Jet Pinched Plasma Arc Lamp And Method Of Forming Plasma Arc

Abstract

A plasma arc is generated between an anode and a cathode in a gas pressurized chamber. The plasma arc is stabilized to give greater safety and concentrated to give greater irradiance by a jet of gas coaxial with and pinching the plasma arc. Means are employed for greatly reducing heat flux through the face of the anode. The gas jet is created by forcing the gas through a nozzle type opening positioned at one of the electrodes and antipodal the other of the electrodes, while a second gas flow cools the chamber walls. The gas is recirculated through a purifying medium to avoid contamination.

Parent Case Text

This application is a continuation in part of the application of Ronald E. Sheets, Serial No. 655,536 filed July 24, 1967 now abandoned.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to plasma apparatus and methods and more specifically to apparatus and methods for concentrating and stabilizing the plasma arc in a plasma lamp.

2. Description of the Prior Art

Devices employing a plasma arc provide compact high energy sources having a plurality of contemporary uses including such diverse ones as solar simulators, search lights, signaling beacons, projector lamps and furnaces.

One type of prior art plasma lamp consists of a hermetically sealed chamber spherical or ellipsoidal in shape having two antipodal cylindrical extensions and made of clear fused silica containing a gas such as argon or xenon. In each of the two cylindrical extensions, there is an electrode usually made of tungsten; supporting structure for the electrode; and connecting rods to external electrical contacts.

It is established plasma arc phenomena that the centerline irradiance of the plasma arc may be increased by cooling the periphery of the arc and by concentrating the arc into a smaller cross-section. Accordingly, most plasma arc lamps are operated with several atmospheres pressure within the chamber to concentrate the arc.

A plasma arc lamp is restricted to use in the vertical position unless the plasma arc is stabilized in some manner. During horizontal operation, an unstabilized plasma arc will tend to rise, and if it contacts the chamber walls, will cause an explosion due to its intense heat.

One type of device presently in use stabilizes the arc by forcing gas tangentially into the chamber, thereby generating a vortex through which the plasma arc passes. The vortex keeps the arc from rising as it traverses from one electrode to another.

The vortex stabilized arc, however, will characteristically dissipate electrodes quite rapidly. The vortex concentrates the plasma arc along its entire length from the cathode electrode to the anode electrode. Due to the concentration of the arc at the anode, a high amount of energy must be absorbed over a relatively small area. This energy absorption results in the vaporization of the anode electrode.

The vortex stabilized arc apparatus of the prior art also fails to benefit from the increased irradiance generated when the periphery of the arc is cooled. The apparatus will generally introduce the gas into the chamber at a point far removed from the plasma arc. By the time the gas has reached the proximity of the arc, it has increased tremendously in temperature and will have little cooling effect upon the arc.

A second type of apparatus stabilizes the plasma arc by use of a magnetic field. Such a device may generate a field by a coil coaxial to the plasma arc and extending between electrodes. The current passing through the coil establishes a magnetic field longitudinal the axis of the plasma arc. This field stabilizes the ions of the arc. Stabilization of the arc in this manner requires additional power capability to maintain the field and additional hardware for the coil. This coil, being coaxial to the plasma arc, also reduces the usefulness of the lamp as a radiance source since it intercepts a portion of the radiance generated.

SUMMARY OF THE INVENTION

The present invention provides a gas pressurized cylindrical chamber formed by two concentric quartz cylinders having an annular passageway between them for passing a gas which supplies the plasma, and which serves also as a coolant. Within the chamber are two electrodes, an anode and a cathode, in circuit with an external high current source for generating a plasma arc between the electrodes. The plasma arc generated in the chamber is stabilized and concentrated and its periphery cooled by a convergent, but non-vertical, hollow jet of gas coaxial to the arc and entering the chamber through a nozzle concentric to the cathode.

The stabilization of the arc allows the plasma lamp to be employed safely in any attitude and also the lamp to be accelerated in a direction normal to the plasma arc.

The concentration of the plasma arc and the cooling of the periphery of the arc increase the irradiance generated by the lamp. Since the arc is concentrated by a gas jet emanating concentric to the cathode, it flares as it nears the anode due to the decreased pinching by the gas jet. This results in longer anode life since the per unit area of energy absorption is reduced from what it would be if the arc were not flared.

An additional benefit derived by the present invention is that the arc is stabilized and concentrated without hardware which would interfere with the radiant output of the arc.

In accordance with another aspect of the present invention, the gas introduced into the chamber through the nozzle leaves the chamber through openings adjacent the face of the anode. That portion of the gas jet which does not pass out of the chamber through the openings is deflected toward the cathode by a deflection rim formed around the anode. The deflected gas forms a toroidal collar around the plasma arc temporarily containing the gas. This feature of the invention prevents the deflected hot gas from directly contacting the chamber walls and depositing vaporized portions of the electrodes thereon or in some instances causing a container wall failure due to the temperature of the gas.

In accordance with yet another aspect of the present invention, the inner cylinder forming the chamber is provided with a plurality of gas ports about its periphery. A portion of the gas coolant flowing in the passageway between the cylinders will enter the chamber through the gas ports and form a boundary layer of moving gas adjacent the inner wall of the cylinder. This moving layer of gas cools the wall of the cylinder and prevents ambient gas within the chamber from contacting the cylinder walls and depositing vaporized electrode material. An alternate embodiment of the present invention introduces this boundary layer gas from openings concentric with the cathode to produce a laminar flow along the chamber walls.

Another aspect of the present invention is the purification of the gas as it is recirculated by placing metallic getters at a point in the gas flow having a temperature at which the getter operates most efficiently.

Another feature of this invention is the provision for initiating the arc by moving the entire cathode toward the anode, while limiting the current which can flow through the lamp circuit.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side elevation view, partially in longitudinal-section of a system including a plasma arc lamp and parabolic reflector;

FIG. 2 is a perspective view of the plasma arc lamp chamber;

FIG. 3 is an end elevation view of the lamp shown in FIG. 2;

FIG. 4 is a sectional view of one embodiment of the plasma arc lamp of FIG. 2, taken on section line 4--4 of FIG. 3, showing one embodiment of the present invention;

FIG. 5 is a sectional view of the embodiment of the plasma arc lamp of FIG. 4 illustrating the electrode coolant flow;

FIG. 6 is a perspective view of the anode assembly of the embodiment of FIG. 4 showing the heat exchanger fins thereon;

FIG. 7 is a cross-sectional view of the cathode taken along line 7--7 of FIG. 4;

FIG. 8 is a cross-sectional view of the anode taken along line 8--8 of FIG. 4;

FIG. 9 is a cross-sectional view of the plasma lamp taken along line 9--9 of FIG. 4;

FIG. 10 is a cross-sectional view of the pump shown in FIG. 1, taken on section line 10--10 of FIG. 11, used to recirculate the gas in the plasma arc lamp;

FIG. 11 is a cross-sectional view taken along line 11--11 of FIG. 10;

FIG. 12 is a longitudinal sectional view of the preferred embodiment of the present invention taken along lines comparable to those which produced FIG. 4, and showing a schematic diagram of a starting circuit used to initiate the arc between the electrodes.

FIG. 13 is a sectional view of the cathode housing of the preferred embodiment taken along the line 13--13 shown in FIG. 12;

FIG. 14 is a sectional view of the cathode housing of the preferred embodiment taken along the line 14--14 shown in FIG. 12;

FIG. 15 is an enlarged sectional view of the cathode tip assembly as shown is FIG. 12;

FIG. 16 is a sectional view of the cathode tip assembly taken along line 16--16 of FIG. 15;

FIG. 17 is an enlarged sectional view of the anode tip assembly as shown in FIG. 12;

FIG. 18 is a sectional view of the anode tip assembly taken along line 18--18 of FIG. 17;

FIG. 19 is a sectional view of the anode tip assembly taken along line 19--19 of FIG. 17;

FIG. 20 is an enlarged partial sectional view of the anode tip assembly as shown in FIG. 12, but with provisions for multiple purifying agents;

FIG. 21 is an enlarged partial sectional view of the anode and cathode tip assemblies of FIG. 12 with the cathode in the arc-initiation position; and

FIG. 22 is an enlarged partial sectional view comparable to FIG. 21, but with the cathode midway between the arc-initiation and steady state positions.

DESCRIPTION OF THE EMBODIMENT OF FIGS. 1-11

Referring to FIG. 1, there is shown a radiance directional system 10 having a gas pressurized plasma arc lamp 12 as its radiance source, a pump 14 for recirculating the gas within the lamp 12 and reflecting means shown as parabolic reflector 13 for directing the radiance produced by the lamp 12. The heat generated by the lamp 12 and radiated to the reflector 13 is removed from the reflector 13 by a cooling means represented by cooling coils 16. The coil 16 forms a helix around the outside surface of the reflector 13. A coolant pumped through coil 16 by a coolant recirculating means (not shown) removes the heat from the reflector 13.

The high current source 18 is provided to sustain the plasma arc within the lamp 12. In one embodiment of the present invention, the source 18 is rated at 30 kw and is operable between 60 and 100 volts DC supplying up to 500 amps.

This embodiment of the lamp 12 has an efficiency of approximately 50 percent generating 15 kw radiant output for a 30 kw input and develops a horizontal intensity of 180,000 candles.

One embodiment of the plasma arc lamp 12 is illustrated in FIGS. 2 through 9. Referring first to FIG. 2, the plasma arc lamp 12 is shown to include gas recirculation inlet port 19, outlet port 20 and electrode coolant recirculation tubes 22. The pump 14 (FIG. 1) receives gas from the lamp 12 via outlet port 20 and recirculates the gas back into the lamp 12 through inlet port 19. A non-conductive electrode coolant such as water is recirculated between the electrodes within the lamp 12 by coolant recirculation tubing 22.

Referring now to FIG. 4, the lamp is shown to include an electrode means for maintaining a plasma arc column 30 comprising cathode electrode assembly 32, anode electrode assembly 34 and a gas pressurized chamber coaxial to the plasma arc 30.

The anode electrode assembly 34 is connected to the high current source 18 by lead 17. The cathode electrode assembly 32 is connected to high current source 18 by lead 15. The anode electrode assembly 34 is electrically insulated from the remainder of the lamp 12 by an insulating collar 21.

The chamber of lamp 12 is translucent and formed by two concentric annularly spaced quartz cylinders 26 and 28. The outer cylinder 26 provides a pressure container for the apparatus and the inner cylinder 28 serves as a heat shield for the cylinder 26. Walls of the chamber are cooled by gas entering the lamp 12 through inlet port 19 and flowing through an annular passageway 29 formed between the cylinders 26 and 28. The inner chamber wall formed by cylinder 28 has a plurality of openings around its periphery represented by openings 46 and 48 to allow a portion of the gas flowing through the passageway 29 to enter the chamber. By way of specific example, in one embodiment of the present invention, there are twenty openings in each of two planes perpendicular to the longitudinal axis of the chamber at points represented by openings 46 and 48.

The outer quartz cylinder 26 is shown in FIG. 4 to have flared ends 25 and 27 inserted into end adapters 63 and 65 respectively. The cost of a commercially available cylinder having the close tolerance required for a proper seal with end adapters 63 and 65 would be quite burdensome. Accordingly, the cylinder 26 is advantageously formed by applying glass blowing techniques to a standard wide tolerance cylinder to flare the ends to the required tolerance.

Cathode electrode assembly 32 comprises a cylindroconical electrode tip 33 advantageously formed of tungsten positioned in the nose of a generally cone-shaped base 36 advantageously formed of copper. The nose section 38 of the base 36 forms a shroud defining a generally conical annulus around the electrode tip 33 leaving an annular funnel-shaped or generally conical passage 37 between them that leads to a convergent nozzle opening 39 circumscribing the conical portion of the tip 33 and terminating short of the end thereof, as shown. Gas exiting generally longitudinally from nozzle opening 39 is thus directed and formed into a convergent, non-vortical sheath moving in the direction toward the arc forming extremity of the cone on the electrode tip, immediately beyond which its momentum causes it to converge further, and thus pinch the arc 30.

Nozzle opening 39 communicates with the passageway 29 formed between the cylinders 26 and 28 by a channel system internal to the electrode assembly 32. Gas flowing through the passageway 29 enters the base 36 through a series of ports represented by port 42. In one embodiment of the present invention, twenty ports are positioned radially within the base 36, as shown in FIG. 7. Ports 42 allow the recirculating gas to enter chamber 44 and then channel 45; channel 45 represents a plurality of cylindrical channels communicating between chamber 44 and the annular funnel-shaped passageway 37. One embodiment of the present invention uses cylindrical channels instead of a funnel-shaped passageway to allow the heat from the plasma arc 30, which is intercepted by the face of the base 36, to be conducted to the interior of the base by the metal webs between the channels. The gas will be seen to be thus directed into the nozzle 39 in a straightforward direction, instead of tangentially, so that its components of velocity within the nozzle are longitudinally of the cathode-anode axis, and radially inward toward that axis.

A second channel system internal to the base 36 allows a coolant to continuously circulate through the interior of the cathode electrode assembly 32 removing heat conducted from the face of the base 36. FIG. 5 shows the electrode coolant flow more clearly. Coolant enters the base 36 from coolant tubing 22 through ports represented by port 66. The coolant flows into chamber 67 removing the heat from the base 36 and flowing out of the base 36 through ports represented by ports 68 to the coolant tubing 22.

The anode electrode assembly 34 is shown in FIG. 4 as comprising a cylindrical side wall assembly 49 made up of an exterior cylindrical wall 49a, and an externally annularly channeled wall 49b immediately inside thereof. At the front or arc end of this wall assembly is a frustoconical end or end wall 50, into which is sunk an annular depression 50a. Towards the center, end wall 50 joins a tubular rearwardly extending core wall 50b, and this core wall 50b and the walls 49a and 49b are fitted into the end adapter 65 (FIG. 4). Seated in the annular depression 50a is a hollow anode face disk 59, which is on the front end of a nipple 61 set into the front end of the anode core wall 50b. A plurality of gas outlet channels 60 communicate from a position adjacent to the periphery of anode face 59 to points within the electrode assembly 34. In one embodiment of the present invention, there are twenty gas outlet channels 60, as shown in FIG. 8. The frustoconical end 50 protrudes about the periphery of the face 59 to form a gas deflection rim 70, as seen in FIG. 6.

Embodied in the cylindrical wall assembly 49 is a heat exchanger 52 shown in FIG. 6. The heat exchanger 52 comprises a network of fins 49c which form a labyrinth of interconnected channels. The fins are formed by oblique slots 49d through spaced radial or circumferential ribs 49e about the cylindrical wall 49b. The oblique slots 49d are preferably offset from rib to rib, so that the flow through the slots will not be straight through, but deflected by the next rib after passing through each slot.

The gas outlet channels 60 leading from adjacent the anode face 59 terminate in the heat exchanger 52. Gas flowing into channels 60 from the chamber will enter the heat exchanger 52 and pass through the labyrinth of channels to leave the lamp 12 through gas outlet port 20 (FIG. 4). In the process of passing through the labyrinth the gas transmits its heat into the fins of the heat exchanger 52 and is cooled. The heat received by the heat exchanger 52 from the flowing gas, and the heat from the plasma arc 30 intercepted by the electrode assembly 34, is conducted internally.

The flow of the non-conducting coolant through the electrode assembly 34 can be seen in FIG. 5 in conjunction with FIG. 4. Coolant enters the lamp 12 through inlet port 62 and circulates through the electrode assembly 34, entering along core 50b to extract anode heat from the latter, then returns outside a dividing wall 50d to sweep and extract heat from heat exchanger walls, and finally entering coolant recirculation tubing 22 through ports represented by port 64. The circulating coolant conducts heat away from the electrode assembly 34.

The arc 30 maintained between the electrode assemblies 32 and 34 is initiated by an arc initiating means shown in FIG. 4 as comprising a displaceable rod 53 positioned in the hollow nipple 61 and core 50d of the electrode assembly 34, and a rod positioning means 55 for advancing the rod 53 through the hollow center 61 to contact the electrode tip 33. Once an arc is established between the rod 53 and the electrode tip 33, the rod positioning means 55 retracts the rod into the electrode assembly 34 drawing the arc with it. Once the rod 53 has been retracted within the proximity of the anode face 59, the arc leaves the rod taking the path of least resistance to the face 59 and is thereafter sustained between the electrode tip 33 and the face 59, as shown.

The rod positioning means 55 comprises a cylinder 54, a piston 56 and spring 58. A fluid or gas entering cylinder 54 exerts a force upon piston 56 which advances the rod 53 against the opposition of loading spring 58. The reduction in fluid pressure within cylinder 54 will allow spring 58 to retract rod 53 to a normal position.

The rod 53 is constructed of two cylindrical sections 71 and 73. The smaller cylinder 71 is advantageously made of tungsten and is gold soldered to the larger cylinder 73 advantageously made of stainless steel. The bimetallic construction allows the cylinder 71 to perform its function as an electrode during arc initiation and yet due to the stainless steel cylinder 73, the intense heat of the arc is not conducted to the rod positioning means 55. It is to be understood that other metals of high temperature resistivity may be substituted for stainless steel in the construction of cylinder 73.

A high current source 18 is connected between electrode assemblies 32 and 34 to maintain the plasma arc 30 between them. The plasma arc 30 (FIG. 4) is stabilized, concentrated, and has its periphery cooled by a gas jet formed coaxial to the arc 30. The gas jet is formed concentric to the electrode tip 33 by a gas flow constricting means comprising the convergent funnel-shaped annular channel 37 between the electrode tip 33 and annulus 38 and flows to the anode face 59 of electrode assembly 34. The gas enters the lamp 12 by inlet port 19 and flows down the passageway 29 with a portion entering the chamber directly through the ports represented by ports 46 and 48. The remainder of the gas enters the chamber through the nozzle opening 39 after passing through the electrode assembly 32. The gas flowing through the passageway 29 acts as a coolant for the quartz cylinders 26 and 28. Pressure within the chamber is maintained due to the flow of gas through ports 46 and 48.

One embodiment of the present invention typically employs xenon gas and maintains a pressure of 14 atmospheres within the chamber. It is understood that the present invention may be practiced in an atmosphere comprised of any inert gas and under any pressure the particular container is capable of withstanding.

The gas jet emerging around electrode tip 33 is in the form of a convergent, non-vertical sheath which constricts the plasma arc 30 severely and as a result concentrates the arc 30 into a small cross-section, thus increasing its radiance. The expression "substantially non-vortical," as used herein may here be defined as excluding a rapidly spinning gas, i.e., where a major portion, or nearly all, of its kinetic energy is involved in spinning motion, but not necessarily excluding some minor degree of swirl, such as would not largely increase the residence time of the gas flowing along the conical cathode to the arc, and thereby permit material heating of the gas. The expression thus denotes herein that a large proportion, if not all, of the kinetic energy of the gas flow is along and toward the axis of the jet, rather than around it. The expression substantially non-vortical, or essentially non-vortical, thus denotes simply that the flow of the gas is primarily, or essentially, if not exclusively, longitudinally of and convergently with the common axis of the arc column and gas jet, in a sense consistent with avoidance of the long average residence time of gas particles following vortical spin paths to the arc as distinguished from paths which are generally longitudinal of the axis of the arc. It should be stressed in this connection that long residence time while undergoing a vortical flow path affords time for undue heating of the gas, and thus detracts from the arc cooling effect that is so important to achieve. As the gas jet flows toward the anode face 59, the pressure upon the arc 30 is reduced owing to expenditure of its kinetic energy through encounter with the arc, and the arc 30 flares, as shown. The flare of the arc 30 allows the face 59 of the anode assembly 34 to absorb the high energy of the arc 30 over a large area, thus reducing the per unit area energy absorption or flux. This important feature of the invention substantially increases the useful life of the anode.

The gas jet flowing from the nozzle opening 39 strikes the anode face 59 and all but a small portion of it passes directly out of the chamber through gas outlet channels 60. The portion of the gas jet which does not enter channel 60 is deflected from the anode face 59 by the deflection rim 70 (FIG. 6) and forms a spinning toroidal collar of gas concentric to the plasma arc 30 shown by arrows 75 in FIG. 4. This gas toroid is very stable and exerts an additional stabilizing force upon the plasma arc 30. The toroidal containment of this circulating portion of the gas jet also prevents the hot gas from directly striking the chamber walls. The temperature of the gas approaches 6,000.degree. F. and if it contacted the walls directly, could result in a failure of the quartz cylinder 28. In addition, the flowing gas will absorb particles from the electrodes which vaporize slightly during operation and will deposit them upon the chamber walls if not deflected.

Another important feature of the present invention is a boundary layer of moving gas adjacent to the inner wall of cylinder 28 formed by the gas which enters the chamber from passageway 29 through ports 46 and 48. This layer of gas serves to both cool the walls of the chamber and provide a protective buffer which prevents the ambient gas within the chamber from contacting the walls of cylinder 28 and depositing vaporized electrode material.

An improved pump 14 for recirculating the gas in lamp 12 is shown in detail in FIGS. 10 and 11 and comprises a centrifugal gas impelling means 101, a switching means 103 responsive to the flow of gas through the pump 14, and a gas filtering means 105.

The centrifugal gas impelling means 101 is provided by a disc-shaped impeller 106. This impeller is affixed to the end of motor shaft 102 which in turn is fixed to the rotor 104 of motor 100. Advantageously, motor 100 comprises an electric induction motor 100 operated on 400 cycle, three-phase AC power.

The recirculating gas of lamp 12 enters the pump 14 through port 108 and passes to the center of the impeller 106. As the impeller 106 is rotated, the gas is accelerated from the center to the periphery of the impeller 106 through a plurality of radial channels in the impeller 106, one such channel being shown at 109. The gas passes through passageway 107, to the chamber 110 containing filtering means 105. Filter 105 removes impurities from the gas acquired during recirculation. The gas then passes through port 112 to chamber 114.

Switching means responsive to the flow of gas through pump 14 comprises a magnetic piston 116 and spring assembly 118 within the chamber 114, and magnetic electrical switch 122. The gas entering the chamber 114 has sufficient energy imparted to it by the impeller 106 to force the piston 116 to the rear of the chamber 114, compressing spring 118. The rearward movement of piston 116 exposes port 120 through which the recirculating gas leaves the pump 14. The piston 116 will remain at the rear of chamber 114 so long as the pumping action of the motor 100 continues. If the gas flow should stop, the loaded spring 118 forces piston 116 to the front of the chamber 114.

Magnetic electrical switch 122 is magnetically responsive to the movement of magnetic piston 116 and remotely controls the high current source 18. The contacts of the switch 122, shown in FIG. 11, are in series with the coil of a control relay (not shown) within the high current source 18. When the contacts of switch 122 are closed, the control relay enables the source 18 and when the contacts are open, the source 18 is disabled. The contacts of switch 122 will be closed when piston 116 is in the rear of chamber 114 and will be open when piston 116 is in the front of the chamber 114.

Accordingly, the source 118 is operable when gas is recirculating through the lamp 22 and is inoperable when gas is not recirculating.

This aspect of the present invention provides a failsafe feature. Without the flow of gas in the form of a gas jet concentric to the plasma arc, the arc would impinge upon the chamber walls during horizontal operation of the lamp, resulting in catastrophic failure. Such an occurrence is automatically inhibited by the present invention since the lamp 22 will not operate due to open switch 122 if gas is not recirculating through the pump 14.

The preferred embodiment of the present invention is illustrated in FIGS. 12 through 22. Referring first to FIG. 12, the preferred embodiment plasma arc lamp 200 is shown to include an electrode means for maintaining a plasma arc 202, including a cathode electrode assembly 204, an anode electrode assembly 206, and a gas pressurized chamber coaxial to the plasma arc 202.

The anode electrode assembly 206 is connected to the high current source 208 by lead 210. The cathode electrode assembly 204 is connected to high current source 208 by lead 212. The anode electrode assembly 206 is electrically insulated from the remainder of the lamp housing 200 by an insulating collar 214.

The pressurized chamber of lamp 200 is translucent and formed by two concentric quartz cylinders 216 and 218. The outer cylinder 216 provides a pressure container for the apparatus, and the inner cylinder 218 serves as a heat shield for the cylinder 216. The outer quartz cylinder 216 has flared ends 220 and 222 which are inserted into a pair of end adapters 224 and 226 respectively. The purpose of the flared ends 220 and 222 is the same as that explained in reference to the flared ends 25 and 27 of the embodiment shown in FIG. 4.

The cathode electrode assembly 204 comprises an electrode tip 228 advantageously formed of tungsten and conically formed at each end. This tip 228 is mounted within a cavity formed by an annular shroud or nose section 230 and a cathode base 232 which has a conical recess to receive the tip 228. The cathode base 232 is mounted on a tubular sleeve 234 which is slidably positioned within a recessed cathode housing 236. This housing 236 has a machined conical recess which is adapted to receive the conical end of the cathode base 232 when the sleeve 234 is retracted to its fullest extent.

The anode electrode assembly 206 comprises a disk-like anode tip 237, which has a recessed semi-spherical face 298 opposite the cathode tip 228 and a generally conical face on the opposite side. This conical face fits closely within the concave side of a generally conical anode-face heat exchanging disk 238, which in turn is mounted with its convex side within a generally conical concavity in the end of a core cylinder 308 mounted inside the main anode heat exchanger 240.

Both the arc gas and an electrode coolant are recirculated through the lamp structure. The gas is introduced into the lamp structure through a gas inlet aperture 242 and passes into an annular cavity 244 to enter the annular space 246 between the cylinders 216 and 218, thereby cooling these cylinders. At the opposite end of the annular cavity 246 the gas flows into an annular cavity 248 in the cathode assembly and then through a series of apertures 250 into an annular cavity 252, at which point the gas flow separates, approximately 95 percent of the gas flow going directly into the pressurized chamber through a series of ports 254 and approximately 5 percent of the gas following a series of tubes 256 and an annular channel 258 to pass through a series of holes 260 and thereby flow longitudinally towards the cathode into the tube 234. From the tube 234, the small percentage gas flow enters the cathode base 232 through a series of channels 262 to flow through an annular convergent or frustoconical annular nozzle 264 defined by and between the forward conical or frustoconical portion of the cathode tip 228, and the opposed frustoconical internal surface of the aforementioned annular shroud 230. It will be noted that the cathode tip tapers substantially conically to a blunt forward arcing extremity and is thus in the form of a frustum of a cone; and that the surrounding conical shroud 230 terminates short of the conical terminus of the cathode, and thus short of this blunt forward extremity, providing an annular or ring shaped, convergent gas discharge orifice at that point. Thus, the nozzle passage 264 and nozzle direct the gas in a convergent, conical, non-vortical sheath along the surface of the frusto-conical cathode and off the end thereof in a still convergent but non-vortical manner so as to impinge directly on and thus pinch as well as cool the column of the arc issuing from the cathode. Attention is drawn to the essentially non-vortical character of the gas flow issuing from the nozzle, which assures only fleeting residence time for the gas within the convergent nozzle, thus minimizing heating of the gas. The termination of the shroud short of the arcing end of the cathode is a feature of importance, since it avoids overheating and burning of the shroud, it removes the shroud from a prominent forward position where it could occlude a fraction of the radiant energy from the most intense region of the arc column, and it also frees the convergent hollow gas jet soon enough that it will still have the necessary pinching effect on the arc, but will be "soft" enough so that after initial impingement on the arc, it will tend more readily to thereafter spread, and so permit the desired final flare of the arc toward the anode. Both the large percentage gas flow entering the chamber through the ports 254 and the small percentage gas flow entering the chamber through the annular space 264 are exhausted from the chamber through an annular recess 266 between the anode tip 237 and a frustoconical end 268 of the anode assembly 206. The gas then flows through the main anode heat exchanger 240 and is exhausted from the plasma arc lamp 200 through an annular channel 270 by a tubular port 272.

The electrode coolant enters the lamp 200 through a central bore 274 in the anode core 308 and initially flows through the anode-face heat exchanger 238 to enter an annular channel 276 around core 308 and inside the cylindrical side wall of heat exchanger 240. The coolant then, following extraction of heat from heat exchanger 240, passes out of the anode assembly 206 through a port 278 and is conducted by a tube (not shown) into a coolant recirculation tube 280. This tube 280 conducts the coolant to the cathode assembly 204 where the coolant passes successively through a pair of concentric annular channels 282 and 284 and is exhausted through a second coolant recirculation tube 286 to an exhaust port 288 on the anode end of the lamp 200.

Portions of both the coolant fluid flow path and the gas flow path are substantially improved in the embodiment of FIG. 12 and following from that which was described in reference to FIGS. 4 through 11. These changes are described in detail below.

FIGS. 12, 13, and 14 illustrate the gas flow channels within the cathode assembly 204. On entering the cathode assembly 204 from the annular space 246 between the cylinders 216 and 218, the gas is circulated into the annular cavity 248. This gas then passes through a series of portals 250, best illustrated in FIG. 13, to enter an annular cavity 290. The metallic webs 292 between the apertures 250 serve to conduct heat from a ring-shaped member 294, which forms the outer wall of the cathode- electrode assembly 204. From the annular cavity 290 the gas flows into an adjoining annular cavity 252 at which point the flow separates, 95 percent of the flow passing through a series of aperatures 254, best illustrated in FIG. 14, and 5 percent of the flow passing through a series of tubes 256.

The larger percentage flow which passes through the apertures 254 enters the cylindrical chamber around the arc 202 in the form of a laminar flow which separates the region adjoining the quartz tube 218 from the region in the vicinity of the arc 202. This flow, therefore, tends to cool the quartz tube 218 and to prevent impurities which form on the electrodes 204 and 206 from reaching the quartz tube 218. The series of ports 254 greatly increases the wetted area past which the large percentage gas flow must flow before entering the arc chamber. This increased wetted area reduces the Reynolds number of the flow and therefore makes this large percentage gas flow in the vicinity of the quartz tube 218 more laminar than would a simple annular opening at the extremity of the cathode assembly.

The small percentage gas flow which enters the series of tubes 256 and flows through the annular cavity 258 and the series of holes 260, flows through the tubular sleeve 234 to the vicinity of the cathode electrode tip 228. This flow is best described in reference to FIGS. 15 and 16. On leaving the tubular sleeve 234, this small percentage gas flow passes into a series of channels 262 and enters the large end of the frustoconical annular space 264. At the small end of this annular space 264 proximate to the outer extremity of the cathode tip 228 the annular space 264 is broken by a series of longitudinal ribs 296. These ribs greatly increase the effective perimeter of the wetted area through which the gas must flow on leaving the cathode tip assembly 204 and therefore, by reducing the Reynolds number of that flow, make the flow more laminar. In addition, these ribs allow easy assembly of the annular nose section or shroud 230 over the cathode electrode tip 228 while still accurately defining the width of the annular nozzle space 264 at the point where the gas exits the cathode tip assembly. Since, in prior art gas jet assemblies, the annular nose section 230 in the vicinity of the cathode required accurate machining to define the width of the annular jet cavity 264, these ribs 296 reduce the cost and complications of assembling the cathode electrode assembly 204.

The highly laminar flow which exits from the annular cavity 264 tends to hug the surface of the cathode electrode tip 228 and this effect allows increased pinching of the arc in the vicinity of the cathode electrode tip 228 while still allowing the shroud 230 to be positioned away from the arc to avoid its overheating.

The preferred means for recirculating the coolant fluid and gas through the anode assembly is best described in reference to FIGS. 12, 17, 18 and 19. The anode tip of the present embodiment, best shown in FIG. 17, has a face 298 which is formed in a frustoconical or disked shape which generally coincides with the shape of a sphere generated around the center of the extremity or tip of the cathode. Such a formation increases the region over which the anode tip 298 and the cathode tip 228 are equidistant, and in which the electric field lines between cathode and anode are of substantially uniform length and spacing, and therefore spreads the arc 202 toward and at the anode footing while it is being concentrated at the extremity of the cathode. The anode footing of the arc 202 is likewise increased by the fact that the low percentage gas flow which emanates from the tip of the cathode assembly 204 is directed into the center of the spherical segment anode tip 237 and produces a higher pressure at the center of this anode tip 237 due to the impact or stagnation of this high velocity jet. This increased pressure on the center line produces an outward radial flow of plasma along the surface 298 of the anode tip 237.

An added advantage of the spherical anode tip 237 is the enhancement of the effective cooling of the anode footing area. The plasma arc 202 will rapidly destroy the face 298 of the anode tip 237 even when the anode footing area of the arc 202 is relatively large if effective means are not utilized to cool the anode tip 237. Since the heat incident at the face 298 is transferred through the anode in a diverging manner due to the generally spherical geometry of the device, each of the isotherms within the anode tip being spherical in shape, the heat flux at the larger rearward face 300 of the anode tip 237 is of reduced unit intensity. The increased radius of curvature increases the surface area at the rear face 300 of the anode tip 237 from which the heat may be extracted. In a similar manner, the concave or disked end of the anode face heat exchanging element 238 which fits with the face 300 of the anode tip 237 further reduces the density of the heat energy such that when the heat flows from the surface 302 of the heat exchanger element 238, it may be extracted over a considerably larger area than the footing area on the divergent arc column on the anode surface 298.

In order to enhance the ability of the coolant liquid to remove the heat from the anode tip 298, an improved anode tip heat exchanging element 238 is utilized in this embodiment. This element is best described with reference to FIGS. 17 and 19. The element fits flush with the surface 300 of the anode electrode tip 237 and incorporates a series of ribs 304 which form circular paths for the cooling fluid. These ribs are slotted in various places to allow the coolant fluid to pass from the center of the heat exchanging element 238 to the periphery. However, the slots 306 are oblique from the radius of the heat exchanger as viewed in FIG. 19. Therefore, the coolant liquid in passing from the center of the heat exchanging element 238 to the periphery is forced by the oblique slots 306 to flow tangentially as well as radially through the heat exchanging ribs 304. This bi-directional flow greatly increases the Reynold's number of the flow of coolant liquid through the heat exchanging element 238 and therefore increases the turbulence of that flow. This increased turbulence, in turn, increases the efficienty of the heat exchanger 238. As shown in FIG. 17, the cylindrical wall 308 forces the coolant liquid which enters the central bore 274 to pass radially and tangentially through the heat exchanger before exiting through the annular channel 276.

The main anode heat exchanger 240 is similar in construction and operation to the heat exchanger which was described in reference to FIG. 6 and has oblique or diagonal slots 308 as shown in FIG. 18 to increase the turbulence of the gas flow through the heat exchanger 240 and thereby increase the efficiency of this heat exchanger. As the gas passes from the forward extremity 310 of the heat exchanger 240 to the rear extremity 312 of this heat exchanger 240, heat is transferred from the arc gas into the coolant liquid in the annular channel 276. The gas therefore goes through a large temperature reduction between the extremities 310 and 312. It has been found, in the past, that contamination of the arc gas, of even a very small percentage, will produce performance degradation and will eventually lead to electrode failure. Metallic getters have been used in the prior art by placing the getter in the arc chamber and depending upon convection currents of the arc gas to bring the gas past the getter material to purify the gas. The present invention utilizes a bed of getter material 314 such as titanium, tantalum or barium, which materials, when properly heated, will combine with the impurities in the gas flow to form oxides, hydrides, nitrides and carbides, thereby removing the impurities from the gas flow. Each such metallic getter has a particular temperature at which it operates most efficiently to purify the arc gas. The getter material in the present embodiment may therefore be carefully positioned in the heat exchanger 240 so that it is placed in a temperature zone in this heat exchanger 240 to provide an optimum gas purification reaction. As an additional feature of this placement of the getter material 314 within the heat exchanging element 240, it is possible to recirculate all of the arc gas through the getter material and therefore assure purification of the arc gas in an organized rather than a random gas flow manner.

Referring to FIG. 20, an alternate method of utilizing the metallic getter in the heat exchanging element 240 allows placement of a variety of metallic getter materials 314, 316 and 318, each of which has a different optimum temperature for reaction with the impurities in the gas flow. Alternately, the materials 314, 316, and 318 may all be formed of one metal, where that metal most effectively combines with different gas impurities at different temperatures. These temperatures again may be selected along the temperature gradient between the extremities 310 and 312 of the heat exchanger 240. It has been found that these beds of getter material 314, 316 and 318 may be formed either by wrapping fine wire formed of the metallic getter within the recesses of the heat exchanger 240 or by placing a semi-porous ring of metallic getter within these recesses.

An additional improvement of the present embodiment is the means for initiating the arc within the arc chamber and is best described in reference to FIGS. 12, 21 and 22. Electrical pulsing techniques which have been utilized in the past to initiate the arc within plasma lamps become increasingly difficult as the pressure within the arc chamber is increased to increase the light output of the arc lamp. Similarly, a movable rod, such as the element 71 as shown in FIG. 4, has the disadvantage that the electrode configurations must be altered to allow utilization of such an element. The present embodiment includes a movable cathode mechanism which allows the entire cathode assembly 204 to be positioned proximate the anode tip assembly 206 to initiate the arc. The configuration of the electrodes at their extremities which determines the arc shape is therefore not limited.

Referring to FIG. 12, the actuating mechanism for advancing the cathode tip assembly 204 toward the anode tip assembly 206 will first be explained. This mechanism is hydraulic and uses the high pressure coolant fluid which typically has a pressure of 200 to 300 psig at the lamp as the driving force. By using the coolant fluid for actuation of this starting mechanism, an interlock is afforded, since the lamp cannot be started without the coolant system operating satisfactorily. The high pressure coolant is fed by means of a high pressure tube 320 into the end of a cylinder 322 in which a piston 324 is slideably mounted. The pressure of the high pressure coolant drives the piston 324 toward the anode assembly 206. When this occurs the piston will engage a cap 326 which is fitted within the end of the sleeve 234, thus driving the sleeve 234 toward the anode assembly 206 and, in turn, bringing the cathode assembly 204 adjacent the anode assembly 206. When the high pressure coolant 320 is removed from the cylinder 322, the piston is biased to return to its retracted position by a first spring 328; and the cathode assembly, with the sleeve 234, is biased by a spring 330 to return to a position wherein the cathode base 232 abuts and fits within the recessed cathode housing 236.

Referring now to FIGS. 21 and 22, the cathode is shown in FIG. 21 in its most extended condition with the cathode tip 228 adjacent the anode tip 237. With the lamp in this condition, an arc is easily generated, even if the pressure within the lamp is extremely high. With the arc thus initiated between the cathode tip 228 and the anode tip 237, the cathode is slowly retracted by the spring 330 into the recessed cathode housing 236, and is shown at a position midway through this travel in FIG. 22. The arc, once it is initiated, is thus drawn across the chamber between the electrodes eliminating the need for high voltage pulses or for irregularities in the anode configuration.

The hydraulic actuation of the starting assembly facilitates the introduction of a starting circuit which is shown in FIG. 12 which automatically places the lamp in a starting mode when the main DC power supply 208 is energized. This starting circuit functions as follows:

To start, an auxiliary power supply 252 is turned on, energizing a relay coil 338 to close a normally open switch 336. The DC power supply 208 is then turned on, which developes the full open circuit voltage of the power supply between the separated anode and cathode by a circuit lead 210 to the anode assembly 206, and a circuit lead 332, the now closed switch contacts 336 of relay 338, and the wire 212, to the cathode. Since the cathode assembly 204 is fully retracted, an arc will not be generated between the cathode tip 228 and the anode tip 237 and no current will be drawn by the lamp 200 from the DC power supply 208. The full open circuit voltage of the DC power supply is therefore presented to a series circuit comprising a lead 240 which is connected to the lead 212, a zener diode 242, a resistor 244, a relay 246, and a lead 248 which is connected to lead 210. The zener diode 242 has a breakdown voltage which is high enough to allow the relay 246 to be energized by the DC power supply 208 only if the DC power supply is operating in an open circuit configuration. For example, if the open circuit voltage of the DC power supply 208 is 160 volts, while the nominal operating voltage under the lamp load is 80 volts, the zener diode 242 advantageously has a breakdown voltage of 130 volts and the relay 246 advantageously operates at 24 volts DC, such that when the DC power supply 208 is operating into an open lamp circuit, the relay 246 will be energized; whereas, when the DC power supply 208 is driving an arc in the lamp 200 the voltage at the zener diode falls between its breakdown level, and, the relay 246 will become deenergized. When the relay 246 is energized, it opens a switch 260 in an auxiliary power circuit from power supply 252, and which includes a normally closed switch 258, opening in response to fluid pressure in a coolant tube 320, and which also includes the coil of relay 338. Switch 336 is thus opened and a by-passing, current limiting resistor 234 cut into the lamp supply circuit. Relay 246 also closes a switch 250 completing a circuit from an auxiliary power supply 252 to a three-way hydraulic solenoid valve 254. This solenoid valve 254, when energized, conducts high pressure coolant liquid from a high pressure coolant liquid source 256 into the tube 320 to advance the cathode assembly 204 toward the anode assembly 206, thus overriding or overcoming the springs 328 and 330. The rate of this advancement is limited by a throttling orifice 257 between the source 256 and the valve 254. Likewise, the application of pressure to the tube 320 opens the normally closed pressure sensitive switch 258 which is in communication with the tube 320. This pressure sensitive switch 258 is connected in series with normally closed contact 260 of the relay 246. The normally closed switches 258 and 260 are series connected between the relay 338 and the power supply 252, such that a lack of pressure in the tube 320 and a deenergized condition of the relay 246 are required before the relay 338 becomes energized to close the normally-open contacts of switch 336. Thus, at the outset, with power supply 252 on, and power supply 208 not yet switched on, switch 258 is closed, relay 246 is deenergized, so switch 260 is closed; and relay 338 is therefore energized and switch 336 closed.

When the DC power supply 208 is initially energized, the relay 246 is in turn energized and closes the switch 250 to actuate the three-way solenoid valve 254 and thus feed high pressure coolant to line 320 to advance the cathode assembly 204 toward the anode assembly 206. Energization of relay 246 also opens switch 260 which deenergizes relay 338 and opens switch 336. Thus, the resistor 334 is added in series to the DC power supply 208 circuit to limit the current which can flow from the cathode tip 228 to the anode tip 237, and also to form a circuit maintaining temporarily the energization of relay 246. It will be noted that the rise in pressure in the system driving the cathode to striking position also acts at pressure switch 258 to open it, so the circuit energizing the relay 238 to close switch 336 is then open at two points, switch 258, and switch 260, in series, and both must close before switch 326 can close to short out current limiter resistor 334. Now, when the cathode tip 228 touches the anode tip 237, current is drawn from the DC power supply 208 through the resistor 334 to initiate conduction through the interface of the cathode and anode. This current conduction through the resistor 334 lowers the voltage impressed on the zener diode to a level below its cutoff voltage, such that initiation of current conduction through the lamp deenergizes the relay 246, thereby closing switch 260. Also, when the relay 246 deenergizes, the switch 250 returns to its normally open condition driving the three way solenoid valve 254 to exhaust the fluid from the cylinder 322 through the tube 320, allowing the cathode assembly 204 to retract. The speed of retraction of the cathode assembly is determined by a throttling valve 262 in the exhaust orifice of the three way solenoid valve 254. With the pressure removed from the cylinder 322, the cathode is withdrawn slowly, drawing the arc 202 across the pressure chamber of the lamp 200. The deenergization of the relay 246 allows the switch 260 to return to its normally closed position. However, the switch 258 remains open until the cathode assembly 204 becomes fully retracted, due to the hydraulic pressure caused by the spring 328 driving the piston 324. When the cathode assembly 204 is fully retracted, pressure within the tube 320 will subside and the switch 258 will return to its normally closed position, completing a circuit through normally closed switch 260 to the relay 338 to energize the relay 338. Energization of the relay 338 closes the switch 336, which allows the DC power supply 208 to directly drive the lamp assembly 200. In this condition the voltage of the DC power supply is below the open circuit voltage due to the lamp load, and therefore the zener diode 242 will continue to maintain the relay 246 in a deenergized condition.

Should the arc 202 within the lamp 200 cease for any reason during the operation of the lamp, the output voltage of the DC power supply 208 will return to the open circuit level and a new starting sequence will begin.

This preferred embodiment lamp described in reference to FIGS. 12 through 22 is capable of operating at higher efficiencies and input power levels, while extending electrode usable life. One such lamp has been operated at an input power level of 50 kw with comparatively short distances between the anode tip 237 and the cathode tip 228, specifically 6 mm between the cathode tip 228 and the frustoconical end 268 of the anode assembly 206. This lamp exhibited an extremely high power density within the arc 202, namely 10,000 W-cm.sup..sup.-2 -ster.sup..sup.-1, while operating at an input voltage of 67 volts and having an arc current of 760 amps.

Besides this capability for increased brilliance, this lamp of FIGS. 12 through 22 extends the usable lifetime of the electrode elements. For example, one such lamp has been successfully operated at an input power level of 30 kw in excess of 400 hours without failure. When this lamp was dismantled, there was negligible degradation of the electrode elements, indicating a probable electrode life well in excess of 400 hours.

Claims

I claim:

1. A method of forming a plasma arc maintained between a pair of electrodes concentrated at one electrode and flared at the other comprising the step of:

applying to the arc a surrounding initially convergent sheath of substantially non-vortical gas travelling essentially longitudinally of the arc with progressively decreasing constrictive force along the length of the arc to increase the radiance at one electrode and reduce the energy flux at the other electrode.

2. A method of stabilizing a plasma arc being maintained between a first and a second electrode comprising the steps of:

flowing a jet of gas from the first electrode to the second electrode coaxial to and around the plasma arc;

withdrawing a portion of said jet of gas from around the periphery of said second electrode; and

deflecting another portion of said jet of gas at the second electrode radially outward and back towards the first electrode so as to form a toroidal ring circulation of gas around the plasma arc.

3. A high intensity plasma arc lamp comprising:

means for providing a gas pressurized chamber;

a first electrode and a second electrode for maintaining a plasma arc, said electrodes being positioned within said pressurized chamber;

means for concentrating said arc along only a portion of its length and stabilizing said arc along its entire length comprising means proximate to said first electrode for introducing a jet of cooling gas coaxial to said plasma arc, said jet of gas cooling the periphery of the arc proximate said first electrode so that the arc is concentrated at said first electrode and flares as it nears said second electrode, the heat energy in the flared portion of the arc being absorbed over a large area of said second electrode, thus reducing the per unit area energy absorption and increasing the useful life of said second electrode;

means proximate to the inner wall of said chamber for supplying a boundary layer of moving gas adjacent to said inner wall for cooling the chamber wall and providing a protective buffer between the chamber wall and said jet of gas.

4. The high intensity plasma arc lamp defined in claim 3 further comprising:

means for deflecting a small portion of the gas jet striking the second electrode for forming a toroidal collar of gas surrounding and concentric to the plasma arc, said collar of gas further stabilizing the plasma arc and containing the hot gases therein to prevent hot gas from directly contacting the chamber wall.

5. Apparatus for generating a plasma arc, comprising:

a transparent gas pressure chamber;

a cathode and an anode spaced apart along a longitudinal axis in said chamber for an arc to be maintained therebetween;

said cathode having a generally conical portion coaxial with said axis, whose conical exterior converges continuously and unbrokenly to an arcing forward extremity facing along said axis toward said anode;

a generally conical shroud encircling said conical portion of said cathode with convergent annular space therebetween to define therewith a generally conical nozzle, said shroud terminating short of said arcing forward extremity of said cathode; and,

channel means directing pressurized gas into said annular space generally and primarily longitudinally of said nozzle;

all in such manner as to form in said nozzle a convergent sheath of gas travelling essentially longitudinally of the nozzle along the portion of the conical cathode protruding therefrom so as to surround the arc and impinge convergently thereon in a region immediately beyond the cathode extremity and thereby locally constrict said arc in said region.

6. The subject matter of claim 5, wherein said annular space, at the gas discharge end portion thereof, is configured with a longitudinally extending interior flow-path-defining structure having an effective wetted perimeter for gas flowing therethrough of sufficient extent to establish laminar flow characteristics in the sheath of gas emitted from the nozzle and impinging convergently on the arc.

7. The subject matter of claim 6, wherein the gas discharge end portion of said annular space contains alternating ribs and channels extending generally longitudinally of said longitudinal axis.

8. The subject matter of claim 5, including flow passages for flowing a pressurized inert gas into said pressurized chamber for circulation therein about said cathode and anode and the arc therebetween, and

means including a flow passage for feeding into said channel means and thence through said nozzle a flow of pressurized inert gas bearing a predetermined ratio to said flow into said chamber.

9. The subject matter of claim 8, wherein said flow passage, channel means, and nozzle are proportioned so that the flow quantity through said nozzle is a fixed minor fraction of the flow quantity directly into said chamber.

10. Apparatus for generating a plasma arc, comprising:

a transparent gas pressure chamber;

a cathode and an anode spaced apart along a longitudinal axis in said chamber for an arc to be maintained therebetween;

said cathode having a generally conical portion coaxial with said axis, whose conical exterior converges unbrokenly to an arcing forward extremity facing along said axis toward said anode; and

means for forming a sheath of gas around said conical portion of said cathode and directing it to flow generally and primarily longitudinally of and radially inwardly toward said axis and then off said extremity of said cathode to impinge on and constrict said arc in a region proximate to the arcing extremity of the cathode.

11. The subject matter of claim 10, wherein

said anode has a generally dished annular face opposite to and coaxial with the longitudinal axis of the cathode and anode, said dished annular face of said anode being of substantially larger area than that of the extremity of the cathode, said dished face being an approximation of a spherical surface whose center of curvature is located substantially at said extremity of said cathode, whereby all points on said dished anode face are substantially equidistant from said extremity of said cathode.

12. The subject matter of claim 10, wherein said pressure chamber includes:

a pair of transparent annularly spaced cylinders fitted at opposite ends to cathode end and anode end adapters;

means for admitting a pressurized gas inside the annular space between said cylinders;

ports through the inner of said cylinders to pass pressurized gas to the interior of said chamber and to form a boundary layer of heat insulating gas along the inside surface of the inner of said cylinders; and

passage means for conveying a portion of the pressurized gas within said annular space to supply said sheath of gas formed around said conical portion of said cathode and which is directed to impinge on said arc.

13. The subject matter of claim 10, wherein said pressure chamber includes:

a pair of transparent annularly spaced cylinders fitted at opposite ends to cathode end and anode end adapters;

means for admitting a pressured gas inside the annular space between said cylinders;

means for supplying a relatively large flow of gas from said annular space to said chamber, including a flow of laminar characteristics adjacent the inner wall surface of the inner of said cylinders; and

means for conveying a relatively smaller flow of gas from said annular space to supply said sheath of gas formed around said conical portion of said cathode and which is directed to impinge on said arc.

14. The subject matter of claim 10, wherein said pressure chamber embodies a transparent cylinder and including:

means for introducing into said chamber a flow of gas adjacent and longitudinally along the inner surface of said cylinder in a region to surround the cathode and the arc between the cathode and the anode to form a boundary layer adjacent said cylinder affording heat insulation therefor.

15. The subject matter of claim 14, wherein said gas introducing means is designed and arranged with a Reynold's Number sufficiently low to cause discharge of said gas with laminar flow characteristics.

16. The subject matter of claim 15, wherein said gas introducing means comprises an annular body positioned inside said cylinder and having gas inlet means and gas discharge means connected thereto, said discharge means comprising an annular series of circumferentially spaced discharge ports facing generally longitudinally of the chamber cylinder and whose wetted port areas at point of discharge are of sufficient perimeter to provide the gas flow discharged therefrom with laminar flow characteristics.

17. Apparatus for generating a plasma arc, comprising:

a transparent gas pressure chamber;

a cathode and an anode disk spaced apart along a longitudinal axis in said chamber for an arc to be maintained therebetween;

said cathode having a generally conical portion coaxial with said axis, whose conical exterior converges to an arcing forward extremity of limited area;

said anode disk having a front arc footing area substantially larger than said arcing extremity of said cathode, and

having a rearward face, also of materially larger area than said arcing extremity of said cathode, and

liquid cooled heat exchange means for extracting heat from said rearward face of said anode disk.

18. The subject matter of claim 17, wherein said arc footing area of said anode disk is on a dished or concave front face which approximates the curvature of a sphere whose center of curvature is located substantially at said arcing extremity of said cathode.

19. The subject matter of claim 18, wherein said anode disk is generally concavo-convex, with the rearward convex face thereof larger in area than the arc footing area on the front face thereof, whereby to effect divergence of heat energy during heat conduction between said faces.

20. The subject matter of claim 19, wherein said heat exchange means comprises a generally concavo-convex heat exchange wall whose concave face conforms to and is in contact with the rearward generally convex face of said anode disk, coolant wall means having a generally concave face positioned at the rear of the generally convex rearward face of said heat exchange wall, and a plurality of concentric radially spaced ribs between said generally convex rearward face of said heat exchange wall and said generally concave face of said coolant wall means spaced rearwardly therefrom, said ribs forming coolant flow passages therebetween, there being diagonal, coolant flow slots through said ribs, said flow passages and slots forming a labyrinth for flow of coolant, and coolant entrance and exit channels, one communicating with a central region of said labyrinth, and one communicating with a peripheral region of said labyrinth.

21. The subject matter of claim 20, wherein the diagonal slots through successive ribs are laterally offset relatively to one another.

22. Apparatus for generating a plasma arc, comprising:

a transparent gas pressure chamber;

a cathode and an anode spaced apart along a longitudinal axis in said chamber for an arc to be maintained therebetween;

said cathode having a generally conical portion coaxial with said axis, converging to an arcing extremity of limited area facing along said axis toward said anode;

said anode having an arc footing area substantially larger than the arcing extremity of the cathode;

means for flowing a gas along said generally conical cathode, thence in a moving sheath surrounding said arc to the footing of the arc on the anode; and

a heat exchanger comprising annularly spaced side walls with gas entrance walls for receiving the arc heated gas from around the footing of the arc on the anode and conveying said gas into one end of the annular space between said side walls;

gas discharge means leading from the opposite end of said annular space; and

a series of annular ribs between said side walls, to define annular gas channels therebetween, there being diagonal gas passing slots in successive ribs to pass gas progressively from channel to channel from said gas entrance walls to said gas discharge means.

23. The subject matter of claim 22, wherein the diagonal slots through said ribs are laterally offset from rib to rib.

24. A plasma arc lamp comprising:

a pressurized arc chamber;

a pair of electrodes positioned within said chamber to maintain an electric arc;

means for flowing a gas through said pressurized chamber to stabilize said electric arc;

a heat exchanger arranged to receive and cool said arc stabilizing gas, said heat exchanger having gas ingoing and outgoing ends and a gas passage therebetween, with a temperature gradient between said ends, said gas passage including a region operating at a temperature at which a selected getter material has a high impurity-removing property; and

a getter material of such operating characteristic located in said gas passage at said region.

25. The subject matter of claim 24, wherein there are a plurality of regions in said gas passage operating at differing temperatures at which different getter materials have optimum effectiveness, with said different getter materials located in respective regions of optimum effectiveness in said gas passage.

26. A plasma lamp comprising:

a transparent gas pressurized chamber;

an anode and a cathode in said chamber, normally separated by a distance to define a predetermined normal arc length therebetween;

a mounting stem for said cathode movable longitudinally on the axis defined by said anode and cathode;

stop means for said stem and cathode when said cathode is retracted to the normal separation distance from the anode;

spring means yielding urging said stem and cathode to return to said stop means when separated therefrom in the direction of the anode; and

means for moving said stem and cathode from said stop means in the direction of said anode to strike the arc.

27. The subject matter of claim 26, wherein said stem and cathode moving means embodies fluid pressure actuated means movable to engage and displace said stem; and

return spring means for said fluid pressure actuated means.

28. The subject matter of claim 26, wherein said stop means embodies a socket into which said cathode is receivable.

29. A plasma lamp comprising:

a transparent gas pressurized chamber;

an anode and a cathode in said chamber, normally separated by a distance to define a predetermined normal arc length therebetween;

means mounting said cathode for movement between a normal position at a predetermined established arc length distance from the anode and a striking position in closer proximity to the anode;

means for circulating a pressurized coolant through the lamp; and

means utilizing said pressurized coolant to move said anode to said striking position, so that said arc may not be struck in the event of failure of circulation of said coolant.

30. A plasma lamp comprising:

a transparent gas pressurized chamber;

cathode and anode electrodes in said chamber, one movable relative to the other on a longitudinal axis between a closed striking position and an operating position spaced apart by a predetermined arc length distance; and

means for moving said movable electrode to either of said two positions, including spring means urging said movable electrode to move to said operating position and pressure fluid actuated means for overriding said spring means for moving said electrode to said striking position.

31. The subject matter of claim 30, including an energizing circuit for impressing a voltage between said cathode and anode electrodes, and an electric starting circuit in conjunction therewith for automatically actuating said pressure fluid actuated means to move said movable electrode to said striking position in connection with impression of said voltage between said cathode and anode electrode.

32. The subject matter of claim 30, including:

an energizing circuit for said cathode and anode electrodes; and,

means for limiting the arc current when the electrodes are positioned in said striking position and the arc is struck, and until said electrodes are repositioned at a spacing distance substantially equal to said predetermined arc length distance.

33. The subject matter of claim 32, including also a sensing means for sensing when said electrodes have become spaced at said predetermined arc length distance, and for thereafter deactivating said arc limiting means.

34. A plasma lamp comprising:

a transparent gas pressurized chamber;

cathode and anode electrodes in said chamber, one movable relatively to the other on a longitudinal axis between a close spaced striking position and an operating position spaced apart by a predetermined arc length distance;

means including a pressure fluid actuated piston for moving said movable electrode from said operating position to said striking position;

return spring means for returning said movable electrode from said striking position to said operating position, and for returning said piston from a position in which it has moved said movable electrode to said striking position, back to an initial starting position;

means including a source of pressure fluid and a valve for conveying said pressure fluid to said piston to actuate said piston to move said movable electrode to said striking position, said valve being operable to depressurize said piston to allow return to said movable electrode to said operating position;

an energizing circuit for said cathode and anode electrodes, including a DC power supply, and a current limiting resistor; and

means responsive to depressurization of said pressure fluid actuated piston to deactivate said current limiting resistor.

35. A plasma lamp comprising:

a transparent gas pressurized chamber;

cathode and anode electrodes in said chamber, one movable relatively to the other on a longitudinal axis between a close spaced striking position and an operating position spaced apart by a predetermined arc length distance;

means including a pressure fluid actuated piston movable along said axis from an initial position, to move said electrode from its said operating position to its said striking position;

return spring means for returning said movable electrode from said striking position to said operating position, and for returning said piston to its said initial position;

means including a source of pressure fluid, and a valve movable to a feeding position for conveying said pressure fluid to said piston to actuate said piston and thereby move said movable electrode to said striking position, said valve being movable to a second position in which it exhausts said pressure fluid from said piston and thereby depressurizes said piston to allow return of said movable electrode and of said piston by said return spring means;

a solenoid for moving said valve between said positions;

an energizing circuit for said cathode and anode electrodes including a DC power supply and a current limiting resistor in series;

a normally open switch in said energizing circuit by-passing said current limiting resistor to deactivate said resistor;

an active circuit loop including said DC power supply, said current limiting resistor shunted by said by-passing switch, and a device conductive at and above a predetermined breakdown voltage which is lower than the open circuit voltage across the electrodes, but non-conductive at the lower voltage impressed thereon when an arc has been generated between the electrodes;

an auxiliary power supply;

circuit means energized from said auxiliary power supply to normally open said by-passing switch;

means responsive to current in said circuit loop for operating said solenoid to move said valve to position to feed pressure fluid to said piston and thereby move the movable electrode to striking position;

means for moving said valve to its said second position upon cessation of current in said circuit loop;

means in said circuit means also responsive to current in said circuit loop to close said by-passing switch; and

a pressure sensitive electric switch operative on said circuit means and responsive to pressurization and depressurization of said piston means to hold said by-passing switch upon and during pressurization of said piston means;

all in such manner that generation of an arc between the electrodes stops current conduction in said circuit loop by reducing the voltage across said conductive device, and thereby causes movement of said valve to its second position to depressurize the piston, and also closure of said by-passing switch to allow full arc current between said electrodes.