US2155237A

US2155237A - Electric discharge device

Info

Publication number: US2155237A
Application number: US175183A
Authority: US
Inventors: Laurence F Perrott
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1937-11-18
Filing date: 1937-11-18
Publication date: 1939-04-18
Anticipated expiration: 1956-04-18
Also published as: DE907676C; FR846160A; GB521850A; US2141933A

Description

April 18, 1939. L, PERRGTT 2,155,237

ELECTRIC DISCHARGE DEVICE Filed Nov. 18, 1937 m B R R R A/VOE cu/mavr 64mm.) M ii 0 m a0- so a0 a0 ma //0 T/ME (sew/v05) Inventor Lawren ce FT Perrotc His Attorhev.

Patented Apr. 18, 1939 V ATENT OFFIQE ELECTRIC DISCHARGE DEVICE Laurence F. Perrott, Revere, Mass., assignor to General Electric Company, a corporation of New York Application November 18, 1937, Serial No. 175,183

3 Claims.

The present invention relates to electric discharge devices and more particularly to improvements in cathodes for such devices. While not limited thereto, the invention is especially applicable to discharge devices of the type which employ a readily ionizable medium such as mercury vapor in the discharge space.

Cathodes comprising alkaline earth materials are highly efiicient in operation where efficiency is measured in terms of emission current per watt of heating energy supplied to the cathode. However, due to the vaporization characteristics of the alkaline earth metals it is impracticable to operate such cathodes at temperatures appreciably above 950 C. At this temperature the emission current which can be derived from a unit of cathode area is so small that a relatively extended cathode surface must be provided if considerable total emission is desired.

While the use of an extended surface cathode is in no way objectionable after normal operation is established, it involves the disadvantage of prolonging unduly the time required to heat the cathode to an effective emitting temperature during initial starting periods. For many applications where instantaneous availability of the discharge device is desirable this is a highly objectionable feature.

It is an object of the present invention to provide a cathode which is as highly efiicient in normal operation yet which is capable of assuming full load within at least a few seconds after its initial energization.

It is a further object of the invention to provide a quick-acting cathode which is characterized by a long commercially useful life.

According to a preferred embodiment of the invention the foregoing objects are accomplished by providing a composite cathode combining an extended surface having an alkaline earth coating with a resistance heater which is of low thermal inertia and which also has an electron emitting portion capable of carrying the full load current of the device for at least a substantial period of time. Preferably the cathode parts are so constructed that while the discharge current may initially be carried by the resistance heater alone, it is eventually transferred to the alkaline earth coated surfaces as normal operating conditions are attained.

The features of novelty which I desire to protect herein are pointed out with particularity in the appended claims. The invention itself, together with further objects and advantages thereon may best be understood by reference to the following description taken in connection With the drawing in which Fig. 1 represents a discharge device suitably embodying my improved form of cathode; Fig. 2 comprises a detailed view in partial section of a cathode constructed in accordance with the invention; Fig. 3 is a plan view looking down on the top of the construction of Fig. 2; and Fig. 4 is a graphical representation useful in explaining the operation of my improved cathode.

Referring particularly to Fig. 1 I have shown a discharge device comprisinga sealed envelope ID, for example, of glass, enclosing discharge electrodes and a quantity of a readily ionizable medium adapted to facilitate the maintenance of an arc-like discharge. comprise, for example, vaporizable substance such as mercury, a fixed gas such as argon, or the combination of a vaporizable substance and a fixed gas.

The electrode structure comprises an anode ll, suitably of graphite, and a composite cathode comprising a hollow member l2 and a resistance heater I3 for helping to maintain the cathode at a temperature of effective electron emission. Lead-in conductors l5, l6, and I1 are sealed through the walls of the envelope for energizing the anode and cathode and for supplying heating current to the heater [3 from a suitable potential source (not shown) connected between the conductors l6 and IT. The details of the cathode construction are shown in Figs. 2 and 3 in which parts illustrated in Fig. 1 are similarly numbered.

In accordance with the invention, one part of the cathode comprises a member or members providing relatively extended surfaces exposed to the discharge space and having a coating of a material which has electron emission character'- istics comparable to those of alkaline earth materials. Such surfaces may be provided in various ways as by the use of vaned or corrugated structures or by utilizing a hollow enclosure having an opening of substantial area therein. In the cathode illustrated in Fig. 2 the latter ar rangement is exemplified by the provision of an elongated hollow metal cylinder 52, for example of nickel, having an open end thereof directed toward the anode II. In order to increase its mechanical strength the cylinder I2 is provided at each end with a circumferentially extending beading or corrugation 19. It is coated either :15 Such medium may interiorly or exteriorly or both with .an alkaline device the cylinder I2 is heated to a temperature of effective electron emission (about 950 C.) by the effects of the arc drop and by the use of a resistance heater in good heat-exchanging relation therewith. Such a heater is illustrated in the present case as a spirally coiled filament l3 having its axis substantially co-linear with that of the cylinder l2, although it is evident that other forms may be employed. For instance, the heater itself may be of complex construction embodying a resistance element and another element indirectly heated thereby.

For the purposes of the present invention, it is desired that the heater combine low thermal inertia with high electron emissivity. The former quality is achieved by limiting the mass and area of the heater as by giving it a filamentary form such as that shown. The quality of high emissivity is best attained by so proportioning the heater as to cause it to operate at a very high temperature (in response to the passage of a-heating current therethrough) and by embodying in it an electron emissive material adapted to function satisfactorily at such temperature. Morespecifically, I prefer to employ for this purpose a material which is. capable of operating without excessive vaporization at temperatures appreciably above the permissible operating range of alkaline earth materials and which is capable at such temperatures of developing for at least substantial periods of time an electron emission current. equal to the full load cur-. rent of the discharge device.

A specific material which admirably fulfills the requirements set forth in the foregoing comprises tungsten, especially when used in connection with an activating coating such, for example,

as thorium oxide, 1. e. thoria. (A tungsten filament coated with thoria has an emissivity at 2000 C. many times in excess of that of tungsten alone). I prefer to use as a heater body, a filament comprising a tungsten mandrel wire overwound with a helix of finer tungsten wire, the whole being coated with thoria and activated by heating in vacuum or in an inert gas at about 2800 C. In Figs. 2 and 3 a filament I3 of this type is shown as being suspended by means of a conducting supporting member 20 which is set into a conducting rod 2| on which the cylinder I2 is mounted.

The operation of the invention may best be understood by referring again to Fig. l in which the cathode is assumed to be of a construction such as'that described. If a discharge producing potential is impressed between the anode H and the cathode l2 simultaneously with the supplying of heating current to the filament l3, the latter element will reach a temperature of effective electron emission very quickly. Due to the low thermal inertia of the filament for example, a temperature of 2000 C. may be attained within at least a few seconds. Under these conditions, the discharge device may almost immediately pass full load current, such current being drawn from the exposed surfaces of the filament even though the cylinder. I2 is at too low a temperature to contribute appreciably to the discharge. However, as the cylinder gradually heats up due to the generation of heat by the filament and byrthe arc discharge, an increasing proportion of the discharge current will be diverted to the activated surfaces of the cylinder.

This diversion of current and its variation with time is illustrated in Fig. 4 in which the curve A represents the portion of the discharge current supplied by the filament l3 and the curve B represents that portion supplied by the cylinder I2, these being plotted against elapsed time following the initial energization of the device. Referring to the curve A, it will be noted that the filament current rises almost immediately (actually within about 1 seconds) to a value corresponding to the full load current of the discharge device. Thereafterthe filament current begins to decline as a result of the increased diversion of current to the more extended surfaces of the cylinder l2. After about sixty seconds, of the total current will be drawn from the cathode cylinder and only the residual 20% from the filament.

The fact that the discharge current is progressively diverted to the cathode cylinder and does not continue from the filament itself is explainab-le in part by the relatively high arc drop required to maintain adequate emission from the filament. Due to the smalltotal area of the filaments emitting surfaces the field free emission of the filament may be less than that required by the discharge. Consequently this deficiency in emission will be compensated by an increase in the cathode drop and a corresponding increase in the total are drop. However, as soon as the cathode cylinder reaches an effective emitting temperature, the discharge current may be supplied from this source with a lower arc drop 1;

than is required to maintain the necessary emission from the filament. Consequently, the arc will seek the path of least resistance and will be transferred at least preponderantly to the cylinder.

It is important to note that the heating current supplied to the filament I3 need not be in excess of that required to maintain the cylinder l2 at the desired temperature during the normal operating period, taking into account the assistance;

mionic emission alone all the electron currenti required during the initial starting period. The deficiency will be made up, however, by field emission due to a temporary increase in the arc drop as previously explained. Because of the exist-.

ence of this mechanism for temporarily enhancing the filament emission, the cathode as a whole may be operated at all times with a heating energy input no greater than that required to obtain effective emission from the alkaline earth material. ciency of such material the cathode will therefore be characterized by a high degree of efiiciency. (Measured in terms of emission current per watt of heating energy supplied.)

It will be understood that the ion bombardment which is incidental to the high are drop obtaining during the initial starting period of the device would tend to destroy the filament if continued for a long period of time. However, due to its relatively short duration its actual destructive eifects are substantially negligible. Thus a filament which might be consumed in a relatively few hours if continuously subjected to ion bombardment, will nevertheless survive many thous- Due to the inherently high emitting effiand starting operations under the special conditions stated herein.

A further advantage which is attributable to the presence of a high are drop during the initial starting period is the effect of the correspondingly high arc losses in decreasing the heating time of the cathode cylinder l2. It will be understood that during such starting period this cylinder receives heat not only by direct radiation from the filament itself but also by radiation from the arc. The latter component is substantially increased by the existence of a high are drop.

While I have referred particularly to the use of thoria as a preferred emitting material for use with a resistance heater it will be understood that other substances may be alternatively employed. For example, one may use for this purpose thoriated tungsten providing a sufiiciently high gas pressure is employed in the discharge device to prevent stripping or de-activation of the filament material by excessive bombardment. One may also employ uranium oxide as a substitute for thorium oxide as a coating material. Furthermore, while I have exemplified the invention by reference to a discharge device employing a single anode, it is equally applicable to devices in which two or more anodes are used. I aim, in the appended claims, to cover all such modifications of structure and use as come within the true spirit and scope of the foregoing disclosure.

What I claim as new and desire to obtain by Letters Patent of the United States is:

1. An electric discharge device comprising an envelope enclosing a quantity of a readily ionizab'le medium, an anode and a cathode of the typewhich requires a continuous supply of heating current thereto, said cathode including a relatively massive structure of high thermal inertia coated with alkaline earth material and exposed to the discharge space and a quick-acting element comprising a heater of low thermal inertia effective to maintain the alkaline earth material at its proper emitting temperature during normal operation of the device, the heater being also exposed to the discharge space and having a coating of a substance having substantially the emission characteristics of thoria whereby it is enabled to supply the full load current of the device during starting periods without excessive vaporization.

2. An electric discharge device comprising an envelope enclosing a quantity of a readily ionizable medium, an anode, and a cathode including a hollow metal structure of high thermal inertia coated interiorly with an alkaline earth material and a filamentary heater effective by virtue of its heating properties to cause the said coated structure to develop substantially full load current during normal operation of the device, the

said heater being exposed to the discharge space and being coated with thoria whereby it is enabled to carry the full load current of the device without excessive vaporization during initial operating periods.

3. A cathode comprising a hollow open-ended metal cylinder having a coating of alkaline earth material applied to the surfaces thereof and a heating filament of overwound tungsten arranged within the said cylinder, the filament being coated with thoria whereby it is enabled to develop the full rated emission current of the cathode during starting periods and being proportioned to heat the said cylinder to a temperature of eifective electron emission during normal operation.

LAURENCE F. PERROTT.