SYSTEMS ENGINEERING APPROACH TO THE DESIGH OF MAGNETS FOR FUSION DEVICES

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SYSTEMS ENGINEERING APPROACH TO THE DESIGH OF MAGNETS FOR FUSION DEVICES

K. Wakefield

To cite this version:

K. Wakefield. SYSTEMS ENGINEERING APPROACH TO THE DESIGH OF MAGNETS FOR FUSION DEVICES. Journal de Physique Colloques, 1984, 45 (C1), pp.C1-107-C1-110.

�10.1051/jphyscol:1984124�. �jpa-00223679�

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JOURNAL DE PHYSIQUE

Colloque C1, supplement au n o I, Tome 45, janvier 1984 page Cl-111

DEVELOPMENT OF SUPERCONDUCTORS FOR FUSION

P. Komarek

Kernforschungszentrum KarZsruhe, I n s t i t u t fiir T e c h i s c h e Physik, Postfach 3640, 0-7500 KarZsruhe, F.R. G .

RGsurnB - Une vaste gamme de travaux de recherche dans le domaine des bobines supraconductrices pour la fusion est actuellement en cours dans le monde en- tier. Dans la plupart des cas, l'effort se concentre sur des projets spgci- fiques ofi le di?veloppement du conducteur est d'une importance centrale.

LeS trois projets appeles "Large Coil Task - IEA", TI5 (tokamak) et MFTF-B constituent un exemple pour des bobines de grande taille. Un r81e important est attribu6 aux facilitgs de mesure permettant 1'6tude d'importantes lon- gueurs (bobinage) de nouveaux conducteurs sous des cond~tions trSs proches de la rkaliti?. La plus grande activitg est enregistrge dans le domaine des conducteurs 2 haut champs, 2 base d'alliages du type A15.

Abstract - A large spectrum on research effort is running worldwide in this area. It is in most cases devoted to specific magnet projects where the conductor development is a major task. Large scale examples are the IEA-"Large Coil Task", the T15-tokamak and the MFTF-B-coils. An important role play test facilities where up to large lengths (windings) of new conductors can be investigated under most real conditions. The largest part of activities can be observed in the area of high field conductors with A15-compounds.

1. INTRODUCTION

The use of superconducting magnets for fusion devices requires designs providing highest reliability during long-term operation. In this respect the behaviour of the superconducting wire in the winding under the real electromagnetic, mechanic and thermal load conditions plays an important role. Due to the fact, that these conditions are especially severe for fusion magnets, the conductor represents a major subtask of the whole fusion magnet development area. Thereby, it is important to rec- ognize that the overall winding and coil design is strongly linked to that of the conductor, especially concerning cooling and mechanical force transmission.

How diversified the philosophies for solutions are, can be demonstrated by looking at the suggestions developed for INTOR by the four participating groups in the Phase IIA of the workshop period /I/. In Fig. 1 t.he cross section parts required for steel are especially indicated. The question of its distribution between coil case, winding and conductor must be solved based on careful stress analysis and implies different results in the layout for different conductor principles as can be seen from the figure. A further characteristic distinction is based on the cooling mode, where several options are possible, with different implications to winding and coil case design. This question is closely related to that of stability against distur- bances and transient and permanent heat loads, where again different philosophies on conditions to be fulfilled exist, so that finally also from this point of view the existence of a broad variety on specific conductors can be explained.

All the conductors of Fig. 1 are advanced in respect to the present state of art, thus they represent only examples for the kind of goals envisaged by the presently running development effort to be discussed in this review. It might also be a desir- able goal of these developments to reduce finally the variety on options to few accepted and broad applicable economic solutions.

2. DEVELOPMENT GOALS

For magnetic confined fusion plasmas there exist at present several promising con--

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1984124

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EUR EUR USSR J

USA USA USA

Fig. 1: TF-coil cross sections and conductor designs for INTOR in accordance to the national proposals for the Phase IIA-workshop period / I / . Especially indicated are the steel parts.

finement configurations. First of all the tokamak system with a combination of toroidal (TF) and poloidal field (PF) coils. For the TF coils high field strengths

(% 11 T) and mechanical forces are the most severe conditions, while the constraints for pulsing and the large physical size are the burdens for the PF-coils. In Europe, the stellarator plays the role for an alternate option with increasing interests since a modular design became feasible. Here, the coils can be DC-operated again, but their complicated shape requires specific precautions concerning force distribution and fabrication. Of complicated shape are also the Yin-Yang coils in tandem mirror devices, the favoured second confinement system in the US-fusion program. Their field level remains moderate _{( 5}8 T) as well as that of the central cell coils, while the high field barrier coils of solenoidal type with field levels as high as possible

( 2 15 T) will be more ambitious.

Thus, for the conductors a large spectrum of specific development goals exist which are tackled by many different programs in the engaged laboratories. These programs are in most cases aiminq directly in the construction of magnet systems for a specific device, thereby the conductor development is a major subtask, being in other cases also of more fundamental kind aiming so far only for prototype or subsize conductors and their tests.

Fig. 2 summarizes the present goals for fusion magnet conductors and indicates which means are tried to reach the different aims within the development. Major emphasis in recent years has been given to the toroidal field coil conductors due to several facts- First of all, different magnet projects (LCT, Tore Supra, T 15) called for the development of advanced conductors as a first reliable step to implement superconducting coils i.n tokamak systems and secondly the desire or need for higher fields than 8 T triggered the basic development of high field conductors for tokamaks and mirrors.

These strong efforts were the reason that the successful development of large Yin-Yang coils for mirrors (also with an advanced Nb-Ti conductor) perhaps didn't get the at-

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Fig. 2: Goals for fusion magnet conductors and means for their achievement.

GOALS

tention as deserved /2/. Not well treated yet was only the development of PF-coils and their conductors. Other reasons than funding problems might have been that prior to the INTOR design no clear understanding about an ultimate need in the next generation of tokamaks was recognized and that the conductor layout depends strongly on the specifications for the coils which can be very different in specific designs.

Looking into Fig. 2, one common aspect dominating the development can be seen, namely that of ambitious fabrication processes which are required. Fig. 3 tries to summarize this fact. All means listed up in Fig. 2 imply a combination of principal decisions concerning stabilizer, strands etc. The fabrication processes following these decisions might require modifications of the previous decisions before (perhaps only after several iterative steps) a useful conductor can be expected. This proced- ure explains also that the development of a new conductor, even if the basic superconductor material is well known like in case of NbTi, is a time-consuming and expen- sive task.

In any case, the above mentioned nature of the development work requires engaged participation of industry in close cooperation with the laboratories envolved. This principle of joint effort is fulfilled in practically all cases of the developments

MEANS Conductors for TF-coils

- high degree of stability

-

low a.c.-losses concerning PF- and plasma transients

- sufficient mechanical strength

- suitability for field levels much more higher than 8 T

- reasonable fabrication costs

- high ratio of cooling surface to matrix cross section

- resistive eddy current paths by strand insulation or cabling on distance and well chosen distributions of the stabilizer

- steel reinforcement, strong matrix - ternary alloying of NbTi for optimized

critical currents at 1'8 K, or

- use of A15-compounds with metallurgical optimization of the material concerning current density versus field and prestrain due to matrix and reinforcement.

-

High current cables for PF-coils - minimal a.c.-losses

- high current carrying capacity ( a ^{50 k ~ )}

- sufficient mechanical strength - reasonable costs

NbTi-conductors with

-

multistage cabling - resistive matrix barriers - resistive strand.surface

- high ratio of cooling surface to matrix cross section

- encapsulation in steel or internal reinforcement.

Conductors for d.c.-operated mirror coils (Yin Yang and central cell coils)

- high degree on stability

-

sufficient mechanical strength - low fabrication costs

- technology developed now

-

Conductors for d.c.-operated mirror barrier coils - high current density up to

2 15 T/4 K

-

sufficient mechanical strength - high degree on stability

- metallurgical optimized A15-material - strong mechanical reinforcement - Al-stabilizer

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JOURNAL DE PHYSIQUE

material-

matrix amount

Fig. 3: Decisions and processes to be combined for design and fabrication of superconductors for fusion magnets

reviewed in the following chapters.

3. CONDUCTOR DEVELOPMENT PROJECTS

3.1 Advanced NbTi-conductors for coils in steady state operation mode

...

The advanced status of fabrication processes for sophisticated NbTi-conductors has been demonstrated in the recent years by the manufacturing of the different conductors for the "Large Coil Task" (LCT) and that for the MFTF-Yin-Yang coils. While all LCT-coils have to meet the same general magnet specifications, the solutions for conductor design are very different. This is due to the fact that the several designers gave different priorities to the constraints on the conductors, e.g. stability, mechanical integrity (also of the winding), electrical insulation capability, a.c.- loss performance concerning PF-transients, and fabrication difficulties as well as costs. Of major influence is the compromise between stability margin including the decLsion on the cooling mode and the a.c.-loss minimization. In Table 1 the different designbasises arecompared using parameters as introduced in /3/. In all cases the chosen rated current is well below the critical one at fixed field and temperature

(CD

<

0.75). However, very different values have been accepted for the stability margin, given by the current safety function, if one compares val-ues for an often used energy input level of 100mJ/cm\ Some designers want to be conservatively safe, toward full cryogenic stability, yielding CS-values far below one, others feel confi- dent that such a disturbance level will never happen, so that their CS-values for 100m~/cm' are larger than one. As long as no clear possibilities for the prediction of disturbance levels for a certain design exist, both ways are not optimal, one may be too risky and the other may be too conservative, thus not economic. Hopefully the operation of the LCT-coils can provide information also in this respect. Only based on that, a further optimization of such conductors, which have sufficiently well advanced state of the art now, seems to be useful.

Tokamaks require superconducting PF-coils whose currents and fields change intime regardless of the length of theplasma burn time finally achievable. In EF-coils e.g..

the currents must be programmed in time as part of the startup cycle and to maintain

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' ) Values not reported at B or T

max nominal are converted, using the formulars from /10/.

-

LCT/JAERI 141

LCT/GD 151 LCT/GE I61 LCT/CH 171 LCT/

EURATOM /8j MFTF- Yin Yang 191

-

'TABLE 1: Comparison of the different design philosophies for the LCT- and MFTF-Yin- Yang coils, based on the two factors CD = Iop/Ic and CS = Iop/~quench(E) as introduced in /3/.

ck A]

I quench(Equench) at B = 8 T 12,5/100m~cm-~

or 10,22/1~cm-~

10,2/100m~cm-~

10,6~/37mJcm-~

13,0/10m~cm-~

11,0/150mJcm-~

cold end Iop* 'recovery

the plasma at equilibrium in the desired position with the correct shape.

For several years only a program running at the LANL was present /11/, now also at JAERI /12/ and recently within Euratom development tasks are in procress. In comparison with the large variety of'conductors for TF-coils the design o2inions are rather uniform here. Pirst of all, the conductor material NbTi has been considered as sufficient so far, because the field level can be kept in the order of 8 T or less. Per- haps stability considerations can indicate the desire for the large temperature re- serve of A15-compounds, but due to their other disadvantages they are not yet considered seriously.Secondly only two main conductor opinions are investigated so far.One is a flat cable for pool boiling anb the other one a forced flow cable in conduitcon- ductor, similar to that for TF-coils. Of course, large emphasize is given on minimization of a.c.-losses by multi-stage cabling processes and several resisitive barriers, mainly CuNi sheets. For the support of the large hoop forces, especially in the large diameter, EF-coils, steel reinforcement, either in form of a core or as an additional U-profile outside the conductor, is foreseen.

The status of the work reported in /12/ indicates that no insurmountable fabrication problems for such conductors exist, but an answer on full size performance tests ,concerning the electrical behaviour is still open. Here the Japanese program is aim-

ing a 20 MJ-prototype coil for tests as it was earlier foreseen in the US-program, too. Within the Euratom Program a task started now jointly by CEA and KfK, aims for coils applicable for Tore Supra and Asdex-U in the late 80's.

I

cs=

OP I quench (E) for

~ = 1 0 0 m J c m - ~ 0,82

1ro

>

¹

>

¹

C 1

<

¹

3.3 High field conductors for TF-coiLz

This field has received major attention in the last years, maybe due to the fact that not only the tailoring of a sophisticated composite with a given superconductinq alloy is requested, even more, the metallurgical investigation and optimization of the superconductor itself and its most suitable fabrication process are still develop ment tasks. This caused many groups carrying out more fundamental investigations on superconductors to become involved, too. All groups are working toward conductors for machines of the "INTOR-generation" for which examples have been introduced by Fig. 1.

The major near term tasks in the different countries can be summarized as

- Euratom: Prototype conductors for NET via tests and use of forerunners in test facilities like "Sultan" /13/ and "HOMER" /14/;

CD = Iop/Ic 0,52

0,76 0,67 0,40

0,50

0,72 BA]

Ic at Bmax Tnominal

19,7

13,3 16,O 33,O

22,O

8,O

r k A l I OP at B~~~

Tnom 10,22

10,2 10,65 13,O

11,O

5,775

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- Japan: Conductors for test module coils (TMC) I and I1 of the JAERI-Cluster test facility

/Is/;

- USA: Conductors for the 12 T coil program with the "High Field Test Facility"

(HFTF) /16,17/;

- USSR: Conductors for the T15-tokamak /18/.

Fig. 4 shows the principle design of conductors under development within the above mentioned programs. From these types the TMC I-conductor has already been used suc- cessfully /19/ and-the MIT-conductor represents an upgrade of the ~7estinghouse/Airco- .type already fabricated for use in the !Jestinghouse LCT-coil. All other types are in different earlier stages of development.

The development contains several major areas, mainly:

- metallurgical optimization of the superconductor;

- integration of the stabilizer in a way with minimal a.c.-losses and optimal cooled surface;

- mechanical reinforcement by steel or strong alloys.

Concerning the first topic, a large effort is being spent worldwide on A15-conductors, mainly Nb,Sn. But also ternary alloyed l3bTi (with Ta or Hf) together with subcooling to superfluid helium temperature (?. 1,8 K ) is investigated seriously as promising alternative. About the cryogenic implications of this alternative, the Tore Supra project will give information.

TMC I TMC I1 MlT G A

Ic ( 6 . T ) l O k A l l o T i L . 2 K 18 k A I I Z T I C Z K 12.5 k A 1 11.5TILK 10 kA 111.5 T11.8K

SULTAN -SIN SULTAN

-

ECN SULTAN -ENEA T 15

KfK-HOMER I Kf K- HOMER

n

KfK-HOMER /SULTAN-Test

0

Cu @ He MO

a

SS 0 N b S n

clodded with Nb-or Cu alloy

L

(6.T) 1.5kAI12TIL.ZK 0,65!4HSTl&.ZK 2 0 k A I l Z T 1 1 . 2 K

Pig. 4: Conductors presently under development toward high field ~~-coils/13,14,15, 16,17,18/.

It is practically impossible to discuss within this short review the development effort on A15-compounds. Concerning the fusion conductors, as introduced by Fig. 1 and 4, most important outcome of the effort should be

- minimized current density degradation due to prestrain,

- cost minimizing fabrication route (beside the "classical" bronze route many other processes like e.g. powder techniques or the modified jelly roll technique look promising, too. )

.

Of course, these problem areas are interconnected as discussed in chapter 2.

Fig.,5 indicates the "hot core" of present discussions and metallurgical investigations. Summarized is in the left part of the figure the jc versus B characteristic of Nb,Sn-conductors fabricated by different methods which are commercial or look promising for commercial use later on. It can be seen, that an important step forward, independent of the process will result from ternary or quaternary alloying. This be- comes especially obvious if one looks at the diagram of Fig. 5, where the stress induced current degradation with and without ternary alloying is shown for bronze route fabricated conductors as example /29/. Due to the fact that steel reinforcement and

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even the stabilizer increase the prestress on the filaments if they are incorporated before reaction, which is necessary at least partially, this kind of behaviour is of great importance for an optimal conductor design. Thus, it is not surprising that so many investigations are devoted to this area. As major progress can be summarized that the physical relations concerning the stress induced degradation could be clari- fied in the recent years and that means (e.g. alloying) are identified to reduce the negative effects / 3 0 / .

3.4 Conductors for d.c. operated mirror barrier coils

...

In this area, the use of AIS-conductors is imperative, due to the very high field level envisaged. With regard to the discussions in the previous chapter, some problems are similar, but the emphasis is different as already mentioned in Fig. 2. Major means, based on present knowledge, will be ternary and quaternary alloying of the A15-compound to optimize the current density at high field, stabilization with A1 and reinforcement similar to that discussed above. The overall layout can be much simpler than for tokamak conductors due to the pure d.c. operation mode. Fi.rst applied development steps have been started for the barrier coils in MFTF-B and possible upgrades / 2 / .

4. CONDUCTOR TEST FACILITIES

During the development phases the testing of conductor parts, subsize and fullsize conductors, is a major task. Beside the evident measurement of the short sample characteristic, testing of following performances is necessary:

- mechanical behaviour concerning tensile strength

-

fatigue limits

current degradation due to mechanical stresses

-

bending characteristic

- electromagnetic behaviour concerning

-

temperature dependence of current density in the requested field regions

-

stability behaviour in the selected cooling mode

-

a.c.-losses for the given B- and B-levels

- integral electromagnetic and mechanical tests by trial windings of the conductors to be investigated.

Thus, a broad variety on test facilities, most of them including superconducting magnets too, is needed for getting reliable data.

Concerning the mechanical testing possibilities, it will become more and more common to combine high field solenoids with a tensile frame to measure the characteristic jc (B, E) of large conductors. Such facilities exist so far only in few places (LLNL, KfK). The investigation of fatigue behaviour, underestimated so far, will be nottriv- ial to predict for the envisaged complicated conductor composites. It will be of great importance, especially for the pulsed PF-coils.

Concerning the electromagnetic behaviour, special arrangements are needed, so that finally in many cases the direct test of trial windings or test coils is preferable.

For that purpose, several medium and large size facilities have been constructed in recent years. They are summarized in Table 2 /31,32,13,12,33,34/. For high field conductor tests in form of trial windings several solenoidal systems and a cluster test facility providing a sufficiently high background field could be brought into operation. Their construction represents already a development task itself for high field technology. Forpulsable conductors only few facilities exist so far, reflecting the smaller emphasis given to that development until now.

5. SUMMARY

The fusion conductor developments represent a major task in the superconductivity programs worldwide. Major emphasis is given to high field conductors, mainly on A15- basis. The nature of the development work requires close cooperation of the laboratories with industry, because sophisticated fabrication techniques play an important role. Test facilities are in operation or under construction which provide sufficient confidence for experimental judgement of the development of subsize and full size con- conductors prior to the construction of large coils.

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0 Nb-0.6NiICu-16 Sn-3 Zn 800" C120h

v Nb-1.6 Ti/Cu-13 Sn 800°C/20 h

+ NbICu-13Sn Irnultifilarnentl 70O0C164 t (composition In wt % )

Fig. 5:

Summary on high field conductor development concerning achievable current densities jc, versus magnetic field B and/or mechanical strain &

/29/.

1. NbTil-Ta (or Hf) at 1.8 K /20/

2. Nb3Sn-ECN-powder-method /21/

3. Nb3Sn- bronze technique + Ti at core and matrix or PIi/zn /27,28,29/

4. ~b,Sn-internal tin diffusion /24,25/

5. Nb,Sn-modified Jelly Roll technique /23/

6. Nb,Sn-external tin diffusion /26/

7. Nb,Al-powder metallurgy /22/

6. ACKNOWLEDGEMENT

The author wishes to thank Dr. R. Flukiger for many valuable discussions.

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TABLE 2: Survey on fusion conductor test facilities (in operation or under construction) /12,13,31,32,33,34/.

7. LITERATURE

1 INTOR Phase IIA-Report, to be published by IAEA, 1983

2 KOZMAN T. A. et al, IEEE-Trans. on Magn., Vol. MAG-=, (1983) 859 3 KOMAREK P., IIR-Refrigeration Science and Technology 1981-6 (1981) 5 4 SHIMAMOTO S. et al, IEEE 79CH1441-5 NPS (1979) 1174

5 HACKLEY D. S., KRUSE G. S., ~ O C . cit. /4/ 1163 6 QUAY R. et al, Loc. cit. /4/ 1154

7 BENZ H. et al, IEEE-Trans. on Magn., Vol. M A G - 2 (1981) 2213 8 SCHMIDT C., loc. cit. /2/ 707

9 HENNING C. D., UCRL-52955 (1980) 10 LUBELL M. S., loc. cit. / 2/ 754 11 ROGERS J. D., LA-9755-PR (1983)

12 SHIMAMOTO S. et al, ANS 5th Top. Meet. on Techn. of Fus. Energy, Knoxville 1983 13 HORVATH I. et al, paper 4P2-13, this conference

14 TUROWSKI P., Fliikiger R., private information 15 SHIMAMOTO S., private information

16 HOENIG M. 0. et al, IEEE-Trans. on Magn., Vol MAG-= (1981) 638 17 ALCORN J. S. et al, loc. cit. /16/ 642

18 CHERNOPLEKOV N. A., loc. cit. /7/ 2158 19 AND0 T. et al, loc. cit. /2/ 312 20 WAKE M. et. al, loc. cit. /2/ 552

21 von WEES A. C. A. et al, loc. cit. /2/ 556 22 THIEME C. L. M. et al, loc. cit. /2/ 567 23 McDONALD W. K., et al, loc. cit. /2/ 1124 24 YOSHIZAKI K. et al, loc. cit. /2/ 1131 25 SCHWALL R. E. et al, loc. cit. /2/ 1135 26 COGAN S. F., loc. cit. /2/ 1139

27 SEKINE M. et al, loc. cit. /2/ 1429 28 KAMATA K., loc. cit. /2/ 1433

29 FLUKIGER R. et al, to be published ICMC 1983 30 FLUKIGER R. et al, loc. cit. /2/ 1441 31 ZBASNIK J. P. et al, loc. cit. /7/ 2226 32 SHIMAMOTO S. et al, loc. cit. /7/ 2230

33 KIM S. H., KRIEGER C. I., McGHEE D. G. loc. cit. /2/ 346 34 TUROWSKI P., KfK 3368 (1982)

Dimensions for TestLoops

150 cm diam.

100 cm diam.

40 cm bore 100 cm bore 100 cm bore 60 cm bore 40 cm bore 30 cm bore 30 cm bore

80 cm bore

45 cm bore 20 cm bore 38 cm bore 25 cm bore Maximal Field

for Tests

10 T 12 T 11.4 T

8 T 8 T 12 T 8 T 10 T 12 T

8 - 9 T

6.5 T, 6 T/S 6.7 T, 10 T/s

10T

+ 0'4T at 4 0 T / s

3

Year of first Operation

1982 1984 1983 1981 1983 1986 1982 1983 1984

?

1980 1982 1979 1984 Type

Cluster ar- rangement of circular coils Split pair solenoids Split pair solenoids Split pair solenoids Solenoidal or 'luster ar- rangement of T15-coils

"lit pair solenoids Solenoid Solenoid Facility

CTF

HFTF SULTAN HOMER

TI5 - SMS

PCTF Pulser C+D

T2S

Location

JAERI

LLNL SIN KfK

Kurchatov Institute ANL JAERI

- - -

(IEN- Saclay