CA1285997C - Plasma gun with adjustable cathode - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

A plasma generating system comprises a plasma gun including a hollow cylindrical anode member, a hollow cylindrical intermediate member electrically isolated from and juxtaposed coaxially with the anode member to form a plasma-forming gas passage through the intermediate member and the anode member, and an axially movable cathode member. The intermediate member comprises tubular segments separated by resilient insulating spacing rings held in compression. Arc radiation is blocked from the spacer rings by meanders in the inter-segment slots and further by ceramic barrier rings. An electric motor or pneumatic piston responsive to a measurement of arc voltage continually adjusts the axial position of the cathode tip relative to the anode nozzle so as to maintain a predetermined arc voltage.

Description

i2 8~ PATENT
ME-3570.1 PLASMA GUM WITH ADJ~STABLE CATHOD~

Backcround of the In~ention .
Plasma guns are utilized for such purposes as thermal spraying which involves the beat softening of a heat fusible material, such as a meta' or ceram~c, and propelling the softened material in particulate form against a surface to be coated. The heated particles strike the surface and bond thereto. The heat fusible material i~ typically supplied to the plasma spray gun in the ' form of pow,der,that"is generally below 100 mesh ~. S. standard screen size to about 5 microns.

In typical placma ~ystems an electric arc is created between a water cooled nozzle (anode) and a centrally located cathode. An inert gas passes through the electric arc and is excited thereby to temperatures of up to 15,000 degrees Centigrade. The plasma of at lea~t partially ionlzed gas i6suing from the nozzle resembles an open oxy-acetylene flame.

. S. Patent 2,960,594 (Thorpe) discloses a ba6ic type of plasma gun. Figure 1 tbereof ~hows a rod shaped cathode 28 and an anode noz~ie 32. Th~ cathode is located coaxially in spaced 2Q relat~on~hip with the anode nozzle operable to maintain a plasma generating arc between the cathode tip and the anode nozzle.

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~E-3570.1 Plasma-forming gas is introduced into an annular space ~0 (Thorpe, Fiy. 1) ~urrounding the cathode. This basic str~cture (without the adjustable cathode or interelectrode segments discussed below) is the type used commercially for such applications as plasma spraying.
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~horpe also depicts in Fig. 1 there~f the mounting of the cathode onto an electrode holder 3 which is threaded into the body of the gun so as to provide adjustment of the position of the cathode.
As indicated at column 6, lines 17-24, initial striking of the lo arc is achieved by screwing the electrode body toward the nozzle and retracting it. An alternative method taught for starting the arc i8 by providing a high frequency source of current. After the arc is struck the same may be ~suitably adjusted" by screwing electrode holder 3. It i~ also indicated that the tip of the electrode may be positioned at a distance away from the entrance of the nozzle. (Column 6, lines 64-66.) However, the ~distance~
is limited to relatively small variations, and there is no teaching or suggestion in Thorpe of what position of the catho~e is suitable or how to determine such a positior..

V. S. Patent 3,627,965 ~Zweig) similarly shows a plasma gun with a threaded cathode holder (Fig. 4) and suggests it may be used to alter the arcing gap. Zweig gives no further enlightenment as to the use of the threaded holder.

V. S. Patent 3,242,305 (~ane et al.) discloses a retract starting 2s torch in which starting of the arc is accompli~hed by a spring urging ar electrode against the nozzle. Retraction to a fixed operating position i~ effected by the fluid pressure of the cooling water.

Zweig also teac~.es feeding powder inside the ~un for spraying.

' ' ~ ~ '' lX~5997 ME-3570.1 It is well known in the art that such internal feed results in buildup of melted powder in~ide the nozzle bore. Therefore the conventional powder feeding method which avoids buildup is accomplished by feeding the powder into the flame near or outside the nozzle exit as illustrated in U. S. Patents 3,145,287 (Siebein et al.) and 4,445,021 (Irons et al.). This location ~sults in reduced uniformit~ and effectiveness in heating the powder.
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A plurality of electrically isolated interelectrode se~ments is o disclosed in U. S. Patent 3,953,705 (Painter). ~-ith reference to the Painter figures these tub~lar segments are positioned between a nozzle assembly 8 and a rear, fixed electrode 12 of a tubular type, it ~eing generally desirable to have the rear electrode serve as the anode. (Column 8, lines 47-57.) Starting i8 achieved by application of 20,000 volts which i8 further increased until the arc occurs. Thus the plasma gun of Painter is for a generally different mode of operation than that of the Thorpe type of plasma gun which has the nozzle as the an~de and operates at up to only about 150 volts (Table III of Thorpe). In the low voltage mode the current is hish, i.e. of the order of hundreds of amperes, and factors such a~ arc length and gas type ~nd gas flow establish the operating arc voltage.

As indicated above and illustrated in the above-mentioned patentc, the plasma-forming gas i8 generally introduced into the vicinity of the upstream electrode. Further gas may be injected at at least one point downstream ~uch a~ i~ shown in Painter.
Other references which show a construction for injecting a ~econd flow of gas are U. S. Patents P.e. 25,088 (Ducati et al.) and 4,570,048 (Poole). Each of these reference~ ~hows a fixed cathode.
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ME-3570.1 Plasma guns generally are capable of operating on an inert gas such as argon or nitrogen as the primary plasma gas. For argon the gas is introduced into the chamber near the cathode through one or more orifices with a tangential component to cause a vortical flow to the plasma. The reason is that, without the vortex, the arc is not carried far enough down the nozzle, resulting in low voltage and low thermal efficiency. On the other hand, radial input is generally selected-for nitrogen because a vortex tends to extend the nitrogen arc a long distance o down the bore of the nozzle causing difficulty in starting the arc.

However, without a vortex for nitrogen, the voltage and efficiency are low. Therefore, an additive gas such as hydrogen -is combined with the nitrogen, having the effect of improving 1S these factors. When argon is used, even with a vortex, the efficiency is undesirably low. ~ydrogen i8 again added where possible, but that gas i8 often considered undesirable as it may cause brittleness in the sprayed coating. Helium is an alternative additive gas but is expensive and less effective.

In view of the foregoing, an object of the present invention is to provide a novel pla~ma generating system and a novel method for maintaining a predetermined arc voltage without the use of an additive gas to the plasma-forming gas.

Another ob~ect i~ to provide an improved plasma ~pray gun 2s including a novel powder injector.

A further object is to provide a novel method for accurately controlling arc length and voltage at efficient levels in a plasma gun.
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~E-3570.1 These and still further object~ will become apparent from the following description read in conjunction with the drawings.

Brief DescriPtion of the Invention The foregoing objects are achieved by a plasma-generatin~ system ~hich comprises a plasma gun that includes a hollow cylindrical anode member, a hollow cylindrical intermediate member electrically isolated from and juxtaposed coaxially with the anode member to form a plasma-forming gas passage through the intermediate member and the anode member, and an axially movable rod-shaped cathode member with an anterior cathode tip. The cathode member is located generally in the plasma-forming gas passage coaxially in spaced relationship with the anode member operable to maintain a plasma generating arc between the cathode tip and the anode member. The plasma generating system further comprises pr$mary ga~ means including a primary gas inlet for introducing pla~ma-forming gas into the plasma-forming gas passage rearwardly of the cathode tip, a source of arc ~ower connected between the anode nozzle and the cathode member, and positioning mean~ for continually adjusting the axial positior. of the cathode tip relative to the anode nozzle so as to maintain a predetermined arc voltage.

In ~ preferred embodiment the intermediate member comprises a plurality of tubular segments and insulator a~emblie~ for - spacing the segment~. The insulator assemblies include a plurality of resilient spacing ring~ held in compression in the gun. A ceramic barrier ring i8 juxtaposed loosely between adjacent segment~ radially inward of each spacer ring to block the spacing ring from radiation from the arc. ~he slot~ between adjacent ~egment~ have meanders therein to block arc radiation from impinging directly on the ceramic barri~e~ ring.

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~8S997 ME-357~.1 Brief Description of the Drawinas Figure 1, comprising Figs. l(a) and l(b) is a longitudinal sectional view of a plasma gun incorporating the present invention.
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Figure 2 is a transverse sectional view in the direction of the arrows along the line 2-2 in Fig. 1. .

Fiqure 3, comprising Figs. 3(a) and 3(b), is a longitudinal sectional view of a plasma gun incorporating further embodiments of the present invention.

lo Figure 4 is a transverse sectional view in the direction of the arrows along the line 4-4 in Fig. 3.

Figure 5 is a longitudinal ~ection of a nozzle with a powder injection port.

Figure 6 is a longitudinal sectional view of a nozzle with a powder feeding assembly incorporating the present invention.

~etailed Description of the Invention An embod~ment of the present invention i8 illustrated in ~ig. 1 which ~how~ a plasma gun generally at 10. There are broadly three component assemblies, namely a gun body as6embly 12, a nozzle assembly 1~ including a tubular nozzle 16, and a cathode assembly 18. Gun body assembly 12 includes a generally tubular segment 2~D adjacent the nozzle asæembly, segment 2~D
constituting an anode. The cathode assembly includes a cathode member 20 that is located coaxially in spaced-relationship with :. .
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~285997 ME-3570.1 anode segment 2~D such as to maintain a plasma generating arc between the cathode tip 22 and the anode in the presence of a stream of plasma-forming gas and a DC voltage. An arc power source is shown schematically at 23. The anode and cathode are s of conventional materials such as copper and tungsten respectively.
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Gun body assembly 12 constitutes the central portion of the gun, excluding cathode ~ember 20. Assembly 12 includes at least one, f and preferably three, four or five generally tubular segments.
Figure 1 shows three such segments 24A, 2~B, 2~C and similar anode segment 2~D (designated collectively herein as 2~) that are stacked to form assembly 12. Segments 2~A, 2~B, 2~C define an intermediate member 26 which excludes anode 24D and contains the rear portion of a plasma-forming gas passage 28 extending therethrough for the arc and its associated plasma ~tream. (The letters A, B, C and D used with component numbers herein indicate, respectively, the rear, rear-central, forward-central and forward component. Also, as used herein and in the claims, the terms ~anterior~, ~forward~ and terms derived therefrom or synonymous or analogous thereto, have reference to the end from which the plasma flame issues from the gun; 6i~ilarly ~posterior~, ~rearward~, etc., denote the opposite location.) Segments 24 are preferably made of copper or the like.

The segments 24 ~re electrically isolated from eacb other by respective dish shaped insulators 30A, 30B, 30C, each having a circular opening axially therein. The inner rim of each insulator is sandw~ched between adjacent segments. An ir._ulator 30D of similar shape fits on the forward end of anode segment 24D. The four stacked insulators form an in6ulating member 30.
These plus a rear body member 32 and a forwardly located washer-shaped retainer 34 are held to~ether with t~ee bolts 36 (only 7 .

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: -il2~35997 ~ ME-3570.1 one such bolt is shown in Fig. 1). The bolted outer rim portions 38A, 38B, 38C, 38D of insulators 30 thus establish the rigidity of the gun body.

For fluid cooling each of the segments 2~ has an annular channel ~ ~0 therein formed by a forward rim ~2 and a rear rim ~4 bounding ,~ the annular channel in the middle of each segment. One such rim, i.e. the forward rim ~2 in each segment in the present example, is of lesser diameter than the other rim ~. A containment ring 4C is brazed to the outer surface of the forward rim ~2 and o against the forward facing surface of the other rim ~, and fits inside of dish shaped insulators 30, thus enclosing annular channel ~0 for coolant, typically water. O-ring seals 51 are appropriately placed between successive segment rims ~2, ~, rings ~6 and dish shaped insulators 30 to retain the coolant.
Conventional connections (not shown) for supplying and re~oving coolant are made with annular channels ~0.

~ozzle assembly 14 comprises nozzle 16 having a nozzle bore 53 therethrough and is held with three insulated screws 55 (one shown in Fig. 1) to retainer 3~ on the forward part of gun body 20 assembly 12. The nozzle bore is aligned coaxially with the rear portion 28 of the gas passsge in the gun body assembly to form the full length of plasma-forming gas passage 28, 63, 53 from the rear body member through to the anterior exit of the nozzle bore.
The nozzle, al~o ~ade of copper or the like, ls electrically isolated from gun assembly 12 including the stacked segments 2~.
Thi~ isolation is accomplished with forward dish-shaped insulator 30D.

Annular channeling 57 is provi~ed in nozzle 16 for coolant.
Coolant ducting in and out of the channeling as well as for the annular channel~ in the stacked segments is`provided in any . .
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~85997 ME-3570.1 convenient and conventional manner (not shown).

The configuration and diameters of nozzle bore 53 are as known-or desired for the purpose such as plasma spraying. In an embodiment described in detail below the bore is enlarged to contain a powder feeding assembly. The diameter of the ~onnecting passage 63 in the forward (anode) segment 2~D may diverge from the desired diameter of rear passage 28 in the other segments in order to match the diameter of nozzle bore 53.

Cathode assembly 18 including cathode member 20 is generally o cylindrical, and the a~sembly is attached rearward of intermediate member 2~ coaxially therewith. A mounting member 48 has a flange 50 which is held to the rear-facing surface of rear body member 32 by three circumferentially spaced screws (one shown at 5~). Member 32 is formed of rigid in~ulatlng material ~uch as machinable alumina. A tubular support member 56 i8 affixed within mounting member ~8 and extend~ rearward therefrom.
The forward part of support member 56 has a flange 58 which sets into a corresponding depression in the rear-facing surface of rear body mem~er 32, thus positioning support menber 56 coaxially within gun body a~sembly 12.

Rear body member 32 has a lateral ga~ duct 62 therein for receiving pla~ma forming gas from a source 6~ of pressurized gas ~uch as argon or nitrogen. The duct leads to an annular ~anifold 66 ln the outer circumference of ~ gas distribution ring 68 situated around the perimeter of an annular gas inlet region 70 or plenum that con~titutes the posterior end of Flasma gas passage 28, 63, 53. Gas distribution ring 68 contain6 one or more ga~ inlet orifice~ 72 (two ~hown) lea2ing from annular manifold 66 into inlet region 70. The orifices may be radial (as shown) as typically required for nitrogen gas~ or may have a - ' ' .. .. .
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12~35997 ME-3570.1 tangential component to form a vortical flow in passage 28, 63, 53 in the manner desired for argon gas. There may be a combination of radial and tangential orifices, and at least ane orifice may have a forward axial slant. Alternatively, ring 68 may be formed of porDus material so as to diffuse the gas into region 70. Gas distribution ring 68 is replaceable so that different plasma-forming gases or arc conditions may be chosen.

Returning to cathode assembly 18, cathode member 20 is shaped as a rod with anterior cathode tip 22 from which the arc extends 0 forwardly to anode segment 24D. The cathode member is approximately the length of the portion of gas pas~age 28 that is enclosed by the three other segments-2~A, 2~B, 2~C. The posterior (rearward) end of the cathode member may be formed as a tapered base 71 and i~ attached by threading 73 coaxially to the anterior (forward) end of a cathode support rod 7~ slidably mounted in support member 56. Support rod 7~ i& free to move axially to locate cathode tip 22 within a range ~etween a maximum extended position 78 (shown by dottea lines) rea; the postericr end of anode segment 2~D and a maximum retracted position proximate the gas inlet chamber. It will be appreciated that the specific range will be as required for the operation that is described below. In Fi~. 1 catbode tip 22 is set for a ~ossible operating condition between the maxima.

Coolant for cathode member 20 i~ provided by coaxial channels in the conventional manner. An axial duct 80 extends from the rear of support rod 7~ into cathode member 20 to a point near cathode tip 22. A long tube 82 is positioned axially in duct 80 forming duct 80 into an annular duct. Connecting pipes (not ghown) for coolant flow in and out are made to tube 82 and duct 80.

As indicated in Fig. 1 respective annular s~o~s 86A, 86B, 86C, . 10 .

12~3S~97 ~lE- 3 57 0 . 1 86D are formed between each adjacent pair of segments 2~ and between anode segment 2~D and nozzle 16, the slot being bounded outwardly by the inner surface 88 of each corresponding dish shaped insulator 30. An intense arc is generated in the passage 28, between cathode tip 22 and anode 2~D. The slots, with a width preferably between about 0.5mm and 3mm, serve to isolate insulators 30 from the degrading effects of the radiation an~
hea-t from the arc and plasma. To further protect the insulators a radial meander 90 is formed in each such slot 86. This is o achieved in the embodiment of Fig. 1 by having in each slot 86A, 86B, 86C an annular shoulder or ridge on the face of one segment encircling the continuous gas passage and a ~orresponding annular shoulder or depression in the surface of the facing segment. ~he ridge and depression create the radial meander 90 which inhibits arc radiation. A similar meander 90D is provided in slot 86D
between forward segment 24D and nozzle 16. ~owever, a different configuration for a slot 86C may exist between forward segment 2~D and forward-central segment 2~C as described immed~ately below.

In a preferred embodiment a second supply of plasma forming gas 96 is introduced into a lateral secondary sas duct 90 forward of the primary gas inlet at manifold 66. As depict æ in Fig. 2 this ~econdary ~upply i8 preferably introduced through a plurality of tangential ori~ices 100 located in the rearwar~ rim ~ZD of forward segment 2~D. Most preferably tangential orifices 100 are oriented ~uch that the extended axes of the orifices are substantially tangential to a coaxial circle of diameter equal to - that of the bore of the anode Fe~ment 2~D in the average location where the arc strikes the anode. For example, the nearest separation S (Fig. 2) between the axis and the circle should be less than about 10 percent of the diameter of the circle. That orientation ~ , 5~97 ME-3570.1 was discovered to be most effective in rotatin~ the arc root at the anode.

An annular groove in rearward rim ~D of segment 2~D in conjunction with a clQse fitting ring 10~ brazed to the rim ~D
encloses a forward annular manifold 106 for the gas. Duct 98 ,~onnects between this manifold and external source 96 of -secondary gas.

Typically the primary and secondary gas sources 6~, g6 sùpply the same type of gas but they may have independent flow controls. It o is also possible, where desired, to u'ilize different gases such as argon for the primary gas and nitrogen for the ~econdary gas.

~or the operation of movable cathode member 20, support rod 7 may be moved axially by any known or desired method, including manually, but preferably by mechanical means such as pneumatically, or with an electrical motor.

In the embodi~ent of ~ig. 1 support rod 7~ is moved and positioned pneumatically. A piston 108 is affixed to the approximate axial midpoint of the support rod concentrically thereto. The piston slides axially within an elongated cylinder 110 that is threaded into the rear end of the mounting member ~8.
The available length of the cylinder is sufflcient for the piston to carry the support rod ~nd cathode the desired range of distance. The maximum extended posit$on (forwardlys shown at 78 for the cathode) i8 established by support member 56 and a forward stop 112 which contact respectively a central flange 114 on support rod 7~ and piston 108. The maximum retracted position (rearwardly) is established by a rear stop 116 wbich contacts pi~ton 108, and by end piece 12~ which contacts a bumper ring 117.
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ME-3570.1 An anterior chamber 118 is formed in cylinder 110 between piston 108 and support member 56. A first pair of 0-rings 120 in support member 56 seal the ante.ior chamber and provide a suide for support rod 74. A posterior chamber 122 is formed in the cylinder between the piston and end piece 12~ screwed onto and ~losing the posterior end of the cylinder. The end piece slidingly engages the support rod with a second pair of 0-rings 126 that seal the posterior chamber and further guide the support rod. A third pair of 0-ring seals on the piston ~lide along the o cylinder wall and provide pneumatic sealing between the chambers 118, 122. Further 0-rings (not numbered) are strategically located to maintain pressurization of the c~ambers.

A forward sas pipe 130 communicates with anterior chamber 118, through mounting member ~8, and a rear gas pipe 13~ communicates with posterior chamber 122 through end piece 12~. The forward and rear gas pipes are connected to a source of pressurized gas 138, desirably compressed air, through first and second sol~noid supply valves 140, 142 respectively. First and second solenoi~
venting valves 1~ 6 are al~o connected to the forward and rear gas pipes respectively to provide selective venting of anterior and posterior chambers 118, }22 to atmosphere.

In operation, to move cathode member 20 rearwardly valve 1~0 ~ 8 opened to allow compres~ed air to be forced into anterior chamber 118 and, ~imultaneou~ly, valve 1~6 i8 opened to vent posterior chamber 122. To ~top, valve 1~0 i8 clos~d. Similarly, to move cathode member 20 'orwardly valve 1~2 is opened (with valve 1~6 closed) to enter compressed air into posterior cha~ber 122 and, simultaneously, valve 1~ is opened to vent anterior chamber 118.
Desirably the first supply and venting valves 140, 1~ are combined mechanically or electrically (not ~hown), as are the .

5~97 ME-3570.1 second supply and venting valves 1~2, 146, such that posterior chamber 122 is automatically vented when the first valve 1~0 is closed and the anterior chamber 118 is automatically vented when the second valve 1~2 is closed.

Figure 3, comprising ~igs. 3(a) and 3(b), shows a~further e~bodiment of a plasma gun utilizing an electri~ potor and other features according to the present invention. Man~ of the features are c,uite similar to those of Fig~ 1 as described above.
Certain differences will become apparent from the following lo description.

An intermediate me~ber 226 i8 formed ~f four tubular segments 22~A, 22~B, 22~C, 22~D which are stacked between insulating spacing rings 230B, 230C, 230D and closely fitted into an insulator tube 231 which i8 held in a metallic outer sleeve 211 which, in turn, is retained in a gun body 212. A similar ring 230A is engaged on the rearward side of rear segment 22~A. The ins~lator tube is formed, for examp e, of glass filled DelrinTM.
The rims 2~2, 2~ of segments 22~ have 0-ring seals (not numbered) in the circumference to seal annular channels 240 in segments 22~ against insulator tube 231. Coolant to annular channel~ 2~0 i8 ~upplied through channeling in in~ulator tube 231, the channeliny comprising a longitudinal duct 40~ in outer sleeve 211 and a lateral duct ~02 leading between duct ~0~ and , 25 e~ch annular channel 2~0. Coolant i~ removed from channels 2~0 through a ~econd set of lateral ducts ~02' diametrically oppo~ite first ducts ~02, thence through a second longitudinal duct ~12 in ~leeve 211 '~o a large hose fitting ~06.

Spacing rings 230 are formed of a resilient material ~uch as polyamide pla~tic and each i8 juxtaposed between adjacent segments 224 for ~pacing the segments. Eac~ 8pacing ring is held ~5~7 ME-3570.1 - in compression between seqments. Thermal barrier rings 233 formed of a ceramic material such as boron nitride that is resi~tant to radiation of th~ arc are juxtaposed one each between each pair of adjacent segments radially inward of the corresponding spacing ring 230, which also sup~orts the corresponding barri~r ring 233. The barrier ring thus further ~rotects the plastic spacing ring from the degrading effects of the radiation, in a~ddition to a meander 290 in the corresponding 610t (as described with respect to Fig. 1?.

o A spacing ring 2308 of similar resilient material is held between forward segment 22~E which, with the nozzle member, forms the anode structure, and adjacent segment 22~D. ~pacing ring 230E
has a radially inward surface with a step 235 therein. A
corresponding barrier ring 233E has a radially outward surface with a second step therein meshed with the first ~tep. The purpose is to provide a path length along the meshing steps that is sufficient to resist electrical breakdown between the adjacent segments in the presence of the high frequency starting voltage.
Also, it is desirable that each pair of rims 2~2, 2~4 be slightly une~ual, for example 0.005 to 0.010 inches different, in dia~.eter to prevent po~sible line-of-sight arcing.

~ach barrier ring 233 has ~ width that is slightly but ~ufficiently less than the space in wbich the ring i8 ~ituated between adjacent segments for freedom to float and compensate for 2s unrestricted thermal expansion of the segments during operation of the plasma gun, without encountering stresse~ that may fracture the ring. Also the width is sufficiently large to block the spacing ring from radiation from the arc, preferably wider than the spacing rings 230 as shown in Fig. 3.

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~5~97 ME-3570.1 An anode nozzle 216 is held in the forward end of gun body 212 by a retainer ring 2~1 fastened to the front of the gun body with threalding 2~3. As in the embodiment of Fig. 1, a nozzle bore 253 and ai rear portion 228 of the gas passage through the stacked segments 22~ form the plasma-forming gas passage. Arc current is conducted from anode 216 through forward segment 224E and gun ~ody 212 to a conventional current ccnnector ~08.

Nozzle 216 has an annular coolant channel ~10 therein, similar to those annular channels 2~0 in ~egments 22~. An irregularly lo shaped portion ~11 of segment 22~ directs flow of coolant to the nozzle wall. Screws (one shown at ~12) affix forward segment 22~ and gun body 212 to outer sleeve 211. Coolant is fed to channel ~10 from longitudinal duct ~0~ which communicates with a conventional connector ~08 attached to gun body 212 for a coolant-carrying power cable which carries coolant as well as the anode current.

Continuing with Fig. 3, rearward of the stacked sesments 224 an elongated gas distribution ring 268 is spaced axially from the rearward segment 22~A by a barrier ring 233A that is cimilar to the other of rings 233 situated between segments. The forward part of distribution ring 268 has at least one gas inlet orifice 272 fed by a ~upply of gas via an annular manifold 266 ~nd a laterally directed gas duct (not shown, the gas ~upply being sim~lar to that in ~ig. 1).

Simil~rly a ~econd ~upply of pla~ma forming gas may be introduced through a passage (not shown) in outer ~leeve 211 to an outer manifold 297 outward of forward segment 22~D, thence through a plurality of outer orifices 298 in segment 22~ to an inner manifold 299 that is adjacent nozzle 216, and inner orifices 300 . .

1~5~ 7 ME-3570.1 in nozzle 216 for introducing the second gas into the forward part of gas passage 228 as described for Fig. 1.

A cathode a~embly 218 of Fig. 3 includes a rod-shaped cathode member 220 which has an anterior tip 222 and is attached at its posterior end to a cathode support rod 27~. The support rod is ~lidably mounted in elongated ~istribution ring 268 which serves as a support member to guide the support rod in its axial path.
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At the rear end of support rod 27~ a plastic cylinder 308 of such a material as DelrinTM is fitted by means of an axial protrusion o 37~ ~,ressed into a hole in the end of support rod 27~ and held with a pin 375. Plastic cylinder 308 rides in an elongate~
hollow cylinder 310 that is attached axially to the rear of gun body 212 by means of a retaining flange 376 that is held with a large retaining ring 378 onto body 212 with a threaded connection lS 379. Plastic cylinder 308 provides a self-lubricated guide in hollow cylinder 310 and support for the rear of support rod 27~.
Flange 376 also retains the components in the gun body inclu~ing holding the spacing rings 230 between segments 224 in co~pression, in cooperation with forward segment 22~E.
Positioning rings 377, 377' aid in positioning components in body 212.

To provide an arc current connection for cathode member 220 and coolant to the gun, a connector block 380 is mounte~ on support rod 27~ near its rear end. ThiC i8 shown further in Fig. 4 which 2s is a cross section of the gun taken at the location of block 380.
Support rod 27~ fits clo~ely through a cylindrical aperture extending through the block.

A nut 382 threaded on the support rod between plastic cylinder 372 and block 380 holds the block Against a~contact flange 38~ on ., .

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-~5~397 ME-3570.1 support rod 27~. The contact surfaces of the nut, flange and rod with the block provide an arc current path to the cathode. The bloçk extends laterally from the support rod through a slot 385 in hollow cylinder 310 to where a ~econd conventional connector 386 for a coolant-carrying power cable is made at the distal end of the block. A second slot 385' in cylinder 310 diametrically ~pposite the first al~o accommodates the block.

Lateral coolant duct 388 leads through the block from cable connector 386 to an annular duct 390 formed between support rod o 27~ and block 380. A short channel 392 leads to the center of support rod 27~ where an axial duct 280 leads coolant to near the cathode tip 222. As in the embodiment of Fig. 1 a long tube 282 provides inlet and outlet channeling for the coolant.

A second annular duct 39~ located between block 380 and support rod 27~ connects axial duct 280 through a second short channel 396 to a small hose fitting 414. The two adjacent annular ducts 390, 39~ are sealingly separated and enclosed by three O-rir.gs ~16. A second small hose fitting ~18 is mounted in the rear of flange 376 and communicates through two fluid orifices ~20, ~21 with the anode power/coolant connector ~08 on the gun body. A
flexible hose ~hown schematically at ~22) attaches between the two small hose fitting~ 18. Thus coolant for cathode 222 is tapped from the inlet at connector ~08 through flexible hose ~22 and into long tube 282 in the cathode ~uppor~ rod 27~ and 2s cathode member 220. Outlet coolant from the outside of tube 282 passe~ to lateral duct 388 and on to cable connector 286.

A cecond large ho~e fitting ~2~ extends rearwardly from block 380 and communicates forwardly with lateral duct 388. A large diameter flexible hose (shown schematically at ~25) attaches between the first and second large hose fitt~gs ~06, ~2~ and ~3S~97 ME-3570.1 passes coolant from nozzle 216 and segments 22~ to block 380 and thuE; out through cable connector 386.

Coolant i5 also directed through ducts (partially shown) to an annular region ~28 formed in the central portion of gas distribution ring to cool the ring.
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Returning to connector block 380, being mounted rigi~ly on cathode support rod 27~ it is moved axially therewith as the cathode member 220 is being positioned. The slots 385, 385' in cylinder 310 are elongated sufficiently to accommodate this o movement.

The width W of block 386 i~ ~lightly less than the inside diameter of cylinder 310 (Fig. 4). The slots 385, 385' are close fitting to the clock on both sides to prevent the block from rotating. The flexible hoses 422, ~25 for coolant between fittings ~06, ~ 18, ~2~ also accommodate to the movement.

Extending rearwardly and axially from a hole in plastic cylinder 308 is a worm gear member 430 which cooperates with a drive gear ~32 associated with a conventional electrically driven linear actuator type of stepper motor ~34 suitably mounted in a rear hou~ing 436 of the gun. Other known or desired coupling meang for a motor may be utilized. Current leads ~38 to the motor selectively drive the motor in forward or rever~e such as to move worm gear ~30 ~xiall~ and thu~ the entire cathode assembly forwardly or rearwardly. The current i~ provided ln respon~e to arc voltage measurement as de~cribed herein.

In Fig. 3 motor ~3~ is shown attached to a mounting ring ~0 in hou~ing ~36 that also supports the posterior end of cylinder 310.
It is further desirable to have conventional~ it Ewitches ' ' ' ' ' ' .- ~ - . . . .
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~35~39~
- ME-3570.1 (shown schematically at ~2) at the rear extremity o~ worm gear member (or other convenient location) to stop current to the motor to prevent overrun of the cathode a~sembly beyond predetermined maximum extremities of axial motion.

As previously indicated, the primary plasma-forming gas is ~ troduced throuqh the forward part of gas distribution ring 268, and the ring also provides a guide for cathode support rod 27~.
It is desirable to force gas between the support rod and the distributor in order to prevent blowback of hot gas and powder lo into the guide area. This is done with a bleed orifice ~
communicating with duct ~26 to an annular opening ~6 formed near the rearward end of distribution ring 268 and a plurality of inwardly directed orifices ~8 leading through the ring.

Although intermediate member 26 or 226 (Fig. 1 or 3 respectively) 1S may be formed of one piece, even of ceramic or the like, several metallic segments are preferred as described herein. It is important that the arc not short over to the intermediate member since uncontrolled arc length and volta~e may ensue. Ceramic is feasible for the intermediate member or its segments but is difficult to cool and may deteriorate in the arc environment.
Thus the segments are best produced from copper or the like. The purpo~e of the several segments is to create increased difficulty for the arc current to traverse the intermediate member to the ~node nozzle.

-5 The po6ition of cathode tip 22 or 222 is chosen in correspondence with the desired predetermined voltage for the arc. The actual voltage i8 measured across the anode and cathode, or acros~ the arc power supply 23 or 223, as shown schematically at 1~8 or 3~8 in Fig. 1 and Fig. 3 respectively. Generally a longer arc corresponds to a higher voltage which also yields a higher ' .

35~9~

ME-3570.1 efficiency in thermal transfer of power to the plas~a stream.
(Thermal efficiency is generally determined by subtracting heat loss to the coolant, i.e. temperature rise times coolant flow rate, from the electrical power input, and taking the ratio of the Idifference to the power input-) .
~t i~Jhighly desirable, for process control purposes, to maintain a constant voltage. These results are achieved according to the present invention by determining the arc voltage and repositioning the cathode member as required to maintain the o desired voltage. This is accomplished ~y moving the cathode member rearward with respect to the nozzle if the actual voltase is low, and forward if the voltage is high.

Preferably the positioning system, such as the solenoid valve control or the electrical motor, is electrically coupled to the voltage measuring system through a controller (shown schematically at lS0 in Fig. 1 and 350 in Fig. 3) and is responsive to the voltage measurement such that a change in the arc voltage results in a corresponding change in the axial position of the cathode tip. This is readily achieved in controller lS0 or 350 with a conventional or desired comparative circuit that provides the difference between the arc voltage and a preset voltage of the desired level. When the difference exceeds a specified differential an electronic relay circuit is closed to send an adjusting current for moving the support rod 2s forward or rearward according to whether the voltage difference 1~ positive or negative. The adjusting current i~ ~ent to the corre~ponding solenoid (Fi~. 1), or to the appropriate win~ing of the motor tFig. 3), as the case may be. The result will be minute (or, if necessary, large) cathode adjustments as any voltage changes take place, for example, from ero~ion of the anode and/or cathode surfaces.

.

.

~2 ~ 5<3~7 ME-357n . 1 Generally the longer arc contemplated for steady state operation - under the present invention is difficult if not virtually impossible to initiate with application of the standard high frequency ~tarting voltage. Therefore, according to a further embodiment of the invention, the cathode member is initially positioned in its extended position (dotted lines at 78 in Fig. 1 ~nd a similar position in Fig. 3) near the anode nozzle. The desired operating gas flows and the arc voltage æource 172 or 372 (Fig. 1 or 3) are turned on, although no çyrrent will flow yet.
o Then, when the high frequency starting voitage is momentarily applied in the normal manner (e.g., by closins switch 173 or 373 in Fig. 1 or 3), the arc will start and arc current will flow.

When the arc has been started ~and high frequency switch 173 or 373 opened), the cathode is then retracte~ to its operating position, indicated approximately ~y its location in Fig. 1 and Fig. 3. By actuating the voltage comparison and responsive circuit, by means of an arc current detector in controller 150 o, 350, the retraction will be automatic. Thus, when the arc initiates, the detector is turned on and will determine that the voltage is too low (due to the short arc) and will immediately signal the movement ~eans to retract the cathode to an operating position corresponding to the pre~et voltage condition.

The arc current may either be preset so that the current assumes the desired value upon startups or the current may be initially set at a low value and brought up after startup ~n the conventional manner or by electronic coordination with the voltage signal.

Power feeding into the plasma may be accomplished in the conventional manner as in aforementioned U.S. Patent No.

4,445,021. However, the plasma gun according to the present ~5~97 ME-3570.1 invention is especially suited for internal feed in the nozzle, where the nozzle also is the anode, without the usual problem of buildup of powder adhering to the nozzle bore. This is apparently due to the controlled location of the arc root cn the anode and to a wiping action of the secondary gas. Fig. S
depicts a nozzle 216' that may be used in place of nozzle 2}6 in ~ig. 3. A powder port 366 therein directs powder from a conventional powder source (not shown) well within the nozzle bore. .

o In a preferred embodiment, control of the arc position with the apparatus and method of the pres-ent invention allows for 2 ~owder feeding assembly to be placed in the nozzle bore. Figure 6 shows a desirable feeding assembly 151 situated in nozzle 216- which may be used in place of nozzle 16 or 216 in Fig. 1 or Fig. 3 respectively. An elongated cylindrical central member i8 pogitioned in the nozzle bore 253 which has an enlarged bore diameter to accommodate the assembly. A cylindrical central ~ember 152 of assembly 151 is held in place with a mounting arm 15~. The plasma flow path is provided in the annular space 156 between central member 152 and nozzle wall 153, the path being split by mounting arm 15~.

It is particularly desirable that the anterior and posterior edges of the cylindrical inner 6urface of each sesment be rounde~
in order to minimize splitting and ~umping of the arc to the 2s lntermediate member. The radius of the rounded edges (~50 in Fig. 3) of between about 1 mm and 3 mm i~ suitable. The radius of the posterior edge (452 in Fig. 3) of the a.ode 6hould be between about 3 mm and 5 mm. These radii were found to be ~uite critical. The edge rounding of the anode ap~arently cooperates with the tangential flow of the secondary gas to provide the ~5'3g7 ME-3570.1 wiping effect to prevent powder buildup when using the powder injection structure shown in Fig. 5.

Coolant ducting 158 is provided in arm 15~ and further ducting 160 in the central member for circulation of liquid coolant such as water, sufficient to prevent rapid deterioration of the assembly components in the presence of the plasma flow. At least the u~stream-edges 162 of the central member and the mounting arm should be gasdynamically rounded to minimize interference with, and cooling of, the plasma flow and erosion of the components~
.

o Central mem4er 152 has a powder port 166 opening folwardly into the center of the plasma stream. This port communicates with a powder duct 168 in the mounting arm, located coaxially in the coolant ducting. The powder duct is connected to a standard or desired type of powder feeder (shown schematically at 170) which supplies plasma powder in a carrier gas.

The apparatus of the present invention is operated generally with parameters of conventional plasma guns except voltage is maintained somewhat higher, a mode which is expected to provide increased thermal efficiency. Prefera~ly the voltage is maintained at a set level between about 80 and 120 volts, the upper limit depending on power supply characteri6tics. For comparison the upper limit for a conventional sun is typically about 80 volts with an additive plasma gas in use. Current may be up to about 1000 amperes, although care ~hould be taken not to exceed a power level that depends on factors such as coolant flows, for example 80 KW. Internal dia.ueters are also conventional. ~ozzle bores may be between about 3.8 mm and 12 mm diameter. A suitable diameter for gas passage 28 in the intermediate member ig about 5 mm; and for electrode member 20 ..

--~5~397 ME-357~.1 about 2.5 mm. A suitable range of travel for the cathode is about 50 mm.

Other variations of the present invention are possible. ~or example, the cathode may be held fixed relative to the gun body, and the assembly of the anode nozzle and the intermediate member may t~en be in ~liding relationship to the gun body. In this arrangement, the gas distribution ring may be fixed with respect to the nozzle and slide therewith. It further may be desirable to fix the gas distribution ring with respect ~o the cathode lo member in order to maintain the gas introduction at an optimum point with respect to the cathode tip, even as the tip is moved.
Thus, in a further embodiment (not shown in the drawings), the axial movement of the cathode assembly in the gun also carries a parallel movement of the gas distribution ring. It is also possible to utilize the motor driving mechanism of Fig. 3 with the forward part of the pla~ma gun construction of Fig. l and, conversely, the pneumatic device of Fig. l with the gun of Fi~. 3.

The apparatus on method of the present invention provides for higher voltage operation than has proven practical in previous commercial plasma guns, especially those used for plasma ~praying. The higher voltage increases the thermal effic~ency of the ystem and allows higher power operation while minimizing the devastating effects of a high current arc on the electrode surfaces. The adjustability of the cathode according to voltage provides for choice of optimum voltage without the need for an additive gas and its attendant di~a~vsntages. It also provides for continual and precision maintenance of a predetermined voltage, particularly with automated control based on voltage measurement. The ~resent invention further allows for simple ~tarting and automatic reaBjustment to the elevated condition, ~35~g~
ME-3570.1 eliminating the difficulties of starting a high voltage arc. Yet other advantages of the system are eviden_ in the foregoins description and further presented below.

It was further discovered, surprisingly, that a highly uniform pla6ma plume issues from the nozzle of the plasma gun of the ~resent invention. This uniformity is an improvement over conven~ional plasma spray guns, such as the Metco Type 9~lB sold by The Perkin-Elmer Corporation, Westbury, New York. The result is a significant improvement in repeatability of plasma spray o coating properties. The uniformity is important for the application of gradated and sequential coating layers, and also of such materials as Metco 601NS plastic-metal powder blends, which are sensitive to uniformity of the plasma conditions.

Improved spray efficiencies were also discovered. For example, in spraying 601~S under similar conditions of powder and flow, the Type 9MB at ten pounds per hour splay rate yields a deposit efficiency of approximately 60~, while a gun according to Pig. 3 of the present invention yields a deposit efficiency of nore than 80%. Additionally, at 20 pounds per hour, the Type SM~ ~roduces virtually no coating while the present gun still yields ~,ore than 75~ deposit efficiency.

When spraying at supersonic velocity, i.e. with a smaller diameter nozzle, quite distinct ~hock diamond patterns are visible, whereas with conventional guns the patterns sre more diffuse. Clear shock patterns are desirable for choosing location of powder injection into the plasma stream.

~he above described construction of the pla~ma gun according to the embodiment of Fig. 3 ig highly desirable with respect to the combination of the ~egments, the res$1ient spacing rings held in .~ , .

1~5997 ME-3570~1 compression, and the ceramic barrier rings. This construction was c3iscovered to allow a practical assembly with insulating components sensitive to arc radiation and to fracture due to thermal expansion, under the severe conditions of the plasma and arc.
.
~hile the invention has been described above in detail wlth reference to specific embodiments, various changes and -I
modifications which fall within the spirit of the invention and scope of the appended claims will become apparent to those skilled in this art. The invention is therefore only intended to be limited by the appended claim~ or their equivalents.

26a . - .
.

.

Claims (31)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A plasma generating system characterized by precision controlling of plasma conditions, comprising a plasma sun including a hollow cylindrical anode member and a rod-shaped cathode member located coaxially in spaced relationship with the anode member operable to maintain a plasma-generating arc therebetween, voltage determining means for measuring an arc voltage between the cathode member and the anode member, and means for continually adjusting relative axial spacing between the cathode member and the anode member so as to maintain a predetermined arc voltage.

2. A gas stabilized plasma generating system characterized by precision controlling of plasma conditions, comprising:

a plasma gun including a hollow cylindrical anode member, a generally tubular intermediate member electrically isolated from and juxtaposed coaxially with the anode member to form a plasma-forming gas passage through the intermediate member and the anode ME-3570.1 member, and an axially movable rod-shaped cathode member with an anterior cathode tip located coaxially in spaced relationship with the anode member operable to maintain a plasma generating arc in plasma-forming gas between the cathode tip and the anode member to produce a plasma stream, the cathode member being located generally in the plasma-forming gas passage such that the cathode tip is movable coaxially within the intermediate member;

primary gas means including a primary gas inlet for introducing plasma-forming gas into the plasma-forming gas passage rearwardly of the cathode tip;

means for connecting a source of arc power between the anode member and the cathode member;

voltage determining means for measuring the arc voltage between the cathode member and the anode member; and positioning means for continually adjusting the axial position of the cathode tip relative to the anode member so as to maintain a predetermined arc voltage.

3. A plasma generating system according to Claim 2 further comprising secondary gas means for introducing plasma-forming gas into the plasma-forming gas passage at a location proximate the anode member.

4. A plasma generating system according to claim 3 wherein a forward annular chamber is formed between the intermediate member and the anode member, and the secondary gas means introduces plasma-forming gas with a vortical flow at the circumference of the forward annular chamber.

ME-3570.1

5. A plasma generating system according to Claim 4 wherein the secondary gas means includes a plurality of tangential orifices having axes substantially tangential to a circle of diameter equal to that of the bore of the anode member at the average location where the arc strikes the anode member.

6. A plasma generating system according to Claim 2 wherein the positioning means includes means for positioning the cathode tip sufficiently close to the anode member for the arc to be initiated in the presence of a high frequency starting voltage, and further includes means for retracting the cathode member after arc initiation to position the cathode tip relative to the anode member so as to establish the pre-determined arc voltage.

7. A plasma generating system according to Claim 2 wherein the intermediate member comprises a plurality of tubular segments and insulating means for spacing the segments, the segments being juxtaposed coaxially and held electrically isolated from each other by the insulating means.

8. A plasma generating system according to Claim 7 wherein the plasma gun further includes a forward segment comprising the anode member and the insulating means comprises a plurality of insulating rings, one such ring being interposed between each pair of adjacent segments and an annular slot being formed between the adjacent segments, each slot being bounded outwardly by the corresponding insulating ring.

9. A plasma generating system according to Claim 8 wherein the width of the slot between segments is between about 0.5 mm and 3 mm.

10. A plasma generating system according to claim 8 wherein, in ME-3570.1 each of said slots formed between adjacent segments, one such segment has an annular shoulder thereon encircling the continuous gas passage and the adjacent segment has a corresponding shoulder depression therein cooperating with the annular shoulder to form a radial meander in the slot such that arc radiation is blocked from impinging directly on the corresponding insulating ring.

11. A plasma generating system according to Claim 7 wherein the segments are three, four or five in number.

12. A plasma generating system according to Claim 7 wherein each segment has a cylindrical inner surface with a posterior edge and an anterior edge rounded with a radius between about 1 mm and 3 mm, and the anode member has a posterior bore edge rounded with a radius between about 3 mm and 5 mm.

13. A plasma generating system according to Claim 7 wherein:

the plasma gun further includes a forward segment comprising the anode member, and includes retaining means for retaining the segments and the insulating means in coaxial relationship;

the insulating means comprises a plurality of resilient spacing means, each spacing means being juxtaposed between adjacent segments for spacing the segments, the spacing means being held in compression by the retaining means; and the insulating means further comprises a plurality of ceramic barrier rings each being juxtaposed between adjacent segments radially inward of a corresponding spacing means.

ME-3570.1

14. A plasma generating system according to Claim 13 wherein each spacing means comprises a spacing ring formed of resilient material supporting the barrier ring.

15. A plasma generating system according to Claim 14 wherein the spacing ring adjacent the forward segment has a radially inward surface with a first step therein, and the corresponding barrier ring has a radially outward surface with a second step therein meshed with the first step so as to provide a path length sufficient to resist electrical breakdown between the adjacent segments in the presence of a high frequency starting voltage.

16. A plasma generating system according to Claim 13 wherein an annular slot is formed between the adjacent segments, each slot being bounded outwardly by the corresponding barrier ring.

17. A plasma generating system according to Claim 13 wherein a space is formed between adjacent segments with the barrier ring having a width sufficiently less than the space to compensate for thermal expansion of the segments and sufficiently large to block the spacing means from radiation from the arc.

18. A plasma generating system according to Claim 2 wherein the positioning means is electrically connected to the voltage determining means and responsive thereto such that a change in the arc voltage is detected by the voltage determining means and the axial position of the cathode tip is correspondingly adjusted to maintain the predetermined arc voltage.

19. A plasma generating system according to Claim 18 wherein the plasma gun further comprises a support rod having an anterior end with the cathode member attached coaxially thereto and a ME-3570.1 rearwardly located tubular support member with the support rod slidably mounted therein, and the positioning means includes drive means for providing axial movement of the support rod in the support member.

20. A plasma generating system according to Claim 19 wherein the drive means comprises a reversible electric motor coupled to actuate the support rod in axial movement.

21. A plasma generating system according to Claim 19 wherein the plasma gun further comprises a closed cylinder extending rearwardly from the support member, and a piston attached concentrically to the support rod and slidably positioned in the closed cylinder thereby forming in the cylinder an anterior chamber and a posterior chamber, and fluid sealing means interposed between the piston and the cylinder, and the plasma system further comprises anterior supply means for supplying fluid under pressure to the anterior chamber and posterior supply means for supplying fluid under pressure to the posterior chamber, such that selective supply of fluid to the anterior chamber or the posterior chamber provides adjustment of the axial position of the cathode tip relative to the anode member.

22. a plasma generating system according to Claim 21 wherein the anterior supply means comprises a pressurized fluid source and a first supply valve connected between the fluid source and the anterior chamber, the posterior supply means comprises the fluid source and a second supply valve connected between the fluid source and the posterior chamber, and the plasma system further comprises a first venting valve connected to the anterior chamber and a second venting valve connected to the posterior chamber, the first and second venting valves being respectively cooperative with the second and first supply valves such that the ME-3570.1 first venting valve is open to release fluid from the anterior-chamber when the second supply valve is open to pass pressurized fluid into the posterior chamber and the second venting valve is open to release fluid from the posterior chamber when the first supply valve is open to pass pressurized fluid into the anterior chamber, the first and second supply valves further being electrically connected to the voltage determining means and responsive thereto such that a change in the arc voltage is detected by the voltage determining means and the first or second supply valve is opened such as to adjust the axial position of the cathode tip to maintain the predetermined arc voltage.

23. A plasma generating system according to Claim 2 further comprising a nozzle member and powder feeding means therein for introducing powder into the plasma generated by the arc.

24. A plasma generating system according to Claim 23 wherein the nozzle member has an inner wall forming a nozzle bore portion of the continuous gas passage, and the powder feeding means includes a feeding assembly mounted in the nozzle bore, the feeding assembly comprising a cylindrical central member and a mounting arm attached between the central member and the nozzle wall to hold the central member substantially in the axial center of the nozzle bore forming an annular flow path for the plasma between the central member and the nozzle wall, the central member and the mounting arm each having a coolant duct therein for circulating liquid coolant sufficiently to prevent rapid deterioration of the central member and the mounting arm in the presence of the plasma, the central member further having an axial powder port therein for introducing powder forwardly into the plasma, and the mounting arm further having a powder duct therein connected to the powder port for conveying powder to the powder port.

ME-3570.1

25. A plasma generating system according to claim 23 wherein the anode member comprises the nozzle member, and the nozzle member has therein a radially directed powder feed port for injecting powder into the gas passage, the nozzle bore portion having a posterior bore edge rounded with a radius between about 3 mm and 5 mm.

26. A plasma generating system characterized by precision controlling of plasma conditions, comprising:

a plasma gun including:

a hollow cylindrical anode member;

a hollow cylindrical intermediate member electrically isolated from and juxtaposed coaxially with the anode member to form a plasma-forming gas passage through the intermediate member and the anode member, the intermediate member comprising a plurality of segments including a forward segment adjacent the anode member, and further comprising insulating means for spacing the segments, the segments being juxtaposed coaxially and held electrically isolated from each other and the anode member by the insulating means, an annular slot being formed between the adjacent segments and between the forward segment and the anode member, the slot being bounded outwardly the insulating means, and each slot having a radial meander therein such that arc radiation is inhibited from impinging on the insulating means;

An axially movable rod-shaped cathode member with an anterior cathode tip, the cathode member being located generally in the plasma-forming gas passage coaxially in spaced relationship with the anode nozzle operable to maintain a plasma generating arc between the cathode tip and the anode member;

ME-3570.1 a cylindrical rear body member positioned rearwardly adjacent the intermediate member and having a cylindrical cavity therein forming an annular manifold axially adjacent the posterior end of the continuous gas passage, the rear body member including a primary gas inlet for introducing plasma-forming gas into the annular manifold;

a secondary gas means for introducing plasma-forming gas into the plasma-forming gas passage at a location between the primary gas inlet and the anode member, including a forward annular chamber in the intermediate member of substantially larger diameter than that of the continuous passage and a plurality of tangential orifices in the intermediate member for introducing plasma-forming gas with a vortical flow at the circumference of the forward annular region;

a tubular support member mounted rearwardly adjacent the rear body member; and a support rod slidably mounted in the tubular support member and having an anterior end with the cathode member attached coaxially thereto, with a drive means coupled to actuate the support rod in axial movement;

the plasma generating system further comprising:

primary gas means including a primary gas inlet for introducing plasma-forming gas into the plasma-forming gas passage rearwardly of the cathode tip;

a source of arc power connected between the anode member and the cathode member; and ME-3570.1 voltage determining means for measuring the arc voltage between the cathode member and the anode member, the drive means being electrically connected to the voltage determining means and responsive thereto such that a change in the arc voltage is detected by the voltage determining means and the axial position of the cathode tip is correspondingly adjusted to maintain the predetermined arc voltage.

27. A plasma generating system according to Claim 26 wherein:

the plasma gun further includes retaining means for retaining the segments and the insulating means in coaxial relationship;

the insulating means comprises a plurality of spacing rings formed of resilient material, each spacing ring being juxtaposed between adjacent segments for spacing the segments, the spacing ring being held in compression by the retaining means; and the insulating means further comprises a plurality of ceramic barrier rings each being juxtaposed between adjacent segments radially inward of a corresponding spacing ring;

each slot being bounded outwardly by the corresponding barrier ring;

a space being formed between adjacent segments with the barrier ring having a width sufficiently less than the space to compensate for thermal expansion of the segments and sufficiently large to block the spacing means from radiation from the arc; and the spacing ring adjacent the forward segment having a radially inward surface with a first step therein, and the corresponding barrier ring having a radially outward surface with a second step ME-3570.1 therein meshed with the first step to as to provide a path length sufficient to resist electrical breakdown between the adjacent segments in the presence of a high frequency starting voltage.

28. A method for generating a precision controlled plasma in a plasma gun having a hollow cylindrical anode member and an axially movable rod-shaped cathode member located in spaced relationship with the-anode member operable to maintain a plasma-generating arc therebetween, the method comprising measuring the actual arc voltage and comparing the same with a predetermined arc voltage, and continually adjusting the relative axial spacing between the cathode member and the anode member so as to maintain the actual arc voltage substantially equal to the predetermined arc voltage.

29. A method for generating a precision controlled plasma in a plasma gun having a hollow cylindrical anode member, a hollow cylindrical intermediate member electrically insulating from and juxtaposed coaxially with the anode member to form a plasma-forming gas passage through the intermediate member and the anode member, and an axially movable rod-shaped cathode member with an anterior cathode tip, the cathode member being located generally in the plasma-forming gas passage coaxially in spaced relationship with the anode member operable to maintain a plasma generating arc between the cathode tip and the anode member, the method comprising:

introducing plasma-forming gas into the plasma-forming gas passage rearwardly of the cathode tip, applying an arc voltage between the anode member and the cathode member to generate an arc therebetween, measuring the actual arc voltage and comparing the same with a predetermined arc voltage, and continually adjusting the axial position of the cathode tip relative to the ME-3570.1 anode member so as to maintain the actual arc voltage substantially equal to the predetermined arc voltage.

30. A method according to Claim 29 further comprising introducing plasma-forming gas with a vortical flow into the plasma-forming gas passage at a location proximate the anode member.

31. A method according to Claim 29 further comprising in sequence, positioning the cathode tip sufficiently close to the anode member for the arc to be initiated in the presence of a high frequency starting voltage, applying the high frequency starting voltage between the cathode tip and the anode member, and retracting the cathode member after arc initiation to position the cathode tip relative to the anode member so as to establish the predetermined arc voltage.