1. Cooling down of W7-X coils with/without casing circulation1

2. KSTAR (in operation since 2008) W7-X (in construction)
CICC coils experience
CICC conductors
in fusion machines :
EAST (in operation)
KSTAR (in operation since 2008)
W7-X (in construction)
SST1 (in construction)
JT-60SA (in construction)
ITER (in construction) 2

3. 3. Cooling down without casing circulation
OUTLINE
1. Coil cooling down
Description and limitations
2. Model of coil cooling
Principle and comparison
3. Cooling down without casing circulation
Cooling duration prevision
4. Current test without casing circulation
Possibility to use nominal temperature and flow? 3

4. Coil cooling down: ramp
AAB10, July 2007, position 2A
The inlet temperature ramp reaches the limit of 2 K/h 4

5. Coil cooling down: mass flow rate
m evaluated from ΔP and from the precise cryogenic measurement 5

6. Coil cooling down: maximum gradient
ΔT in-out reaches the limit of 40 K on the casing 6

7. Coil cooling down: He power
Power extraction through helium circulation 7

8. Coil cooling down: total power
Total coil energy loss 8

9. Coil cooling down: limitations
Helium inlet temperature limiting parameters:
Maximum gradient in the coil: 40 K
Maximum rate of inlet temperature decrease :
2 K/h between 300 and 100 K
4 K/h between 100 and 50 K
10 K/h below 50 K
Refrigerator power
Mass flow rate
Nitrogen power 9

10. 3. Cooling down without casing circulation
OUTLINE
1. Coil cooling down
Description and limitations
2. Model of coil cooling
Principle and comparison
3. Cooling down without casing circulation
Cooling duration prevision
4. Current test without casing circulation
Possibility to use nominal temperature and flow? 10

11. Model of coil cooling: motivation
Motivation for a model
Research interest
Validation of cryogenic inertia
Better understanding of phenomena and limitations
Calculation of the coil cooling time
Engineering interest
Design support for coils, cryogenic loops and cryoplants
Projection to W7X operation 11

12. Model of coil cooling: simplification
Simple isothermal assumption:
T(winding+insulation) = Toutlet winding
T(casing) = Tcasing skin = Toutlet casing 12

13. Model of coil cooling: mass flow rate input
A linear mass flow rate is imposed
Refrigerator mass flow rate (two coils):
K K 13

14. Model of coil cooling: power input
Power available for one out of two coils
Refrigerator power is K 14

15. Model of coil cooling: principle
Principle of the simple model
winding
Tw(t)
Tinlet(t-dt)
casing
Tc(t)
Max power
Tinlet
40 K gradient
Max ramp
Tinlet(t)
WHe w=mw.cpw.(Tw-Tinlet)
WHe c=mc.cpc.(Tc-Tinlet)
Wconduction w/c
Wradiation
Tc(t+dt)
Tw(t+dt) 15

16. Model of coil cooling: comparative power
Difficulty to know coil emissivity and LN2 radiation 16

17. Model of coil cooling: in/out temperatures
Cooling temperatures shape is good, timing is not 17

18. Model of coil cooling: gradient
Model ΔTHe does not reach the limit of 40 K 18

19. 3. Cooling down without casing circulation
OUTLINE
1. Coil cooling down
Description and limitations
2. Model of coil cooling
Principle and comparison
3. Cooling down without casing circulation
Cooling duration prevision
4. Current test without casing circulation
Possibility to use nominal temperature and flow? 19

20. Cooling without casing: motivation
Motivation for an experiment:
Research interest
Usefulness of casing circuit
Validation of cool-down previsions from model
Design support for coils, cryogenic loops and cryoplants
W7-X interest
Casing cooling malfunction of a coil
Corrosion
on AAC52 20

21. Cooling without casing: inertia
Non-planar coil inertia
Coil average diameter
2.3 m
Conductor total length
945 m
Conductor total
870 kg
Insulation (Epoxy resin)
540 kg
Case (ss)
3640 kg
Total mass
5300 kg 21

22. Cooling without casing: maximum gradient
Maximum 40 K gradient is always limiting
Average speed > 1K/h 22

23. Cooling without casing: power
He power is not a limitation: mass flow rate is. 23

24. Cooling without casing: temperature ramp
dT/dt [K/h]
He outlet and coil casing have similar temperatures
=> Uncertainty on w/c conduction is not a problem

25. Cooling without casing: duration
Mass flow rate
Max gradient ΔT
Duration
1-> 3 g/s
30 K
380 h
35 K
350 h
(proven)
40 K
312 h
(reference graph)
1,5-> 3,5 g/s
336 h
300 h
280 h
Normal cooling down is ~220 h

26. 3. Cooling down without casing circulation
OUTLINE
1. Coil cooling down
Description and limitations
2. Model of coil cooling
Principle and comparison
3. Cooling down without casing circulation
Cooling duration prevision
4. Current test without casing circulation
Possibility to use nominal temperature and flow? 26

27. Current test wo casing: previous test
AAB14 current test on 10/05/2007
Reduced (2h) and stopped (3h) casing cooling
Asymptotic temperature: +3.2 K
Observed time response of ~2 hours
Limitation: experiment is too short 27

28. Current test wo casing: temperature step
Long time refrigerator He temperature regulation ?
LHe test
Use of LHe should be avoided
Minimum duration 6h before and after (stable T)
Close busbars circuits
Total power should be reduced
Use other coil as complementary mass flow
Constant mass flow rate, stable power for the liquefier 28

29. Current test wo casing: cryogenic circuit
W7-X Test Facility cryogenic circuit 29

30. Temperature outlet prevision
Current test wo casing: T prevision
Temperature outlet prevision
Experimental evidence using AAB34:
Nominal outlet winding T is 0.04 K above inlet T
Winding extraction ~1 W (+ joints)
Nominal casing extraction is 15 W (current or not)
Calculation using outlet temperatures without joints
Worst scenario without casing circulation:
All energy is forced in the winding => maximum outlet T.

31. Current test wo casing: T prevision (2)
With nominal mass flow rate (0.6 g/s/DL),
the maximum inlet temperature before a quench is ~5.4 K.
With nominal temperature 5.7 K,
flow must be increased from 0.6 to ~0.8 g/s/DL

32. Other questions – IPP decisions
Current test wo casing: questions
Other questions – IPP decisions
Question
Pro / Con
AAB31 alone or together with another coil? F
A second coil can be installed, but cooling of both will be slow
Start of cool down trial beginning at LN2-temp
Not in accordance to the future operation scenario
Will one hour current test be enough?
Interest to raise temperature by steps
Finish the test by fast discharge or quench?
Might help to get knowledge about the thermal behavior in case of need
End with normal current test with casing cooling
Validation of the coil performance

33. CONCLUSION 1. Coil cooling down 2. Model of coil cooling
Description and limitations
2. Model of coil cooling
Principle and comparison
3. Cooling down without casing circulation
Cooling duration prevision
4. Current test without casing circulation
Possibility to use nominal temperature and flow? 33