Cooling down of W7-X coils with/without casing circulation

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

2. 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. 1. Coil cooling down OUTLINE 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, 11-20 July 2007, position 2A The inlet temperature rampreaches 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. 1. Coil cooling down OUTLINE 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 • Researchinterest • 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): 46 g/s @ 300 K - 15 g/s @ 4.5 K 13

14. Model of coil cooling: power input Power available for one out of two coils Refrigerator power is 200 W @ 4.5 K 14

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

16. Model of coil cooling: comparative power Difficulty to know coil emissivity and LN2radiation 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 ΔTHedoes not reach the limit of 40 K 18

19. 1. Coil cooling down OUTLINE 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: • Researchinterest • 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 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 Normal cooling down is ~220 h

26. 1. Coil cooling down OUTLINE 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. Current test wo casing: T 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. Temperature outlet prevision

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. Current test wo casing: questions Other questions – IPP decisions

33. 1. Coil cooling down CONCLUSION 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