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

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. 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

Coil cooling down: ramp AAB10, 11-20 July 2007, position 2A The inlet temperature ramp reaches the limit of 2 K/h 4

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

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

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

Coil cooling down: total power Total coil energy loss 8

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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.

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

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

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