US1987998A - Discharge tube - Google Patents

Description

Jan. 15, 1935 D. v. EDWARDS AL 1,987,998

DI SCHARGE TUBE Filed July so, 1952 INVENTOR S DWI! M zdwm MM/(M Patented Jan. 15, 1935 UNITED STATES PATENT OFFlCE DISCHARGE TUBE Donald V. Edwards, Montclair, and Earl K. Smith, ,East Orange, ,N. J., assignors to Electrons, Inc.

of Dela'waraa corporation of Delaware Application July 30, 1932, Serial No. 626,619 6 Claims. (Cl. ate- '15) This invention relates to gaseous discharge tubes andis of peculiar'utility in connection with the type of discharge tube known as control rectifiers in which the starting of the'gaseous' discharge betweenthe cathode and anode is controlled by a third element.

In this type of rectifierit is important that the characteristics of the tube shall remain'uniform throughout its life and also among similar tubes when such tubes are manufactured in quantity. It is of especial importance that the starting of the tube shall, for a given potential applied to the grid, occur at the same point on the curve of cathode-anode voltage in tubes which are in tended to replace other tubes of like construction, and that such point shall not be subject to shifting during the life of the tube, especially when the tube has stood idle without load, but with filament lighted for some time.

One of the causes of such shifting of control and variation of characteristics is the deposit of emissive materials such as barium, strontium, calcium, sodium, or their salts, upon the surfaces of normally non-emissive electrodes, such as grid or plate.

Since barium, strontium or calcium is usually a constituent of the cathode, and sodium salts are often present in the electrodes or gases, it is desirable that they be prevented from depositing on the non-emissive electrodes. However, the heating to which the electrodes are subjected to remove gases therefrom does not ordinarily remove these substances, since barium oxide and sodium chloride, in which form these substances are usually present, are not vaporized from electrodes by heat alone unless the heat exceeds the melting point of nickel, which is commonly used as the electrode material. Hence, in the ordinary process of evacuation, in which the temperature must be kept below that of the melting point of nickel, more or less barium oxide, sodium chloride or the like, will remain.

It is the object of our invention to remove vaporizable electro-positive substances which would remain on normally non-emissive electrodes after the usual evacuation process.

The invention will be explained with reference to the drawing, in which is shown diagrammatically a control rectifier and an apparatus employed in practicing our invention. In this drawing, 1 represents a control tube having the usual cathode 2. 3 is a grid or control element, and 4 is the anode.

In evacuating tube 1 and its electrodes, the cathode 2 may be heated from a current source 5 with or without a potential on the anode from a source 6 and the electrodes may also be heated inductively in the usual manner, the gases being removed meanwhile by a pump connected to a stein '7. I 7' After the tube 1 has been evacuated, the oathode 2 is disconnected, as by opening switch 8, an inert gas such as argon is introduced through the stem 7, and a suitable source of periodically reversed current 9 is connected between the grid 3 and the anode 4, by suitable means such as a switch 10. A series resistance 11 is introduced into the circuit for the purpose of limiting the current fiow. Under ordinary conditions we prefer a gas pressure of about 5 millimeters and a voltage in the grid-anode circuit sufiioient to cause an arc discharge between points on the grid 3 and the anode 4 where there are particles of emissive material. Ordinarily the current is limited to from 2 to 3 amperes. The current, applied voltage, and gas pressure depend to some extent on the electrode construction and the current rating of the tube.

The are so established vaporizes the emissive material at the spot where it first forms and then moves from one particle to another over the surfaces of the electrodes until all deposits have been vaporized. The vapors finally condense on the tube walls or are absorbed by getter substances which may be present in the tube.

The gas employed should be such that the ionization potential thereof is greater than the ionization potential of the objectionable electropositive metals in order that the metals may ionize in preference to the gas and support the arc.

The applied potential and the gas pressure therefor should be so chosen that they will not support an arc after all traces of electro-positive metals have been removed from the grid and plate. In practice this condition is indicated by the are automatically changing to a glow discharge.

Tubes treated in this manner have been found to be free of objectionable vaporizable substances and will preserve their characteristics, and tubes made in quantities will have uniform characteristics.

We claim:

1. The method of removing deposits of highly electro-positive metal from non-emissive electrodes in a discharge tube, which comprises applying a periodically-reversed potential between two such electrodes with the cathode disconnected, admitting an inert gas to the tube at a pressure which will support an arc discharge between a particle of said electro-positive metal on one electrode and the other electrode, and causing the arc to change to a glow discharge after saidv deposits have been vaporized.

2. In an electron discharge tube containing an emissive electrode and a plurality of non-emissive electrodes, the method of cleaning the nonemissive electrodes of deposits of emissive material, which comprises filling the tube with inert gas and vaporizing said deposits by establishing an arc discharge therebetween, the emissive electrode being unheated and disconnected.

3. In an electron discharge tube containing an emissive electrode and a plurality of non-emissive electrodes, the method of cleaning the non-emissive electrodes of deposits of emissive material, which comprises filling the tube with a rare gas having a greater ionization potential 'thansaid deposits and vaporizing said deposits by establishing an arc discharge therebetween.

4. The method of cleaning normally nonemissive electrodes of surface deposits of emissive material, which comprises causing an are between said electrodes to travel over their surfaces from one particle to another of the emissive material until all of said material has been vaporized.

5. The method of cleaning normally nonemissive electrodes of surface deposits of objectionable electro-positive substances, which comprises causing an are between said electrodes to travel over their surfaces from one particle to another of said substances and causing the arc to change automatically to a glow discharge when all of said substances have been vaporized.

6. The method of removing deposits of objectionable electropositive substances from nonemissive electrodes in a discharge tube, which comprises applying a periodically-reversed potential between two such electrodes with the cathode disconnected and admitting a rare gas to the tube, the combination of the gas pressure and potential being chosen to strike an arc between points on said electrodes where there are deposits of said substances, said gas having a higher ionization potential than the substances to be removed.

DONALD V. EDWARDS. EARL K. SMITH.

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