CA1229139A

CA1229139A - Correction coil assembly for nmr magnets

Info

Publication number: CA1229139A
Application number: CA000480185A
Authority: CA
Inventors: Mark E. Vermilyea
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1984-07-05
Filing date: 1985-04-26
Publication date: 1987-11-10
Also published as: US4509030A; IL75443A; DE3570798D1; JPH0334843B2; IL75443A0; EP0167128A2; JPS6124209A; EP0167128A3; EP0167128B1

Abstract

CORRECTION COIL ASSEMBLY FOR NMR MAGNETS
ABSTRACT OF THE INVENTION
An NMR magnet correction coil comprises a plurality of cylindrical coil forms coaxially disposed with radially exterior coil forms possessing saddle coils so as to provide slits in the exterior forms.
The outermost coil formed is wrapped with a non-magnetic wire in order to keep the forms in a fixed relation-ship. If three or more coil forms are employed, the inner forms are wrapped with a band of material which also serves to provide a spiral flowpath for cryogens such as liquid helium. A pair of keys is provided for tieoff and insulation of the outer wire band, as well as for positioning of the entire assembly.

Description

RD-15,604 ~29~3g CORRECTION COIL ASSEMBLY FOR NOR MAGNETS

BACKGROUND OF THE DISCLOSURE
The prevent invention relates to correction ; coils employable in nuclear magnetic resonance (NOR) imaging systems. More particularly, the present invention relates to a coil assembly including axial and tray verse magnetic field correction coils which are configured in an assembly which prevents relative motion between the coils even at cryogenic temperatures while allowing accurate positioning of the coils as a unit.
10~n NOR imaging systems, particularly those employed for medical diagnostic purposes, it is nieces-spry to provide a highly uniform and high strength magnetic field. Superconducting magnet coils provide a desirable method for achieving such a field. Super con-dueling magnets offer a particular advantage in that once energized, no electrical power is needed to ; maintain the resulting magnetic field. However, NOR
imaging impose strict requirements upon the magnetic field uniformity. In order to reduce the presence of image artifacts, magnetic fields exhibiting spatial ; variations of only a few parts per million are desired.
However, even slight manufacturing variations in the construction of the main magnet can adversely effect magnetic field uniformity. Accordingly, correction coils are generally required to provide corrective magnetic field components. Typically the correction coils carry much less current than do the main magnet coils. Adjustment to the main field provided by correction coins are typically achieved by selecting appropriate current levels and current polarities for ~D-15,604 Lo the correction coil general, correction coil comprise coils or coil sets which are either axisymmetric or axiperiodic. Axisymmetric correction coils typically comprise coil loops which completely surround a cylindrical support form in the circumferential direction. These coils are I; particularly desirable in adjusting certain axial - gradients of the magnetic field. On the other hand, axiperiodic coils are typically configured as arcuate 10 . segments joined by axial segments and are commonly referred to as saddle coils. These coils are provided to correct other axial components of the magnetic field within the cylindrical volume. However, of course, the spatial distribution associated with the axial coil corrections and those associated with the saddle coil corrections are significantly different.
Since axial correction coils completely surround the cylindrical coil form upon which they are disposed, it is not possible to employ coil forms with axial slot which extend completely through the form.
however, appropriate choices for transverse or saddle shaped correction coils are seen herein as being capable of providing an opportunity for constructing coils with slotted forms.
The reasons which motivate the use of superconductive main magnet coils also provide motivation for the construction of superconductive correction coils. While it is be desirable to include axial and transverse correction coils on a single form, the wiring patterns are generally too complicated to manufacture on a single form to the precision required.
Accordingly, multiple forms are employed. However, it must be borne in mind that the electrical conductors RD-15,604 - ~L2~913~

preferably ccmpri~e superconductive material and accordingly should be cooled to cryogenic temperatures during operation. Therefore, it is preferable to have all of the winding in a clove contact as possible S with cryogenic coolant such as lugged helium. Such assemblies are operated at temperatures of approximately 4.2K. The intimate cryogenic fluid contact with the coils is advantageous in preventing : and/or minimizing the effects of quench conditions.
over, as indicated above, there are stringent requirements with respect to manufacturing tolerances.
The correction coils must not only be disposed on their respective forms in fixed positions which prevent conductor motion, but the assembly of distinct coil forms must also be held together in a fashion which prevents relative motion between inner, middle and outer coil forms. The correction coil sets must therefore be accurately positioned with respect to one another and Allah with respect to the main coils themselves. This insures that the geometric center for each coil set is known. This is a necessity for field Jo corrections. Furthermore, in the case of multiple coil forms, the assembly must be configured so that the outer forms do not lock against one another thereby permitting motion of the inner coil form. It is therefore seen that it is necessary to simultaneously achieve intimate contact between the correction coil conductors and the coolant fluid while at the same time insuring that relative motion between the coil forms is prevented. It must be further borne in mind that these requirements must be met for a coil assembly which is manufactured under room temperature conditions but which is to be immersed in a cryogenic fluid.

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RD-15,604 ~22g~39 SUMMARY OF THE INVENTION
In a preferred embodiment of the present invention a correction coil assembly for NOR magnets comprise a set ox three epoxyfglas~ fiber cylindrical coil from deposed coccal with respect to one another. Axi~ymmetric coil are disposed on the inner form while radially exterior form are slotted and include tray verve or saddle shaped coil structures, All coil forms, save the innermost one, are slotted.
; 10 The first (radially innermost) slotted coil form is expanded and slipped over the axial coil form and is affixed thereto by means of a non-magnetic band which it tightly wrapped around the slotted coil form to securely hold it against the first form. The band posy sues a coefficient of thermal expansion which is greater than that of the material comprising the first unspotted) coil form. The banding tensile strength ;, and attachment considerations may be effectively I' eliminated however because pins which temporarily hold the banding in place can be removed and their function supplanted by the presence of one or more overlying coil forms. Therefore, the tensile strength of the banding must also be considered since it is subject to greater stress under cryogenic conditions than during manufacture and assembly at room temperature conditions. In the preferred embodiment of the present invention a third coil form including saddle coils is also included. This latter coil form comprises the ; second slotted coin form employed. In this embodiment, the radially innermost slotted coil form is held again t the axial coil form by means of the above-mention~d band which is wrapped around the outer portion of the first slotted form. The second slotted I: .

RD-15,604

2~91;~

form it then slid over the banded structure and is icily further banded with a wire wrapping which preferably comprises a material which has the same thermal and mechanical properties as the S superconductive material included in the coils on the worms. The temporary pins holding the inner banding may then be removed as the function of the banding has been supplanted by the overlying form. The innermost coil form i axially longer than either of the two lo slotted coil forms which bear the saddle coils.
Accordingly, this additional length provides room for the inclusion of a key assembly which acts to align the coil forms in both an axial and a circumferential direction. Furthermore, this key provides means for alignment of the completed correction coil assembly with respect to the main magnet coils themselves.
Another object of the invention which is achieved by the present correction coil is the formation of a I` structure which is relatively thin in the radial direction. In this regard, it should be borne in mind that the assembly of the present invention is to be inserted in a cryogenic vessel. In order to minimize the volume of cryogenic liquid employed, it is therefore seen that the "build out" in the radial direction should be as small as possible. The reasons for not being able to achieve such a small build out using a single coil form have already been considered.
Accordingly, it is an object of the present invention to provide a correction coil assembly which it particularly useful in cryogenic environments.
It is a further object of the present in-mention to provide correction coils for NOR medical diagnostic imaging systems.
.
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RD-15,604 ~,',229~3~

It i another object of the present invention to provide a correction coil assembly in which relative motion Betty cowl form is prevented.
: It it yet another object of the present invention to provide a magnetic field correction coil assembly for a superconductive magnet in which there is ; provided a large degree of contact between the cornea-lion Casey and the cryogenic fluid.
Lastly, but not limited hereto, it is an object of the present invention to provide a correction coil assembly which is readily alienable both in manufacture and during installation in a cryostats DESCRIPTION OF THE FIGURES
The subject matter which it regarded as the invention 18 particularly pointed out and distinctly claimed in the concluding portion of the specification.
The invention, however, both as to organization and method of practice, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawings in which:
: : Figure 1 is an isometric view of an axial coil set and coil form employed in the assembly of the prevent invention;
Figure 2 illustrates a transverse coil form in the process of being slipped over the axial coil form of Figure l;
Figure PA illustrates a transverse coil form ; in place over an axial coil form and further thus tryout the use of flat banding material to hold the transverse coil form against the axial coil form;
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R~-15,604 ~2~L39 Figure 3B illiterate the same structure as shown in Figure PA except that it it rotated through an angle ox approximately 180~;
Figure PA illustrates the structure shown in figure PA with the addition of a second slotted coil form together with a final layer ox wire wrapping to hold the entire structure together;
Figure 4B illustrates the same structure as shown in Figure PA except that it is rotated through an angle of approximately lRO;
figure 5 is an isom trig view particularly illustrating the alignment kiwi structure of the present invention;
Figure PA is a schematic view illustrating, in a developed format, the transverse coil winding pattern employed on the first slotted coil form; and Figure 6B it a schematic view illustrating, in developed format, transverse coil winding pattern employed on the second slotted coil form.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is illustrated in the Figures described above. These Figures illustrate a particular embodiment of the present invention.
However, it should be understood that its applicability extends to general coil configurations and not only to those coil configurations shown herein for purposes of illustration comprehension and completeness. More particularly, other arrangements of axial and trays-verve saddle coils may be provided.
Figure 1 illustrates a typical innermost (axial) coil and coil form employed in thy assembly of RD-15,604 ~9~3~ -the pronto invention. In particular, axial coils L
Lie, aye Lowe Lowe aye Lob Lob Sub Lo and Lob, are shown. The particular coil designations do not have any direct relevance to the present invention and are provide merely for convenience. These coils are axial coils and are mounted on coil form 10 which preferably comprises a composite of epoxy and glass fiber material. Coil form 10 is mounted on mandrel 103 which is supported by stand 100 having vertical end members 101 and 102, as shown. One of the most important aspects to note in Figure 1 is that it would not be possible to incorporate an axial slit in coil form 10. If such a slit were present, the axial coils shown would not be held in a rigid condition. Since lo these coils preferably comprise superconductive material, it is important that relative motion between the conductors or between the conductors and the coil form not be permitted. Such motion creates friction which produces heat which increases the risk of I; 20 localized quench conditions.
However, correction of magnetic field non uniformities generally requires more than the kind of correction which can be provided by axial coils alone. More particularly, saddle coils are required.
It is important to realize that the selection of correction coils for NOR imaging systems is not a straight forward matter. The proper design of transverse coils dictates that the coil patterns exhibit certain circumferential periodicities.
Fortunately, coil patterns and periodicities have been found such that certain sets of transverse coils can be laid out on the same coil form so that an axial slit is permitted. These coils are therefore seen as having RD-15,604 ~2g~39 the advantage that, even though placed on a cylindrical coil form, an axial slit may nonetheless be provided in this for without the necessity of correction coil conductor crossing the slit or gap. Furthermore, because of the complexity ox the coils and the desire to align them properly with respect to one another various correction coil sets are required to be duped on distinct, but aligned coil forms.
Accordingly, Figure 2 illustrates first transverse coil form I with slot 27 therein (see Figure PA) in the process of being slipped over coil form 10 on which the axial windings are disposed. The transverse or saddle coils are in contrast disposed on coil form 20. Slit 27 enables form 20 to be spread slightly and slipped over coil form 10. However, as is indicated below, the restoring force associated with this spreading operation is not sufficient to hold the assembly together.
Figure particularly shows transverse coils : lea' Lea' Lab Lob Lea and Lob (see below for a discussion of the coil labeling scheme). All of the : : coils on form 20 include connecting leads snot shown) connecting all coils subtending the same Jo : circumferential angle and disposed on the same axial end of the form to one another (so as to act as coil sets) and to exterior terminal connection port 23.
Furthermore Figure 2 illustrates that coil leads 11 : from the axial coils extend along coil form 10 in an I: axial direction and terminate in temporary cable 30 bundling 12. Likewise conductors from connection port : 23 terminate in a temporary cabling bundle 22. Again it should be noted that the conductors in the axial and transverse coils shown in the Figure it the present I: :

.' "`' RD-15,604 2Z9~39 invention preferably comprise superconductive material.
The Amy holds true for the conductors in bundles 12 and 22.
Figure PA illustrates the same configuration shown in Figure 2 with several expense. Firstly, coil form 20, which is preferably shorter in length -I than golf form lo is now shown in its final position, the o'er having been spread at slot 27 and slipped over coil or 10. Secondly, the structure shown in Figure I ha been slightly rotated in a counterclockwise direction 80 as to provide a view of slot 27. As ; indicated above, it is seen that the saddle coils are ; disposed 80 as not to cross axial slot 27. Thirdly, the assembled structure is shown wrapped with banding material 25 which is preferably, wrapped in a spiral pattern. Banding 25 is preferably affixed at either ; end thereof with temporary holding pins aye and 26b.
These pins are temporary in that they may be removed at a later point in assembly without loss of structural rigidity. Banding material 25 preferably comprises a non-magnetic material such as aluminum. Furthermore, inch the glass fiber forms shrink much less than most metals under cryogenic conditions, metal banding material 25 should exhibit sufficient tensile strength.
Furthermore, the design of the banding structure is complicated by the fact that each axially slit (saddle coil) form must be banded independently to avoid frictional locking together of two forms and consequent ennui from base form lo Metal banding is preferred over a glass fiber band because of its low cost, availability and ease of application and disassembly. In any event metals employed for such banding purposes must be non-magnetic, because of the ' Jo RD~15,604 9~39 environment in which it it I be employed. The flat band structure also provide an advantage in that it result in a very small radially outward build. Again as pointed out above this minimizes coil form volume and core pondingly the volume of cryogenic fluid u Ed for cooling the superconductive windings.
It is also seen that two additional trays-verse coils Lea, and Lob are now visible in Figure PA
(in comparison with Figure 2). A more thorough under-lo standing of the transverse coil winding pattern is best had from an examination of Figures PA and 6B below.
; Furthermore, the discussion below with respect to these figures also provides an appropriate description of the coil naming (subscripting) scheme employed herein.
lo Figure 3B provides an isometric view of the same structure shown in Figure PA except that the structure is rotated in a counterclockwise direction through an angle of approximately 180. Accordingly, Lowe Lea, Lye, and Lob are now visible. It is also important to note the presence of key assembly Sheehan in Figure 3B. Key assembly 50 provides means for aligning coil forms Lo and 20 in both axial and circumferential directions. Additionally, key assembly 50 alto provides an alignment mechanism which is of use when the correction coil assembly it inserted into the cryostats with the main magnet coils, This key assembly is therefore seen to be useful in providing an earth based referenced frame for the correction got} asset-by. This alignment is crucial in matching the action of the correction coils with any inherent in homogeneity in the main magnet coils. A more detailed view of key assembly 50 is provided in Figure 5, which is discussed below.

RD-15,604 29~

Figure PA illustrate, by way of example, a finally assembled correction coil in accordance with a preferred embodiment of the present invention. In particular, a second transverse correction coil form 30, with its own 810t 37, has been slipped over coil form 10 and 20. Slot 37 in coil form 30 is aligned with connection port 23 in coil form 20 and disposed so that temporary pins aye and 26b extend through the slot 37. Additionally, coil form 30 is provided with notch 33~ to accommodate connection port 23. Note also that the cabling from port 23 together with the cabling associated with the saddle coils on form 30 are disposed in slot 27. Accordingly, Figure PA
illustrates the presence of temporary cabling bundles 15 12, 22 and 32. Also visible through slot 37 is banding 25.
Lastly, and importantly, it is to be noted that the radially outermost coil form (30 here) is wrapped with a wire material. In the present invention thy maternal preferably comprises the same superconductive wire employed in the correction coils themselves. This final banding must be tied off :, :
without inducing stress concentrations high enough to break it upon cooling. The material selected for this final banding is important. A high ratio of strength to thermal contraction is desired. Additionally, non-magnetic material must be employed. An excellent ; non-magnetic material which is available for these purpose i the very superconductor with which the I; 30~ coils are wound that it, a Niobium-Titanium superconductor matrix which includes a ratio of copper Jo superconductor of 2:1. For such a material its contraction from 300K to 4.2K is only about 2.9 miss :, RD-15,60~
13~

per itch, compared to 4.4 miss per inch or aluminum.
Howevax, its yield tress has been measured at 55,000 pat, compared to 40,000 pi for aluminum. However, the circumferential contraction of the inner coil Norm is only about 2.2 mill per inch so that the thermal stress induced in the banding it approximately Lowe psi which is comfortably below the yield stress of either material. wound wire banding 35 is preferably chosen since it is easy to tie off the material without inducing strew concentrations which are high enough to break it upon cooling. Tipoff points are provided at key assemblies 50 as more particularly described in Figure 5. Additionally, wire banding 35 is desirable since it can be tied off without increasing the radial build problem discussed above. This ensures a final ; assembly which it relatively thin.
Additionally, Figure PA illustrates the I; presence of a second set of transverse correction coils, namely coils Lowe LEA LO LEA Lug LOB' LAB' and ~XB' Note that the subscripts designating the , different coils on coil form 30 are all Capital letters. This helps to distinguish these transverse coils from eke transverse coils mounted on coil form 20. Also, if one considers symmetry with respect to a central plane (not shown) which is perpendicular to the cylindrical axis one finds that the transverse coils on one side of this plane of symmetry possess second subscripts designated "A" while transverse coils disposed on the opposite side of this plane possess second subscript designated "B". Similarly with respect to the transverse coils disposed on coil form 20 (Figures PA and 3B) the corresponding transverse coils possess subscripts "a" or "by', also depending .

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RD-lS,604 2 9~L3'9 upon ho which wide of this plane on which they are disposed. Additionally the elongate rectangular coils include a subscript designation in which the first letter of thy ~ub~crip~ is either ox" or "y" (on coil form 20~ or "X" or "Y" (on coil form 30). The saddle coils which more closely approximate a square are designated with subscripts whose first letter is either "a', "b", I or "d" (on coil form 20) or "A", "B", I or "D" (ox coil form 30). It it thus seen that lo all of the transverse coils are designated by wholly alphameric subscripts, while the axial coils shown in Figure l include subscript in which the first kirk-ton it always numeric. In this way all ox the coils employed in the embodiment illustrated herein are readily distinguished and located. It should be understood however that connections between pairs of transverse coils do not necessarily follow from the labeling scheme descried. For example, on coil form 20, coils Lab Lob, Lob and Lob are connected in series with appropriate winding polarities. Lob and Lye are similarly connected to form a distinct transverse correction coil set. The tame pattern also holds for the o her transverse coils shown. See also the discussion below with respect to Figures PA and 6B.
It is also seen that figure 4B illustrates the same structure a shown in Figure PA except that the entire structure is rotated in a counterclockwise direction through an angle of approximately 180. Also more particularly visible in Figure 4B are key as embowels 50 which particularly illustrate the final wire wrap 35 which it tied off to these keys.
Thy structure of the keys is more portico-laxly illustrated in Figure 5. In particular, key 55 -, .

RD-15,604

3~3 is shown bolted to the axially extending portion of coil form 10 by moans of bolt 53 and washer 54. Key 55 compare a nonmagnetic material such as aluminum or a glass fiber composite material. Key 55 is preferably in the o'er of a cross the top of which extends axially into Nash in coil forms 20 and 30, a shown. Round wire banding 35 is wrapped several times around in~ulative screw 52 which is affixed to key 55 by means of screw 51. rushing 52 provides a desired degree of tress relief for the wire banding. Wire banding 35 is further wrapped around insulative binding post 57 which it affixed to key 55 by means of screw 56.
Additionally, it is desirable to include Belleville washers on screw 56 to ensure clamping power even under cryogenic conditions. All screws, bolts, bushings, binding posts and washers shown in Figure 5 comprise non-magnetic material. Additionally key 55 may alto be glued to form 10 for additional strength.
In order to tie off wire banding 35, any insulation on 20 the end of the wire is stripped off and the tie off wrap 58 is soldered. Furthermore, a coating of glue is preferably applied to the solder joint in order to ; ensure that wire band 35 is not electrically connected in any way to key 55, particularly if it comprises aluminum. This insulating function is very important since the keys are used to align the assembly within the cryostats Accordingly, key 55 would be in contact with the metal cryoqtat vessel. Accordingly, a closed circuit path could be formed which would include wire banding 35. Such a situation is highly undesirable.
Figures PA and 6B are provided to indicate several possible forms of transverse coil winding patterns. They are also provided to indicate the "'" ' l .
RD-15,604 Lo relative orientations of the coil forms shown in Figures PA, 3B, PA and 43. Figure PA and 6B alto proviso an opportunity to view the layout of all of the tran~ver e coil forms at one time. The scheme for coil labeling hs0 already been indicated above.
Wow the structure shown in Figures PA and 4B illustrate preferred embodiment of the present invention in which two sets of transverse correction coils are employed, it is also possible to employ only a single transverse coil form. In the case of a single transverse form, coil form 20 and its associated coils along with aluminum banding 25 are not employed.
Instead, the radially outermost coil form 30 is directly disposed around coil form 10 with thy outermost banding comprising wire wrap 35 as described above. In this case banding 25 is not required since qpllt coil form 20 is not present. However, if ; desirable, such banding is employed in order to provide a spiral coolant flow path between the coil form Such a configuration provides greater protection against quenching. In the case of multiple transverse coil forms, one preferably employs a flat band between every pair of transverse coil forms.
From the above, it should be appreciated that the correction coil assembly of the present invention provides a structure which is particularly useful in cryogenic environments. The structure provided exhibit a small radial extent so as to minimize the required volume of liquid coolant. The present in-mention is also seen to prevent relative motion between the coil forms so as to preserve design conditions.
The present correction coil assembly also provides a large degree of contract between the correction coils ,, RD-15,604 ~2~29~

and the cryogenic coolant. The present invention is alto particularly advantageous in that it provides a mean to align the entire correction coil a3semb1y not only with respect to the various coil forms themselves, but alto with no poet to external and main magnet assumably. Additionally, the present desist permits ample assembly, disassembly and reassembly, if necessary, for accurate positioning of the forms with respect to one another. Furthermore, the structure of the present invention is particularly adapted for manufacture at room temperature conditions and for use at cryogenic temperatures without inducing undue amounts of thermal stress.
While the invention has been described in detail herein in accord with certain preferred embody-Monet thereof, many modifications and changes therein may be effected by those skilled in the art. Accord-tingly, it is intended by the appended claims to cover all such modifications and changes as fall within the true spirit and scope of the invention.

Claims

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

1. A correction coil assembly, especially for use in superconductive magnets and other cryogenic environments, said assembly comprising:
at least one electrical coil disposed on a first cylindrical form;
at least one transverse coil disposed on a second cylindrical coil form, said second form having a longitudinal slit therein, said second form being disposed coaxially about said first form and over said coil disposed on said first form;
an insulated, non-magnetic wire having a coefficient of thermal expansion greater than that of the material of said second coil form, said wire being tightly wrapped around said second coil form so as to securely hold said second form against said first form under cryogenic conditions; and a pair of insulative supports disposed at and fixed to each end of said assembly, each end of said wire being attached to a distinct one of said sup-ports.

2. The assemble of claim 1 further includ-ing:
at least one transverse coil disposed on a third cylindrical coil form, said third form having a longitudinal slit therein, said third coil form also being disposed coaxially between said first and second coil forms;
a flat, non-magnetic band, insulated from said coils on said third coil form, said band being wrapped around said third coil form so as to be disposed between said second and said third coil forms.

3. The assembly of claim 2 in which said band comprises aluminum.

4. The apparatus of claim 2 in which each end of said band is affixed to a distinct end of an underlying coil form.

5. The assembly of claim 4 in which said band is affixed by means of pins disposed through said slit in said second form.

6. The assembly of claim 2 in which said third coil form comprises glass, fiber and epoxy composite material.

7. The assembly of claim 1 wherein said wire comprises superconductive material.

8. The assembly of claim 1 in which said electrical coil on said first form and said transverse coils on said second form comprise superconductive material

9. The assembly of claim 1 in which said first form comprises glass fiber and epoxy composite material.

10. The assembly of claim 1 in which said second form comprises glass, fiber and epoxy composite material.

11. The assembly of claim 1 in which said insulative supports comprise:
a T-shaped key member affixed to said first form with an end thereof extending into a mating notch in said second form so as to provide axial and cir-cumferential alignment of said second form with respect to said first form;
an insulative bushing affixed to said key member for attachment of said wire.

12. The assembly of claim if in which said T-shaped key member comprises aluminum.

13. The assembly of claim 1 further including:
at least one transverse coil disposed on a third cylindrical coil form, said third form having a longitudinal slit therein, said third coil form being disposed coaxially between said first and second coil forms;
a flat, non-magnetic band, insulated from said coils on said third form, said band being wrapped around said third coil form so as to be disposed between said second coil form and said third coil form;
a T-shaped key member affixed to said first coil form with an end thereof extending into a mating notch in said second and third coil forms so as to provide axial and circumferential alignment of said second and third coil forms with respect to said first coil form; and an insulative bushing affixed to said key member for attachment of said wire.