US3538375A

US3538375A - Vapor fed liquid-metal cathode

Info

Publication number: US3538375A
Application number: US720694A
Authority: US
Inventors: Wilfried O Eckhardt
Original assignee: Hughes Aircraft Co
Current assignee: Raytheon Co
Priority date: 1968-04-11
Filing date: 1968-04-11
Publication date: 1970-11-03
Anticipated expiration: 1987-11-03

Description

Nov. 3, 1970 w. o. ECKHARDT 3,538,375

VAPOR FED LIQUID-METAL CATHODE Filed April 11, 1968 Wilfried O. Eckhordt, INVENTOR.

vALLEN A. DICKE, Jr.,

AGENT.

United States Patent 3,538,375 VAPOR FED LIQUID-METAL CATHODE Wilfried 0.. Eckhardt, Malibu, Calif., assignor to Hughes Aircraft Company, Culver City, Calif., a corporation of Delaware Filed Apr. 11, 1968, Ser. No. 720,694 Int. Cl. H01j 1/12 U.S. Cl. 313-346 11 Claims ABSTRACT OF THE DISCLOSURE A liquid-metal arc cathode is fed with metal vapor. The cathode has a wall which forms a transient condensation surface. Temperatures and pressures are maintained so that transient condensation occurs on these surfaces to permit an arc to run upon the transiently condensed liquid metal.

The invention described herein was made in the performance of work under a NASA contract and is subject to the provisisions of Section 305 of the National Aeronautics and Space Act of 1958, Public Law, 85-568 (72 Stat. 435; 42 U.S.C. 2457).

BACKGROUND OF THE INVENTION This invention is directed to the field of arc cathodes, and especially liquid-metal arc cathodes wherein the metal provides electrons (from an are spot) for are current.

The prior art liquid-metal arc cathodes primarily are directed to pool cathodes wherein the arc strikes on the pool surface or at the juncture between the pool and a wall which confines the pool. When the pool is large, it is gravitationally retained between the walls and thus such structures are not useful in non-gravitational environments, and specifically such large pool cathodes cannot be used for space thrusters. For non-gravitational use, a small pool cathode which is force fed and has a small enough pool so that it can be-retained by adhesive and cohesive forces, is satisfactory.

All pool cathodes which employ a liquid-metal juncture with a pool-defining wall have a linear pattern of arc spot activity because the are spot is restrained to act at the juncture of the pool with the pool-keeping wall. This results in a high thermal input density to the narrow line representing the wall and pool juncture, with consequent high temperatures and high atom evaporation rate from the adjacent liquid-metal pool surface. In those cases Where it is desirable to maintain the electron to atom emission ratio as high as is practical, this high local thermal input density is undesirable. This localization is obviated when no specific liquid-to-wall line'is provided, but transient vapor condensation provides a larger area upon which the are spot operate. Such operation provides lower average thermal input density to minimize evaporation.

SUMMARY In order to conveniently understand this invention, it can be stated in essentially summary form that it is directed to a vapor fed liquid-metal cathode. The vapor fed liquid-metal cathode is operated at such temperature and pressure that transient condensation occurs from the vapor being fed to the cathode, which condensation is positioned for arcing activity.

Accordingly, it is an object of this invention to provide a liquid-metal cathode which has a wall therein on which liquid-metal vapor can transiently condense to provide a zone on the wall for are spot activity. It is a further object of this invention to provide a liquid-metal cathode which is fed with vaporized liquid metal, and wherein a certain amount of the vapor is condensed upon condensation walls so that are spot activity can take place anywhere there is vapor condensed upon the condensa- 3,538,375 Patented Nov. 3, 1970 tion walls. It is a further object of this invention to provide a liquid-metal cathode which operates at such temperatures and pressures so that when liquid-metal vapor is fed to the cathode, transient condensation occurs thereon to permit are spot activity in the areas of such transient condensation. It is a further object of this invention to provide a cathode which is fed by vaporized liquid metal so that the cathode is independent of positioning and is not influenced by gravity direction or the absence of gravity. It is another object of this invention to provide a vapor fed liquid-metal cathode which is useful in mercury are devices, including space thrusters, mercury arc rectifiers, switch tubes and thelike. Other objects and advantages of this invention will become apparent from a study of the following portion of the specification, the claims and the attached drawings.

BRIEF DESCRIPTION OF THE DRAWING The single figure of the drawing is generally a section through a cathode suitable for feeding with vaporized liquid-metal, together with the feed structure therefor.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to the drawing, the cathode is generally indicated at 10 and its feed system is generally indicated at 12. The cathode 10 is useful in many environments which require an arc cathode. This utility extends from an electron source in an ion thruster, such as is disclosed in H. R. Kaufman Pat. No. 3,156,090, to usage in electric current switching and rectifying devices. In the case of use as an electron source for ionization in the Kaufman thruster, the liquid metal supplied by the feed system also satisfies some or all of the need for ionizable material. However, as set forth more completely below, the vapor fed liquidmetal cathode can be operated at a higher electron to atom emission ratio so that in operation of such Kaufman thrusters an additional inlet for propellant, beyond that represented by atoms discharged from the face of the cathode 10, may be necessary.

Higher thruster efliciency and lower specific thermal loading results from thermally decoupling the pool-keeping zone from the evaporator zone, as compared to a small liquid-fed pool cathode, because the required electron to atom emission ratio may be obtained with less vapor flow construction, and thus less plasma loss, downstream from the electron emitting zone.

It is possible to achieve such high electron to atom emission ratios that only a fraction of the total expellant flow needs to be fed through the cathode. This is a consequence of the reduced thermal power input density of the vapor fed cathode. The resulting design freedom for the expellant injection pattern results in additional thruster efiiciency improvements as well as complete interchangeability between the vapor fed liquid-metal cathode and other cathodes, without any thruster modifications.

In the case of electric switch tubes and rectifiers, the cathode 10 supplies the electrons for the arc with a minimum of atomic vaporization which is discharged into the general atmosphere within the tube.

Cathode 10 has a face 14 in which is located a recess which provides transient condensation surface 16. In addition, a plug is inserted interiorly of the transient condensation surface 16 in order to provide a facing transient condensation surface 18. These two surfaces are preferably curved surfaces of revolution so they provide a diverging nozzle to provide optimum expansion of vaporized liquid metal which moves past them. Annular feed channel 20 opens between

surfaces

16 and 18 so that vapor may be fed adjacent the surfaces. The divergence of these surfaces from the throat at the opening of feed channel 20 provides maximum velocity of the vapor away from face14, in a direction normal to face 14. One or more feed tubes 22 are connected from the back of cathode to annular feed channel 20. The abovedescribed construction is the preferred construction, but ordinary single divergent nozzle construction, either conical or curved, is also useful. Additionally, rectangular or slot cathode openings, again preferably divergent, may find utility in specific applications.

A convenient way of feeding liquid metal to the cathode 10 is generally indicated at 12. However, the structure is merely illustrative and the many other equivalent structures can be used instead. Cylinder 24 contains liquid mercury or other liquid metal 26 in its bottom. Piston 28 is positioned within the cylinder so that when piston 28 is forced downward, the liquid metal 26 within the cylinder is pressurized. The piston 28 can be forced down by any convenient means, and in the illustrated embodiment, gas under pressure is admitted at inlet 30 to pressurize the interior of the cylinder above the piston. Flow is measured by indicator 32 which reads the position of piston 28 within the cylinder. Indicator 32 is a commercially available dial indicator which indicates by the position of its hand with respect to its dial the relative position between the end of its probe and its housing. Since the probe of indicator 32 touches piston 28, and the housing of indicator 32 is mounted upon cylinder 24, the position of its hand on its dial face indicates the position of the piston with respect to the cylinder. By computing these readings with respect to time, the flow rate in outlet tube 34 is readily and accurately determined.

Outlet tube 34 conveys the liquid metal 26 from the interior of cylinder 24. Vaporizer screen 36 is positioned within tube 34. The screen is such that it is not wet by the liquid metal and thus minisci of the liquid metal are exposed between the wire meshes of the screen and are directed downstream of the mesh to the vapor filled portion of th tube downstream from the vaporizer mesh. The relationship between the surface characteristics of a liquid and a solid against which the liquid lies determines whether or not the liquid wets the solid. In the case of mercury against glass, the miniscus formed is convex on the gas side, as compared to the concave miniscus of water against glass when viewed from the gas side. In the present case, the material of the screen is chosen with respect to the liquid-metal material that the liquid metal does not wet the screen. Thus, the minisci between the screen wires are convex, as viewed from the vapor side of screen 36. Heater 38 is positioned around tube 34 downstream from the vaporizer mesh, all the way from the vaporizer mesh to the cathode body. Heater 38 acts to supply the heat of vaporization, to cause the liquid metal to vaporize from the minisci in the vaporizer mesh, and thus the evaporization rate is controlled by the temperature of heater 38. Furthermore, heater 38 maintains the vapor filled portion of the feed tube at above the temperature adjacent the vaporizer mesh so that the liquid-metal vapor is superheated to prevent substantial condensation upon the tube walls. Furthermore, the temperature of the cathode 19 is controlled by heat exchange jacket 40 which also maintains the temperature in the annular feed channel and feed tube 22 above the metal condensation temperature. Thus, all of the surfaces adjacent the vaporized liquid metal are above the temperature at the vaporizer mesh, in order to prevent condensation of liquid-metal droplets upon the walls. Thus, the temperature of all the walls facing the vapor are maintained sufiiciently high to prevent condensation in multi-atomic thicknesses. However, at least the transient condensation surfaces 16 and 18 are maintained at a sufficiently low temperature that a partial mono-atomic coverage can be attained.

Metal which is liquid at a reasonable, preferably room, temperature is preferred, a d While m rcu y is the pr 4 ferred liquid metal for use in the supply of vaporized liquid metal to the cathode 10, cesium, lithium andgallium are also examples of suitable materials. Thus, with respect to the exemplary figures given below, mercury is the metal used in the illustration.

When the cathode 10 is placed in an operative environment, and mercury is the liquid metal, temperatures at the vaporizer mesh typically range from 180 to 250 C. with corresponding pressures of mercury vapor just downstream of the mesh from 10 torr to torr. Since the pressure in the arc chamber at face 14 is in the order of 1() torr, the pressure drop is principally taken up in feed tube 22 and annular feed channel 20. This is a convenient way to supply mercury vapor, for while it is possible to place the vaporizer mesh at the throat immediately below transient condensation surfaces 16 and 18, in such case the temperature at the vaporizer mesh must be kept very low in order to prevent excessive evaporation at the arc chamber pressure.

When cathode 10 is placed into a suitable environment, at a reduced pressure such as at 10 torr, together with an anode and a suitable source of electric current and an igniter, and vapor feed is started, the vapor from vaporizer 36 proceeds up to feed tube 22 to a position adjacent the transient condensation surfaces 16 and 18. Transient condensation of the liquid-metal vapor on the walls occurs all along the channels up to and including the

surfaces

16 and 18 to permit an electric are spot to act thereon. Transient condensation refers to the maintenance of the walls at such a temperature that a partial coverage with condensed atoms of the liquid metal results. Other factors, in addition to surface temperature, which affect this coverage are the arc emission current and the metal vapor flow. In any event, when transient condensation occurs, there is a surface coverage of condensed atoms on the surface which provides a less than unity coverage of a mono-atomic layer of the

surfaces

16 and 18. The arc acts preferentially as close to the face 14 as there is adequate liquid-metal fil-m deposition.

A very wide range of temperatures for the cathode transient condensation surfaces and the vaporizer mesh are possible for proper operation. As previously stated, the transient condensation surfaces must be at a higher temperature than the vaporizer in order to maintain steady flow conditions. A film of liquid metal can condense upon the transient condensation surfaces even though they are at higher than equilibrium condensation temperature at that pressure. The equilibrium condensation temperature is the temperature at which the atoms condensing from the vapor equals the number of atoms evaporating to the vapor. The equilibrium condensation temperature is, thus, necessarily lower than the transient condensation temperature, because at equilibrium condensation temperature, condensed atoms can reach unity on the surface, and can build up into multi-layers and into droplets. In fact, higher than equilibrium temperature is preferred in order to prevent droplets from condensing, for only a thin condensed film is necessary for operation in the manner described. In various different applications of the cathode 10, different operating constraints would call for different operating conditions. In thrusters, in order to reject are heat, a selected cathode temperature is typically 300 C. Once the cathode temperature is chosen, the vaporizer temperature must be below that value, but the exact value of the vaporizer temperature is determined by the flow impedance between the vaporizer and cathode face.

Heat exchange jacket 40 controls the temperature of the passages within the cathode and of the transient condensation surfaces. At high are current, cooling will be necessary to maintain the transient condensation surfaces at a proper temperature. However, at low arc current, heating may be necessary, depending upon radiation and con duction losses. If the temperature of the transient condensation surfaces is too high for a given liquid-metal vapor flow rate, transient condensation does not occur and the arc will not run.

There is no fixed line between regions with adequate liquid-metal film and the adjacent regions with inadequate liquid-metal film on the transient condensation surface for are activity. As a result, the are spot does not run in a line as is characteristic of a pool placed adjacent such a surface. Instead, the are spot randomly occurs at the juncture between such condensed liquid metal and the adjacent wall. Thus, since the transient condensation is random, virtually the entire wall over a period of time is available to define an are spot location with transiently condensed liquid metal. Thus, a larger area of wall is active and heat discharge into the cathode structure is spread over a larger area. This results in reduced local temperatures with higher electron to atom emission ratios, the ratio extending to at least ten electrons per atom for mercury when the cathode temperature is 300 C. In operating such a cathode with vapor feeding at 300 C., the heat input into the cathode from the arc is in the Order of watts of heating power per ampere of current in the arc. This represents an improvement as compared to forced liquid-fed small pool cathodes operating at the same temperature.

When the cathode is used in a Kaufman-type thruster, the higher ratio of electrons per atom provides increased ionization ability when operated at a given temperature, thus providing flexibility in choosing either higher temperatures, together with a lighter cathode, or choosing a secondary inlet for the ionizable material, rather than use only the liquid-metal vapor emitted from the cathode as the ionizable material.

This invention having been described in its preferred embodiment, it is clear that it is susceptible to numerous modifications and embodiments within the ability of those skilled in the art and without the exercise of the inventive faculty.

What is claimed is:

1. A cathode, said cathode being positionable in an arc chamber for supplying electrons to an electric arc:

said cathode comprising a transient condensation surface on said cathode;

temperature control means connected to said transient condensation surface for maintaining the temperature of said transient condensation surface so that transient condensation of liquid-metal vapor can continuously occur on said transient condensation surface;

vaporized liquid-metal feed means connected for continuously directing vaporized liquid metal adjacent said transient condensation surface so that liquid metal continuously transiently condenses on said transient condensation surface so that an electric arc can continuously run on transiently condensed liquid metal.

2. The cathode of claim 1 wherein said cathode has a face and said transient condensation surface is angularly positioned with respect to said face.

3. The cathode of claim 2 wherein said transient condensation surface is a surface of revolution.

4. The cathode of claim 3 wherein said transient condensation surface is a divergent nozzle.

5. The cathode of claim 3 wherein there are first and second facing transient condensation surfaces annularly positioned adjacent said face of said cathode and a feed tube is connected between said liquid-metal vapor feed means and between said first and second transient condensation surfaces.

6. The cathode of claim 1 wherein said vaporized liquid-metal feed means comprises a vaporizer for heating and vaporizing liquid metal.

7. The process for supplying electron emission material to an electric arc comprising the steps of:

providing a transient condensation surface for the transient condensation thereon of a thin film of liquid metal;

continuously feeding vaporized liquid metal adjacent the transient condensation surface;

continuously controlling the temperature of the transient condensation surface for continuously maintaining the transient condensation surface at transient condensation temperature; and

continuously transiently condensing liquid metal on the transient condensation surface.

8. The process of claim 7 wherein said controlling step comprises maintaining the transient condensation surface at a temperature above the equilibrium condensation temperature at the pressure adjacent the transient condensation surface.

9. The process of claim 8 wherein said feeding step comprises the feeding of superheated vaporized liquid metal past the transient condensation surface.

10. The process of claim 9 wherein the step of feeding superheated vaporized liquid metal comprises vaporizing liquid metal away from the transient condensation surface, superheating the vaporized liquid metal and expanding the superheated vapor as it is transported from the point of vaporization past the transient condensation surface.

11. The process of claim 7 wherein the step of feeding vaporized liquid metal adjacent the transient condensation surface comprises superheating the liquid-metal vapor, expanding the superheated vapor in a divergent nozzle, the surfaces of which comprise the transient condensation surface.

References Cited UNITED STATES PATENTS 3,370,198 2/1968 Rogers et al 315-111 RAYMOND F. HOSSFELD, Primary Examiner U.S. Cl. X.R.