DESIGN AND CONSTRUCTION DETAILS OF THE 7,4 TESLA SUPERCONDUCTING METASTABLE MAGNET FOR THE ELMO BUMPY TORUS PROOF-OF-PRINCIPLE PROGRAM

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DESIGN AND CONSTRUCTION DETAILS OF THE 7,4 TESLA SUPERCONDUCTING METASTABLE

MAGNET FOR THE ELMO BUMPY TORUS PROOF-OF-PRINCIPLE PROGRAM

S. Ackerman, E. Kimmy, D. Lieurance, T. Mann

To cite this version:

S. Ackerman, E. Kimmy, D. Lieurance, T. Mann. DESIGN AND CONSTRUCTION DETAILS OF

THE 7,4 TESLA SUPERCONDUCTING METASTABLE MAGNET FOR THE ELMO BUMPY

TORUS PROOF-OF-PRINCIPLE PROGRAM. Journal de Physique Colloques, 1984, 45 (C1), pp.C1-

185-C1-188. �10.1051/jphyscol:1984138�. �jpa-00223693�

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D E S I G N AND CONSTRUCTION D E T A I L S OF T H E 7 a 4 T E S L A SUPERCONDUCTING METASTABLE MAGNET FOR THE ELMO BUMPY TORUS PROOF-OF-PRI NC I PLE PROGRAM*

S.L. Ackerman, E.R. Kimmy, D.W. L i e u r a n c e a n d T.L. Mann

General Dynamics Convair D i v i s i o n , San Diego, C a l i f o r n i a 92238, U.S.A.

S s m 6

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L a r e a l i s a t i o n d ' u n e b o b i n e s u p r a c o n d u c t r i c e f o n c t i o n n a n t d a n s l e mode d e r e f r o i d i s s e m e n t m d t a s t a b l e , a v e c u n e d e n s i t d mayenne d e c o u r a n t d e l ' o r d r e d e 1 0 . 0 0 0 ~ / c m ~ e t u n champ m a g n e t i q u e d e 7,4 T, demande d e s c a l c u l s p r 6 c i s e t u n e c o n s t r u c t i o n s o i g n d e . L e s a s p e c t s les p l u s i m p o r t a n t s c o n c e r n a n t les j o i n t s d u c o n d u c t e u r , l a b o b i n e , l ' i s o l a t i o n , les s o n d e s v o l t m d t r i q u e s d e d d t e c t i o n d u c h a n g e m e n t d ' b t a t d u b o b i n a g e , l e s s u p p o r t s en titane e t le dewar, s e r o n t d i s - c u t & e n c o n n e x i o n a v e c les m d t h o d e s d e f a b r i c a t i o n e t d ' a s s e m b l a g e . L e s r d s u l - t a t s d e s essais d u p r o t o t y p e d ' u n b o b i n a g e m i r o i r d e p r o d u c t i o n s o n t i n c l u s . A b s t r a c t

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The c r i t i c a l n a t u r e o f a s u p e r c o n d u c t i n g m a g n e t , o p e r a t i n g i n a m e t a s t a b l e c o o l i n g mode a t 7.4 T a n d a t an o v e r a l l w i n d i n g c u r r e n t d e n s i t y o f a b o u t 1 0 , 0 0 0 ~ / c m 2 , demands r i g o r o u s d e s i g n t r e a t m e n t , c o n t r o l l e d , h i g h - q u a l i t y p a r t s c o n s t r u c t i o n , a n d s u p e r i o r workmanship. D e t a i l s s u r r o u n d i n g t h e d e s i g n a n d t e s t i n g o f c o n d u c t o r splices, c o n d u c t o r a n d b o b b i n i n s u l a t i o n , v o l t a g e t a p s f o r q u e n c h d e t e c t i o n , c o l d mass s u p p o r t s , a n d d e w a r d e s i g n w i l l b e d i s c u s s e d a s t h e y r e l a t e t o f a b r i c a t i o n a n d a s s e m b l y . T e s t d a t a f o r t h e p r o d u c t i o n p r o t o - t y p e m i r r o r c o i l i s a l s o i n c l u d e d .

Introduction

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Transfer of technology and skills from the Oak Ridge National Laboratories to industrial team members was fundamental to the Elmo Bumpy Torus Proof-of-Principle (EBT-P) program. This was especially true with respect to the mirror coil component of the system. ORNL had spent several years developing a design, building, and winding two mirror coil cold mass units.l General Dynamics was responsible for applying the knowledge thereby acquired to make the cold mass assembly production-ready and to design a structurally sound and thermally efficient dewar system to support and house it. A secondary objective was to develop plans, techniques, and processes for low- cost replication of 36 magnet coils by average technicians in a production environment. These requirements were satisfied by the high performance of the ORNL cold mass units with the General Dynamics-fabricated dewar system and by the operation of the production prototype coil wound by General Dynamics.

System Requirements - The EBT-P experiment imposed five major requirements on the magnetic system:

1. The 36 toroidal mirror coils must produce a 3.3 T on-axis field in the throat of the coil when the RF resonance (ECRH) system is operated at 60 GHz.

2. The cumulative error field must not exceed a AB/B of 1 X 10-4 when averaged around the torus.

3. The superconducting windings must be within 7 cm of the vacuum surface.

4. The coil must withstand 2.5 W of x-ray heating at 60 GHz.

5. The magnet quench detection system must differentiate between plasma ring collapse and a critical event.

In addition, the coils were to be capable of operating at a field equivalent to the 90 GHz resonance. The magnetics design meeting these requirements has an inner radius of 24 cm, an outer radius of 34 cm, and a width of 21.6 cm. At 90 GHz, the peak field is 7.4 T and the conductor current density is approximately 12,500 A/cm2.

Conductor Design -The conductor is a copper stabilized niobium titanium rectangular monolith, 0.29 X 0.5 cm. The low copper-to-superconductor ratio of 3:1 was chosen t o maintain a high critical current margin. When operating at 7.4 T, the coil is not cryogenically stable; therefore the excess superconductor serves to raise the tolerance of the conductor to local thermal inputs. The resulting minimum critical current is 2,800 amps at 7.4 T while the operating current is approximately 1,840 A. The conductor is coated with copper oxide to maximize the insulation characteristic and the tran- sient heat flux.2

Winding Configuration

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The entry to the winding is accomplished with a single pancake spiral; the lead entry occurs at the center of Layer 32 (see Fig. 1). The layer portion was wound helically. This winding scheme, developed by ORNL, was chosen to minimize radial error. A General Dynamics study concluded that the radial field error for joggle- style winding exceeds 3 G, while the helical winding chosen for the General Dynamics cold mass will have an error less than 1 G.

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*Research sponsored under General Dynamics Convair Division Contract YOEZOSR with McDonnell Douglas Astronautics Company under Contract 22X-21099C with Union Carbide Corporation under Contract W-7405-eng-26 with the office of Fusion Energy, U.S. Department of Energy.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1984138

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JOURNAL DE PHYSIQUE

It :K

INCOMING ENTRY

PANCAKE SPIRAL .A0

fl

^N ^L ^O ^N ^,LAYER ³²

LAYER 1' \CURRENT.CARRIING ELEMENT 3554.1

Fig. 1. Winding configuration.

Insulation Scheme - The only internal layer-to-layer and turn-to-turn insulation is formed by a barber-pole wrap of woven nomex tape that provides 50% coverage around the conductor. There are five-ply build-ups of thin, (two slotted - one solid - two slotted) G-IOCR sheets between the pancake spiral and the layer winding and over the 316L stainless steel bobbin sidewalls and inner ring. These promote cooling by providing ventilation flow paths and a small plenum of helium against the coil case. The corners of the bobbin and other critical areas are additionally insulated with polyimide (kapton) pressure-sensitive tape.

Cold Mass Supports - The cold mass supports shown in Fig. 2 are made from extra-low interstitial (ELI) grade titanium. The center web of each is pinned and bolted to the cold mass; the ends are match-drilled and bolted on assembly into the dewar. This precision fit is required to prevent any hysterisis in positioning, which could invalidate alignment of the magnets and therefore contribute to the field error. This design results in a very efficient, radiation- resistant, high strength, and low heat-loss support system.

The diamond web shape serves to maximize the thermal conduction path length. Also, when the center web cools and contracts, the diamond is effectively elongated to compensate for shrinkage between the two room-temperature ends. This reduces thermally induced stresses a t the ends and results in a rigid support system capable of carrying an out-of-plane fault load of 73,000 pounds. Peak load is carried by the bottom support. It accounts for 51% of the in- plane load and 34.5% of the out-of-plane load.

Liquid Nitrogen Shield - The LN2 shield shown in the magnet exploded view, Fig. 3, is made from 0.081-cm copper sheet. The attached piping cools the surface by conduction. The shield is split circumferentially to minimize the magnetic forces associated with eddy currents during a rapid discharge. It also thermally intercepts the various conduction paths. Specially designed multi-layer aluminized kapton insulation provides additional thermal shielding. This material meets the radiatipn design life based o n 108 rads.

Vacuum Dewar -The dewar, made from 304L stainless steel, is key-shaped. One side is left open, permitting complete assembly of the cold mass, LN2 shield, supports, and helium stack prior to their insertion (see Fig. 4). Ports for helium, nitrogen, instrumentation, helium vent, and the vapor cooled lead occur near the top of the dewar. The inner ring is spin-formed to minimize spacing and avoid a weld at this critical stress corner. Table I summarizes the calculated heat load of the magnet system for the proposed 90-GHz operating mode.

Winding Process - Except for the production oriented techniques and equipment used to wind the prototype coil, the winding sequence was evolved by O R N L ~ . The insulation winder was designed and manufactured by General Dynamics. A sensor monitored the nomex woven tape application and sounded a n alarm if spacing tolerances were ex- ceeded. The tape wrapping device can accommodate varying conductor feed rates and permits side-loading of the conductor.

A practice winding was performed prior to winding the production prototype. Manufacturing and quality assurance personnel adhered to the very detailed procedure that resulted. Tests were conducted at the completion of each layer and several Hy-Pot insulation resistance tests were performed to verify the absence of shorts between conductor turns and winding layers. A tension load of 175

':,

pounds was maintained during winding to ensure confor- mance to the required 1,214f 8 turns (1,218 actual) and 68:22 i 0 . 3 6 cm coil diameter at the end of layer 31 (achieved diameter within 0.05 cm of nominal). See Fig. 5.

Table I. Steady-state liquid helium heat load per coil (90 GHz).

Refrigeration Liquefaction Load Svstem (vapor cooled leads) X-ray heating 10.0 W

Supports (conduction) 4.25 W

Stack losses 4.23 W 6.4 I/hr

instrumentation wiring 0.30 W Radiation from cold

wall (through MLI) 0.23 W

S~lices 0.03 W

Fig. 21 cold mass supports attached to cold mass. 3554-3

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VAPOR COOLED LEAD LHe LEVEL SENSORS

SUPPORT STRUT

(COPPER) 2 PL 3554.4

Fig. 4. Completed magnet before insertion in dewar.

Fig. 5. Pancake winding in place and - - Fig. 3. The dewar is designed for efficient final assembly. start of layer winding.

Splice Development

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Considerable effort was expended to ensure a reliable conductor splice. Emphasis was placed on refining the manufacturing techniques for a very simple overlap design developed by O R N L ~ that relied totally on a good physical bond between solder and conductor. These techniques were evaluated by subjecting 40 splice samples to tests including fatigue and ultimate load testing at 4K degrees. Significant features of the final procedure include:

1. A 10-degree cut on conductor ends (see Fig. 6). 4. Helium purge on oven, chloride flux, and wire 2. Acid etch cleaning of conductor. brush shim solder.

3. Copper wire spacers (5 mil) between conductors. 5. White gloves and dedicated tools.

Nine additional samples were prepared with the above features, and the test results are shown in Table 11. The splice used in the prototype was subjected to a 535-pound proof load test for added assurance. Two more sample splices were made immediately before and after the coil splice. These were subjected to an ambient temperature ultimate load test and demonstrated separation loads at 1,100; 1,080; 1,070; and 1,075 pounds.

Voltage Taps

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~ n a l ~ s e s 4 revealed that voltage data from five specific locations within the winding were required to enable the quench detection system to identify a true magnet quench condition versus a false signal that would occur when the plasma ring in the torus collapsed. The voltage tap design required:

1. Redundant taps at each location. 4. Protection against physical damage.

2. Three levels of electrical isolation to preclude shorts. 5. Freedom of movement to accommodate thermal ex-

3. Ease of manufacture and test. pansion and contraction.

The design that evolved satisfied the above requirements and is depicted in Fig. 7. The conductor is a multiwrap insulated, 30-gauge, stranded wire soldered to the side of the superconductor and encased in 0.635 cm kapton tape. It is routed to the outside of the winding through 1.27 cm wide channels of solid G-10 material. No problems with the voltage taps were encountered during winding and testing of the prototype magnet.

Prototype Testing - Testing of the EBT-P prototype mirror coil was accomplished in a large open dewar at the ORNL Fusion Energy Division EBT-P Magnet Test Facility. At the onset, the magnet was charged at about 1 A/sec up to the 60-GHz operating point of about 1,220 A. This initial charging was followed by additonal ramp rate testing to 1,220 A at 2 A/sec. The magnet was then manually dumped and subjected to coil heating tests to simulate the effect of x-ray heating in the conductor pack. The heating was produced inductively by imposing a 0.5-Hz sine wave on the base operating current and incrementally increasing its peak amplitude from k 5 A to +20 A. At each step, the heat was applied for a sufficiently long period to allow the coil pack to stabilize. At +20 A, 20 W of heating was produced.

IO.DEG TYPICAL/ /

0.15 MM SOLDER STRIP

(EXPLODED VIEW) 3554.7

Fig.6. 10-degree cuts and 0.13 mm clearance provide optimal joint flexibility and strength, respectively.

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JOURNAL DE PHYSIQUE

Table 11. Stress load test.

Specimen Test Ultimate

No. Temperature Load (Ib) Ambient

Ambient Ambient 4K 4K 4K 4K 4K 4K Fatigue-tested prior to ultimate test.

114 U(. WlDE KAPTON TAPE

LEAD COVER STRANDED WIRE

6.10 COVER

OVER (2) 112 IN. WlDE SLOTS (2) UPPER LAYERS

(LOOSE FIT) SLOTTED INSULATlON

CONDUCTOR (50% NOMEX WRAP)

OUNOANT VOLTAGE TAPS 3554-8

Fig. 7. Voltage taps.

Charging rate tests from the 60-GHz level to the 90-GHz field operating point were then conducted. This was the initial investigation of the ability to achieve peak field of 7.4 T. After achieving peak field without any difficulty, the magnet was held at current for one half hour and then ramped down to the 60-GHz level. Accelerated charging cycles from 60 GHz to 90 GHz were then performed, again increasing the charging rate in 2 A/sec increments. After a manual discharge, the radiation simulation tests were repeated at the 60-GHz level. A maximum heat input of 20 W was achieved without any impact on the coil operation. Reference 5 describes the test results of the first ORNL mirror coil.

Conclusions

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From the above data, it can be concluded that the development and construction of the EBT-P prototype superconducting magnet was highly successful. The tests further verified that meticulous fabrication practices can result in efficient superconductng magnets. When operated at high current densities and high Tesla values, these magnets are suitable for tokamak test devices. The production cost for certain applications can be substantially lower than that of more conventional cryo stable magnet designs.

References

1. Ballou, J.K., et al, IEEE Publication 81CW171522, p 543.

2. Kim, In Kun, Cryogenic Conference ICEC9, Kobe, Japan, 1982.

3. Saunders, J.L., and Lieurance, D.W., IEEE Proceedings, 1982 Applied Superconductivity Conference.

4. Arrendale, H.G., et al, Fifth Topical Meeting on the Technology of Fusion Energy, American Nuclear Society (ANS), April 1983.

5. Lue, J.W., et al, CEC, August 1983.