US2053501A

US2053501A - Thermionic gaseous discharge rectifier

Info

Publication number: US2053501A
Application number: US8435A
Authority: US
Inventors: Perey L Spencer
Original assignee: Raytheon Production Corp
Current assignee: Raytheon Production Corp
Priority date: 1935-02-27
Filing date: 1935-02-27
Publication date: 1936-09-08
Anticipated expiration: 1953-09-08

Description

ZJQSEEM Sept S, WEQQ P. L. SPENCER THERMIONIC GASEOUS DISCHARGE RECTIFIER Filed Feb. 27, 1955 INVENTOR PERCY LSPENCER BY 82 ATTORNEY Patented Sept. 8, 1936 UNITED STATES 2,053,501 THERMIONIG GASEOUS DISCHARGE RECTIFIER Percy L. Spencer, West Newton, Masa, assignor to Raytheon Production Corporation, Newton, Mass., a corporation of Delaware Application February 27, 1935, Serial No. 8,435 6 Claims. (01. 250-2'7.5)

This invention relates to thermionic rectifiers, and more particularly to such rectifiers in which the cathode is heated to temperature of thermionic emission solely by means of the discharge 55 current.

An object of this invention is to devise such a rectifier which may be subjected to a large number of starting operations and still possess a life sufiiciently long for commercial purposes.

Another object of my invention is to produce such a device in which the voltage drop is low.

A further object of my invention is to devise a simple, effective and durable structure which will accomplish each of the above purposes.

The foregoing and other objects of my invention will be best understood from the following description of an exemplification thereof, reference being had to the accompanying drawing wherein the figure is a side view of an embodiment of my invention, with certain parts thereof in cross-section.

In gaseous space discharge devices using thenmionic cathodes, such cathodes are usually heated to temperature of thermionic emission by heating 1% current passed through the cathode by an independent source of current or by a separate heating element. In order to simplify this type of device, it is desirable to eliminate the independent heating arrangement and cause the discharge current which passes through the device to raise the cathode to temperature of thermionic emission as well as maintain it at said temperature during operation. It is further desirable that the above simplification be accomplished without the sacrifice of low voltage drop and consequently relatively high efficiency which is obtained with the independently heated type of cathode. In order to obtain such low voltage drop, it is desirable to have the cathode coated with materials whichemit electrons at relatively low temperature, such as, for example, the oxides of the alkaline earth metals. If it is attempted to heat a coated cathode to temperature of thermionic emission by the discharge in which the starting 5 current is allowed to impinge upon substantially the entire coated cathode, the coating is subjected to severe bombardment by relatively fastmoving positive ions, which in a relatively short time destroys the coating and ends the useful life of the tube. In accordance with my invention I have discovered that the desired results can be secured if a small coated cathode is used, of which the major portion is protected during starting from bombardment by positive ions and 55 the starting current is allowed to impinge only upon a projection on said cathode arranged so that this starting current can heat said cathode to temperature of thermionic emission.

In the drawing I have shown a rectifier incorporating the above principles and consisting of a 5 glass envelope I having a reentrant stem 2 with a press 3 at the upper end thereof, said press carrying a cathode 4 and a plurality of anodes 5.

The cathode 4 consists of a few turns of small refractory metal wire, such as, for example, tan- 1o talum wire. In a practical embodiment of my invention I have used tantalum wire of .0"3 inch diameter. The cathode 4 carries at its upper end a projection 6 which is a continuation of the wire forming the turns of said cathode. The 15 cathode is coated with an electron emissive coating, preferably in the manner as will be described below. Closely surrounding the turns of the cathode is a shielding sleeve 1 made of any suitable metal, such as, for example, nickel. This 6 sleeve I is supported on the press 3 by means of a tubular projection 8 formed on said press. The projection 5 extends from the cathode beyond the upper end of the shield I.

Each anode 5 is made of a short rod of a re- 5 fractory conducting material, such as, for example, graphite or tantalum. Each anode 5 is supported by an insulating sleeve 9, preferably of lava, which in turn is supported by a tubular glass projection l0 formed on the press 3. The 30 cathode 4 isprovided with a lead-in conductor l I which is sealed in the press 3 and extends through said press and reentrant stem 2 to the exterior of the tube. Each anode 5 is likewise provided with a lead-in conductor l2 sealed in the press 3 and 35 extending therethrough and through the reentrant stem 2 to the entrance of the tube. The tube I may be provided with a conventional base l3 having a plurality of external contact prongs. The anode conductors l2 are connected to corre- 4o sponding anode contact prongs l4, and the cathode conductor II is connected to the cathode contact prong l5.

Although my device may operate satisfactorily with various types of electron-emissive cathode coating, I prefer to use a special coating process which produces a cathode surface which is very durable and able to withstand discharges of arc intensity for relatively long periods. I preferably first oxidize the surface of the cathode in 5 any suitable manner as by heating in a gas flame. Thereafter I coat the cathode with a mixture of solutions of alkali metal compounds, preferably the nitrates of barium, strontium, potassium, and if desired other alkaline earth or alkali metals. These compounds are bln-ned into the surface of the metal by an intense flame which is preferably a reducing flame. I have found that a flame produced by illuminating gas and air'is satisfactory for this purpose. The burning process is carried out until the surface of the cathode appears to become white, encrusted and rough while the metal itself becomes brittle. This coating and burning process may be repeated several times.

The cathode coated in the above manner is mounted in the tube as described above. The tube is then evacuated in accordance with-the usual practice. During the evacuation the oath-- ode is subjected to further treatment to convert the coating thereof to its final form. In order to do this, a discharge is passed through the device sufficient to heat the cathode to a relatively high temperature at which the coating on the cathode is further broken down. Under these conditions the cathode is no longer brittle, and the coating is presumably reduced to the oxides of the alkali metal together with possibly some pure alkali metal impregnated into the surface of the cathode.

The tube as constructed in the above manner is freed of occluded gases in accordance with the usual practice, and is then filled with a gas at sufficiently high pressure to produce copious ionization therethrough upon the passage of a discharge through the device. I preferably use a rare gas, such as, for example, neon, helium, or argon, or a mixture of such gases at a pressure of about five millimeters of mercury. Due to the comparatively low ionizing voltage of argon, I prefer to use argon as the gaseous filling.

The tube which I have illustrated may be connected in any suitable circuit, such as that illustrated diagrammatically in the drawing. A transformer l6 may have its primary II connected to a source of alternating current, and its secondary l8 may be connected at its opposite ends to the two anode prongs M. The center of the secondary winding may be connected through a suitable load I! to the cathode prong I 5.

In accordance with my present understanding of the theory of operation of the device, the tube operates substantially as follows. When the transformer I6 is energized, a voltage is impressed upon the device, between each of the anodes 5 and cathode 4. Under these conditions, a glow discharge will start through the device, and the glow current will concentrate upon the exposed areas of the anode 5 and upon the projection 6 of the cathode l. The spacing between the shield I and the turns of the cathode is sufliciently close so that this glow discharge current does not impinge upon any part of the cathode except said projection 6 The voltage drop which occurs through the tube during the starting op- ,eration is relatively high, and a considerable amount of heat is*liberated at the cathode in the vicinity of the projection 6. This heat travels down to the relatively small cathode l, and quickly heats it to a temperature at which said cathode emits thermionically. Since the turns of the cathode are connected in series with the projection 6,- the current which flows to said projection 6 likewise flows through the turns of the cathode l, and this current likewise tends to heat the cathode to its operating temperature. As soon as the cathode is heated. to temperature of thermionic emission, the coated turns start to emit electrons and the current then no longer concentrates solely upon the projection 8 but is drawn from the coated turns of the cathode. Under these conditions the drop through the tube falls to a low value. In practical embodiments of my invention I havesecured voltage drops of about ten volts at a load of about forty millianaperes. It will be seen that, due to the presence of the shield I, the main coated portion of the cathode is not sub- .jected to positive ion bombardment during starting, this bombardment being confined substantially solely to the projection 6. The fact that any coating which exists on the projection 8 may be eventually dislodged therefrom, due to such bombardment, has no detrimental effect upon the operation of my device inasmuch as it operates satisfactorily without any coating on said pro ection 6. Furthermore, the shield 1 prevents radiation of heat from the coated portion of the cathode, and therefore but a comparatively small amount of energy is necessary to maintain the cathode at its operating temperature. Thus during operation a negligible amount of energy is utilized in keeping the cathode hot, and thus the voltage drop through the tube can be very low and still provide sufficient energy to keep the cathode at its proper temperature. It is probably due to this arrangement that the voltage drop through the tube can be reduced to the low values which I have been able to obtain with my device.

This invention is not limited to the particular details of construction, materials or processes as described above, as many equivalents will suggest themselves to those skilled in the art. It is accordingly desired that the appended claims be given a broad interpretation commensurate with the scope of the invention within the art.

What is claimed is:

' 1. An electrical space discharge device comprising an envelope containing an ionizable gas, electrodes adapted to support an ionizing discharge through said gas, one of said electrodes being a thermionic cathode, said cathode having thereon a coating of electron-emissive material and adapted to be heated to temperature of thermionic emission solely by the discharge, means electrically insulated from said coated cathode to prevent the glow discharge which occurs during starting from impinging on said coated cathode, and a projection electrically connected to said cathode and projecting beyond said means on which projection the starting glow discharge current impinges.

2. An electrical space discharge device comprising an envelope containing an ionizable gas, electrodes adapted to support an ionizing dis charge through said gas, one of said electrodes being a thermionic cathode, said cathode having thereon a coating of electron-emissive material and adapted to be heated to temperature of thermionic emission solely by the discharge, a shield surrounding said coated cathode and electrically insulated therefrom to prevent the glow discharge which occurs during starting from impinging thereon, and a projection electrically connected to said cathode and projecting beyond said shield on which projection the starting glow discharge current impinges, said shield having an opening through which the discharge can pass when said cathode has been heated to temperature of thermionic emission.

3. An electrical space discharge device comprising an envelope containing an ionizable gas, electrodes adapted to support an ionizing discharge through said gas, one of said electrodes being a thermionic cathode, said cathode having thereon a coating of electron-emissive material and adapted to be heated to temperature of thermionic emission solely by the discharge, a metal tubular shield surrounding said coated cathode and electrically insulated therefrom to prevent the glow discharge which occurs during starting from impinging thereon, and a projection electrically connected to said cathode and projecting beyond said shield on which projection the starting glow discharge current impinges, said shield having an opening through which the discharge can pass when said cathode has been heated to temperature of thermionic emission.

4. An electrical space discharge device comprising an envelope containing an ionizable gas, electrodes adapted to support an ionizing discharge through said gas, one of said electrodes being a thermionic cathode, said cathode having alkaline earth metal compounds burned into its surface and adapted to be heated to temperature of thermionic emission solely by the dis-- charge, a shield surrounding said coated cathode and electrically insulated therefrom to prevent the glow discharge which occurs during starting from impinging thereon, and a projection electrically connected to said cathode and projecting beyond said shield on which projection the starting glow discharge current impinges, said shield having an opening through which the discharge can pass when said cathode has been heated to temperature of thermionic emission.

5. An electrical space discharge device comprising an envelope containing an ionizable gas, electrodes adapted to support an ionizing discharge through said gas, one of said electrodes being a thermionic cathode, said cathe having alkaline earth metal compounds burned into its surface and adapted to be heated to temperature of thermionic emission solely by the discharge, a shield surrounding said coated cathode and electrically insulated therefrom to prevent the glow discharge which occurs during starting from impinging thereon, and a projection electrically connected to said cathode and projecting beyond said shield on which projection the starting glow discharge current impinges, said shield having an opening through which the discharge can pass when said cathode has been heated to temperature of thermionic emission, said cathode being sufliciently small and said shield preventing loss of radiant energy from said cathode sufliciently to produce a discharge voltage drop of the order of the ionization voltage of the gas or less.

6. An electrical space discharge device comprising an envelope containing argon at a pressure of about five millimeters of mercury, electrodes adapted to support an' ionizing discharge through said argon, one of said electrodes being a thermionic cathode, said cathode having thereon a coating of electron-emissive material and adapted to be heated to temperature of thermionic emission solely by the discharge, a shield surrounding said coated cathode and electrically insulated therefrom to prevent the glow discharge which occurs tiuring starting from impinging thereon, and a projection electrically connected to said cathode and projecting beyond said shield on which projection the starting glow discharge current impinges, said shield having an opening through which the discharge can pass when said cathode has been heated to temperature of thermionic emission.

PERCY L. SPENCER.