being coated withrefractory oxide and having its edges rounded whereby from the tube is precluded.

2. An electric discharge device comprising an evacuated receptacle which terminates in electron emission a reentrant stem, a cathode and a cooperating anode contained in the receptacle, a support for the cathode comprising a. metal tube which fits over the reentrant stem and is secured thereto, 5 at the other end said cathode being mounted of the tube remote from the stem, said-tube being coated with chromium oxide and havlng its edges roundedwhereby electron emission from the tube is precluded.

In witness whereof, I have hereto set my hand this 30 day of June, 1927. WALTER HOFMANN.

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