US3708704A

US3708704A - Thermionic cathodes for mhd generators

Info

Publication number: US3708704A
Application number: US00178881A
Authority: US
Inventors: B Zauderer
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1971-09-09
Filing date: 1971-09-09
Publication date: 1973-01-02
Anticipated expiration: 1990-01-02

Abstract

Efficiency of non-equilibrium magnetohydrodynamic generators is markedly increased by provision of thermionically emitting cathodes. Cesium in concentrations 0.1 to 0.3 percent improves plasma conductivity, and when deposited on hot tungsten vastly improves thermionic emission but only in absolute concentrations which, near atmospheric pressure, greatly exceed 0.1 to 0.3 percent. Cesium is applied topically in sufficient concentrations to tungsten surface either from dispenser arrangement in which cesium passes through tungsten cathode face, or through apertures in channel wall adjacent to tungsten cathode face; amounts used are such that, after passage into main gas stream and mixture therewith, cesium concentration is still within permissible limits for improving plasma conductivity. Alternatively, generator is operated at high gas pressure (e.g., 5 atmospheres) so that the requisite absolute cesium concentration for thermionic emission is only the permissible 0.1 to 0.3 percent of the concentration of the pressurized plasma. The Invention herein described was made in the course of or under a contract or subcontract thereunder, (or grant) with the Department of the Navy.

Description

FIFSSUZ 3R Zauderer 1 Jan. 2, 1973 54] THERMIONIC CATHODES FOR MHD GENERATORS [75] Inventor: Bart Zauderer, Bala Cynwyd, Pa. [73] Assignee: General Electric Company [22] Filed: Sept. 9, 1971 [21] Appl. No.: 178,881

3,355,605 11/1967 Okress Primary Examiner-D. X. Sliney Att0rneyAllen'E. Amgott et al.

[5 7] ABSTRACT Efficiency of non-equilibrium magnetohydrodynamic generators is markedly increased by provision of thermionically emitting cathodes. Cesium in concentrations 0.1 to 0.3 percent improves plasma conductivity, and when deposited on hot tungsten vastly improves thermionic emission but only in absolute concentrations which, near atmospheric pressure, greatly exceed 0.1 to 0.3 percent. Cesium is applied topically in sufficient concentrations to tungsten surface either from dispenser arrangement in which cesium passes through tungsten cathode face, or through apertures in channel wall adjacent to tungsten cathode face; amounts used are such that, after passage into main gas stream and mixture therewith, cesium concentration is still within permissible limits for improving plasma conductivity. Alternatively, generator is operated at high gas pressure (e.g., 5 atmospheres) so that the requisite absolute cesium concentration for thermionic emission is only the permissible 0.1 to 0.3 percent of the concentration of the pressurized plasma. The Invention herein described was made in the course of or under a contract or subcontract thereunder, (or grant) with the Department of the Navy.

7 Claims, 6 Drawing Figures 30 PUMP 3.?

655/044 M /5 ii 20" I I l \\L\\\\\\\\\\ N05L (5A5 PfiE/ON/ZER :24 c0020? [I] PR5$UR 3 HEAT L SOURCE 32 COMP/M350 l /0 Jo I 34 THERMIONIC CATHODES FOR MHD GENERATORS BACKGROUND OF THE INVENTION 1. Field of the Invention This invention pertains to magnetohydrodynamic generators.

2. Description of the Prior Art confined to subsonic plasma flows, because stagnation pressures of 100 atmospheres are required for super- The addition of cesium vapor as a seeding material to 10 SUMMARY OF THE INVENTION It has been found that the electrode voltage loss which is subtracted from the induced voltage, and appears as a reduction of output voltage, is of the order of 100 volts in conventional MHD generators, which do not have high-density thermionic cathodes. It has been reported (Advances in Electronics and Electron Physics, L. Marton Editor, Academic Press, New York City, 1962, page 147) that pure tungsten electrodes heated between l,200 and 1,500 K. will emit up to 50 amperes per square centimeter in a cesium-seeded noble gas atmosphere if the arrival rate at the tungsten surface is greater than 10 cesium atoms per square centimeter per second. Unfortunately, it has been found that in non-equilibrium MHD generators it is desirable to have a very low cesium proportion, e.g., 0.1 to 0.3 percent of the total atoms present for two reasons. First, the elastic electron-cesium cross section is very large compared to the electron-noble gas cross section, which causes a large cesium concentration to reduce the conductivity of the plasma, which is inversely proportional to the elastic cross section. Secondly, a non-equilibrium plasma in a magnetic field is susceptible to an ionization instability which sharply reduces the effective plasma conductivity. Experimental evidence (Vitshas, A. F. et al., Electricity from MHD, Vol. 1, page 29, International Atomic Energy Agency, Vienna, 1968) indicates that if the cesium is fully ionized, which implies a low cesium concentration of less than 0.2 percent, the effect of the ionization instability on the conductivity is markedly reduced. But for effective thermionic emission the cesium density, regardless of the total gas pressure, must be a minimum of about 10 per cubic centimeter. Now, dividing Avogadros number by the gram molecular volume gives a density of about 2.7 X lmolecules per cubic centimeter at N.T.P.; and at e.g., l,900 K and 1 atmosphere pressure the density will be about 4 X 10 molecules per cubic centimeter. Thus the minimum cesium concentration for thermionic emission will be about 2.5 percent of the total much higher than the concentration desirable for good plasma conductivity. I have found two basic ways of providing cesium concentration adequate for thermionic emission, while maintaining a cesium concentration in the active volume of plasma which is compatible with good plasma conductivity.

The first method is to operate the noble gas atmosphere well above atmospheric pressure, atmospheres or higher, which will raise the total gas density to about 2 X 10 or more, making the cesium concentration of 10 only 0.5 percent of the total. This is sonic operation at Mach 1.5 to 2.5, and at such high pressures the conductivity of helium is too low to produce non-equilibrium ionization even at magnetic fields as high as 10' gauss. Argon can harbor nonequilibrium ionization at such pressures, but even full ionization of the cesium would still result in a high cathode sheath drop.

The second method, which has several possible embodiments for execution, is to provide a source of high density of cesium vapor adjacent to the thermionic cathode surface and allow the cesium vapor thus introduced to mix with the working gas so that its concentration in the plasma in general is within the permissible limits for high plasma conductivity. This I do by feeding cesium through a porous tungsten plug, or, alternatively, by feeding cesium vapor through apertures in the channel wall adjacent to the heated tungsten cathode surface so that a sufficiently high concentration of cesium vapor will be provided to the tungsten surface, but the cesium concentration will fall elsewhere through the working gas to a suitably low value. Basically, I propose a non-equilibrium distribution of cesium vapor to provide different concentrations as required at different locations. This method, because it affords flexibility to meet different conditions, is a convenient manner of practicing my invention. The choice between the use of a porous dispensertype cathode and provision of cesium vapor through separate apertures is best determined by design considerations.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 represents schematically and partly sectioned an MHD generator.

FIGS. 2, 3, 4, 5, and 6 represent various cathode structures suitable for practicing my invention in a generator as represented in FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 represents generally schematically, and in section, MHD apparatus of the kind to which my invention is applicable. A heat source 10 is connected thermally to a store 12 of noble gas, under pressure, which is seeded by cesium seeder l3 with cesium to promote ionization. These are purely conventional, and represented by simple rectangles. The seeded noble gas passes through a duct 14, shown in section and may pass through various preionizing means 16, although this is optional, and into an MHD channel 18, also shown in section. As is well known in the art, the MHD channel must be insulating and refractory to as high a temperature as possible. It is therefore usually made of ceramic material best meeting these requirements. It is hornbook doctrine in MHD that materials are not available which will withstand the temperature at which gases become highly conductive from thermal ionization; and therefore it is necessary to employ lower temperatures, e.g., 2,000 K, at which materials can survive, and to produce conductivity in the gas by a variety of stratagems. Seeding with a material such as cesium is a practical necessity in order to promote at least an initial ionization in the gas in order that the various other methods for increasing the ionization may function effectively. But since some cathodes according to the teachings of my invention will add some cesium to the already seeded gas, the proportion of cesium initially seeded by cesium seeder 13 may be toward the low side of the desirable 0.1 to 0.3 percent range. Pole pieces and 22 of a magnet produce a magnetic field transverse to the direction of gas flow, parallel to the plane of FIG. 1. Anodes 24 are mounted in the back wall of the channel; while these are represented as subdivided only in the direction of gas flow, there are some advantages to subdividing them also at right angles to the direction of gas flow that is, the long strips shown may be cut horizontally (in FIG. 1) to form a larger number of smaller electrodes. These are all of a suitably refractory metal, must all have separate connections extending backward through the channel wall, and are preferably designed to be relatively cool since it is desirable that, being anodes, they not emit electrons. An exit tube 26 for discharge of the gas is also represented. Exit tube 26 leads to a cooler-filter 28 where the cesium vapor is condensed to a liquid, the cesium droplets (and any accidental particulate matter in the gas) being filtered out. The liquid cesium is returned to cesium seeder 13 via conduit 30, which is also provided with a branch outlet 32 which will be used only for embodiments (such as those represented in FIGS. 3, 4, 5, and 6) which require to be supplied with cesium. The coolerfilter unit is conventional in the art; and a pump 33 is provided for returning the liquid cesium under suitable pressure to permit its being fed to the various stores where it is required. A compressor 34 receives the cooled and filtered gas and returns it to the store 12 of noble gas under pressure. Thus the system is completely closed, except for heat, the working materials being recirculated.

FIG. 2 represents a cathode suitable for use in a first embodiment of my invention in which the cesium present in the gas is only that contributed by the seeder l3, and the pressure of the noble gas is made sufficiently high that the absolute concentration of cesium which is necessary for good thermionic emission is only the 0.1 to 0.3 percent which is desired for high and stable conductivity. A tungsten plug 40 is represented inserted through the wall of channel 18, this wall being the one which does not appear in FIG. 1 because channel 18 is there represented sectioned so that the wall which would be nearer to the viewer of FIG. 1 has been removed by the sectioning. It is this latter wall which is represented in FIGS. 2, 3, 4, 5, and 6. The part of tungsten plug 40 which is external to channel 18 is represented as provided with heater elements 42 to indicate the provision of means for adjusting the temperature of plug 40 so that its upper surface will have a temperature suitable for thermionic emission. However, it must be recognized that the channel 18 will contain gas at a high temperature and therefore it is possible that it will be necessary (depending upon the thermal flow characteristics of the particular apparatus) to cool plug 40 to adjust its temperature to the desired value. Thus heater elements 42 should be regarded as representing temperature adjusting means which may heat or cool, rather than exclusively as heating means, even though the conventional resistor symbol has been used for simplicity of illustration. The shape of plug 40 in the cross section which does not appear in FIG. 2, since it would be normal to the paper, depends upon the electrode pattern chose; in general, for each anode 24, there should be opposed to it a cathode surface of approximately the same shape. The relation between cathode and anode is one to one, with each electrode of either kind having its separate connection external to channel 18.

In this embodiment the entire system operates at a pressure such that the requisite concentration of cesium to promote adequate electron emission from cathode plugs 40 is introduced by cesium seeder 13; but the pressure in the system is so high that this concentration constitutes only the desirable 0.1 to 0.3 percent concentration. The pressure required is about 5 atmospheres. The reason this tour de force is actually operative is that the cesium supply required by the cathodes is an absolute value, substantially indepen dent of pressure; but the concentration limit desirable for good plasma conductivity is relative, that is, a fraction of the total gas concentration. Hence what one does in practicing this form of the invention is to alter the total gas concentration so that the absolute concentration required for thermionic emission promotion becomes only the fraction desirable for high plasma conductivity. While the apparatus for operation under pressures of 5 atmospheres is naturally heavier and somewhat more complex than that for operation at lower pressures, the size of generator for a given output is somewhat reduced, so that this method merits consideration depending upon the operating requirements.

The remaining figures are embodiments of various cathode electrode arrangements in execution of the second basic form of my invention, in which the concentration of cesium vapor adjacent to the thermionically emissive face of the tungsten is adjusted to a desirable value independently of the concentration of cesium provided by cesium seeder 13.

In FIG. 3, a plug 44 of porous tungsten is inserted through the wall of channel 18, as solid plug 40 is inserted in FIG. 2. The lower end of plug 44 is fitted into the upper end of a cylinder 46, in whose lower end (in FIG. 3) a piston 48 moves. The rod of piston 48 has a collar 50 against which a spring 52 bears, tending to push piston 48 into cylinder 46, the opposite end of spring 52 bearing against a stop 54. Cylinder 46 is filled with a charge of cesium 56 which is kept molten, heater elements 58 being represented as surrounding the cylinder 46 to indicate the possible need to supply heat for this purpose. The part of plug 44 which lies on the side of the wall of channel 18 which outside of the channel is represented as provided with heater elements 42, as in FIG. 2. However, it must be recognized that the channel 18 will contain gas at a high temperature and it is possible that it will be necessary (depending upon the thermal flow characteristics of the particular apparatus) to cool plug 44 and, possibly, even cylinder 46 to adjust its temperature to the desired value. Thus heater elements 58, like 42, should be regarded as representing temperature adjusting means which may heat or cool, rather than exclusively as heating means, even through the conventional resistor symbol has been used for simplicity of illustration. The shape of plug 44 in the cross section which does not appear in FIG. 3, since it would be normal to the paper, depends upon the electrode pattern chosen, just as for plug 40 of FIG. 2. In operation of the embodiment represented in FIG. 3, the cesium 56 is kept molten, preferably at 500 to 700 K, and plug 44 is kept so that its upper surface is at a temperature preferably in the range 1,300 to 1,400 K. The molten cesium 56 cannot flow directly through the capillary pores in plug 44, but it vaporizes at its contact with plug 44 and the vapor flows through the pores of plug 44 and appears at the upper face of plug 44 where it promotes thermionic emission from the hot face, from which it evaporates but is constantly replaced by flow of further cesium vapor through plug 44 from the molten charge 56. The concentration of cesium vapor immediately adjacent to the upper face of plug 44 will in general be above the maximum desirable for good plasma conductivity; but because the surfaces of all the cathodes are only a fraction of the total channel surface, the total cesium vapor introduced from the cathode surfaces will not raise the total cesium concentration in the flowing plasma above the desirable maximum of about 0.3 percent. Thus this and succeeding embodiments provide cathodes having a cesium supply sufficient for high density of electrons emitted without raising the total cesium concentration in the plasma impermissibly. While it has not been represented, to avoid complication of the drawing, conduit branch 32 of FIG. 1 may in practice be connected to return condensed cesium to the interior of cylinder 46. Indeed, it would be possible to replace cylinder 46 by a closed chamber fed cesium at the desired pressure, and eliminate piston 48 and spring 52 which serve merely to produce the desired pressure. However, in developmental work, the spring and piston offer the advantage of ready adjustment of the cesium pressure by adjustment of the force produced by spring 52.

FIG. 4 represents a modification of the embodiment FIG. 3 in that cesium 56 is maintained in a boiler 60 where cesium is actually vaporized and passes into a housing 62 which conducts the vapor to plug 28, through which it flows with effects as described for FIG. 3. It has possible advantages of slightly more simple mechanical execution in not requiring a moving piston; and it would be possible to design it so that heat flow from plug 44 to cesium 56 would be negligible, making the rate of cesium vapor flow more completely dependent upon the rate of heat supply by heater elements 58, and thus more readily adjustable. As with FIG. 3, cesium from conduit branch 32 may be fed back to boiler 60.

FIG. 5 represents a still further modification of the embodiment of FIG. 3 in which a tungsten plug 64 is provided with tubular holes 66 through which cesium vapor flows from liquid cesium 56 in a boiler 68 which also serves as a housing to lead the vapor to the openings of holes 66. This embodiment may be preferable if there is any difficulty in a given installation with solid contaminants depositing upon porous plugs 44 and clogging their pores. The general mode of operation of this embodiment is similar to that of the embodiments previously described.

FIG. 6 represents a further mechanical modification of the structure of cathodes operating according to my basic teachings. Here wall 18 of the chamber is pierced with slots 70 located upstream (as indicated by the arrow showing the direction of plasma flow) of solid tungsten plugs 72. Cesium vapor, produced from a store of molten cesium 56in boiler 74, passes through slots and is blown by the moving plasma across the emissive face of plug 72, promoting thermionic emission in the manner previously described. It should be observed that this embodiment lends itself particularly well to the use of a single cesium boiler connected to a number of slots feeding a number of cathodes, and thus permitting some simplication in apparatus design. Cesium from conduit branch 32 may be fed back to such boilers as 68 and 74.

It is evident that the various embodiments I have shown, or others embodying the same principles but modified within the skill of the art, may each, in specific circumstances, be the one to be preferred for a particular apparatus design.

The net virtue of any of these embodiments of my invention is that they permit efficient continuous operation of MI-ID generators by permitting one to obtain simultaneously the high plasma conductivity which is requisite to high efficiency and high output and the lowered electrode drop which is also necessary to high efficiency.

Other particular known embodiments of the various components of the embodiments of MHD generators that I have disclosed may also be employed in the practice of my invention. For example, in Electricity from MHD," International Atomic Energy Agency, Vienna, I968, Volume I, Pages 397-408, a paper by Barouezec and Salvat discloses thermionic cathodes shaped like circular cylinders, provided with internal ducts for bringing cesium to the cathode surface, and suspended inside the MHD duct so that it is subject to free flow of plasma over almost its entire surface, except for very small supports. The object of interest to these authors was avoidance of the extreme concentration of current which the Hall effect produces in the edge of substantially orthogonally bounded electrodes, such as metal strips. However, there is no reason why such a cathode should not be applied in the practice of my invention.

Generalizing the description of the embodiments, and employing reference numbers from the drawings, 12 is a source of noble gas under pressure, 10 is means for heating the noble gas, 13 is means for providing cesium vapor in the noble gas, 18 is a channel of refractory insulating material which receives the gas moving as a result of its initial pressure and guides it normal to the magnetic field produced between

pole pieces

20 and 22, and between pluralities of anode electrodes 24 and cathodes of the various forms shown in FIGS. 2 through 6.

The various containers for cesium S6 and the vapor passages associated with them are cathode cesium means. Tungsten provided with interior passages is represented by 44 and 64, those of 44 being sufficiently small to stop the passage of liquid molten cesium, which in FIG. 3 is in contact with the tungsten, and in FIGS. 4 and 5 is apart from the tungsten so that only its vapor enters the interior passages. References 70 are apertures in the wall of channel 18 for discharging cesium vapor into the immediate vicinity of the exposed surfaces of the cathode electrodes.

What is claimed is:

1. A magnetohydrodynamic generator comprising:

a. a source of noble gas under pressure b. means for heating the noble gas c. means for providing cesium vapor in the said noble gas d. a channel of refractory insulating material connected to receive the noble gas moving responsively to its pressure, after it has been heated and provided with cesium vapor, and guide it along a path normal to e. a magnetic field transverse to the said path, and

between f. a plurality of anode electrodes and a plurality of cathode electrodes supported to present conductive surfaces to the gas in its motion along the path and having electrical connection to the exterior of the channel g. gas exit means connected to the end of the path in the channel;

in which h. the surfaces of said cathode electrodes exposed to the moving gas in its motion along the path are of tungsten at a temperature of at least 1,400" K and immediately adjacent to the said exposed surfaces the concentration of cesium vapor is at least 10 atoms per cubic centimeter; and

i. the molecular concentration of cesium in the flowing gas is between 0.1 and 0.3 percent of the total flowing gas.

2. A generator as claimed in claim 1 in which j. the concentration of cesium in the noble gas as received by the channel is at least 10 atoms per cubic centimeter, and j k. the pressure of the gas in the channel is such that the concentration of cesium atoms is from 0.1 to 0.3 percent of the total number of gas atoms.

3. A generator as claimed in claim 1 in which l. the concentration of cesium in the noble gas as received by the channel is less than 10" atoms per cubic centimeter, and

m. cathode cesium means are provided with inject additional cesium vapor immediately adjacent to the exposed cathode surfaces to provide a concentration of at least 10" cesium atoms per cubic centimeter, and

n. the concentration of cesium atoms in the gas which enters the gas exit means is not more than 0.3 percent of the total number of gas atoms.

4. A generator as claimed in claim 3 in which 0. the said cathode electrodes comprise tungsten provided with interior passages extending to the surface exposed to the moving gas, and

p. the cathode cesium means comprise a store in which cesium is kept molten connected to the said interior passages at a part of the tungsten not exposed to the moving gas.

5. A generator as claimed in claim 4 in which q. the interior passages are sufficiently small to stop the passage of liquid molten cesium and r. the liquid molten cesium is in contact with the tungsten.

6. A generator as claimed in claim 4 in which 5. the molten cesium is apart from the tungsten and its vapor enters the interior passages.

7. A generator as claimed in claim 3 in which t. the cathode cesium means comprise a store of cesium and means for vaporizing the cesium, connected to u. apertures m the channel wall ad acent to the exposed surfaces of the cathode electrodes for discharging the cesium vapor into the immediate vicinity of the exposed surfaces.

M-m mma) PATENT @FFICE m5 wmmmmm Patent No. 3, 708, 704 Dated January 2, 1973 Inventofls) Bert (nmi) Zauderer w I I I 4 H 1' I Column 8, line 4, change "with" to whlch Signed andvsea' led this 17th day of April 197$ (SEAL) Attest:

EDWARD M.PLETCHER,JR. i ROBERT GOTTSCHALK Attesting Officer Commissioner of Patents

Claims

2. A generator as claimed in claim 1 in which j. the concentration of cesium in the noble gas as received by the channel is at least 1017 atoms per cubic centimeter, and k. the pressure of the gas in the channel is such that the concentration of cesium atoms is from 0.1 to 0.3 percent of the total number of gas atoms.
3. A generator as claimed in claim 1 in which l. the concentration of cesium in the noble gas as received by the channel is less than 1017 atoms per cubic centimeter, and m. cathode cesium means are provided with inject additional cesium vapor immediately adjacent to the exposed cathode surfaces to provide a concentration of at least 1017 cesium atoms per cubic centimeter, and n. the concentration of cesium atoms in the gas which enters the gas exIt means is not more than 0.3 percent of the total number of gas atoms.
4. A generator as claimed in claim 3 in which o. the said cathode electrodes comprise tungsten provided with interior passages extending to the surface exposed to the moving gas, and p. the cathode cesium means comprise a store in which cesium is kept molten connected to the said interior passages at a part of the tungsten not exposed to the moving gas.
5. A generator as claimed in claim 4 in which q. the interior passages are sufficiently small to stop the passage of liquid molten cesium and r. the liquid molten cesium is in contact with the tungsten.
6. A generator as claimed in claim 4 in which s. the molten cesium is apart from the tungsten and its vapor enters the interior passages.
7. A generator as claimed in claim 3 in which t. the cathode cesium means comprise a store of cesium and means for vaporizing the cesium, connected to u. apertures in the channel wall adjacent to the exposed surfaces of the cathode electrodes for discharging the cesium vapor into the immediate vicinity of the exposed surfaces.