Experimental results of domestic testing of the pool-boiling cooled Japanese and the forced-flow cooled Euratom LCT coils

Experimental results of domestic testing of the pool-boiling cooled Japanese and the forced-flow cooled Euratom LCT coils H. Tsuji, S. Shimamoto, A. Ulbricht*, P. Komarek*, H. Katheder*, F. WiJchner* and G. Zahn* Japan Atomic Energy Research Institute, Naka Fusion Research Mukaiyama, Naka-machi, Naka-gun, Ibaraki-ken, 3 1 1 - 0 2 Japan *Karlsruhe Nuclear Research Centre, D - 7 5 0 0 Karlsruhe, FRG

Establishment,

Institute for Technical Physics, PO Box 3640,

Received 16 April 198& revised3 June 1985 The Large Coil Task (LCT), an international project of technology development under the auspices of the lEA, has been performed to develop large superconducting toiroidal coils for tokamaks. The participants are the USA, Switzerland, Euratom and Japan. Among the six coils being developed under this programme, domestic single coil tests of the pool-boiling cooled Japanese coil and of the forced-flow cooled Euratom coil were carried out successfully in June 1 982 and in April 1 984, respectively. These two LCT coils are the first to compare experimentallythe characteristics of pool-boiling cooled and forced-flow cooled large coils for a tokamak. Major results obtained in the two domestic tests are described to compare the two designs. Keywords: superconducting magnets; LCT coils; magnets

Design work on fusion reactors has clearly shown thc neccssity of largc superconducting magnets lot tokamak reactors. Therefore, under the auspices of the IEA a contract was signed in 1977 l~r a programme of research and development on superconducting magnets for fusion power( Large Coil Task. LCT) by four parties, USA. Japan, Euratom and Switzerland. The US, acting as an operating agent, built and is now operating the Large Coil Test Facility (LCTF) in which six superconducting magnets can be tested in a toroidal arrangement Three magnets were manufactured by [JS industry, three further magnets were developed and delivered by the other participants, Japan, Euratom and Switzerhmd. Five magnets use NbTi as the superconducting material and one uses Nb3Sn. Three magnets operate in a pool-boiling mode, three in a forced-flow cooled mode. The Japanese LCT coil belongs to the first group, the Euratom LCT coil to the second group. Both magnets were tested in domestic test facilities before they were installed into the LCTF. A discussion comparing the results obtained in these tests is the subject of this Paper.

Features of the coils Thc design features of the pool-boiling cooled Japanese (JA) coil and the forced-flow cooled Euratom (EU) coil, developed according to the same specificationsL are different in the way lhc final goal is approached. The guiding principle in the EU coil design was to find a way of setting up a mechanically rigid, electrically predictable coil, and for the JA coil the guiding principle was to lind a way of getting a cryogenically stable coil. As a resuh of both dcsigns carried out at the Kernlk)rschungszenlrum Karlsruhe (KfK) in collaboration with German industu 0011-2275/85/100539-13 $03.00 © 1985 Butterworth8 Co (Publishers) Ltd

(Siemens at Erlangen and Vacuumschmelze at Hanau) and the Japan Atomic Energy' Research Institute (JAERI) in collaboration with a Japanese company {Hitachi Lid at Hitachi), parameters for each coil were fixed as listed in Table 1. In terms of magnet design both coils have similar characteristics, such as the number of Ampere turns and winding current density. Major differences of the design principles arc forcedflow cooling lot the EU coil and pool-boiling cooling IBr the JA coil. Figure 1 shows both LCT coils during installation in the domestic test facilities. In the Euratom design supercritical helium is maintained at a subcooling temperature (3.8 K) at the inlet to ensure the critical current margin of the NbTi forced flow cooled conductor. The wetted perimeter was optimized to provide the highest possible stability for a given copper cross-section, The operating inlet pressure can be varied from 6 to 15 bars* with a flow rate of 300 g s -~. The maximu m allowable pressure was 25 bars. To decrease thc pressure drop during cooldown and operation, the cooling channel length was shortened by using a double pancake winding with two conductors wound in handL According to the Japanese design the aim was to achieve at least cryogenic stability with cold end recovery'. For this purpose, a specially treated roughened surfacc was developed for the JA conductor. To set high heat transfer characteristics (Figure 2}, 3-4 times higher than that of a fiat surface, the JA coil achieved full cryostability3,4. The cross-sectional views of the JA and the EU conductors are shown in Figure 3. A comparison of the

• 1 bar= 100 kPa

Cryogenics 1 9 8 5 Vol 25 October

539

Domestic testing of LeT coils." H. Tsuji et al. Table 1

Major features of Japanese and Euratom LCT coils JA-LCT

Basic specification Maximum magnetic field Coil external size Similar features Superconductor Rated current Number of turns Winding

EU-LCT

8.0 T 4.6 r e x 3 . 6 m

8.0 T 4.6 m x 3 . 6 m

Nb-Ti 10.22 kA 658 20 double pancakes

Nb-Ti 11.4 kA

Self-inductance Structural material

24.2 M A m -2 (8T grade) 30.3 MA m -2 ( 5 T grade) 2.0 H a t 4 . 2 K SS 304 LN

Assembly Winding weight Total weight

Bolts with seal welding 18 000 kg 39 000 kg

Winding current density

Different features Coil cooling Operating temperature Operating pressure Test pressure Helium inventory Conductor position in pancake Grading Number of double pancakes Number of cooling paths Current density in conductor copper Joule heating in copper at 8 T and rated current Designed dump voltage Test voltage

Pool cooling 4.2 K 1.2-1.5 bar 8.8 bar 2200 dm 3 Edgewise 2 (8 T, 5 T) 20 1 winding, 1 case 44 MA m-2 (8 T grade) 52 MA m-2 (5 T grade) 231 W m-1 1.0 kV 3.0 kV, 300 K He

compositions of the two windings shows (Figure4) nearly the same area fractions of NbTi and helium. The major difference is that the pool-boiling cooled JA winding has a large fraction (54%) of copper, while the EU winding shares 30% of this area with the stainless steel conduit and the Vacromium centre tape. The result is higher cryogenic stability of the pool-boiling cooled coil due to the lower Joule heat generation in the stabilizing copper and a higher mechanical rigidity of the forced-flow cooled coil by appropriate impregnation of the winding pack Alternatively the pool-boiling cooled coils can be discharged with a longer time constant (20 s for the JA coil) than that of forced-flow cooled coils (7 s for the EU coil). This fact also leads to lower discharge voltages for pool-boiling cooled coils ( 1 kV for the JA coil) than those for forced-flow cooled coils (2.5 kV for the EU coil).

Cryogenic systems to be used in the domestic coil tests For testing of the coils, both JAERI and KfK constructed their own domestic test facilities, SETF and TOSKA, respectively. The helium cryogenic system of SETF is composed of a 1.2 kW, 350 dm 3 h -1 refrigerato?. The cryogenic system of TOSKA uses two existing refrigerators(Linde550 W, equivalent to 150 dm 3 h -1 and MesserGriesheim 400 W, equivalent to 85 dm 3 h -1) and a cold helium pump to generate the required mass flow6. Flow schematics of the cryogenic supply systems for the coils are shown in Figure 5. For initial cooling from 300 K, both coils are directly connected with the helium cryogenic system by the supply and return helium gas flows. After the JA coil was cooled down, liquid helium stored in a 5000 dm 3 dewar was used to fill the coil with liquid helium. Then, the liquid level in

540

Cryogenics 1 9 8 5 Vol 2 5 O c t o b e r

588 7 double pancakes (two conductors wound in hand) 25 MA m -2 1.59 H at 4.8 K DIN 1.4429, similar SS 316 LN Bolts with seal welding 18 000 kg 39 000 kg Forced cooling 3.8 K 6-15 bar 25 bar 630 dm 3 Flat-wise No grading 7 28 windings, 2 case 83 MA m-2 5 1 8 W m -~ 2.5 kV 10.0 k V a t 5 K

the coil was controlled and sustained by the refrigeration mode of the helium cryogenic system. For the EU coil, also after cooldown, a 3-piston helium pump with heat exchangers immersed in a 3000 dm 3 liquid helium dewar was used to circulate supercritical helium through the forced-flow cooled coil in a closed secondary loop. In case of a dump in TOSKA, helium can be stored in a cold buffer tank (capacity 1.85 m 3) up to a pressure of 18 bars. With this method the dump gas quantities have been handled without helium losses.

Initial checkouts In initial checkouts, the vacuum tightness and the high voltage characteristics of the two coils were measured. A summary of the results is given in Table2. Neither of the coils showed cold leaks during cooldown. The leak rates were comparable and reproducible throughout the tests. The excellent electrical insulation properties of the forced-flow cooled coil are impressive. This property will be important for the operation of large superconducting magnet systems. Fast discharging of superconducting magnets electrically connected in series requires a higher discharge voltage as if every magnet has its own dumping system. More vapour-cooled leads are necessary. Therefore, a higher discharge voltage reduces the number of vapour-cooled leads, a considerable source of cryogenic losses.

Cooldown behaviour The major results of the cooldown tests of the JA and the EU coils are summarized in Tabh, 3 and Fig. 6. The total flow rates through the winding and coil case were almost the same for both coils: however, the distributions of flow

Domestic testing of LCT coils." H. Tsuji

Figure l b Germany

Euratom

LCT coil at KfK,

Federal

et al.

Republic of

thcrmal resistance to the winding- Therefore, the warmer parts of coil case corners are not critical lot cooldown and operation if the thermal stresses remain within their acceptable lilnits. As lbr temperature c o n t r o l the guideline for the m a x i m u m telnperature difference in the whole coil was 9(~ K in the case of the JA coil. The inlet temperature of the hclium gas v, as c o m p u t e r controlled~, based on the maximun~ tenlperature in the coil, and the m a x i m u m actual temperature difference was 95 K, By this active c o o l i n g the ,IA coil had been cooled down within 120 h I

I

)

I

I''

J

,

,

,

,

,

,

2

4

6

8

I0

12

14

.

1.4

Figure 1 a Japanese LCT coil at JAERI, Japan

on tile hydraulic paths were different. In the case of the JA coil. the pressure drop in thc coil was negligible and a helium flow of34 g s -~ was possible in tile JA w i n d i n g at a m a x i m u m pressure of 1.5 bar from the b c g i n n i n g of cooldown. A mass flow of only 6 g s -~ in the structural cooling circuit was sufficient, because the coil case was strongly cooled from the inside surface by the helium flow in the winding. The helium mass flow rate in tile w i n d i n g of the EU coil was limited by the prssure drop of the flow' channels. A typical flow rate in the w i n d i n g was 18 g s -~ and the m a x i m u m pressure drop was 8 bar at the b e g i n n i n g of cooldown. The pressure drop characteristic, as a tunction of mass flow. ~as the same as calculatedv. A mass flow of 18 g s - ' , which was three times its much its that of the JA coil case, was provided for the cooling c h a n n e l s of the structure. The area cooled was only ~30% of that of the JA coil case. Cooling just the outside surface of the case by tile i n n e r c h a n n e l s provides sufficicnt

I 2

! 0 6

o 4

021 o

i/,

o

Tconducto r - TL.He (deg)

Figure2 coil

Heat transfer from the conductor surface of theJA-LCT

Cryogenics 1985 Vol 25 October

541

Domestic testing of LCT coils." t4. Tsuji et al. Cu stabilizer

i

NbTi-Cu strand (23 mm diameter)

- -

IV, bZ.6mm I

a

surface (I.2 mm height)

--Roughened

Solder --Cu plug(4.2 x 4.9ram 2)

German Code 1.4429 conduit (0.8 mmt) ~Vocromium (CrNi SS) spacer (0.6 mint)

from the refrigerator. The result was a loss of 106 W at the rated current of 10 kA. Design values for the heat load at 10 kA were 7 W radiation, 31 W conduction, 11 W Joule heat generation at the internal joints, a n d 27 W from the current leads, 76 W in total. The m e a s u r e d value, 106 W, was ~ 3 0 W higher than this estimate. This difference is c o n s i d e r e d to be caused by a higher c o n s u m p t i o n due to radiation a n d some a d d i t i o n a l heat input from the current leads. In the case of the EU coil the system, having reached a helium inlet t e m p e r a t u r e of~---'8K a n d a case t e m p e r a t u r e ioo -

NbTi O" O

.~;~-~i.i~"r~ [~ i.~.2.i.~i'~ ~..~ ~ - . - . .

vo cromium cor e

-

I

.,th Kapto° ,o,,

L

40 0 mm

~.J \

in the mid-plane

NbTi

7.4%

Copper

Copper 80-

6.5%

31.0%

53.8%

b 60-

Insulator 10.1% Helium 21.3 %

4.0Solder etc. 7.6% Insulator 7.1% 20-

Figure 3 Schematic cross-section of (a) the JA-LCT (8T grade) and (b) the EU-LCT conductors. View of (c) the JA-LCT and (d) the EU-LCT

0

I

SS CrNi 30.1%

Helium 24.1%

_

a

b

Figure 4

which was one o f the objectives of the experiment, In the case o f the EU coil, the m a x i m u m a l l o w a b l e t e m p e r a t u r e difference within the coil was limited to 50 K as specified by the coil manufacturer. To have enough margin in case of failure of the computer, the m a x i m u m a d m i s s i b l e t e m p e r a t u r e difference was reduced to40 K, After 140 h o f cooldown, t e m p e r a t u r e sensors at the u n c o o l e d connection plates between both halves of the case prevented further t e m p e r a t u r e decrease. The c o m p u t e r control was m o d i f i e d to ignore high t e m p e r a t u r e readings at the c o r n e r part o f the coil case. This increased the cooling rate and 203 h after the start of cooling the w i n d i n g b e c a m e superconducting. Steady-state

Vacuum tank

i a

a

i

heat load

Alter c o o l d o w n o f the coil liquid helium was transferred from the 5000 d m 3 d e w a r in the case o f the JA coil. At the b e g i n n i n g o f this transfer, the m a x i m u m t e m p e r a t u r e o f the coil w a s ~ 2 0 K. F o r the first transfer, 2530 d m 3 helium were transferred at a flow rate of 720 d m 3 h - l : consequently, 1200 dm ~ remained in the coiL which resulted in an efficiency of 47%. D u r i n g the experiment, three other transfers of liquid helium were carried out a n d the efficiency in these cases varied from 56 to 71% because the coil was completely cold, T h e steady-state heat load of the JA coil was m e a s u r e d when the coil was d i s c o n n e c t e d

542

Composition of JA-LCT and EU-LCT windings. (a) Poolboiling cooled JA-LCT (8T grade); (b) forced-flow cooled EU-LCT (no grade)

Cryogenics 1985 Vo125 October

Cold

I&ir

20K ]SHE

Vacuum tank

Winr---;ing

i

I

b Figure5 Crgogenicflowsheetsfordomestictestingofthe(a) poolboiling cooled JA--LCT and the (b) forced flow cooled EU-LCT

coils

Domestic testing of LCT coils. H. Tsuji Table 2

e t al.

Initial checkouts of JA and EU coils. Determination of leak rates and electrical insulation properties

Test

Boundary conditions

Coil

Temperature (K)

Leak rate

Coil + all components in insulation vacuum

JA EU

300 300

JA EU Coil alone

High voltage

4.2 10

JA EU

Winding + vapour cooled leads + s.c. bus against coil case

Pressurea (bar) VCL+B coil

300 300

JA EU

20 5~

Value measured

1.8 1.3

1.8 6.5

2 x 10 -7 mbar dm 3 s-1 6 x 10 -7 mbar dm 3 s-1

1.25

9.5

No cold leak 8 x 10 4 mbar dm 3 s-~

---

-6.5

-5 x 10 -8 mbar dm

1.2 1.2

1.2 6

d.c. 3 KV, 25 M D d.c. 7 KV, 3 Gf~ d.c. 10 KV, 2.5 GFt

S-1

~(VCL + B), Vapour cooled leads + s.c. bus bVCL + B were filled with LHe 4.2 K

o f ~ 4 0 K, was switched tar cooling using the pump. Alter 30 h the final steady-state c o n d i t i o n was attained. Typical t e m p e r a t u r e values u n d e r this c o n d i t i o n were 4.8 K for the w i n d i n g a n d 6.5 K for the case with the warmest t e m p e r a t u r e of 45 K m e a s u r e d at the h o r i z o n t a l bottom plate of the case. This area received direct radiation from the 300 K outside wall. At the b e g i n n i n g of the EU tests, He oscillations were observed in at least one of the cold return gas lines (from the storage dewar and the current leads, respectively). These oscillations caused high -cryogenic losses. With full refrigeration capacity (600 W) a depletion of up to 50 dm 3 in the storage dewar was observed. However, it was possible to correct this situation and to run the system without oscillations. Typical values of o p e r a t i o n are shown in Fig,re 7. E x t e n d e d i n s t r u m e n t a t i o n was installed tar reliable a n d safe o p e r a t i o n o f the coil. Only two out of 305 sensors failed d u r i n g the tests. Not all sensors were positioned such that an accurate calculation of the t h e r m o d y n a m i c p e r f o r m a n c e was possible in some cases. It was possible to operate the coil at much lower mass flow rates than originally estimated ( ~ 60 g s - ' instead of l(X)-IS0 g s 1 and 300 g s-L respectively, at T O S K A and the O a k Table 3

Ridge N a t i o n a l L a b o r a t o u , ORNL). Therefore, the helium p u m p operated with low efficiency ,7 and the orifices for mass flow m e a s u r e m e n t with low pressure p and thus reduced accuracy. F o r these reasons only a general t h e r m o d y n a m i c analysis could bc p e r l o r m e & which does not allow us to indicate individual loss values for each componenC The losses of the coil itself were m e a s u r e d with a d e q u a t e accuracy. 1"he values are given in F&,,re 7. The static losses of the EU coil were measurcd from 115 to 127 W d e p e n d i n g on the o p e r a t i o n p a r a m e t e r s used. Loss sources were estimated its lbllows: residual radiation, 74 W: residual c o n d u c t i o n by support structure, 6 W: other 30o ~ •"a u

\\'~'~

\\\',,~..

-

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\\\

I L ..J ,~ 7 - - Temperature of

"-.,Q,

\JAfIow~"\

,

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,oo

. . . . . Helium flow rate ~"l Maximum temper, I [ -ature of winding

("

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co,,

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30

EU carl' t e m p e r a t u r e

-

....

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coitco.,dge

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IO

=

Cooldown properties of the two coils 0 JA-LCT

EU-LCT

Figure 6 34 g s-1 6 g s-1

C>0,~7g S-I~ Current

70W

leads

96 96 95 95

K K K K

_< 4 0 _< 4 0 40 80

K K K K~

t.He .-~aI0 w ~ =con-

4.72 K K I Winding 4.72

.. " s t o n f ~

I

~ 1.5 bar --

12 bar 8 bar

18 May, 1982 120 h

15 March, 1984 203 h

Cooldown time Starting date 3 0 0 K to s.c.

o

200

P BSW,a

[ Linde refrigerator I

Helium pressure Maximum inlet pressure Maximum pressure drop

150

18 g s-1 18 g s-1

Temperature difference Guiding value within winding Winding coil case Maximum within winding Winding coil case

I00

Cooldown time (h) Results of cooldown of JA-LCT and EU-LCT coils

Helium flow rate Winding (typical) Coil case (typical)

50

5

I gr "l

14.72K Coil case

bar K

[

20g s-~

115W 60 g sq Vacuum tank

3000- liter Dewar

q-o increase the rate of cooldown temperature sensors in the poorly cooled coil case edges were removed from the 4 0 K condition in agreement with the coil manufacturer

~K

40go-'

rpump, coil= 410W Q

totol'~lines-

= 570W

Lcurrent leads = 85 * 70W 1

Figure 7 Thermal balance during 10 kA operation (indicated values are average of some 10 kA operations)

Cryogenics 1985

Vol 25 October

543

Domestic testing of LCT coils." H. Tsuji

e t al.

losses were negligible. The amount of unexpected losses is about the same as for the JA facility. One significant result of this experiment was that 91% of the heat load was removed by cooling of the coil case, where a relatively high outlet temperature was acceptable. The heat load of the winding was as small as 12 W. This value confirmed the high thermal resistance between the coil case surface and the winding mentioned before. Therefore, coil case losses and the coil case temperature distribution had very little influence on the operation of the superconducting winding

Flow control of the EU coil Fourteen control valves were attached at each inlet of the EU coil winding (Figure8) to keep a balance between the flow rates through different channels. Most of the channels have a length of 245 m, but four channels in the two outside pancakes have a shorter length of~160 m. The flow rate of some particular channels might have been unbalanced during cooldown due to an accelerated decrease in temperature and friction, respectively. It was found by actual cooling that such an increasing imbalance in the flow rate did not happen and there was no necessity for active control of the inlet valves. This result was due to close thermal coupling between pancakes, and the temperature of any channel did not decrease faster than that of the others. Actually, the temperature of the short channels was slightly higher than that of the others due to the heat input from the side plates of the coil case. The estimated time constant of the thermal coupling between conductors is ,~10 s, therefore, this coupling is quite effective lbr slow phenomena such as initial cooling, With the experience presently available it seems unnecessary to control helium flow rates in many channels of the winding as long as the pancakes of the winding have good thermal contact with each other. Flow rate measurements in suitably chosen winding parts will be indispensable for safe operation of large forced-flow cooled magnets.

Charging of the coils Both theJA and the EU coils were successfully charged up o-N~-=~

I T R 810

0 o ,~,.o o ~o0 f,..) ~1- ~.

He i n l e t

544

C r y o g e n i c s 1 9 8 5 Vol 2 5 O c t o b e r

Main data of the charging tests

Current tested Ratio to the design current Current sweep rate Maximum magnetic field Stored energy Training or quench Number of chargings up to 10 kA Wire movement during charging Heat load at 10 kA Maximum stress in the conductor Maximum stress in the coil case measured

JA-LCT

EU-LCT

10.2 kA 100%

10.0 kA 88%

2.8 A s-1 6.4 T 106 MJ None 4 A few 106 W 176 MPa 78 MPa

3.3 A s-1 5.6 T 80 MJ None 9 None within measuring accuracy 127 W No data measured 174 MPa

to 10 kA without showing any signs of training or quenching Major results are summarized in Table4. For the EU coil the test current of 10 kA (88% of the rated current) was limited by the maximum capacity of the power supply. Before 10 kA charging of the JA coil the reliabilities of the test facility and of the protection system were confirmed by preliminary fast discharge tests conducted up to 4 kA, The detection level of a normal transition was 200 mV with 1 s delay time, measured as unbalanced voltage at the centre tap of the whole winding. During the first charging to 10 kA of the JA coil some small voltage spikes around 5 mV. probably generated by wire movements, were measured through a low pass filter (30 Hz). They occurred especially in the current range > 8 kA. However, it was impossible to separate the voltage spikes into an inductive and a resistive part. However, the JA coil showed no sign of normal transition during charging to 10 kA. Finally, at 10.2 kA, a maximum field of 6.4 T with stored energy of 106 MJ was generated. In the case of the EU coil, a system check-out was performed by manually triggered dump tests up to 5 kA before charging to 10 kA, The detection level for normal transitions was 50 mV with 1 s delay time measured as unbalanced voltage at the centre tap of each double pancake, which constitutes 1/7 of the whole winding. During the first charging of the EU coil no resistive voltages caused by conductor movements were observed within the measuring accuracy. The sensitivity was 0.4 mV which would correspond to a normal region of 2 cm length. An existing disturbance energy level was found by measuring the noise of acoustic emission. It was probably caused by micro cracks in the epoxy resin during ramping up and down of the current. But no influence on conductor stability was observed. Finally, at 10 kA, the EU coil reached a field of 5.6 T with stored energy at 80 MJ. During charging of the JA coil at the maximum current sweep rate given in Table 4, no temperature increase was observed within the winding immersed in liquid helium. Within the measuring accuracy the EU coil did not show losses in addition to the static losses during charging It was also impossible to observe within the given accuracy, a.c. losses during ramping up and down. Ohmic losses of the 13 conductor connections were smaller than 1 W and negligible9.

Mechanical behaviour of the coils

=TR 8 0 0

Figure 8 Hydraulic paths of the EU-LCT coil with instrumentation 19

Table4

their

Good mechanical performance is one of the most important characteristics of large superconducting coils.

Domestic testing of LCT coils: H. Tsuli et al. •

T

A/ (mm)

Scale

A/ (F'n)

/ //'°" ~

I

1401~m 470M.m

I"II

330~m

1440~m • Scale 1.0

.580~m (8e

"

(IC

50 0

b 3O

o.5

,4

_sce,e

- 8 0 # m ~ I0

50

,....., C

,,ooo.o,o0: d

Relative displacement between winding and inner ring. (Value measured at the lower apex in theJA-kCTcoil was omitted because the sensor was wrong.) (a) EU-LCT coil; (b) JA-LCT coil. Relative displacement between the winding and the side plate.

---P'-

I00

/2 (xlO6A z )

Figure10 Changing of the sprmg constant for the Lorentz forces in the coil case above 7 kA. O, Gap width dis 609 dms, D, D-shape deformation dis 402 pot; A, outer flange opening in the straight D-section

Figure 9

(c) EU-LCT coil; (d) JA--LCT coil

F r o m this point of view, the relative d i s p l a c e m e n t s between the wind;rig iuld the case and the strains of the coil ca,sc w'ere m e a s u r e d for the JA a n d EU coils. Additionally. the str;lins of the JA w i n d i n g illld the delt)rmalion of the EU coil bore were inea,sured. The m e a s u r e d relative disphicemcnts betwccn the winding and the inner ring tire shown in Ftkcures 9a and h Ior lhe ,IA and E l i coils al 10 kA, The gap width b e t w e e n the w i n d i n g and the case i n n e r ring increases as a result of hoop forces. Thc m a x i m u m di,splaccmenL I).66 ram, between the w i n d i n g and the case inner ring o f the JA coil a p p e a r e d at the top part o f the w i n d i n g while for the l ! t l coil the m a x i m u n l displacement. 1.44 inllL

appe;lred in thc in;d-plane. U n d e r these h o o p forces, a ,singlc solid l ) - s h a p c d w i n d i n g lends to be d e f o r m e d into a circuhir shape. Thcretore, in this re,spccL the distribution of the EU d i s p h l c e m c n t s could be reasonable, ,st,ggesting that the whole w i n d i n g is a solid structure The relative di,splacerncnt pallcrn of lhe JA coil shows a rathcr c o m p l i c a l c d distribution. Thc m a x i m u m displacemeril ;it the top part could be thought to be due to reduced `support of the w i n d i n g by the outer ring. It could h a p p e n if the w i n d i n g was riot acting as a m e c h a n i c a l l y solid body with high stiffness with respect to b e n d i n g forces. Howeveit rcal reasons lor this b e h a v i o u r have not been clarified yeL F'or the EU coil the nl;ixinlunl gap o f 1.44 ran1 in the midplane of the straight `section call be exphiined asstlming a 10 timc,s s m a l l e r radial stiffness thail theoretically predicted. A constant c o n d u c t o r pretension a p p l i e d during m a n u f a c l u r i n g of thc p a n c a k e s leads to a radial c o m p r e s s i o n d c p c n d i n g ori the radius of curvature. This l'ilCl wits c o n f i r m e d by it separate e x p e r i m e n 0 °. Modifications in c o n d u c t o r design and a p p l i c a t i o n of a d d i t i o n a l radial c o m p r c s s i o n d u r i n g the windirig process will help to overcome this p r o b l e m in futurc windings. The rehltive d i s p l a c e m e n t values of the JA coil in the n l i d - p l a n c werc ().2 and 0.34 ii1111 al a rated current of 10 kA.

In this re,spect, the gap distribution indicates a more `solid characteristic of the EU w i n d i n g than thai of the JA w i n d i n g F~ure 10 shows the F d e p e n d e n c e of the gap width between the w i n d i n g a n d the case inner ring, the D-,shape deflection of the coil inner bore, a n d the gap width between the two flanges in the `straight `section to prcvcnt overstre,s,sing of the flange bolts. The m a x i m u m deflection of the coil inner bore was -0.85 m m at the rated current of 10 k/k As shown clearly in Figure 10, the m c c h a n i s m of force transfer changes above a current level of 7.5 kA. No e x p l a n a t i o n can yet be given lor this rest;It. The EU coil has siccl bhidders filled with p r e s s u , i z c d epoxy resin to fix the w i n d i n g within the coil case. No gap was m e a s u r e d between the w i n d i n g and the outcr ring of the coil case, which m e a n s that the coil is in good conlacl with the miter r i n g In the case of the JA coil, a thermal shrink-fit t e c h n i q u e was applied. T h e coil case was heated tip to 330 K belore the w i n d i n g was inserted and, in addition, wedge-shaped spacers were driven between lhc w i n d i n g and the case outer r i n g However, lk)rce t r a n s m i s s i o n from the w i n d i n g to the coil case was poor as will be described later. The reason for this might be attributed to the characteristics of the w i n d i n g itselt~ As to the rclative d i s p h l c e m c n t between the w i n d i n g iuld the case `side plate, as shown in Fi
scheme, l-lov~cvclt the relative disphicements in the case o f the JA coil, ;is she,an in Fig,ltr¢'s 9c and U. c a n n o t be e x p l a i n e d by a simple solid block model. The `side `surface of the v~inding moxcd inward and p r o d u c c d an increased gap width rather than e x p a n d i n g outward like a `solid bh)ck, This rc,sult ,seeln,s to indicate that the local Lorcntz force squeezed each c o n d u c t o r towards the centre of the w i n d i n g at the side `surface of the w i n d i n g Thus, it must be

Cryogenics 1 9 8 5

Vol 2 5 October

545

Domestic testing of LCT coils." H. Tsuji et al. JA-winding c o Coil s ~

winding

Inner ring

!] 'I

Outer ring

50 M P a as far as the a z i m u t h a l stresses were concerned. The m a x i m u m yon Mises stress on the conductor, calculated in the analysis, is 108 MPa, including the a d d i t i o n a l b e n d i n g stress o1"25 M P a a n d s i d e - c o m p r e s s i o n stress o f 46 MPa. M e a s u r e d results, as shown in Figure 12, which were c o m p a r e d with the calculated a z i m u t h a l stresses of ~ 5 0 MPa, varied widely from 20 to 176 MPa. To clarify the reason for these differences, an a d d i t i o n a l stress analysis was carried out u n d e r the following artificial c o n d i t i o n s 1 2 3

Figure 11 Schematic cross-section through an LCT coil for explanation of the deformation of the JA and the EU windings exposed to Lorentz forces t-20

1/10 stiffness of winding: modification of interface between winding anti coil case: 1/10 b e n d i n g stiffness of the winding.

However, up to now, no result has shown a better agreement with the m e a s u r e d stress distribution, a n d the work has to be continued. F o r the stress levels of the coil case, Figures 13a a n d b c o m p a r e the analyses a n d the m e a s u r e d results o f the JA a n d the EU tests. Strain gauges attached to the JA coil case m e a s u r e d the stress c o m p o n e n t for e a c h sensor along a p r e - d e t e r m i n e d direction. F o r the JA case it was pointed out that the m e a s u r e d stress distribution had the s a m e pattern as that calculated. However, the

Figure 12 Azimuthal stress distribution in the JA winding. All values in MPa i

a s s u m e d that the c o n d u c t o r s in the w i n d i n g do not form a solid m e c h a n i c a l body as in an i m p r e g n a t e d w i n d i n g p a c k This is also visible from Figures 9a a n d b. Figure 12 shows the a z i m u t h a l stress levels m e a s u r e d on the c o n d u c t o r o f the JA coil. F o r this purpose, strain gauges were attached to the side surface of the conductor. The strain value at each point was m e a s u r e d a n d the stress values were c a l c u l a t e d b a s e d on the strain-stress characteristics m e a s u r e d in verification tests n. The m a x i m u m stress of 176 M P a was m e a s u r e d in the mid-plane, but not in the i n n e r m o s t turn. W i t h i n the i n n e r m o s t turn, relatively high stresses, 100-150 MPa~ were m e a s u r e d at the top a n d b o t t o m parts o f the winding. Stress analysis carried out for the detailed design, based on F E M calculations with b e a m a n d truss, indicated a rather m o n o t o n o u s stress distribution a r o u n d

546

Cryogenics 1 9 8 5 Vo125 October

.

m

~4',' ".'

b Figure 13 (a) Stress distribution in the JA coil case (0, stress in the azimuthal direction; r, stress in the radial direction). Measured and calculated values (in parentheses) are given in MPa. (b) von Mises stresses and principal stress directions on the coil case surface of the EU coil. tel, Measured values; ©, FEM calculations (all values are given in MPa)

Domestic testing of LCT coils: H. Tsuji et al. measured values of stresses were lower lhari the calculated ones. This result c o r r e s p o n d s to the result of higher c o n d u c t o r stresses. T h e reason may be insufficient force t r a n s m i s s i o n from the w i n d i n g to the coil case. However, its m e c h a n i s m seems to bc different from the EU case, where non-linearity was observed in the m e c h a n i s m of Iorce t r a n s m i s s i o n b e g i n n i n g tit 7.5 kA current (Figure 10). Up till nov+ this b e h a v i o u r has not been understood. Additionally, principal strain(stress) directions have been calculated for the Ei, J coil case (fT
2

the JA coil showed higher stresses at the w i n d i n g and lower stresses at the coil case than calculated in the analysis. The reason for this has not yet been clarified, but it seems to be insufficient m e c h a n i c a l c o u p l i n g between conductors, resulting in less solid block-like b e h a v i o u r of the whole w i n d i n g pack:

maximum

temperature

I0

i

5

1

Table 5

I

I

I

\

\ E 12

0

20

40

Time (s) Figure 14. Variation of conductor temperature by 100% fast discharge of the JA-LCT coil

18I u3 16. 84

B

64

Data of fast discharge tests

Discharged current 10.2 kA Discharged energy 106 MJ Time constant 20 s Maximum terminal voltage 1.0 kV Energy loss 1.9 MJ Maximum pressure increase 1.2 bar Maximum conductor 24 K temperature Number of dumps from 1 rated current 10 kA

2

d

tests

JA-LCT

I

20

/

It is one of the i m p o r t a n t objectives of L(TT development to verify the reliability of the coil when ils energy undcrgoes a fast discharge for the p u r p o s e of proteclion. The coil protection is designed to d u m p the coil when a p r e - d e t e r m i n e d level of n o r m a l resistivity is reached. The LCT coils are basically designed for d.c. o p e r a t i o n s plus p o l o i d a l field coil transients. A d d i t i o n a l pulsed field losses are generated during a fast d u m p o f the coil. Actual discharge tests were m a n u a l l y triggered and not forced by the detection e r a n o r m a l transition. The coil current was transmitted by a power switch to a resistor a b s o r b i n g the stored coil energy. M a j o r results of the discharge tests tire s u m m a r i z e d in Table5. The m a x i m u m discharge voltages were l.[) a n d 2.2 kV for the JA coil and the EU coil, respectively• The t e m p e r a t u r e increase of the JA coil due to fast discharge was m e a s u r e d at the i n n e r m o s t turn of the w i n d i n g and the result is shown in F~gure 14. Other t e m p e r a t u r e sensors located in the low field region a p p a r e n t l y did not show an increase in temperature. The

3

i

v

discharge

tit the

Conductor current (kA)

the EU coil showed more solid block-like characteristics than the JA coil, resulting in better agreement between analysis and measurements. However, the stiffness of the w i n d i n g in the straight section of the D - s h a p e was much smaller t h a n a s s u m e d in the analysis, restllting in the larger gap ( w i n d i n g - c o i l case) and p r o b a b l y also higher stresses at the straight part of the coil case.

Fast

increase w a s m e a s u r e d

lower position'c" as indicated in Figure 14. After initiation of a dump, the reading at "c" increased f o r 6 s at the rate of 0.04 K s-~ and then suddenly increased to 24 K after lb; s. The time of occurrence of m a x i m u m t e m p e r a t u r e was delayed at higher positions. Therelorc, the m o v e m e n t of helium b u b b l e s was considered to have some rclation to this result. Actually, at 'c', helium bubbles in the cooling c h a n n e l s moved upwards from the low-field part to the high-field part. A s u d d e n increase of the w i n d i n g temperature was p r o b a b l y initiated b x a transition from nucleatc to fihn boiling rest.lting in a noNnal transition o f the conductor. However, the c o n d u c t o r length b e c o m i n g n o r m a l c o n d u c t i n g was limited and the total a m o u n t of energy gencrated by a n o r m a l zonc was much smaller than the total discharge losses described later. The temperature increase of the JA coil case was not m e a s u r e d due to a lack of sensors. The t e m p e r a t u r e increase of the EU coil is shown in F~gu#w15. The time scale is > 100 times larger than thai ill

2 EU-LCT 10.0 kA 80 MJ 7 s 2.2 kV 1.5 MJ 3.9 bar 8 K 6

E I0;

5 44 2~ 01~

~_ 0 Time o f t e r dump ( h )

Figure 15 Temperature of the winding and coil case after fast discharge of the EU coil, A, Coil current; B, coil case temperature; C, winding temperature

Cryogenics 1985

Vol 25 October

547

Domestic testing of LCT coils: H. Tsuji I

I

et al.

I

2.0 800

1.5 6O0

/_¢

A

I.O

-

/

0

400

//

E

.=

z

/

/I

0.5

200

0

I

20

40

60

t 80

IO0

0

Monuol dump current (%) 16 Comparison of measured (-- • --) and calculated ) clump losses of the JA coil. Time constanL 20 s

Figure

(

JA conductor EU conductor

estimations including higher losses caused by imperfections in cable fabrication '3. About 2/3 of the total losses were dissipated in the coil case, about 1/3 in the winding 14. For the JA coil this ratio was 1:1. Comparison of average field values for the single coil test and tests in a toroidal arrangement shows that the total dump losses are nearly equal. Considering the transient behaviour of the cryogenic system, the average heat transfer power during dumping was ~I(XI kW for the JA coil. The energy was immediately transferred with this power to the helium bath. To keep the pressure below a given level(Figurel8), a high mass flow of cold gas had to be removed from the coil. The more complicated hydraulic system of the EU coiL combined with a cold buffer tank (Figure5), allowed the storage of dissipated energy loss at different temperature levels. This is possible because of the thermal resistance existing between the coil case and the winding as mentioned before. The greatest portion of the losses was directly stored by the enthalpy of the coil case at a temperature level of 17 1L while the winding losses were stored at 7 K. mainly by the inner energy increase of the supercritical helium at a relatively high pressure level. This pressure level was controlled at ~10 bars and by opening of the valve to the cold buffer as can be seen in Figure 18. With an averaged power of 210 W the coil was re-cooled in the refrigerator mode, the re-cooling power was determined by setting the pressure level in the pump dewar(Figure5) at a value keeping the refrigeration system in a steady-state condition. With these principles applied. six dumps can be handled without any loss of helium and with economic re-cooling,

Pulsed field by self discharge <" ; "

4-1

Bd

8 p ~ Major pulsed field by poloidal coils 3Figure 17 Orientation of poloidal field and toroidal field changes for the JA and EU conductors

Figure 14. A temperature increase up to 15-17 K was measured in the coil case due to eddy current losses. The temperature increase measured at the winding surface was rather delayed from the time of discharge. The peak temperature of only 6-7 K occurred after 20-30 rain. The temperature profiles were determined by heat diffusion between the warmer coil case and the winding. Dump losses in the JA coil were measured by tile decrease of helium level in the coil. For the EU coil these losses were determined by the integration of cooling power needed lbr re-cooldown. A comparison of the calculated and measured JA coil losses shows good agreement (Figure 16). The JA conductor was designed to have small losses for poloidal field transients (Figure 17) 12. Therefore, higher losses caused by dumping field transients in a toroidal arrangement had to be accepted. The EU conductor was optimized to have the potential of a low loss conductor. Unfortunately, the measuring accuracy at low currents was not very good. For the value given in Table5 a tolerance of 20% must be assumed. The winding losses arc about three times higher than expected by the

548

Cryogenics 1 9 8 5 V o 1 2 5 O c t o b e r

g

)

g u

.E

2-

m

~ o

0

/ 50I

0

I

I

!

I

I

I

I

F

I

I

0

Time (s)

Figure 18 Pressure increase during fast discharge. Opening of the relief valve to B310. O; EU-LCT, Qm = 1.5 M J, dump of 80 MJ ( : = 7s); 0 : JA-LCT, O,n = 1.g M J, dump of 106 MJ ( r = 20s)

Domestic testing of LCT coils. H. Tsuji et al.

Stability of the coils

Current

With the rest(Its of stability tcsts on the JA coil it was verified that the coil could recover by itself from a halfturn n o r m a l c o n d u c t i n g zone. F o r the EU coil, no n o r m a l coriducting zone was gcnerated: the coil proved to bc stable cven in the case of a fast discharge tit an increased inlet t e m p e r a t u r e o1"6.0 K. F o r the stability tcsts of the JA coil, resistive heaters werc attached to the c o n d u c t o r tit the i n n e r m o s t lurrl of the w i n d i n g in the m a x i m u m magnetic field. The typical hcating power was b.6 kW for a d u r a t i o n o1"400 ms, which c o r r e s p o n d e d to l.S J cm 3 cnergy input into the conductor. The n l a x i n l u l l l t e m p e r a t u r e n l e a s u r c d was 13 K for a coil current of 0 A, 15.5 K at 6 kA, and 20 K at 10 kA. The additiorial iricreasc of t e m p e r a t u r e in the charged m o d e was d e c to Joule hcating by the coil current. t{xcn in the case of the 1() kA slab(lit\ task thc tempcralUI'C Of the condtlctol-. 0.2 s after t e r m i n a t i o n of heating. %.~rc|]Id()x~]] t'ro]]] ~[ ill(ix(nil(In] xaluc of 20 K to 4 K within I s of a t t a i n m e n t of its peak temperature. The vohage nlctistltcd during that tirnc is shown in l~i<<4m'c 1?. Recovel y from the n o r m a l state was not initiated by both cold cnds as could h a \ c been expected by the col&mad recoxcry theorem. One reason for this was lhal the magnetic fich_t of 6.4 T tit both ends of the heatcd zone was higher than that tit the central park i.e. 5.5 T. The othcr marc important, rcason was that thc .IA coil was fully cryostablc u n d c r this condition rather than just stable due to cold-end rccoxcty, which v~as the inininlLiill goal of stability design. To coilfirill this resulL the lifetime of the normal zone generated was il~eastncd bv changing the length of the heated ilorn'~al /one. The rncasurcd lilL'timc was alrnosl c()nslant for the heated length, from 0.7 to 4 in, shov+ing u n c o n d i t i o n a l recover)' due to the cryostability of the JA coil. T h e restllls of the stability test were o b t a i n e d by varying the coil current from 2 to 10 kA: the absolute limit of the stability was estimated at 12.6 kA at 7.9 T. This test restllt fully agrees with earlier calculations ~-~ which del]ned the stability limit at 12.8 kA tit S T. In the case of it forced-flow cooled coil a definite heat int-mt trite the ctlrrcnt carrying c o n d u c t o r can only take phice bx eddy currcnt healing. T h e invcstigalions shov,ed thnt salL?ty precautions, instilatioFl problems and excessive Lore(it/ forces did nol allow a pulse coil to bc rnotnltcd inside or oulsidc the winding. Thcrclorc. the stability properties of the [ n r a t o n l k(']" c o n d u c t o r were investigated in a separate cxperimenk in ~ h i c h the b o u n d a r i e s of critical energy input ~,,cre d e t e r m i n e d ~6. The b c h a v i o u r o f h m g n o r m a l regions was investigatcd by c o m p u t e r calculations only. As long as the energy input

Heated zone

7 t-' ~

++i4 VH~-29

'

ITemperoture

sens°rs (TH)

29

+ 0

VHm-30

.I : , /,

Ik~-'OI~ t-'i-~ . {'~

~'~ ~ g, } ~

~ _ ~

eC) ~

Voltage

! : !N

,. , :

. . . . . . . . .

; ;11

', : ', ', I~',t

t i

Q.+i3 profile

after

.olf turn heeling

E .~

. . . . . . . . . . . .

i V~

H,eoier curre,n, Time (200 ms/div)

Figure 19

Results of stability test of the JA-LCT coil

L

,VV'H~?, H,'-~t

I

]

f ~

Operation ~ point

col line

Temperature margin

2 CISieMy ~ 3

a Stekly~ 12

v

I 1.4

0

t/,,]~

/

J

_ - - i ~ 3 ~ < z l ~ i ~ . ~ / ~

I

I

5.8K

4.8K I I I 5

i 4

I

I

/

Toske (self f i e l d )

LCTF

5.7K 6.OK (torus f i e l d ) I I I I 6 -}

T(K) Figure 20 Load lines of the TOSKA and LCTF facilities with/- and T-margins m the Ic/T c diagram does not increase the helium temperature up to To(critical temperature), the c o n d u c t o r recovers. Recovery d e p e n d s on many p a r a m e t e r s (length, field distribution and place of the n o r m a l c o n d u c t i n g region, h y d r a u l i c b o u n d a r y conditions). Some trends in the b e h a v i o u r of n o r m a l regions of the EU c o n d u c t o r have been discussed p r e v i o u s l y : . Also: the calculated m a x i m u m press(ire is m o d e r a t e (<< 4 bar) if normal regions tip to the full length of a cooling path (% 250 in) recover. Non-recovery u n d e r the same c o n d i t i o n s leads to a m a x i m u m pressure of ~50 b a r after 3 s TM which has been taken into account in w i n d i n g design and coil protection. Two 500 W gas heaters were installed tit the inlets of one d o u b l e p a n c a k e (two hydraulic paths) to be c a p a b l e of increasing the gas t e m p e r a t u r e and generating a n o r m a l c o n d u c t i n g region. To find out the limits of stability the helium was heated to 5.7 a n d 6.0 K, respectively. With these increased tcmperaturcs the EU coil v, as charged, o p e r a t e d a n d d u m p e d without any deterioration of performance. To discuss this in a more quantitative way, the simple Stekly parameter, oc and a critical current versus temperature d i a g r a m (Figure 20) can be used. If one indicates the T O S K A test results as triangles in this figure and takes into account the heat transfer coefficient of only 0.024 W cm 2 K ~ in a c c o r d a n c e v, ith the small mass flow rate used (2.1 g s ~ He), 7+= 6 K. ~ in this case would be very large, ~24. Thus, the coil could be o p e r a t e d far below cryogenic stability, very close to its critical data. F o r the tests envisaged at O R N L an inlet t e m p e r a t u r e of only 3.8 K. but a higher field(8 T) a n d it higher c u r r e n t ( l 1.4 kA) tire lorescen. Still the triangle for the lIT margin will be hu'ger. Increasing the mass flow rate to 10 g s -~ will result in a heat transfer coefficient of ~ 0 . 1 1 W cm 2 K reducing the o~ value to ~ 3 . still not i n c l u d i n g cold end recoxery. Thus, sufficient lnargin lot a.c. loss heating in poloidal field tests can be predicted. The heat diffusion insidc the winding of the EL: coil ,aas investigated by a heatcr pulsc of 73 W tar It)00 s. F&,ure 21 shows the maxinaun~ tcmpcraturc incrcase at the outlets and the time passed from the b e g i n n i n g of heating to the peak tcmperature. In this test the helium from only two charu-icls out oi28 was heated in one o f t h c two outsidc double pancakes. The average t e m p e r a t u r e increase of the w i n d i n g v, as 0.2 K. The heat capacity of the w i n d i n g is 22 kJ K ~ at 5 K. thus this t e m p e r a t u r e increase c o r r e s p o n d c d to 5 kJ. O f the 73 kJ encrgy input, 14 kJ reached the outlets of the heated chanriels. 29 kJ the outlets of the nori-heatcd channels. 5 kJ were used for the

Cryogenics 1 9 8 5

Vol 2 5 October

549

Domestic testing of LCT coils." H. Tsuji

et al.

Table 6 Comparison of pool-boiling cooled and cooled coils

tmax'~ "~ 0.4

Te m p e r o t u r e ~ z ~

I'-- O~"-~Heoling I

173Wx iOO~Ots

_

O. 3

.

/,

~o

O. 2

Outlet

Tmax

.

.

.

.

?000

.

,,.

"~"~r'" tm"x

v{

21500

•

-I000

~"

o

'1" ~ --

o .~,=~'.,., 7

6

.o~-,, 5

4

. .... 3

2

o I

Simplicity of conductor design Simplicity of winding design Cost of conductor fabrication Cost of winding fabrication Expenses for quality assurance Possibility of internal shorts High voltage performance Vacuum tightness Thermal efficiency during steady refrigeration Stiffness of the winding Possibility of internal disturbance Stability against external heat input Self-recovery from normal state Pressure increase due to fast discharge Reliability in case of cryogenic system down

forced-flow

Pool cooleda 2 3 2 3 2 4 3 3 2

Forced cooleda 4 2 4 2 4 2 1 2 4

4 3 2 2 2

3 2 2 4 2

2

2

~1, Very good; 2, good, 3, -% 4, bad

Poncoke number of exits

Figure 21 Heat diffusion in the EU winding, t v Time of flow for helium from inlet to outlet long conductor length; t 2, time of flow for helium from inlet to outlet short conductor length

temperature increase of the winding, and the other 25 ld seemed to be dissipated to areas outside the winding. It can be seen from Figure 21 that thermal diffusion in the radial direction is not faster than the heat transport by the flowing helium. In the case of a transition to normal conductivity cooling by heat diffusion from the surrounding turns can be neglected, because the heat generated by the quenched conductor (,~280 W m -~ for the domestic test) is much higher than the heat transferred into the winding by diffusion (~8 W m-~). Therefore, diffusion cannot support conductor stability. It is only sufficient for long term equalization, for example, to distribute locally generated pulsed field losses across the winding pack The attempt to generate definite normal regions in the conductor by gas heating with short pulses (~1 s) failed. The reason was the opening of a back pressure valve which was loaded too weakly and prevented the heated gas from entering the conductor.

Conclusions Major results obtained in the domestic tests of the poolboiling cooled JA LCT coil and the forced-flow cooled EU LCT coil are summarized and evaluated. These two LCT coils were the first to demonstrate the feasibility of large superconducting coils for tokamak machines, although based on completely different approaches (pool-boiling and forced-flow cooled systems, respectively). It has been recognized that these single coil tests, performed as preliminary tests for the six coil tests at the Oak Ridge National Laboratory have furnished much significant technical information for future developments. Regarding the comparison of pool-boiling cooled and forced-flow cooled systems, a major topic of this Paper, it is too early to draw final conclusions. An attempt to compare the designs is made in Table 6 as far as the information obtained by the domestic tests is concerned. As a summary of Table 6 it could be suggested that poolboiling cooled coils could be used with a tokamak constructed now and with a toroidal coil size equal to or less than that of the LCT coils and with specifications comparable to those given for LCT. This would result in a

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simple and cryostable toroidal coil system and would require less cost than using lbrced-flow cooled coils based on the present technology. However, forced-flow cooled coils can be recommended lbr large tokamaks whose toroidal coil sizes are larger than that of the LCT coils or whose maximum fields are > 8 T or whose a.c. loss requirements are more demanding. Large magnet systems require an electrical connection of magnets in series and, therefore, higher discharge voltages to avoid the cryogenic losses of many vapour-cooled leads. Also, this condition can be better fulfilled with forced-flow cooled systems. For these reasons, a higher potential of development can be predicted for forced-flow cooled coils. A constraint on the economical operation of such coils is the optimization of the cryogenic supply system. This is the first attempt to compare both cooling systems based on technical inlormation from LCT coil experiments. It should contribute to a more detailed comparison after the six coil tests have been carried out at ORNL, in which three pool-boiling cooled coils (two US and one Japanese) and three forced-flow cooled coils (US, Swiss and Euratom), each developed on a clearly different concept, will be tested jointly.

Acknowledgements The authors would like to thank Professor Dr W. Klose of KfK, and Drs S. Mori, Y. lso, Y. Obata and M. Tanaka of JAERI for their continuing encouragements of this programme which was conducted under the auspices of the IEA. The authors deeply regret the early death of Professor K, Yasukochi and Professor W. Heinz both of whom continuously stimulated the work by their ideas and support. All staff members of both laboratories are gratefully acknowledged for all their work contributing to the development and test of the coils. The work for the EU coil was performed under the auspices of the European Fusion Technology Program. References

1 Superconducting Toroidal Field Coil, Large Coil Program, Revision E. Technical Specffication TS-147(X)-Ol, ORNL, USA ( February 1980) 2 Krauth,H. et al. Proc 9th Syrup on Engineering Problems of Fusion Research Chicago. USA ( 1981) 2027 3 Shimamoto,S, et aL Proc 8th Svmp on Engineering Problems q[" Fusion Re,~eurch San Francisco, USA (1979) 1174

Domestic testing of LCT coils," H. Tsufi et al. 4 5 (~ 7

9

10

Shimamoto, S. et al. Crvogenic~ 11984) 25 227-233 Shimamoto, S. et al. Proc 9d7 Syrup on Engineering Prohh'ms qf Fusion Research Chicago. (1981) 2019 Herz, V~. et al. ISre 1 ( 7 : ( 1# P,utter~orlhs. (;uildt~rd. I.'K (1984) 382 Ilerz, W. et al. Proc 1( "L'('IO gutterv, orths. G ui[dlbrd, UK(1984) g3 rad~ E. i! al. Proc I('EC 9 Buttcrworths. Guildlk~rd, UK ( 19~21 93 Albrecht, C, et al. J Phv~iqm, (1984) CI 607 Herz. W~ el al. Testing of lhe }_:_uratom LCT coil in the T O S K A l~acililv Proc IJth SOFT Varese (1984)

II 12 13 14 15 16 17 IS I t)

Yoshida, K. et al. :ldv ('mog Erie Mat.~ (1982) 28 781 Tsuji, H. et al. IEEE Trans (1981) MAG-17 42 Herz, W. et al. Results of tile Test of the [~tlropean LCT coil in the T O S K A facility Proc .4S(" San Diego, USA 11984) Sehmidk C. IEEE Trans {1983) MAG-19 707 Nishi, M. et al. g Physique (1984) CI 131 Schmidt, ( . ('(vo,k,cnic~ (1984) 24 <~53 Rie~ R. 11:'1:t: Fra,.~ (lgSI) MAG-17 20~)7 Krafft. G. et aL Procl('ECb; 1PC Science and Technology Press. (}uildford. UK (1981)) 33{) Albrecht, C. el al., Siemens A.G.. LCT-Euralonl. Endg(fltiger Ent,~urI~ Abschlussbcricht (19S4) 3 Band A

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