US3453469A - Multi-level vacuum pumping system for plasma containment device - Google Patents

Description

MULTI-LEVEL VACUUM PUMPING SYSTEM FOR PLASMA CONTAINMENT DEVICE I of 4 Sheet Fileq April 27, 1966 0 mm TA v mm vL 3w m V o I a' DE E mm N3 GR 3N NNN ATTORNEYS July 1, 1969 CANN ET AL 3,453,469

MULTI-LEVEL VACUUM PUMPING SYSTEM FOR PLASMA CONTAINMENT DEVICE Filed April 27, 1966 Sheet 2 of 4 GOR ONL. CANN 2 ROBERT HARDER A T TORNEYS July 1, 1969 1 CANN ET AL 3,453,469

MULTI-LEVEL VACUUM PUMPING SYSTEM FOR PLASMA CONTAINMENT DEVICE Filed April 27, 1966 Sheet 5 of 4 A T TORNE July 1, 1969. CANN ET AL 3,453,469

MULTI-LEVEL VACUUM PUMPING SYSTEM FOR PLASMA CONTAINMENT DEVICE Filed April 27, 1966 Sheet 4 of 4 I N VEN TORS' ORDON L. CANN G BY OiBOBEg L. EARDER $478 3 \M A T TORNEYS United States Patent Int. Cl. H01j 1/50 US. Cl. 313-7 1 Claim ABSTRACT OF THE DISCLOSURE Plasma containment apparatus utilizing a multi-level vacuum pumping system which provides more effective vacuum pumping by removing a portion of the material from the chamber at higher pressures than the prevailing chamber pressure is disclosed. A plurality of flow restrictors and vacuum pumping means are employed to produce the required difierential pressures.

This application is a continuation-in-part of our copending application bearing Ser. No. 458,837, filed May 20, 1965, and entitled Plasma Arc Electrodes.

This application relates to plasma containment devices and more particularly to improved vacuum pumping means and methods for use therewith.

In our applications Ser. Nos. 457,414 and 457,746 plasma containment devices are disclosed comprising generally an evacuated chamber, a radial arc electrode assembly at one or both ends of the chamber with means to introduce ionizable material into the chamber through these electrode assemblies, and magnet coils to provide an axial magnetic field. A column of high temperature and pressure plasma extends down the magnetic field from the electrode assemblies, but proper operation requires that the surrounding pressure in the vacuum chamber be kept very low. Since ionizable material is continually being introduced into the chamber at the electrodes a high capacity pumping system is required to remove all of this material from the chamber. It has now been found possible to remove some of this material at a pressure greater than the prevailing pressure in the vacuum chamber. This is highly desirable because less energy and less complex pumping equipment is required to remove a given mass of a material at a higher rather than a lower pressure.

It is accordingly an object of the present invention to provide improved vacuum pumping means and methods for use in plasma containment devices. It is a further object of the invention to provide vacuum pumping means and methods whereby part of the gaseous contents of a vacuum chamber are pumped at a pressure higher than the prevailing chamber pressure. Further objects will become apparent with the description which follows.

FIGURE 1 is a schematic cross sectional view of a plasma containment apparatus useful in connection with the present invention.

FIGURE 2 is a cross section of an electrode assembly utilizable in containment devices of the type depicted in FIGURE 1.

FIGURE 3 is a partially schematic sectional view of one embodiment of the invention, and

FIGURE 4 is a sectional view of a further embodiment of the invention.

FIGURE 1 shows a form of plasma containment apparatus, more fully described in our copending application 458,837 in which the present invention may be usefully employed. It includes a chamber 210 which is evacuated by pump 212 and which contains magnet coils ICC 214, 216, 218 and 220 which are energized in the same direction by power supplies 222, 224, 226, and 228. Water cooling may be provided for the magnet coils as shown by elements 230, 232 and 234. Located within coils 214 and 220 are arc electrode assemblies 236, 238, each of which includes at least a cathode 240 and an anode 242 which are connected to a power supply 248 as well as a gas supply channel 244, which is fed from a source 246 of argon, hydrogen or other ionizable gas. The illustrated apparatus is particularly adapted to form a confined, rotating column of high temperature plasma extending from electrode assembly 236 to electrode assembly 238 and having an internal radial electric field. A related form of plasma containment apparatus, more fully described in the copending application 457,414 of the joint applicant Gordon L. Cann, employed only a single electrode assembly 236 or 238.

FIGURE 2 is a cross sectional view of an electrode assembly useful as element 236 or 238 in FIGURE 1. Cathode 10 is a tapered piece of tungsten and, like all other electrodes in the figure, will normally be axially symmetric. It is mounted in a metal heat sink 14, which, in turn, is mounted at the end of a cathode support and cooling water conduit 21 which is sealed into phenolic support block 18. A cathode cooling water inlet, not shown, communicates with chamber 24 and in turn with an unshown outlet to provide water cooling for cathode 10. A boron nitride or beryllia insulator 26 surrounds cathode 10 and leaves the tip portion exposed. A concentric cathode bufier electrode 28, surrounds cathode 10 and is supported with respect to the cathode and insulated therefrom by insulator 26. As shown, the cathode buffer 28 is tapered internally and defines a chamber 40 surrunding the tip portion of the cathode which is substantially enclosed except for an aperture 30 in the cathode buffer, which is aligned with cathode 10 and positioned slightly in front of the cathode tip. illustratively, the diameter of this aperture may be 0.1 inch.

Cathode 10 and its heat sink 14 are bored to receive a tubular pressure tap 32 located within the cathode water conduit 21. Cathode 10 also contains one or more small channels or passage 34 which connect the pressure tap to the exterior surface of the cathode forward of the cathode insulator 26. The cathode insulator 26 is also provided with a gas passage, or preferably a plurality of circumferentially disposed passages 36 which communicate with the front face of the cathode insulator and are connected to a feed tap 38 in support block 18. Either passages 34 or passages 36 may be used to introduce a fluid to the space adjacent to the cathode, but it is generally preferable to introduce a fluid through passages 36, and to use passages 34 for measuring the pressure adjacent to cathode 10.

Tungsten cathode buffer electrode 28 is attached to and is in thermal contact with a hollow heat sink and cooling assembly 50 which is connected to an electrically conducted water inlet tube 54. The corresponding water outlet is not shown.

Cathode buffer electrode 28 is surrounded by a series of alternating concentric electrodes and insulators 122. The outermost insulators 122 serves to insulate and support a tungsten anode buffer electrode 60, which is concentrically located about the cathode and cathode buffer. A water cooled copper anode assembly 70 is mounted on the outside of support block 18 and is electrically insulated from anode buffer 60 and heat sink 50 by insulators 72 and 74. Anode 70 has at its forward end a cylindrical inner surface 76 which illustratively may be 2 inches in diameter and is separated by a small annular space from a cylindrical outer surface of anode buffer 60. Illustratively, the forward surfaces of the cathode buffer 28, electrodes 20, insulators 122, anode buffer electrode 60, and anode 70, may lie on a common plane as shown. There is a radial passage 84 in anode 70 which opens into the annular space 78 and communicates with an anode pressure tap 86. A magnet coil 90 is shown which is insulated from the anode 70 by an insulator 88 which surrounds the anode and also covers the front face thereof. Insulator 88 also prevents arc attachment to the face of the anode, which would cause very rapid erosion. A conventional power supply may be used to operate magnet 90. A water cooling assembly 94 is positioned so as to cool the forward portion of the magnet and also the face of the anode, each of which is likely to be exposed to high temperatures in the operation of the device. A suitable DC power supply 100 and switch are connected between cathode cooling conduit 21 and anode assembly 70.

A series of passages 124, extend outward from cathode chamber 40, passing through electrodes 120 and insulator 122. These passages do not lie in planes passing through the axis of the device, but are canted to give a tangential velocity to gas issuing therefrom in a direction consistent with the magnetic field. A tube 82 connects a vacuum pump 126 with annular space 78. Vacuum pump 126 has a beneficial effect when used in conjunction with the containment device of FIGURE 1. It is desirable to have a relatively high mass flow of gas through cathode chamber 40 and passages 124 to assist in cooling the electrodes. However, this gas must be somehow removed from the chamber containing the apparatus in order to permit continuous operation. Much less energy is expended in removing this gas at the relatively high pressure encountered at space 78 than at the relatively lower pressure encountered in chamber 210.

FIGURE 3 shows one half of an embodiment of the invention corresponding to an actual experiment. However, it will be understood that no attempt has been made to show all structural details such as cooling, passages, fasteners, shields, welds, and the like. The omitted portion of the apparatus may be a mirror image of the portion shown. The vacuum chamber consists of a central tube 150, approximately 30 inches along with an inside diameter of 3.9 inches connected through insulator 152 to vacuum end chamber 154. This configuration permits magnets 156 to be placed outside the vacuum rather than inside as shown in FIGURE 1. Central tube 150 is preferably made of water-cooled copper to permit dissipation of large amounts of heat and is electrically floating with respect to end chamber 154. Except for the need to dissipate heat, tube 150 could be made of ceramic or other insulating material. Electrode assembly 36 is generally similar to the one described in connection with FIGURES 1 and 2 except that it includes a water-cooled magnet coil 90 which performs a function similar to the end coils 214 and 220 in FIGURE 1 in extending the magnetic field through the electrode assembly, and includes an electrically floating anode shield 160 placed immediately in front of the anode 70. The latter element is more fully described in an application by the present inventors filed approximately simultaneously with the instant one and entitled Plasma Arc Electrodes With Anode Heat Shield, Ser. No. 545,701, filed Apr. 27, 1966.

Each end chamber 154 and thus central tube 150, is evacuated by a pump 12. A single pump may be connected and paralleled to both end chambers. A water cooled vacuum separator 100, surrounds electrode assembly 236 and includes an aperture 102 positioned a short distance in front of and in alignment with anode shield aperture 101. Vacuum separator 100 is supported and electrically insulated from chamber 154 by insulator 153. Vacuum separator 100 defines an inner chamber 106 which does not communicate with end chamber 154 except through aperture 102 which is slightly larger than the anode diameter. Inner chamber 106 is connected to a pump 110 which is separated and distinct from pump .4 12. This pump can be connected to evacuate both inner chambers 106.

When the illustrated apparatus is operated in the manner described in connection with FIGURE 1, a plasma column is formed at electrode assembly 236 and extends into central tube 150. It is believed that there is a longitudinal flow of plasma along the outside of the plasma column and in the direction of the electrode assembly. This flow is passed by aperture 102 but intercepted by the anode heat shield 160 or the anode itself. Regardless of theory, it has been found that the pressure in inner chamber 106 will greatly exceed the pressure outside the inner chamber and a large portion of the gas flow which must be evacuated from the device can be withdrawn by pump 110 at a much higher suction pressure than exists at the inlet to pump 12. As previously explained, it is much more efficient to remove gas at a higher rather than a lower pressure. In addition, vacuum separator 100 and particularly the area surrounding aperture 102 serves to absorb some heat which would otherwise have to absorbed by other parts of the apparatus already burdened by a high heat load. The portion of vacuum separator 100 surrounding aperture 102, can be constructed in the same manner described for a similar element in our simultaneously filed application relating to improvements in Plasma Arc Electrodes With Anode Heat Shield, Ser. No. 545,701, filed Apr. 27, 1966.

In one experiment, utilizing the embodiment of FIG- URE 3, .0237 gram per second of hydrogen was introduced at each electrode assembly, the magnet coil and are assemblies being de-energized. Under these conditions the pressure inside inner chamber 106 was 100 microns and the pressure outside was microns, the difference being presumably caused by a higher pumping capacity of pump 110 as compared to pump 12. When a plasma column was formed by energizing magnets 156 and 90, to create a 20,000 gauss field in tube and an 8,000 gauss field at the electrode assembly, and 200 amperes of current at 68 volts was forced between each cathode and anode, the pressure in the inner chamber 106 rose to 550 microns while the pressure outside was 200 microns. This clearly demonstrates that the pressure increase inside inner chamber 106 is caused by the dynamics of the plasma discharge and not simply due to differences in pumping capacities of the two pumps nor to the fact that the feed gas is introduced to the system inside chamber 106. In a different experiment, the hydrogen flow rate was increased slightly to .028 gram per second, the magnetic field inside tube 150 was increased to 26,000 gauss and the magnetic field at the electrode assembly was increased to approximately 9,500 gauss. Under these conditions, the pressure within chamber 106 was 1900 microns while the pressure outside remained at 220 microns. This represents a pressure ratio of 8.6 between the pressure existing generally in tube 150 and end chamber 154 and that within the inner chamber 106.

FIGURE 4 shows in a highly schematic fashion a still further embodiment of the invention. The apparatus is generally similar to that of FIGURE 3 except that pumping ports or plenums are defined in tube 150 by circular restrictors provided between some of the magnet coils. The circular restrictors, preferably water-cooled metal, are seen to be adjacent each port on the side of the port nearest the outer end of the device. The outer restrictors 200 have the smallest internal diameter, intermediate restrictors 202 have a larger diameter, and inner restrictors 204 have the largest diameter. Restrictors 200 are functionally similar to vacuum separators 100 in FIGURE 3. Each plenum or port is connected with its own vacuum pumping system, except the corresponding symmetrically positioned ports may be connected to a common pump as shown. Thus plenums 205 are connected to a common pump 110 and plenums 206 are connected to common pump 208. In a similar manner the two outlets serving plenums 210 are connected to a common pump 212 and the two outlets serving plenums 214 are connected to a common pump 216. Each restrictor intercepts a portiton of the longitudinal flow of plasma along the outside of the plasma column contained in tube 150 during operation of the device. The plasma recombines and is neutralized at the surface of the restrictors and is removed through the associated pumping system. In this way the total pumping effort required to maintain the vacuum chamber evacuated is distributed over a large number of pumps. The distribution of pumping load among the various pumps can be controlled through variation of the diameters of the various restrictors. In FIGURE 4 pump 216 must be adapted to pump against a low suction port pressure. However, the pump inlet pressure will be progressively higher at pump 212 and 208 and will be highest at pump 110 which will also handle the largest mass flow. In this way the material necessarily introduced into the plasma through feed passages 36 is removed in the most efiicient possible way. Furthermore, much of the thermal energy in the plasma which would otherwise be dissipated at or near the electrode assemblies is removed by the various restrictors. By spreading the thermal load over more elements, the load on each one is reduced and operation at higher power levels becomes possible.

While the present invention has been particularly described in terms of specific embodiments thereof, it will be understood that in view of the present disclosure numerous modifications thereof and deviations therefrom may now be readily devised by those skilled in the art without yet departing from the present teaching. Thus, for example, symmetry about a linear axis is preferred, but departures from this symmetry can be tolerated and the axis itself can be curved since the discharge will follow the magnetic field whether straight or not. Similarly the magnetic coils can be other than solenoidal or cylindrical, as by being wound on conical forms, and permanent magnets may be used within the limits of their technology. Other variations in size, configuration, operating conditions and the like are encompassed by the invention and the invention is accordingly to be broadly construed and limited only by the spirit and scope of the claim now appended hereto.

What is claimed is:

1. Plasma containment apparatus comprising:

(a) a vacuum chamber;

(b) first vacuum pumping means to evacuate said vacuum chamber;

(c) magnetic means to form a longitudinally continuous magnetic field along a line within said chamber; (d) at least one plasma arc generator disposed within said magnetic field on said line and substantially symmetrical thereabout, said generator comprising a central cathode electrode and an anode electrode encircling said cathode, said generator including at least one passage terminating between said cathode and anode, gas supply means to introduce a plasma forming gas through said passage and power supply means to maintain an arc discharge between said anode and said cathode;

(e) multiple, annular, axially positioned flow restrictor means positioned on said line intermediate said first pumping means and said generator, said flow restrictor means sealed to said chamber but electrically insulated therefrom, said annular restrictor means having internal apertures successively larger in accord with the increasing distance of said restrictor means from the nearest plasma arc generator, said restrictor means being adapted to impede the flow of gaseous matter to said first pumping means; and,

(f) second pumping means comprising individual pumping means associated with each of said restrictors, each of said pumping means being positioned to evacuate volumes of said chamber adjacent the side of said restrictor associated therewith in closest proximity to the nearest arc generator.

References Cited UNITED STATES PATENTS 4/1959 Beam et al 313-7 10/1961 Dandl 313231 X US. Cl. X.R.

1 UNI'IEI) S'IA'IES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,453,460 Dated July 1, 1969 Inventor(s) (iordon I. Cann It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

In the drawings, Sheets 1-4:

(a) At the top of each sheet, cancel "G. L. CANN ET AL" and substitute therefor -G. L. CANN-;

(b) At the lower righthand corner of each sheet;

cancel "INVENTORS" and substitute therefor -INV ENTOR; and cancel "ROBERT L. HARDER".

Column 1, lines 4 and 5, cancel "and Robert L. Harder, Altadena, Calif. assignors" and substitute therefor -assignor.

SIGNED AND SEALED MAY 261970 (SEAL) Attest:

WILLIAM E. suauYwR. EdwardM. Fletcher, 11'. Commissioner of Patents Attesting Officer

Images