Device for treating metal surfaces with a magnetically impelled arc and a magnetically impelled arc producing device

Aluminum alloy is a preferred material in sheet form for lithographic sheets and foil for electrical capacitors due to its cost-effectiveness. However, the lithographic plate and the condenser film must be properly granulated or roughened. This increases the surface area of ​​the disc or foil. We will explain this later. Lithographic plate refers to aluminum substrate not yet coated with a photosensitive resin (protective lacquer). Economy refers to the number of prints that can be made before a replacement is required with a single protective lacquer coated lithographic plate. Granulation means the roughening of a metal surface. It can serve a variety of purposes, such as cleaning, surface preparation for bonding to another surface, stress relief, and other properties-changing effects. As we will discuss later, these processes can be performed sequentially, continuously.

Granulation of the aluminum sheet is the first step in the production of a photo-coated sheet that produces image areas and non-image areas with a desired hydrophobic and hydrophilic properties. Although aluminum alloy is used, commercially available lithographic plates made of aluminum alloy are called aluminum sheet or aluminum foil for the sake of brevity, partly because the nearly pure aluminum, such as the 1050 (99.5%) alloy, is the preferred material for electrochemical, partly because pure aluminum is known to be unsuitable for lithographic plates.

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- 3 Lithography used for offset printing typically has a layer of photosensitive organic composition on one side of the sheet. This layer allows photocomputing or reproduction of a print image. After the image has been formed, the particulate substrate onto which the layer is deposited carries the image areas and simultaneously forms a hydrophilic background for the stone printing process in areas without image.

The particulate substrate, which is bare in the non-image area, should be hydrophilic to strongly repel greasy printing ink. The photosensitive layer must adhere strongly to the particulate aluminum carrier both before and after exposure. Therefore, it is essential that the particulate carrier is very stable, both mechanically, in terms of wear, and chemically, particularly against alkaline media.

To obtain hydrophobic and hydrophilic properties, a particulate aluminum sheet is uniformly coated with a photosensitive resist material, which is exposed to photoresist (actinic) radiation beamed onto the resist through a blanket corresponding to the image to be printed. After irradiation, relatively more soluble areas should be easily removable from the substrate by a development operation to form hydrophilic non-image areas without leaving any residue. The bare substrate must be highly hydrophilic during operation and exhibit sufficient repulsive effect on greasy ink.

The cost of manufacturing a lithographic sheet includes the cost of producing a suitably priced alloy film, which preferably has a very uniform microstructure, which is created by controlled manufacturing processes, such as rolling and heat treatment, to ensure a consistent behavior during electrochemical etching. Conventionally, the smoother the microstructure of the controllably pelleted film, the more suitable the pellet film as a lithographic plate.

Along with Alloy 1050, Alloy 3003, 1100 and 5XXX are widely used. The alloys 5XXX are specifically formulated to produce a lithographic plate as described in Rooy et al., U.S. Patent No. 4,902,353. This patent is incorporated by reference in its entirety as part of this application. Although the cost of such alloys per se is relatively low compared to the value of the printed material made from a photoresist coated lithographic plate, lithographic plates are still considered expensive and there is a general need for more economical lithographic plates.

The cost of lithographic sheeting is largely due to the cost of granulating the aluminum sheet. The purpose of this pelletization is to ensure that the aluminum sheet is free from defects and to allow sufficient resolution of the print to be made up to hundreds or even thousands of prints before the lithographic plate is replaced in the printing press. Such defect-free pelletization is presently advantageously accomplished by selecting an alloy particularly suited for electrochemical etching that is precisely controlled by the composition of the bath and by well-defined process conditions. Together they result in very uniform granulation or roughening. The optimum aluminum alloy is expensive not only because of the special treatment required to create the desired microstructure due to the topography of the print surface, but also because it requires precise control of chemical etching as well as chemical bath assembly and maintenance. Disposal of exhausted baths further increases costs.

These considerations point in the direction of finding a non-chemical solution for granulating the aluminum sheet or aluminum foil used as lithographic sheet. In general, however, non-chemical granulation, i.e. mechanical granulation, is too uneven. This is not only because it is relatively rough compared to electrochemical etching, but also because it is difficult to control. A solution is still being sought to produce controllably granular surfaces without the use of an electrochemical process.

Such a controllable pelleting process is described in Applicant's U.S. Patent No. 5,187,046. This patent relates to the use of one or more individual electrodes which, in at least one embodiment, create helical traces or raster-type pellets on a plate attached to a rotating drum. This process is relatively slow and the pelleting effect is not quite uniform.

The device of the present invention provides an arc-grained or micro-sweetened, substantially flat surface on an aluminum alloy sheet or sheet which is provided with a relatively fine and slightly uneven microstructure by moving an arc drawn between the aluminum sheet and a closed continuous loop electrode. Closed and continuous means that the top of the electrode loop is formed by an infinite circle, oval or other suitable shape, so that a continuous electrode is created for arc movement when the arc is moved by a magnetic field. Therefore, a continuous magnetic structure is positioned close to the electrode loop to move the arc discharge on the electrode loop. The speed at which the arc discharge passes around the electrode results in a faster pelletization process than single strand electrodes using a helical or raster type. In addition, the resulting particles may be smoother.

The shape of the continuous electrode loop may be an open-centered oval or an open-centered ellipse, or preferably a parallel-loop having straight sides, in which the ends of the sides are joined by curved or other suitable-shaped sections.

It is therefore an object of the present invention to provide a non-chemical, non-uniform (unevenly uniform microstructure of an electrochemically etched lithographic plate) surface which is particularly suitable as a lithographic plate, but may also be used for other purposes such as condenser foil. increased surface area. It is a further object of the present invention to provide an apparatus for such granulation. In the case of lithographic plates, the surface is made photosensitive in order to obtain a photoresist coated lithographic plate for offset printing.

A further specific object of the present invention is to provide a lithographic sheet having a fine microstructure with a granular arc discharge, which is only slightly less uniform than an electrochemically etched aluminum surface. Surprisingly, this surface, when coated with a phosphate-free coating, is well suited as a carrier for resist. The process avoids the uncontrollable inherent feature of mechanical pelleting, eliminating the need for chemical baths, so they do not need to be maintained and often costly to dispose of.

According to the invention, this object is solved by the apparatus comprising:

at least one electrode having a closed loop portion and a continuous closed loop tip projecting laterally from the general plane of the body portion;

means for providing an arc discharge between the electrode tip and a workpiece;

a magnetic device having a first closed-loop pole member disposed about and substantially concentric to the closed-loop tip of the electrode, a second pole member having polarity opposite to the first-pole member and a substantially concentric continuous surface disposed within the closed-loop electrode tip; and means for applying voltage to the pole members, whereby the arc discharge between the electrode tip and the workpiece moves continuously around the electrode loop.

In a preferred embodiment of the invention, a moving metal plate is continuously fed to a processing drum, for example, a metal support cylinder located beneath the above pelleting device. After drawing an arc between the moving plate and the electrode, the arc discharge is magnetically moved around the electrode loop. The plate is granulated with arc discharge as it moves between the cylinder and the electrode. If both sides of the plate are to be granulated, a second support roller and head may be disposed so that the other side of the disc is between the second cylinder and the second head, as shown in Figure 5 and described in more detail below.

The continuous electrode and the magnetic device are disposed in the direction of the geometric axis of the metal cylinder such that the moving arc discharge transverse to the plate traveling between the cylinder and the electrode. This arrangement allows the surfaces of lithographic plates, capacitors, other tape and plate-like materials and many other products to be treated while the surfaces are moving in an action line and the arc speed is matched to the line speed. The arc discharge velocity around the electrode depends on the strength of the magnetic field (which should be as constant as possible), the amount of electric current flowing through the electrode and the arc, the arc gap length, the material to be treated, the electrode material and the type of shielding gas used.

The problem is solved with respect to the magnetically driven arc discharge device comprising:

an electrode having a closed-loop configuration and a continuous electrode tip projecting laterally from the general plane of the electrode loop, and an electric arc discharge is formed between the electrode tip and a large surface area conductive surface;

a continuous magnetic device disposed adjacent to the electrode and the electrode tip and having at least one pole member disposed within the electrode loop and electrode tip and at least one and a substantially concentric first closed-loop, second pole member of polarity opposite to the first pole member; and a pole member outside the electrode tip, generally opposite the inner pole member, which surrounds the continuous electrode tip and introduces magnetic flux at the electrode tip, which flux moves the arc discharge between the continuous electrode tip and a conductive surface along the continuous electrode tip and the large surface area.

A further object of the present invention is to provide packaging foil and sheet for automobile body panels. condenser film and a granulating system for treating, cleaning and / or etching other metals and materials, such as roll mills, for various applications. Since this is not a chemical process, there are no problems with controlling and handling the quality of the chemicals.

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10 and more objects and advantages of the present invention and of its further objects and advantages thereof will be described in more detail with reference to the drawings, in which the same element is represented by two like reference numerals, of which

First Fig. 4A is a plan view of a magnetically driven arc discharge workpiece for granulating a conductive surface;

Second Figure 1 is a sectional view of the workhead of Figure 1, taken along line III of Figure 1, a

Third Figures 1 and 2 are schematic diagrams of a continuous row comprising the workhead of Figures 1 and 2 and a metal cylinder which commutes the discharge current while simultaneously conveying the disc to the workhead;

4th Fig. 4A is a schematic of a multi-head unit for handling both sides of an advanced continuous sheet material, i

5th Figure 4 is a modified version of the apparatus of Figure 4;

6th FIG. 4A is an embodiment of the present invention in which the coil of a magnetic device driving the granular arc is disposed within the boundaries of the electrode;

7th FIG. 2A is another embodiment of the present invention in which the coil of the arcuate discharge magnetic device is disposed outside the electrode and in the general plane of the electrode.

8th Figure 1 is a schematic diagram of one half of a continuous electrode and a permanent magnet structure according to the present invention;

9th Fig. 4A is a schematic diagram of one side of a continuous electrode and a permanent magnet structure according to the present invention with a compact head.

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The objects and advantages of the present invention are achieved in a preferred embodiment by granulating the surface of an aluminum plate with the arc discharge device shown in Figures 1 and 2.

These figures show a working head 10 having a continuous or closed loop electrode 12. The electrode center 13 of the electrode 12 is open and the electrode has a lower electrode tip or arc. As shown in Figure 2, the electrode has a main body portion, and the tip 14 of the electrode protrudes laterally from the general plane of the main body portion and is in a plane parallel to the general plane of the workpiece 15. As the 2. The electrode tip 14 is shown vertically downward toward the workpiece. Thus, the bottom continuous edge of the electrode tip 14 points downward.

The electrode 12 can be made of a single piece of metal, such as copper, to form a continuous loop and electrode tip. Preferably, the side portions of the electrode are parallel, and the ends of the parallel side portions are connected by arcuate end portions, thereby providing a continuous path for the magnetically driven arc discharge, not shown. The upper surface 12a of the electrode is depicted in Figure 1 by a dotted line. The electrode has the shape of a clamping head and includes a cooling pipe 26 and an outer casing 30. If the electrode 12 is made of a material that cannot be easily and economically formed as a continuous piece structure, the electrode 12 may be formed from relatively short, non-illustrated segments which are then suitably bonded together to complete the electrode loop.

A hollow ferromagnetic magnetic member 18 passes through the plane of the electrode 12 and the open center 13 of the electrode 12. The hollow magnetic member 18 can be used to conduct a gas current in the vicinity of the electrode tip 14 and the movable arc discharge formed between the electrode tip and the workpiece surface 15. The magnet member 18 may distribute a controlled atmosphere through its open lower magnet member 18a to the arc discharge site, although the arcing effect produced by the arc discharge may be generated under normal atmospheric conditions. As shown in Figures 1 and 2, the gas can be supplied to the magnet member 18 through one or more pipe fittings 19. The lower end 18a of the magnet member 18 serves as the inner magnetic pole of the arc-driven magnet. This is discussed in detail below. If a controlled atmosphere is not required, a ferromagnetic plate placed vertically instead of 18 may be used as the internal magnetic pole. The ferromagnetic tag refers to any material that conducts the magnetic flux so that magnetic poles are formed at the opposite ends of the members made of such ferromagnetic material.

Figures 2 and 9 schematically illustrate a sectional view of an electromagnetic coil 20 tightly wound from insulated wires positioned behind (above) the electrode 12. The coil 20 is disposed around the hollow magnetic member 18 and is preferably wound so that its shape is substantially the same as that of the electrode 12. Electrode 12 coils 20 at positions 1 and 2.

5A, electrically connected between the power lines 22 and the hollow magnet 18, which are electrically connected to the electrode. Just like this one.

As illustrated in FIG. 6B, a plurality of such power lines 22 are connected to the electrode 12. This facilitates a smooth current supply to the electrode, thereby reducing the possibility of uneven electrical resistance in the electrode loop, resulting in an uneven current discharge along the electrode length during the arc pelletization process. Figure 2 is a schematic diagram of the power source 23 connected to the power line 22.

Also shown is a vertical tube 24 on one side of the coil in Figure 2. In fact, there are two such tubes 24, as shown in FIG. The tubes 24 introduce refrigerant into and out of the workhead to cool the workhead. One vertical tube 24 conducts refrigerant into the work head 10, while the other tube 24 conducts refrigerant. The coolant in the workhead 10 is guided by a horizontal cooling tube or conduit 26 located in the upper portion of the electrode 12 and in the recess 27 below the coil 20. The ends of the cooling pipe 26 are connected to the inlet and outlet pipes 24 respectively.

The conductive parts of the workhead 10 are separated by electrically insulating insulating bodies 28. The insulating bodies 28 further prevent high frequency energy from being scattered from the electrode 12 when and when high frequency energy is used to form the arc. The preferred material for the insulating bodies 28 is ceramic.

The workpiece 10 further comprises an outer circumferential ferromagnetic sheath 30 which includes elements of the workpiece 10 and serves as part of the magnetic field of the present invention. Preferably, the casing 30 comprises two substantially parallel side panels 30a depicted in dashed lines in FIG.

The opposite ends of the side panels 30a are joined by two C-shaped or arc-shaped panel members 30b. However, the casing may also be a one-piece or any other type of structure that suitably encloses the elements of the workpiece 10 and provides a path for guiding the magnetic flux generated by the coil 20.

At the bottom edge of the housing 30 is an inwardly facing plate 32 having a central panel 33 open to receive the electrode tip or electrodes 14. The sheet 32 ​​forms a second magnetic pole for conducting the magnetic flux between the inner edges of the sheet 32 ​​and the first magnetic pole 18a, the magnetic end 18a of the hollow magnetic member. You can also create these magnetic poles with permanent magnets. We will discuss this later.

The upper edge of the housing 30 rests on an upper solid sheet 34 and is suitably secured thereto. The plate 34 completes the housing of the workhead 10 and passes through the pipe couplings 19, the power lines 22 and the pipes 24. The sheet 34 may be a one-piece or multi-piece structure. The plate 34 engages the upper end of the magnet member 18 and thereby deflects the magnetic field generated by the coil 20 to provide a continuous path between the magnetic end 18a and the magnetic pole formed by the plate 32. The material formed by the casing 30, the sheet 32, the sheet 34, and the magnetic member 18 is ferromagnetic, so that a coil (i.e., a typical iron core) is formed around the coil 20 with its opposite north and south magnetic poles It has 18 magnets at the lower end. In this way, the magnetic flux generated by the coil 20 passes through the lower electrode tip 14 of the electrode 12.

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The elements of the working head 10 are held together by the casing 30, the top plate 34 and the inner magnet member 18. For example, the casing, plate, and inner solenoid may be welded and the power wires 22 may be threaded, as shown in Figure 2, so that nuts can be screwed together to secure the insulating sleeves and other insulating bodies 28 when the wires are suitably they are connected to the electrode 12 as shown in FIG. Similarly, the insulating sleeves arranged around the vertical tubes 24 may be connected to the upper panel 34.

As shown in Figures 1 and 2, the elements of the workpiece 10 are generally positioned close to each other to make the device compact. Such a device is easy to operate and the workpiece can be easily mounted for arc discharge purposes.

2 shows that the distance between the electrode tip 14 and the lower edges 32 of the lower plate 32 and the lower edges of the solenoid 18a is much greater than the distance between the electrode tip 14 and the surface 15 of the workpiece to be shot. If the plate and the metal structure of the magnetic member 18a are too close to the electrode 12, the arc will jump over these metal structures rather than on the surface to be treated. This can be avoided by the fact that the plate 32 and the tip 18a have the same potential, all electrodes 12. Such an embodiment is shown in Figure 9 and will be described in detail below.

Preferably, the working head shown in Figures 1 and 2 is applied in a continuous row in which 36 rolls of a conductive material are wound and 3-5 rolls are made. . Referring to FIG. In this way, the arc-granulation of the material can be carried out on a mass production basis. After the pelletization is completed, the plate is rewound to the pickup location 38 for rewinding.

Between the electrode tip or electrode 14 and a current conducting surface (the surface of the workpiece 15 in Fig. 2 and the surface of the plate 40 in Figs. 3-5), sufficient electrical potential is applied between the electrode and the current conducting surface to form an arc. For this purpose, the surface must be electrically connected to one of the terminals of the power source 23 (Fig. 2). THE 3.

As illustrated in FIG. 6, this is created by a support roller 42. The support roll 42, formed by a metal roll, engages the surface of the plate 40. The support roller is connected to the ground as shown in the figure. The ferromagnetic material of the structures surrounding the coil 20 deflects the permanent magnetic field generated by the current through the coil 20 to the inner edges of the lower panel 32 and to the lower end 18a of the inner magnet member 18. In this way, opposite, north and south magnetic poles are formed on opposite sides of the electrode tip 14. The opposite north and south magnetic poles create a right-angled magnetic flux with the perpendicular flow of arc discharge current entering and exiting the moving plate 40. The interaction between the magnetic flux and the discharge current creates a driving force that acts on the arc in a direction perpendicular to both the discharge current and the magnetic flux. 1. In FIG. 4, this force is either clockwise or anticlockwise by the DC voltage source 50 in the coil 20 at the current 51 • · · · ·

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Depending on the direction of current applied to the wires 17 (Fig. 1) and the direction of the arc discharge current (inward or outward in the plane of the figure) (or electrode 12 depending on both). In this way, the arc discharge is wrapped around the continuous extension of the loop of the electrode tip 14 and this arc discharge serves to granulate the surface of the plate 40 as the plate passes along the electrode tip at the arc discharge. The arc discharge moves through the plate in two ways, so that the face of the plate opposite the electrode tip is treated twice by the arc discharge as it moves at the moving arc discharge. Since the arcuate discharge paths on the plate are preferably parallel, the treatment at the width of the plate is the same as if the parallel sides of the electrode were to reach the edges of the plate or extend beyond the edges of the plate.

Continuing with FIG. 3, to maintain proper electrical contact between the plate 40 and the power source and to position the plate relative to the workhead 10 at a treatment station 41 or pelletization station or location, the plate can be guided by two deflection rollers 44 to the support roll 42 support rollers which cover the plate over a substantial portion of the surface of the metal cylinder. The metal cylinder is held parallel to the electrode tip 14, and the plate 40 engages and surrounds the surface of the cylinder so that the surface of the plate is maintained parallel to the electrode tip as the plate advances along the cylinder and toward the cylinder.

In addition, tensioning of the plate 40 ensures intense contact with the surface of the metal cylinder of the support roller 42, such that the plate passes with a smooth surface at the electrode tip 14, thereby ensuring constant arcing of the surface of the plate.

The intense contact between the sheet and the metal cylinder also reduces the possibility of the sheet overheating and melting (which can affect the properties of the sheet material) as the heat of the sheet is transferred to the roll. Preferably, the material of the cylinder of the support roll 42 is a good heat conducting and good conductive metal such as copper, aluminum or a copper plated roll so that the heat of the sheet and the sheet is transferred from the sheet to the roll and the electrical resistance between the sheet and the roll is on hold.

If the support roller 42 is at ground potential, the plate will be at ground potential as it moves through the roller. In this way, the arcuate arc and the arc discharge are continuously maintained between the plate and the electrode 12 as the plate passes through the cylinder and one of the terminals of the power source 23 is connected to the ground. A slider contact (not shown) can be used to connect the roll directly to ground or other suitable potential. This contact provides a constant electrical potential for the plate at the electrode tip 14. This potential cannot be created if the current is conducted through the roller bearings and bearing housings.

Figure 4 shows arc discharge granulation of both sides of a moving disk at successive top and bottom treatment stations 41 spaced apart. Each station has 10 work heads. In the view of Figure 4, the workheads at the top of the figure roughen one side of the plate passing by the workheads. As the plate moves downward to a lower treatment station 41, it has the other side facing the lower workpieces 10, i.e., the particle that has been granulated at the upper stations is the inside of the plate when it reaches the lower rollers. Thus, when the particle surface reaches the lower rollers, it is in contact with the lower rollers, while the outer side of the plate is free for arc discharge granulation by the lower work heads 10.

Referring now to Figures 3 and 4, the cabinet structures 45, shown only schematically, encapsulate the work heads and support rollers to maintain the desired atmosphere.

Figure 5 illustrates a treatment station 41 and a frame structure 46 for holding and containing the guide roller 44 for guiding the plates 40 in the structure.

As described above with reference to Figure 2, the coil 20 is positioned behind the electrode 12 and is approximately centered around the inner magnet member 18. As shown schematically in Figures 6 and 7, the coil 20 may be positioned either within the boundaries of the electrode (Figure 6) or outside the electrode and in the general plane of the electrode (Figure 7).

The rolled aluminum sheet has a typical surface roughness of 0.25-0.75 µm and has an oxide film, which can vary in thickness over a wide range. This roughness manifests itself in substantially parallel grooves formed on the surface of the disc, created by grinding lines on the rolls of the disc producing roll. Roughness peaks are relatively low and the trenches between them are not deep.

Therefore, the surface of the sheet is relatively smooth, so that roughening is required to increase the surface.

The basic procedure is manual and consists of adjusting the electric current to a value sufficient to produce the desired arc at the electrode tip 14. The power source 23 that supplies the arc discharge current and voltage to the electrode 12 can be a conventional commercially available power source or a special power source. The arc length between the electrode tip and the plate and the open circuit voltage may vary. 10 A voltage of between about 1000 V and 1000 V is applied, depending on the material to be treated, the degree of granulation desired and the speed of the material passing at the electrode tip. The current may be between 10 and thousands of A depending on the length of the loop well, the desired arc speed and the desired degree of granulation. Typical parameters for granulating an aluminum plate surface at a speed of 6 m / min made of 1100 alloys: the arc gap is approx. 2.54 mm (0.100 ''), arc voltage 35 V and arc current 500 A.

When the plate 40 is moved into position (Figure 3-5), magnetizing current is applied to the coil 20 and arc discharge occurs. The position of the workpiece (s) 10 is adjusted to a predetermined clearance relative to the plate to maintain arc discharge as the plate passes through the workpiece (s). The exact conditions for setting the magnetically driven arc discharge, the disc transfer rate, and other operating details are set according to the specific application.

According to the invention, a permanent magnet may be used instead of the coil 20 if the magnetic field strength need not be adjusted by simply controlling the current supplied to the search threads te21.

The use of a permanent magnet eliminates the need for the coil 20, its power source 50 and its wires 51. The outer casing structure consisting of the casing 30, the panel 32 and the panel 34 may be a permanent magnet together with the magnet member 18 and the required magnetic poles are formed on both sides of the electrode tip 14. Figure 8 is a schematic diagram of one half of a continuous electrode and permanent magnet construction on one side of centerline 53. More specifically, there is an iron member 30A around an electrode 12 which terminates near the electrode tip 14. At the ends of the iron member 30A, the north and south magnetic poles are formed by a permanent magnet 52 located in the iron member. The permanent magnet 52 may be positioned anywhere within the element, or the entire iron member 30A may be a permanent magnet.

Figure 9 schematically depicts a compact workpiece 10A in which the electrical potential of a continuous loop grating electrode 12 is equal to the potential of the continuous north / south magnetic pole ends of an iron sheath 30B. As in Figure 8, only one half of the continuous electrode and magnetic structure is shown.

Continuing with Fig. 9, this shows the electrode 12, which is formed as a hollow structure for passing a refrigerant such as water. The hollow electrode contacts the outer iron sheath 30B while the coil 20 is behind (above) the electrode. When current is applied to the coil 20, the ends of the iron member 30A adjacent to the electrode tip become the opposite north and south magnetic poles.

The arc discharge particle surface produced by the workhead of the present invention consists essentially of a plurality of closely spaced, rounded tips or ridges that increase the surface area. The enlarged surface can be chemically treated to provide a durable coating on the rounded tips if the plate is to be used for lithographic purposes. 5. In the case of the apparatus shown in FIG. 1B, the lower deflector rollers 44 may be placed in a water bath (for boiling) or in an electrolyte bath for anodic oxidation or nitride.

The coil 20 may be wound as a single unit structure or may consist of multiple sections which are suitably connected and held together. In either case, the magnet structure has an open center, and is otherwise shaped like the electrode 12, so that the flux generated by the magnet structure can drive the arc discharge at the electrode tip 14 around the electrode tip loop.

Although the invention has been described in connection with its preferred embodiments, the appended claims are intended to cover all embodiments thereof.