Abstract: A plurality of billets are inductively heated in a staged process wherein the output current of a power supply is repeatedly time shared among a plurality of induction coils within which the plurality of billets have been placed. The time periods of the applied current to each coil become sequentially shorter over the total heating time of a billet to allow magnetically induced heat to conduct to the center of the billet during the dwell periods between applied electrical current periods. This maximizes the efficiency of the output of the power supply while melting of the outer regions of a billet is avoided in a process wherein the billets do not have to be moved during the overall billet total heating time.

Abstract: A method and apparatus for heating specimens in wells of a metallic specimen carrier. The specimen carrier is heated by applying resistive heating directly to the carrier. An AC source and transformer may be used where the specimen carrier is in series with a single turn secondary winding of the transformer. A magnet may be placed in each well and excited by the heating current to provided a stirring effect.

Abstract: A plurality of billets are inductively heated in a staged process wherein the output current of a power supply is repeatedly time shared among a plurality of induction coils within which the plurality of billets have been placed. The time periods of the applied current to each coil become sequentially shorter over the total heating time of a billet to allow magnetically induced heat to conduct to the center of the billet during the dwell periods between applied electrical current periods. This maximizes the efficiency of the output of the power supply while melting of the outer regions of a billet is avoided in a process wherein the billets do not have to be moved during the overall billet total heating time.

Abstract: The invention provides an improved induction heating coil, an induction heating device including the induction heating coil and a method for heating a workpiece with the induction heating device. The induction heating coil for heating a workpiece includes an elongated solenoid-type induction heating coil and at least one pancake-type coil positioned within a portion of the elongated solenoid-type induction heating coil, where the solenoid-type coil and the pancake-type coil are in electrical connection with each other.

Abstract: An induction heating device having a hollow cylindrical body made of electrically conductive material, at least one primary induction coil and at least one auxiliary induction coil, the at least one primary induction coil and the at least one auxiliary induction coil being arranged in the cavity of the cylindrical body such that, by means of the at least one primary induction coil and the at least one auxiliary induction coil, first and second magnetic fields, respectively, can be produced in the cylindrical body. The at least one auxiliary induction coil and the at least one primary induction coil operate such that the second magnetic field, at least in one area of the cylindrical body, counteracts the first magnetic field. A corresponding method of operating such an induction heating device, and also a processor for processing a recording medium, having such an induction heating device, are also provided.

Abstract: An first induction hardening coil assembly according to one embodiment of the present invention comprises a coil having a top surface, an underside surface, and an inner coil curvature disposed between the top surface and the underside surface. The inner coil curvature extends approximately 180 degrees around a coil axis. The coil further includes a first end section adjacent a first end of the inner coil curvature which extends between the top surface and the underside surface. A second end section adjacent a second end of the inner coil curvature extends between the top surface and the underside surface. A support arm is connected to the coil and is constructed and arranged for providing an electrical connection between the coil and a source of electrical current. The support arm includes a current-in portion connected to the underside surface adjacent the first end section and a current-out portion connected to the top surface at a location which is between the first and second end sections.

Abstract: A device configured to be heated to a temperature sufficient to fuse an image forming substance to a sheet includes a hollow roller made of a conductive material and a coil arranged in a hollow portion of the hollow roller. The coil is configured to carry an electrical current that induces a current in the conductive material of the hollow roller such that the hollow roller becomes heated so as to fuse toner to a sheet. The coil is mounted on a member that is disposed in the hollow portion of the roller. The member includes various combinations of features that permit a reliable and safe operation of the induction heating apparatus such as recess and projections formed in the member to separate respective turns in the coil, a hollow center of the member that can be ventilated with a ventilation fan, and predefined gaps being maintained between the inside surface of the hollow roller and an outer surface of the coil.

Abstract: An induction heating systems using a plurality of zones in a coil provides selective heating control for better uniformity especially for semiconductor and other thin film processing applications. By arranging the zones to have different resonance frequencies, the power supply may control the various zones by altering its frequency output. The power supply may also act to control the differential heating by switching among zones in conjunction with the frequency control, by sweeping through a variety of frequencies, by simultaneously providing power over different frequencies, by altering the residence time at each frequency, or by outputting different powers to each frequency or the like. Each zone may thus be tuned as appropriate to achieve the desired induction heating characteristic.

Abstract: A heating apparatus includes a heating element having a conductor, at least a part of which is an electrically closed circuit along which an eddy current flows; a container for accommodating the heating element; a magnetic field induction section for induction-heating the heating element; and a high frequency power supply device for supplying high frequency power to the magnetic field induction section. The heating element is induction-heated by an AC magnetic field generated by the magnetic field induction section.

Abstract: An induction heating type fixing device for an electrophotographic printer, facsimile apparatus, copier or similar image forming apparatus and an induced current generating member therefore are disclosed. A coil is wound round a bobbin applied with an adhesion preventing material. After the coil has been adhered by nonconductive adhesive by impregnation, the coil and bobbin subassembly is inserted into a hollow cylinder body formed of a heat-resistant insulating material. After the coil has been adhered to the cylindrical body, the bobbin is pulled out to produce the induced current generating member.

Abstract: A device configured to be heated to a temperature sufficient to fuse an image forming substance to a sheet includes a hollow roller made of a conductive material and a coil arranged in a hollow portion of the hollow roller. The coil is configured to carry an electrical current that induces a current in the conductive material of the hollow roller such that the hollow roller becomes heated so as to fuse toner to a sheet. The coil is mounted on a member that is disposed in the hollow portion of the roller. The member includes various combinations of features that permit a reliable and safe operation of the induction heating apparatus such as recess and projections formed in the member to separate respective turns in the coil, a hollow center of the member that can be ventilated with a ventilation fan, and predefined gaps being maintained between the inside surface of the hollow roller and an outer surface of the coil.

Abstract: The object of the present invention is to provide an induction heating fusing device in which the rotary heating member, such as a fusing roller or fusing belt, may be easily replaced. In order to attain this objective, the induction heating fusing device has a core, which forms a closed magnetic circuit, that comprises a first core component that runs though the empty space of the fusing roller, and a second core component that exists outside the empty space of the fusing roller, said core components being separable. The fusing roller, first core component and induction coil are housed in a casing, and comprise a fusing unit that may be replaceably mounted to the main unit of an image forming apparatus. During replacement of the fusing unit, the second core component remains in the image forming apparatus. The induction heating fusing device also has a pressurization means that causes the end surfaces of the first core component and the end surfaces of the second core component to be in pressure contact.

Abstract: An apparatus such as a glue gun system that utilizes either a stick or solid beads of meltable material such as glue. A motor in the interior of a body of the glue gun drives a drive member. The drive member forces the meltable material towards a nose assembly on the forward end of the body. An inductor and susceptor are provided in the nose assembly to transfer heat to the meltable material. The susceptor has multiple folds to increase the surface area of the susceptor and the rate of heat transfer to the meltable material. Electronics control the drive member and control the power to the inductor.

Abstract: An apparatus such as a glue gun system that utilizes solid beads of meltable material such as glue. The glue gun has a housing that contains a pervious barrier. The pervious diaphragm separates the beads from an airstream that is used to deliver the beads from a hopper to the housing. A load level control detects varying levels of the beads in the housing. A motor in the interior of the housing drives a feed screw. The feed screw forces the beads towards a nose assembly on the forward end of the housing. A heating element is provided in the nose assembly. A co-axial hose delivers the beads from a hopper to the housing through an interior passage. An exterior annulus of the hose delivers the separated airstream away from the housing. Electronics control the rotation of the feed screw and controls the power to the heating element. A hopper contains beads of the meltable material.

Abstract: A molten metal reactor (10) includes a reactor vessel having a heating section (11) formed from a dielectric material. The reactor vessel also includes a reaction section (12) connected to heating section (11). An induction heating coil (18) is associated with the reactor vessel heating section (11) and produces an electromagnetic field within a field area (19). A portion of the field extends through the heating section (11) for inductively heating a reactant metal (14) contained in the heating section, while a dielectric spacing material (25) positioned around induction heating coil (18) isolates electrically conductive materials from the induction heating field (19). A circulating arrangement (20) circulates molten reactant metal between the heating section (11) and reaction section (12) to react waste material introduced into the reactor in the reaction section, outside of the induction heating field (19).

Abstract: An induction hardening apparatus for inductively heating and quench hardening a crankshaft includes an arrangement of two workstations similarly configured and a robotic device indexing the crankshaft from a first workstation to a second workstation. The induction hardening apparatus is designed with a single induction coil located at the first workstation for the sequential induction heating and quench hardening of the pins of the crankshaft. At the second workstation, a single induction coil is used for the bearing surfaces of the crankshaft. One feature of the present invention is that the induction coils do not contact the surfaces of the crankshaft which are being inductively heated and quench hardened. Crankshaft dimensions and geometry are prograrmned into servodrive systems which move the corresponding coil in X and Y directions accurately tracing the orbit or path of each pin and each bearing surface.

Abstract: A getter heating coil assembly producing a focused magnetic field for heating objects in close proximity to the coil assembly. The coil assembly comprises electrical conductors wound on a coil bobbin, the coil bobbin having a central spindle around which the conductors are wound. The wound bobbin is assembled with a magnetic field concentrator, said field concentrator having a central core extending from a base, and a side wall extending from the base to enclose the wound bobbin. The central core of the field concentrator fits within the spindle on the bobbin and extends through it. The coil bobbin remains exposed on one side for emission of focused electromagnetic energy when alternating current is supplied to the coil windings. A button of high magnetic permeability material may be attached to the field concentrator for focusing the emitted electromagnetic energy toward the center of the coil.

Abstract: An induction heating device for surface treating the teeth (11) of a mechanical part on which each tooth (111) has a projecting tip (111s) and a tooth root linking the tooth to an adjacent tooth, wherein an in duction coil (12, 12') is spaced from the tooth tips and powered by at least one high-frequency current source. The coil consists of a set of conductive segments (24, 24') individually orthogonal to the generatrix (d, d') of the teeth, located above respective tooth tips and interconnected via connecting segments (25, 25').

Abstract: A method and hot rolling apparatus for producing a hot rolled steel sheet include a roughing rolling mill to roughing roll a heated slab into a sheet bar, at least one solenoid-type induction heater arranged for re-heating the sheet bar over its entire width, and a finish rolling mill arranged to finish roll the re-heated sheet bar into a hot rolled steel sheet having a predetermined thickness. An initial temperature before rolling is set to be low, and re-heating of the sheet bar is carried out around the middle of the hot rolling apparatus. The thermal energy required for rolling is thereby reduced as a whole without damaging the quality of the resulting hot rolled steel sheet. A descaling apparatus is provided prior to the finish rolling mill to remove oxide scales on surfaces of the sheet bar, whereby the resulting hot rolled steel sheet is free of scale flaws and exhibits superior surface properties.

Abstract: This invention pertains to an electromagnetic induction fluid heating apparatus equipped with a heating element constructed from a conductive material installed in a fluid flow passage, a coil installed around this heating element, and a high frequency electrical current generator for this coil. In particular, this heating element is a layered component that allows electrical conduction between metallic plates. This heating element is formed so that electrical current vortices occur throughout this layered component. By forming a fluid flow passage that allows mixing within this layered component, the electrical power efficiency becomes 100%. Moreover, this high frequency electrical current generator is an invertor that uses semiconductor power devices such as SIT, B-SIT, MOSFET, IGRT, and MCT, etc. When the preferred PWM system (Pulse Width Modulation) is used, the heating efficiency (affected by the efficiency of the invertor, etc. matched with this layered component) exceeds 90%.

Abstract: An induction coil for inductively heating electrically conductive materials includes a plurality of individual coil turns, each turn lying in a plane substantially perpendicular to a longitudinal axis of the coil and comprising an electrical conductor formed into an annulus. The conductor has first and second terminals for connecting the turn to an electrical circuit. The first and second terminals are adjacent each other at a preselected circumferential position on the annulus and are physically and electrically isolated from each other. The first terminal of one turn is located adjacent and electrically connected to the second terminal of an adjacent turn. The first terminal of a selected one of the plurality of turns forms a first coil terminal and the second terminal of a different selected one of the plurality of turns forms a second coil terminal.

Abstract: A single turn induction heating coil provides a varied magnetic flux density for heating a portion of a load having a thermally and magnetically responsive sealant and tubing. The varied interior diameter of the cylindrical coil structure increases the magnetic flux density at the open ends of the coil or wherever desired, causing the ends of the magnetically responsive materials contained within the coil to be heated at a rate comparable to the middle portion of the coil. Consequently, the sealant and tubing recover faster and more uniformly avoiding the high incidence of tubing "flare up" or "flip back" common with typical single or multi-turn induction coil heaters. In the preferred application, heating produces a complete fluid block in a cable load section within the coil without overheating or damaging the load.

Abstract: A heating machine for heating an ingot of a metal such as a magnesium alloy. The heating machine has a heating chamber equipped with a RF induction coil capable of swinging about a horizontally axis. The coil is placed horizontally, and the metal ingot is fed into the coil from an introduction chamber by a pushrod. Stoppers prevent the ingot from falling from the RF induction coil when it is switched from its horizontal posture to its vertical posture. The coil is wound on a cylindrical member. A rotatable hollow shaft is firmly mounted to the holder in a perpendicular relation to a substantially axially central portion of the cylindrical member. A part of the coil is inserted in the hollow shaft via an insulating material. The rotatable hollow shaft is swung by a swinging mechanism. The induction heating coil is wound on a ceramic holder. A metal ingot is inserted in the holder with a given clearance.

Abstract: An electric cooker is provided, which can grill foods by heat rays from a metal heating member using induction heating and is hardly affected by drops from the foods. This electric cooker comprises a work coil wound in a substantially pipe-like shape for generating an alternating magnetic flux, a metal heating member having a substantially pipe-like shape disposed inside and coaxially with the work coil for being heated by the alternating magnetic flux generated by the work coil so as to generate heat rays, and an insulator having a substantially pipe-like shape disposed between the work coil and the metal heating member. A wire mesh for supporting the foods is disposed adjacent to the upper opening of the insulator. A receiving pan is disposed adjacent to the lower opening of the insulator for receiving drops from the foods. The metal heating member preferably has a shape suitable for collecting the heat rays to the upper opening.

Abstract: Thermoplastic welding is an emerging technology targeted at significantly reducing the manufacturing cost of aerospace structure by eliminating fasteners and the touch labor associated with fasteners to prepare, install, and inspect the assemblies. Quality welds are highly dependent upon achieving appropriate temperatures everywhere along the bond line. The present invention is a system that uses a secondary coil to detect misalignment between the induction coil and the susceptor which would result in an excessive bond line temperature (in portions of the weld) and to provide control signals for the coil or parasitic compensation to avoid the overheating.

Abstract: A wrap tool having first and second coil regions and flange regions for inducing oscillating magnetic fields to fuse a plastic coupling having heating elements to a substrate. The first and second coil regions are wrapped around a selected plastic coupling and substrate to induce fusion currents causing melting and fusion of plastic pipes inserted into a magnetically energizable pipe coupling. The flange regions of the wrap tool are positioned adjacent each other overlappingly or inner surface to inner surface, permitting opposite current coil portions to cancel the other's magnetic fields and reduce coil inductance.

Abstract: Described is a low-loss induction coil for heating and/or melting metallic materials, the coil having windings formed by lengths of hollow tubing carrying a fluid coolant. In the central zone of the induction coil, the current is carried by hollow conductors made of copper which at the same time form the hollow tubing. Fitted at least in the windings at one end of the coil is a current-carrying element in the form of at least one braid whose individual conductors are insulated from each other, while remaining windings are designed as hollow conductors connected to the current-carrying element. The use of braids as the current carrier leads, at the end of the coil, to a reduction in eddy-current losses caused by transverse magnetic fields, while the use of hollow conductors in the rest of the coil results in the mean distance between the current flux and the metallic material being heated being kept at the minimum and losses due to this distance thus kept low.

Abstract: A heating assembly 1 for transferring liquid metal 28 has a closed cross-section and includes a refractory tube 4 wrapped in an insulating layer 5 which is in turn surrounded by an inductor 3 enclosed in refractory concrete within a metal shell, the insulating layer 5 consisting of an insulating refractory concrete layer 5a. The heating assembly 1 may be used in a liquid metal holding furnace 9 provided with a supply of pressurised gas Pr for producing metal parts.

Abstract: A drink can induction heating apparatus includes a ferrite core having a plurality of projected portions and recesses formed between the adjacent projected portions. When a drink can is set in a position to be heated, each of the projected portions is oriented toward a longitudinal axis of the drink can and a particular portion of a circumferential wall of the drink can. The apparatus further includes a saddle-shaped heating coil which is partly received in the recesses of the ferrite core and covers a region around the foregoing particular portions of the drink can in a non-contact manner, and an inverter for feeding the AC power to the heating coil. The inverter is arranged to produce the AC power at a resonance frequency which corresponds to one of 5 kHz to 15 kHz where equivalent resistance values of an aluminum can and a steel can approximate to each other.

Abstract: A hinge ball for a variable capacity wobble plate compressor includes a central through hole 27 for permitting a shaft 5 to loosely extend therethrough, and an outer peripheral wall 9b in a convex form for being slidably fit with an inner peripheral wall 43a in a concave form of a boss 43 of a drive hub 41 rotatively fitted on the shaft 5. The shaft 5 is higher in hardness than the hardness of the boss 43 of the drive hub 41, and the outer peripheral surface 9b in sliding contact with the inner peripheral surface 43a of the boss 43 of said drive hub 41 is lower in hardness than the hardness of the shaft 5 and the boss 43 of the drive hub 41 or equal in hardness thereto. The hardness of the inner peripheral wall 9a defining the central through hole 27 is made higher than the hardness of the boss 43 of the drive hub 41.

Abstract: An induction heater for heating a portion of a load comprises a side entry coil driven by a source of alternating current, with a flux concentrator located at the coil opening. The coil produces a magnetic field in the region occupied by the flux concentrator and the heated portion of the load. The flux concentrator extends along the opening of the coil and is fabricated of high permeability, low loss material, such as nickel-zinc ferrite, so as to enhance the uniformity of the magnetic field generated within the coil. In the preferred application, heating produces a complete fluid block in a cable section within the coil without overheating or damaging the cable.

Abstract: In order to determine the contents of solids in a heated elongated slug of thixotropic metal having a longitudinal axis, there is an apparatus comprising an inductive heating coil with electrical winding. Moreover, a sensor includes at least one measuring coil arranged between the heating coil and the slug. This sensor provides an output signal which corresponds to the contents of solids demanded.

Abstract: The high-frequency inductor heating tube (1) for solder injectors is furnished as a hollow cylinder formed at one end as a cylindrical receiver tube. Windings of an inductor (2) are disposed over length of the cylindrical receiver tube. A solder injector (13) made of a highly heat-conducting base material and covered with a ferromagnetic layer (16) is inserted into the cylindrical receiver tube. Claws (8) of a wire spring (9) inserted into elongated holes (7) disposed in a wall of the cylindrical receiver tube lock the solder injector (13) in the cylindrical receiver tube.

Abstract: A high-frequency induction heating coil is provided which enables a semiconductor single crystal in the process of growth to incorporate impurities uniformly therein, permits ready adjustment of the heat distributing property, and precludes the discharge of electricity across a slit. The high-frequency induction heating coil comprises a pair of annular conductors 21 and 22, a pair of power source terminals 23a and 23b for feeding a high-frequency electric current to the pair of annular conductors 21 and 22, and a plurality of small coils 24a through 24f and 25a through 25f having the pair of annular conductors as opposite electrodes and projecting toward the axis of the pair of annular conductors extending from a first annular conductor 21 to a second annular conductor 22.

Abstract: A fluid cooled RF transmission cable, transformer primary or secondary winding, and induction heating coil incorporating litz cable is disclosed. The heating coil comprises: a litz cable including a bundle of mutually electrically insulated, intermixed wire filaments, and a coolant tube, surrounding the litz wire, for conveying a fluid for removing heat generated by the litz cable. Also a combined coolant and electrical connector for providing an electrical connection and coolant to an inductive heating coil including a coolant tube and a litz cable housed inside the coolant tube is described.

Abstract: In a billet induction heating device, a plurality of heating coil units having round holes different in diameter from one another are arranged in such a manner that the central axial lines of the round holes are in parallel with one another and are in a vertical plane, and moving means supports the heating coil units thus arranged. Whenever a billet is given which is different in size from the one which has been processed, among from the heating coil units one suitable for the given billet is selected, and the moving means is operated to move the heating coil units until the selected heating coil unit reaches the position where it can accommodate the given billet. This arrangement makes it possible to achieve the replacement of the heating coil unit quickly and readily.

Abstract: Electrically conductive tubing for realizing induction coils having improved efficiencies comprises an outer peripheral portion of conductive material defined by a plurality of side walls and a single hollow passageway. Each side wall has an inner and outer surface. At least one side wall has a curved outer surface. The hollow passageway is defined by the inner surfaces and can be either centered within the tubing or spaced apart from the center of the tubing. The present invention also defines an induction coil and an induction furnace having an induction coil which utilizes such tubing. The curved outer surface has a radius of curvature which is defined substantially by a function of the tubing width in the axial direction, and spacing between turns of the induction coil. The cross section of the outer surface of the outer peripheral portion of the tubing taken along a transverse axis of the tubing defines a geometric figure.

Abstract: A method of case hardening a steering gear rack bar having a plain cylindrical barrel portion and an integral toothed portion on which are formed a plurality of rack gear teeth each having a predetermined helix angle relative to a longitudinal centerline of the rack bar. The method includes forming an induction coil having a linear section and a spiral section with a helix angle equal to the helix angle of the rack gear teeth, locating the rack bar inside the induction coil with the barrel portion thereof juxtaposed the linear section of the induction coil and with the spiral section of the induction coil surrounding the toothed portion, rotating the rack bar about its longitudinal centerline while passing alternating electric current through the coil to concurrently induction heat both the barrel portion and the toothed portion, and quenching the rack bar after the alternating current is turned off.

Abstract: A godet for guiding and heating an advancing yarn, which includes a shaft mounted for rotation about its axis, and a casing coaxially and releasably mounted to the shaft. An induction coil heating system is fixedly mounted between the shaft and the outer wall of the casing, and the heating system includes a tubular core having a plurality of induction coils mounted thereon. A plurality of annular ribs are positioned on the core and on the inside of the outer wall of the casing for providing an advantageous magnetic flux path between a core and casing.

Abstract: Induction heating is utilized to heat polymer matrix composites containing electrically conductive fibers. This is accomplished with an apparatus having a mandrel for supporting a composite workpiece and a helical induction coil disposed around the mandrel. The mandrel, workpiece and induction coil are disposed in an autoclave. The mandrel is a hollow, porous member having a port formed therein which is connected to a vacuum. A vacuum bag is hermetically sealed on the mandrel so as to define an enclosure over the workpiece. A power source is connected to the induction coil and, when activated, causes the coil to generate an oscillating magnetic field lying along the longitudinal axis of the mandrel. The magnetic field induces heat-generating eddy currents in the fibers of the workpiece which are oriented orthogonally to the magnetic field.

Abstract: Induction heating is utilized to heat polymer matrix composite fiber strands containing electrically conductive fibers. This is accomplished with an apparatus having a helical induction coil and a matching helical guide tube situated adjacent to the induction coil. Composite fiber strands are drawn through the guide tube which is made of a non-electrically conductive, nonmagnetic material. Electrically conductive rollers, positioned next to the entrance and exit of the guide tube, form an electrical circuit with the electrically conductive fibers. This permits currents to be induced in the fibers when the coil is activated, thereby heating the fiber composites. The guide tube can be thermally insulated and flushed with an inert purge gas. Alternatively, the guide tube can be made of an electrically conductive or ferromagnetic material so that heat producing currents are induced in the guide tube, and the composite fiber strands are heated by conduction.

Abstract: The invention provides a system and method for joining plastic members including induction heaters fabricated from a conductive wire coated with a ferromagnetic material and providing ratios of at least 4 to 1 between the areas over the spaces between wires and the area of the wires surrounding the open spaces; the heaters being located preferably between plastic members to be joined. The heaters may be located adjacent the two ends of a plastic connector adapted to receive a pipe in at least one and probably two ends with the heaters being activated preferably by induction heating to produce welding of the plastics to one another. One form of heater is fabricated on a hollow cylindrical mandrel having helical grooves in which the wire is wound and having radial holes arranged in axially extending rows spaced circumferentially about the mandrel so the axially extending wires may be spot welded to the wires in the helical grooves at each hole.

Abstract: Solenoid coils are provided that produce relatively low irradiation of electromagnetic fields with intense fields internally thereof for purposes of heating a metallic element, preferably a ferromagnetic element having a Curie temperature selected for the purpose for which the coil structure is to be employed, the coil structure in one embodiment comprising two coaxial coils of substantially equal diameter interconnected to produce in response to an alternating current being applied thereto out of phase magnetic fields that do not substantially interfere with one another as a result of the spacing between the coils. In a second embodiment the two coils are located between two additional coils that buck axial fields thereby together with fall off of the fields as the cube of the distance from the main coils provide a structure with low external radiation.