US 3515932 A
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June 2, 1970 H'.;J.=f KING HoLLowl cATHonE PLASMA GENERATOR Filed April 27, 1967 Ana/445% United States Patent Oiiice 3,515,932 Patented June 2, 1970 U.S. Cl. 313-339 4 Claims ABSTRACT oF THE DISCLOSURE A hollow cathode body defines an enclosed chamber having an orifice. The chamber wall adjacent the orifice is coated with a layer of nickel encapsulated cathode oxide mix. A heater in the chamber causes the coating to emit electrons while a vapor is introduced into the chamber thus generating a plasma which protrudes through the orifice. The plasma protrusion provides a high current density electron source for an adjacent anode. The vapor escaping through the orifice forms a column of gas between the cathode and anode which is ionized by the electrons to produce a so-called plasma bridge therebetween which allows much more current to ow than would be expected in a norm-al vacuum diode in the same cathode-anode spacing. The preferred cathode uses are as a neutralizer for an ion beam emanating from an ion thruster and as the primary electron source in a Kaufman type ion thruster (see `U.S. 3,262,262).
BACKGROUND This invention relates to cathodes and in a preferred embodiment to a plasma generator.
Previous neutralizers have vemployed a thermionic emitter as the electron source. Such a conventional oxide cathode is relatively simple and functional; however, it suffers from sputtering due to ion bombardment. As a result of the sputtering, the useful lifetime of such thermionic emiters is serverely limited.
Another proposed electron source for neutralizating an ion beam is the liquid mercury cathode described in copending application Ser. No. 476,810, filed Aug. 5, 1965 by W. O. Eckhardt and assigned to the same assignee as is the present invention. This cathode has more than adequate lifetime, however, its use would increase the complexity of the system.
SUMMARY In order to aid in the understanding of this invention, it can be stated in essentially summary form that it is directed to a hollow cathode plasma generator. The cathode has a hollow body defining an enclosed chamber which has an orifice opening from the chamber. The chamber is coated on the interior with thermionically emissive material and contains a heater. Means is included for appliying a potential difference between the heater and the emissive layer so that a discharge can be initiated.
It is therefore a primary object of the present invention to provide a cathode which is simple, rugged, has long life, and consumes small amounts of power and propellant.
It is another object of the invention to provide a plasma generator having an exposed plasma surface. This plasma is very dense whereby the generator is useful, for `example, as a light source, a microwave harmonic generator, and in other applications where -a simple plasma generator is required.
It is a further object of the invention to provide a vacuum diode which allows high current flow.
These and other objects and advantages of the present invention will be more fully understood by reference to the following detailed description when read in conjunction with the attached drawings, wherein like reference numerals refer to like elements.
BRIEF DESCRIPTION OF THE DRAWING FIG. l is a partly cross-sectional, partly schematic, diagrammatic view of the cathode of the present invention, and
FIG. 2 is a cross-sectional view of another cathode embodiment according to the present invention.
FIG. 1 shows a cathode 2 and an anode 4 with a voltage source 6 and switch I8 connected therebetween for applying a potential difference between the two electrodes. The electrodes 2 and 4 are adapted to operate in vacuum such as in outer space or in an evacuated envelope (see FIG. 2).
DESCRIPTION The cathode 2 is formed as a hollow cylinder defining an enclosed chamber 10. 'Ihe en d wall of the cathode 2 which faces the anode 4 is provided with an orifice 12. The interior surface of the wall of the chamber 10 adjacent the orifice 12 is coated with a layer 14 which comprises a triple carbonate emissive mix such as is used to coat oxide cathodes. Preferably the mix includes barium, strontium, and calcium oxide particles which have been nickel encapsulated. Any material which thermionically.
emits electrons can be used, however, low work function materials such as are specified above operate at a lower temperature and hence at a lower input power. A heater 16 is centrally mounted within the chamber 10 by means of the insulating heater leads 18 which pass through the end of the cathode 2 opposite the orifice 12. A heater power supply 22 and a switch 24 are provided for electrically heating the filament of the heater 16. A Voltage source 26 and a switch 28 are provided for applying a potential difference between the heater 10 and the conducting wall of the cathode 2. The heater 10 radiates power to the cathode body and to the layer 14 raising the temperature of the emissive mix to such a' temperature (a brightness temperature of about 900 C.) that free barium is released at the surface of the layer 14 so that the work function is very low thus making it easy for the surface of the layer 14 to emit electrons. Mercury (or -any other suitable material) vapor is introduced into the cathode chamber 10 by means of a vapor reservoir 30, a valve 32 and a feed line 34 connected to a wall of the cathode 2 to provide fluid communication between the reservoir 30 and the chamber 10. A vapor flow impedance device can be used in the feed line 34, if desired, with the valve 32 open to assure continuous replenishment and operation at the desired pressure. Sufficient vapor is introduced to raise the pressure to the desired value. A pressure for mercury vapor on the order of about l Torr. has been used successfully. A DC voltage is then applied between the heater 16 and the body of the cathode 2 by means of the voltage source 26 and switch 28 so that the heater acts as an anode for a discharge between the layer 14 and the heater 16. A relatively intense mercury discharge occurs and a plasma 36 is formed. A plasma tip 38 which protudes through the orifice 12 contains both electrons and ions and acts as a high current density electron source (cathode) for the anode 4. Once the current flow to the anode 4 is established, the switch 28 can be opened since this external current is self-sustaining. Another method of initiating the initial discharge is to apply a very high voltage between the anode 4 and the cathode 2. In this case the voltage source 26 and switch 28 are not needed, and in fact the heater need not now be within the chamber 10. However, this method is less convenient and reliable than the previously described method.
The mercury vapor escaping from the orifice 12 forms a column of gas particles between the cathode 2 and the anode 4 which particles are ionized by the passage of electrons through this region. This so-called plasma bridge changes the potential distribution from the vacuum diode condition thereby effectively presenting a highly conductive region between the cathode 2 and the anode 4 and allows much more current to ilow than would normally be expected in a vacuum diode of the same cathode-to-anode spacing. This feature of the present invention is particularly important when the present invention is used to neutralize the ion beam from anion thrustor where it is desirable to locate the electron source outside the ion beam but still inject the electrons into the beam through a low potential drop.
A great variety of vapor compositions can be used in the cathode of this invention. In the preferred embodi- 4ment the cathode is operated using mercury vapor at a pressure of about 1 Torr. and with the cathode at a ternperature of about 900 CB. A heat shield 40 is preferably provided around the cathode 2 to reduce radiant heat loss. In the embodiment shown in FIG. 1 the heat shield 40 comprises a thin nickel sheet wrapped around the cathode 2.
FIG. 2 shows an embodiment according to the present invention which is identical to that of FIG. l except for the inclusion of a layer of insulating material 42 on the surface of the cathode 2 which faces the anode 4. The layer 42 can be coated onto the surface of the cathode by sputter deposition, plasma spraying, or painting with a colloidal dispersion, etc. Ions from the plas-ma column strike the surface of the layer 42 and build up a surface charge which charge repels or slows down additional ions directed toward the cathode 2 to reduce erosion due to sputtering. FIG. 2 further shows an evacuated envelope 44 in which the electrodes 2 and 4 are mounted. The envelope 44 is not necessary, of course, when the cathode 2 is used as an ion beam neutralizer or as an ion thrustor on a spacecraft. The envelope y44 is useful for such applications of the invention as a light source, for example.
Further, Iwhen the cathode 2 is used for neutralizing an ion beam, there is no need for the anode 4, the voltage source 6, or the switch 8. The ion beam which is to be neutralized, such as a mercury ion beam emanating from an ion thrustor, would be analagous to the anode 4 and would attract electrons from the plasma tip 38 through the plasma bridge thus achieving the neutralization.
What is claimed is:
1. A cathode comprising:
a hollow cathode body defining an enclosed chamber;
a first wall portion of said cathode body dening an 4 orifice extending therethrough to provide uid communication with said chamber;
a layer of electron emissive material coating which thermionically emits electrons on the interior surface of the wall of said chamber adjacent said orifice;
a heater mounted within said chamber for energizing said material so that it emits electrons;
means for applying a potential difference between said heater and said layer to initiate a discharge therebetween; and
means for introducing a gas into said chamber, said means for introducing a gas comprising a closed feed means including a reservoir at a higher pressure than the gas pressure in said chamber and connection means between said reservoir and said chamber, said connection means controlling gas flow into said chamber;
said hollow cathode body being positioned in a space which is at a lower pressure than the gas pressure within said chamber so that said emitted electrons ionize said gas to generate a plasma in said chamber which plasma protrudes through said orilice so that said cathode operates in an environment where gas only ilows out of said orifice in said wall of said cathode body.
2. The apparatus according to claim 1 wherein:
said means for applying a potential difference between said heater and said layer, applies a potential which makes said heater positive with respect to said layer.
3. The apparatus according to claim 2 in which said emissive material comprises nickel-encapsulated particles of barium oxide, strontium oxide, and calcium oxide.
4. The apparatus according to claim 3 including a layer of insulating material covering the exterior surface of said first wall portion whereby sputtering of said first wall portion is reduced.
References Cited UNITED STATES PATENTS 1,929,369 10/1933 Found 313-339 X 2,106,855 2/ 1938 Pearcy 313-212 3,084,273 4/1963 Abdelaziz 315-111 X 3,253,180 5/1966 Huber 313-6'3 X 3,346,750 10/1967 Huber et al 315-111 X RAYMOND F. HOSSFELD, Primary Examiner U.S. Cl. X.R.