Title:
SERIES-CONNECTED THERMIONIC ENERGY CONVERTERS
United States Patent 3863081


Abstract:
A series-connected assembly of thermionic devices for the generation of electrical power having a series of cylindrically spaced apart, axially aligned, heated cathodes, a tubular cooled anode surrounding each cathode, interelectrode leads connecting each anode to its adjacent cathode, an ionization promoting vapor filling the interelectrode gaps characterized by shielding members positioned between the cathodes and the interelectrode leads and their adjacent higher potential cathodes, a tortuous path communicating between the interelectrode gaps of succeeding devices for circulating the ionization promoting vapor and an insulating member axially engaging the end of each cathode for supporting and centering the cathodes within the anodes.



Inventors:
Jules, Edwin (Baltimore, MD)
Levedahl, William J. (Baltimore, MD)
Application Number:
04/384079
Publication Date:
01/28/1975
Filing Date:
07/21/1964
Assignee:
TELEDYNE, INC.
Primary Class:
Other Classes:
376/321, 976/DIG.317
International Classes:
G21D7/04; H01J45/00; (IPC1-7): H01J45/00
Field of Search:
310/4
View Patent Images:
US Patent References:
3211930Thermionic energy converter1965-10-12Clement et al.



Primary Examiner:
Tubbesing T. H.
Assistant Examiner:
Potenza J. M.
Attorney, Agent or Firm:
Fleit, Gipple & Jacobson
Claims:
1. A series-connected assembly of thermionic devices for the generation of electrical power comprising:

2. A series-connected assembly of thermionic devices for the generation of electrical power comprising:

3. A series-connected assembly of thermionic devices for the generation of electrical power comprising:

4. The combination, with a plurality of substantially identical thermionic energy conversion devices for the generation of electrical power each including a tubular cooled anode, a cylindrical heated cathode mounted within said anode, and an interelectrode gap between said anode and cathode filled with an ionization-promoting vapor in operation, comprising:

5. In combination, a plurality of thermionic devices for the generation of electrical power, said devices being connected electrically in series and each comprising:

6. A series-connected assembly of thermionic diodes for the generation of electrical power comprising:

7. The combination, with a plurality of substantially identical thermionic energy conversion devices for the generation of electrical power each including a tubular cooled anode, a cylindrical heated cathode mounted within said anode, and an interelectrode gap between said anode and cathode filled with an ionization-promoting vapor in operation, of means for connecting said devices electrically in series with substantially equipotential connections between the anode of one device and the cathode of the next succeeding device and for accurately centering the cathode within the anode of each of said devices to provide a preselected interelectrode gap therebetween, said means comprising:

8. In combination, a plurality of thermionic devices for the generation of electrical power, said devices being connected in series and each comprising:

9. In combination: a plurality of axially aligned thermionic diodes for the generation of electrical power, said diodes being connected in series and each comprising:

10. A series-connected assembly of thermionic devices for the generation of electrical power comprising:

11. The combination, with a plurality of substantially identical axially aligned thermionic energy conversion diodes for the generation of electrical power each including a tubular cooled anode, and a cylindrical heated cathode mounted within said anode, with an interelectrode gap between said anode and cathode filled with an ionization-promoting vapor in operation, of means for connecting said diodes electrically in series with substantially equipotential connections between the anode of one diode and the cathode of the next succeeding diode and for accurately centering the cathode within the anode of each of said diodes to provide a predetermined interelectrode gap therebetween, said means comprising:

12. In combination: a plurality of axially aligned thermionic diodes for the generation of electrical power, said diodes being connected electrically in series and each comprising:

13. In combination: a plurality of axially aligned thermionic diodes for the generation of electrical power, said diodes being connected electrically in series and each comprising:

14. A series-connected assembly of thermionic devices for the generation of electrical power comprising:

15. A series-connected assembly of thermionic devices for the generation of electrical power comprising:

16. A series-connected assembly of thermionic devices for the generation of electrical power comprising:

Description:
This invention relates to thermionic energy converters and more particularly to improved constructions for such devices for promoting uniformity in the output power obtainable from such devices and for facilitating the interconnection of a plurality of thermionic devices in series.

Among the important considerations which affect the electrical power output obtainable from thermionic devices such as diodes are those of interelectrode spacing in a single diode and interelectrode connections between successive series-connected diodes. The phenomena of electrical power generation by the thermionic mechanism are essentially thermodynamic. Thus, in the determination of the structure by which interelectrode separations are maintained and interelectrode connections are made, thermodynamic factors are exceedingly influential. Indeed, the thermal gradients in a thermionic power generation system are determinative of the electrical gradients. The electrical gradients on the other hand, if not properly controlled, can alter the essential thermal gradients and adversely effect the performance of the system.

The interelectrode spacing between anode and cathode in a thermionic diode is critical. If it is not uniform the power density in the diode is altered and the power output of the diode either declines or builds to destructive peaks in particular areas of the diode. For example, if one portion of the emitter surface on the cathode moves closer to the collector surface of the anode in such a device during operation, either through structural failure or through lack of precision in the construction of the device, the increased local thermal gradient across the interelectrode gap at that portion leads to a localization and concentration of current flow. As a result, one portion of the cathode and of the anode may become overloaded to the detriment of the thermodynamic properties of the device and other portions of the anode and cathode then fail to function at peak efficiency.

In the generation of higher potentials by thermionic diodes it is common to connect a plurality of diodes in series. Any two such series-connected thermionic diodes are capable of developing a significant potential difference between them. Electrons emitted from the lower potential emitter or from the interelectrode lead connected to it can arc across to the emitter of the next succeeding diode resulting in a short circuit which greatly reduces the useful power output. This problem can become far more serious when many diodes are connected in series, for it is possible to develop an arc along the entire series-connected path from one end to the other through the cathodes and thereby effectively to short-circuit the entire chain of diodes.

This invention has a principal object to maximize the electrical power obtainable from thermionic energy converters through the provision of constructions which promote uniformity of power output within and among a number of devices in a system.

An additional object of this invention is to provide new and improved thermionic diode constructions which properly balance the thermodynamic and electrodynamic principles upon which the operation of such devices are based to improve the power distribution and power density and thereby to increase the operating efficiency of such devices.

This invention has as a still further object to provide means to maximize the electrical power obtainable by thermionic diodes by preventing arc-over between the electrodes of series-connected diodes.

It is a still further object of the invention to provide thermionic diodes with improved means for maintaining the interelectrode spacing therein both constant and uniform to improve the efficiency and power output obtainable therefrom.

By way of a brief summary, the present invention may be embodied in thermionic diode constructions which act simultaneously to center a cathode within a surrounding anode and to prevent arc-over between series-connected diodes while maintaining consistant thermal gradients between the cathodes and the anodes. In one form the invention employs a metallic back-emission shield, a relatively cold member interposed between two csthodes. The back-emission shield may comprise a disc-like member connected to one anode and interposed between two cathodes at different potentials. This back-emission shield is sufficiently thick to conduct any heat which is either received by it from radiation or generated within it by small electrical currents outward to the relatively cool collector or anode with little temperature gradient. The back-emission shield is at essentially the same temperature and electrical potential as is the anode. An interelectrode lead attached to the collector at the same point as the back-emission shield and thus at the same electrical potential with it is connected to the cathode of the next succeeding thermionic diode. The interelectrode lead may have a much smaller cross-sectional area than that of the interelectrode spacer to inhibit the flow of thermal energy therethrough to the anode with which it is connected. Thus, a substantial thermal gradient develops across the interelectrode lead. An insulating member mounted at the central portion of either the back-emission shield or the interelectrode lead has a configuration which mates with that of the cathode mounted within the first anode and centers it axially therein.

Any electrons emitted by the interelectrode lead or by the cathode to which it is connected and travelling in the direction of the adjacent cathode of more positive potential are collected by the back-emission shield and are returned via an internal low resistance conducting path producing a low-loss internal electrical loop. Only electrons emitted by the back-emission shield itself can reach the adjacent more positive emitter. However, since the back-emission shield is at approximately the same temperature as the anode to which it is connected, comparatively few electrons are emitted from it and the short circuiting effect is negligible.

To provide proper out-gassing between the series-connected diodes and effective pressure control of the ionization vapor, flow passages are provided through the back-emission shield and the interelectrode lead. These passages may be located eccentrically such that the ionization vapor in passing through them follows a tortuous route. Alternatively, the passages may be located near the perimeter of the shield and lead in positions not aligned between successive cathodes. This avoids the formation direct line-of-sight emission path between successively interconnected emitters without restricting the flow of the ionization vapor through the series-connected diodes.

Although the scope of this invention is defined in the claims appended hereto, further details of the invention as well as additional objects and advantages will be more readily understood in connection with the following description taken together with the annexed drawings in which:

FIG. 1 illustrates a longitudinal cross-sectional view of an assembly for the generation of electrical power constructed in accordance with this invention and comprising a plurality of series-connected thermionic diodes;

FIG. 2 is a view similar to that of FIG. 1 showing an alternate embodiment of the invention; and

FIG. 3 is a similar view of a still further embodiment of the invention.

Turning attention now to FIG. 1, an assembly may be seen in which a plurality of thermionic diodes 10a, 10b, 10c, and 10d are connected in series. Throughout the following description the same reference numbers are employed to identify corresponding elements in the separate diodes with letter subscripts to designate the particular diodes with which these elements are associated. Each diode includes a tubular anode 11 of circular cross-section having a collector surface 12 on the inner side thereof. Within each of the diodes is located a cylindrical cathode 13 which may have a special emitter coating 14 on the external side thereof. In the embodiment shown the cathode is heated by self-contained fissionable material bearing fuel solid 15.

The nature of the fuel solid 15 as well as its structure and containment within the cathode 13 are not the subject of this invention. Preferably these portions of the diode are constructed as described more fully and claimed in a copending application Ser. No. 384,080 filed in the name of James Monroe on July 21, 1964.

The process by which large amounts of energy are released through nuclear fission reactions is now quite well known. In general a fissionable atom, in this case within the fuel solid 15, absorbs a neutron and its nucleus and undergoes a nuclear disintegration. This produces fission products of lower atomic weight having great kinetic energy, and two or more neutrons also having high energy. The kinetic energy of the fission products is quickly dissipated in the fuel as heat. If, after this heat generation, there is at least one net neutron remaining which induces a subsequent fission, the fission reaction becomes self sustaining and the heat generation by the fuel solid 15 is continuous. The heat generated internally raises the surface temperature of the cathode 13 to a level above 1,500°K, at which temperature substantial numbers of electrons are emitted by the cathode 13 or its emitter coating 14. These electrons travel across the interelectrode space 16 to the collector surface 12 of the anode 11 thereby transporting a current and producing a substantial potential difference between the cathode 13 and the anode 11. To assist in the thermionic mechanism it is preferable for the interelectrode gap 16 to be filled with the vapor of an alkali metal, such as cesium, at a pressure of approximately 0.5 millimeters of mercury.

Despite the narrowness of the interelectrode gap, which may be on the order of 0.01 inch to 0.03 inch, for example, the anode 11 should remain at a much lower temperature than that of the cathode 13 for best results. Therefore, cooling means are provided which are represented in this case by a cooling jacket 20 surrounding the anodes 11. A coolant is circulated through the jacket 20 to maintain the anodes at a temperature between 600°K and 1,300°K.

It is necessary that the anodes 11 be electrically isolated from each other. For this purpose annular insulators 23 are positioned between adjacent anodes 11 and may be sealed to one or both. A layer 24 of electrical insulation, preferably formed of a sleeve of Al2 O3, also surrounds all of the anodes 11. Insulating layer 24 is, however, preferably quite thin, on the order of 0.01 inch, to promote the rapid transfer of heat therethrough. A metal sheath 25 surrounds the insulating layer 24 and is in direct contact with the coolant in jacket 20. The insulating layer 24 may be bonded either to the anode 11 or to the metal sheath 25 or to both.

In the practice of this invention each anode 11 is provided at one of its ends with an integral back-emission shield 30 which is formed of a high conductivity material, preferably the same material employed for the anode itself, and is shaped into a somewhat disc-like configuration. Shield 30 is of substantial cross-section so that despite its proximity to cathode 13, it is at approximately the same temperature as anode 11 with a comparatively small internal thermal gradient.

In this embodiment the back-emission shield 30 has a central aperture 31 within which is positioned a flanged support member 32. Support member 32 is preferably formed of a material such as sintered Al2 O3 which is a good electrical insulator. Support member 32 includes a central neck 33 of reduced cross-sectional diameter which fits within the aperture 31 in the back-emission shield. Within neck 33 an axially centered bore 34 of still smaller diameter is provided.

It is to be noted that cathodes 13 carry on their lower domed end caps 35 a circular extension 36 which fits closely within the bore 34 of the insulating support member 32. Either bore 34 or extension 36 or both may be tapered slightly to effect a precise axial alignment between the members thereby determining the concentricity of one end of each cathode 13 within its surrounding coaxial anode 11 and fixing the interelectrode gap 16. Although the entire surface of each of the cathodes 13 including the end caps 35 is at a temperature elevated substantially above that of the associated anodes, the combination of a thermal insulating support member 32 within a shield 30 of high conductivity and broad cross-sectional area results in the interposition of a relatively cool member between cathodes 13a and 13b and between cathodes 13b and 13c and so on down the series.

Also in accordance with this invention an interelectrode lead 40 is provided to connect the anode 11 of each diode to the cathode 13 of the next succeeding diode. Interelectrode lead 40, which is somewhat conical in shape, is affixed at its outer diameter to the anode 11 at the point of juncture 41 between the anode and the back-emission shield 30. At its innermost diameter the interelectrode lead 40 terminates in contact with a circular flange 42 on the cathode 13 in the next succeeding diode, establishing an equipotentiality between the anode of one diode and the cathode of the next.

The cross-sectional current-carrying thickness of interelectrode lead 40 is, however, thinner than that of the back-emission shield 30, and preferably is as thin as possible consistent with the magnitude of the currents which it must conduct. Thus, a substantial thermal gradient develops across the interelectrode lead 40 between its hot innermost end at 42 and its cooler outermost end at 41. In this embodiment the interelectrode lead 40 also functions as a structural centering member to locate the uppermost end of each of the cathodes as seen in this illustration and thereby to determine with precision the thickness of the interelectrode gap 16.

In filling the interelectrode gaps of the respective diodes with vapors of cesium or other alkaline metals it is desirable to have some continuity between the several diodes so that the pressure of the ionizing vapor may be controlled in all diodes simultaneously and with the same structure. The back-emission shield 30 is therefore provided with at least one port 43 through which the cesium vapor may flow and the interelectrode lead 40 is also provided with a similar port 44. These ports are positioned eccentrically with respect to one another to provide a tortuous path for the flow of the cesium vapor thereby to minimize the possibility of the formation of line-of-sight arcs through the cesium vapor from cathode to cathode.

Attention is directed to the alternate form of the invention shown in FIG. 2. The form and arrangement of the components of this assembly are similar to those in the preceding embodiment and for this reason corresponding elements of the assembly are identified by the same reference numbers as are employed in connection with the first embodiment. A major difference present in this second embodiment resides in the use of the back-emission shield 50 to determine the axial orientation of the cylindrical cathodes on both sides of it. Thus, the disc-like emission shield 50 incorporates at the central portion thereof a recess 51 within which is positioned a cup-shaped support member formed of an electrically and thermally insulating material. The axial extension 54 on the cathode 13 above fits tightly and precisely within the cavity 55 formed in the insulating support member 52. The interelectrode gap 16 and particularly its uniformity is determined in part by the precision of the centering of the cathode extension 54 within the central bore 55 in the insulating member 52.

The configuration of the back-emission shield 50 which provides the recess 51 on one side of the shield results in a dome-like knob 56 on the opposite surface. Over this knob is fitted a plate-like layer 57 of an insulating material. This latter layer 57 fits within the flanged ring 58 projecting symmetrically from the top end of the next succeeding cathode. It can thus be seen that the back-emission shield 50 with its insulators 52 and 57 determines the interelectrode spacing of the cathode structures on both sides of it.

In this embodiment the roughly conical interelectrode lead 60 is not employed in a dual capacity to assist in the centering of the cathode. Interelectrode lead 60 is again attached to the anode 11 at its point of juncture 61 with the back-emission shield 50 and extends into electrical contact with the upper surface of the next succeeding cathode. Also because of the reduced cross-sectional current-carrying thickness of the interelectrode lead 60, a substantial thermal gradient is developed across it although it maintains the anode 11 to which it is connected at a potential equal to that of the next succeeding cathode 13.

A tortuous path for the flow of ionization-promoting vapors between diodes is provided by one or more channels 65 in shield 50 and one or more channels 66 in interelectrode lead 60. This tortuous path, as in the previous example, results in a path between adjacent diodes which is not on a line-of-sight between respective cathodes.

A still further embodiment is shown in FIG. 3 wherein, once again, similar reference numbers are employed to identify elements of the combination which are substantially identical with corresponding elements in the preceding illustrations. In this embodiment the relatively cool back-emission shield 70 has only the one function of preventing short circuiting emission paths between cathodes and does not participate in the centering of the cathodes 13 within the anodes 11.

The centering function is fulfilled in part in this embodiment by the interelectrode connector 71 which is separated from the back-emission shield 70 by a narrow space 72. The interelectrode connector 71 extends from the lower end 73 of the anode 11 to which it is connected into engagement with the rolled-over lip 74 on the top surface of the cathode 13 in the next succeeding thermionic device. This engagement not only connects the upper anode 11a electrically to the next succeeding cathode 13b, but also centers the lower cathode 13b in place, determining the uniformity of the interelectrode gap 16b.

The upper cathode 13a is centered by means of an insulating support member 75 which, nested within a recess 76 in the upper end of the lower cathode 13b, receives within its bore 77 a centering projection 78 on the lower end of the upper cathode 13a, thereby fixing the upper cathode 13a on the axis of symmetry of the assembly. A path for the circulation of ionization vapors through the series-connected diodes avoiding line-of-sight routes between successive cathodes may be provided in this instance by nonaligned apertures 80 and 81 through the back-emission shield 70 and the interelectrode connector 71 respectively.

It is to be noted that the back-emission shield 70 terminates at its innermost portion short of contact with any other member. Specifically, it does not support or contact the insulating support member 75. This arrangement results in the back-emission shield 70 having a somewhat smaller thermal gradient and cooler average temperature than in the previous two examples of the invention.

At this point it should be apparent that there have been shown and described thermionic diode assemblies which satisfy the objectives of the invention and provide the advantages referred to above. While but three examples have been illustrated herein, it is to be understood that these examples are offered as illustrative of the invention and not necessarily to limit the scope thereof. Other variations and modifications of the invention in addition to those specifically shown and described herein will doubtless occur to those skilled in the art to which the invention pertains and the appended claims are intended to encompass all such variations and modifications as fall within the true spirit and scope of the invention in its broader aspects.