Transient Heat Transfer Model and Stability Analysis of He-Cooled SC Cables

1. Transient Heat Transfer Model and Stability Analysis of He-Cooled SC Cables Pier Paolo Granieri 1,2, Marco Calvi 2, Panagiota Xydi 1 Luca Bottura 1, Andrzej Siemko 1 (1 CERN, 2 EPFL) Acknowledgments: M. Breschi , B. Baudouy, D. Bocian CHATS on Applied Superconductivity 2008 Workshop KEK, Tsukuba, Japan, October 30 – November 1, 2008

2. Outline • Beam Induced Quenches • Model Description • Heat Transfer to helium • Results • Impact of the Model Parameters • Systematic Calculation of the Stability Margins • Conclusions & Perspectives PP Granieri - Transient Heat Transfer and Stability of He-cooled SC Cables

3. Beam Induced Quench Beam loss: quench of the magnets Interruption of the LHC Protection system (Beam Loss Monitor) BLM reaction thresholds: energy deposition  stability of the magnet The estimation of the stability against transient distributed disturbances is the aim of this work PP Granieri - Transient Heat Transfer and Stability of He-cooled SC Cables

4. From 1-D to 0-D Model Follow-up of the work presented at CHATS 2005 1: • 1-D THEA model 2 (non-uniform T and I distribution) • strands lumped into a single thermal component 3 (1) • The longitudinal dimension can then be neglectedfor distributed energy depositions (e.g. beam loss)  0-Dnimble model • Large He contribution Particular focus on it 1 L. Bottura, M. Calvi, A. Siemko, “Stability Analysis of the LHC cables”, Cryogenics, 46 (2006) 481-493. Presented at CHATS-2005, Enschede, Netherlands. 2 L. Bottura, C. Rosso, M. Breschi, “A general Model for Thermal, Hydraulic, and Electric Analysis of Superconducting Cables”, Cryogenics, vol. 40 (8-10), 2000, pp.617-626. 3 M. Breschi, P.P. Granieri, M. Calvi, M. Coccoli, L. Bottura, “Quench propagation and stability analysis of Rutherford resistive core cables”, Cryogenics, 46 (2006) 606-614. Presented at CHATS-2005, Enschede, Netherlands. PP Granieri - Transient Heat Transfer and Stability of He-cooled SC Cables

5. 0-D Model Description In the longitudinal direction: Cable cross-section: micro-channel He channel through the cable insulation (4) Subscripts : s – strands He : He in the cable i : insulation b : external He bath p : contact (wetted) perimeter h : heat transfer coefficient (5) 4 ZERODEE Software, CryoSoft, France, 2001. PP Granieri - Transient Heat Transfer and Stability of He-cooled SC Cables 5 B. Baudouy, M.X. Francois, F.P. Juster, C. Meuris, “He II heat transfer through superconducting cables electrical insulation”, Cryogenics, 40 (2000) 127-136.

6. Previous studies allowed determining that the most relevant parameter for stability calculations is the heat transfer coefficient betweenthe strands and the helium filling the interstices among them. Its theoretical evaluation represents a complex task. It depends on several parameters, which are often unknown and difficult to measure. A careful research in the literature has been carried out to develop the heat transfer model. It consists of the composition of different terms related to the different He phases : L. Bottura, M. Calvi, A. Siemko, “Stability Analysis of the LHC cables”, Cryogenics, 46 (2006) 481-493. Heat Transfer to Helium Superfluid helium THe < Tλ At the beginning of the thermal transient the heat exchange with He IIis limited by the Kapitza resistance at the interface between the helium and the strands. The corresponding heat transfer coefficient is: (6) 6 S.W. Van Sciver, “Helium Cryogenics”, Clarendon Press, Oxford, 1986. PP Granieri - Transient Heat Transfer and Stability of He-cooled SC Cables

7. Heat Transfer to Helium Normal helium Tλ< THe< TSat When all the He reaches the Tλ, the He Iphase starts growing and temperature gradients are established in the He bulk. A thermal boundary layer forms at the strands-He interface, where a heat diffusion process takes place in a semi-infinite medium: (7) After the boundary layer is fully developed, the heat transfer mechanism is driven by a steady state heat transfer coefficient hss. The transition is approximated in the following empirical way: (8) 7 L. Bottura, M. Calvi, A. Siemko, “Stability Analysis of the LHC cables”, Cryogenics, 46 (2006) 481-493. 8 V.D. Arp, “Stability and thermal quenches in forced-cooled superconducting cables”, Proceeding of 1980 superconducting MHD magnet design conference, Cambridge (MA): MIT, 1980, pp. 142-157. PP Granieri - Transient Heat Transfer and Stability of He-cooled SC Cables

8. Heat transfer to helium refers to a constant temperature of the strands To generalize to the case of an arbitrary temperature (or heat flux) evolution at the strands surface, consider the following heat conduction problem: θ(x,t): He temperature profile in the cable’s transversal direction Ф(t)=Ts(t)-Tλ: temp. of the strands – temp. at the λ transition The heat flux absorbed by the helium in the inlet surface is proportional to local temperature gradient: The implementation is ongoing… PP Granieri - Transient Heat Transfer and Stability of He-cooled SC Cables

9. Heat Transfer to Helium Nucleate boiling THe = TSat Once the saturation temperature is reached, the He enters the nucleate boilingphase. The formation of bubbles separated from each other occurs in cavities on the strands surface. The transient heat exchange during this phase is very effective: (9,10) 9 C. Schmidt, “Review of Steady State and Transient Heat Transfer in Pool Boiling He I, Saclay, France: Int.l Inst. of Refrigeration: Comm. A1/2-Saclay, 1981, pp. 17-31. 10 M. Breschi et al. “Minimum quench energy and early quench development in NbTi superconducting strands”, IEEE Trans. Appl. Sup., vol. 17(2), pp. 2702-2705, 2007. PP Granieri - Transient Heat Transfer and Stability of He-cooled SC Cables

10. Heat Transfer to Helium 13 C. Schmidt, “Review of Steady State and Transient Heat Transfer in Pool Boiling Helium I, Saclay, France: International Institute of Refrigeration: Commision A1/2-Saclay, 1981, pp. 17-31. Film boiling Efilm = Elim Afterwards the vapour bubbles coalesce forming an evaporated helium thin film close to the strands surface, which has an insulating effect. An energetic criterion composed with a criterion based on the critical heat flux has been used 11. Then adapted to the case of narrow channels (reduction of the critical heat flux with respect to a bath). When the energy transferred to He equals the energy needed for the film boiling formation Elim, the heat transfer coefficient drops to its film boiling value: hfilm boiling= 250 W/m²K 14 M. Nishi et al., “Boiling heat transfer characteristics in narrow cooling channels”, IEEE Trans on Magnetics, Vol. Mag-19, No 3, May 1983 12 W.G. Steward. , “Transient helium heat transfer: phase I- static coolant”, International Journal of Heat and Mass Transfer, Vol. 21, 1978 11 M. Breschi et al. “Minimum quench energy and early quench development in NbTi superconducting strands”, IEEE Trans. Appl. Sup., vol. 17(2), pp. 2702-2705, 2007. PP Granieri - Transient Heat Transfer and Stability of He-cooled SC Cables

11. Heat Transfer to Helium Gas Energy deposited into the channel = Elat The film thickness increases with the strands temperature.Since the whole He in the channel is vaporised (film thickness equal to the channel width), the worsening of the heat transfer observed in experimental works 15 is taken into account allowing transition to a totally gaseous phase. This is described by a constant heat transfer coefficient extrapolated from experiments: (16,17) hgas= 70 W/m²K 15 Y. Iwasa and B.A. Apgar, “Transient heat transfer to liquid He from bare copper surfaces to liquid He in a vertical orientation – I: Film boiling regime”, Cryogenics, 18 (1978) 267-275. 16 M. Nishi, et al., “Boiling heat transfer characteristics in narrow cooling channel”, IEEE Trans. Magn., vol. 19, no. 3, pp. 390–393, 1983. 17 Z. Chen and S.W. Van Sciver, “Channel heat transfer in He II – steady state orientation dependence”, Cryogenics, 27 (1987) 635-640. PP Granieri - Transient Heat Transfer and Stability of He-cooled SC Cables

12. Summary of the Heat Transfer Model He II He I Nucleate Boiling Film Boiling Gas PP Granieri - Transient Heat Transfer and Stability of He-cooled SC Cables

13. Impact of the Heat Transfer Model • Different heat transfer models have been considered: • I. an unrealistic one only using the Kapitza heat transfer coefficient • II. the full model as previously described • III. the full model without the external reservoir and without boiling phases • Time scales of the heat transfer mechanisms: • 400 μs : Kapitza regime • then λ-transition (+ boiling) • 5 s: heat transfer through the insulation • The enthalpy reserve (between Tbath and Tcs) of the cable components gives the lower and upper limits of the stability margin • Approach based on the response of the components, not just on their enthalpy PP Granieri - Transient Heat Transfer and Stability of He-cooled SC Cables

14. In the longitudinal direction: micro-channel He channel through the cable insulation Impact of the Cooling Surface The influence of the He micro-channels located between the cables in the coil and of the He channels network through the cable insulation is unknown. This parametric study allows to investigate the effectiveness of these heat transfer mechanisms. • The wetted perimeter pi,b between the insulation and the external bath is important for long heating times (pi,b = 35 mm is an unrealistic case corresponding to an entirely wetted insulation; 8.2 mm corresponds to the case of effective μ-channels, while 2.6 mm corresponds to μ-channels not effective at all); • The actual size of the He channels network (represented by G) starts playing a role for values above 10-5 m5/3; • In the calculations a conservative case has been considered, where pi,b = 2.6 mm and G = 0 m5/3. Pi,b effectiveness of the μ-channels QHeII Pi,b QHeII effectiveness of the He channels in the cable insulation PP Granieri - Transient Heat Transfer and Stability of He-cooled SC Cables

15. Systematic Calculation of the Stability Margins Stability margins of cable 1 (inner layer of the main dipoles) as a function of heating time, for different current levels: • Slower is the process, higher is the impact of helium • For low currents and short heating times the stability margin curve gets flat, since the “decision time” is much greater than the heating time I PP Granieri - Transient Heat Transfer and Stability of He-cooled SC Cables

16. “Decision Time” The “decision time” (time the system waits before “deciding” to quench or not) is noticeably longer at low current regimes “decision time (I = 1850 A)” PP Granieri - Transient Heat Transfer and Stability of He-cooled SC Cables

17. Systematic Calculation of the Stability Margins Stability margins of cable 1 as a function of magnetic field (ranging from 0.5 T to the peak field for a given current) and for several current levels: • The stability margin for a given cable has been estimated as a function of the heating time, current and magnetic field. This allows interpolating the results at any field for any cable in the magnet cross section I PP Granieri - Transient Heat Transfer and Stability of He-cooled SC Cables

18. Systematic Calculation of the Stability Margins Stability margins of all the CERN LHC cables working at 1.9 K as a function of heating time: • The stability of all the CERN LHC superconducting cables working at an operational temperature of 1.9 K has been numerically computed with respect to the actual range of beam loss perturbation times PP Granieri - Transient Heat Transfer and Stability of He-cooled SC Cables

19. Systematic Calculation of the Stability Margins The stability margin and the quench power needed to quench the cable are estimated for different heating times up to the steady-state value: • The model developed links the transient to the steady state regime: for long heating times the power needed to quench the cable decreases and reaches asymptotically the steady state value PP Granieri - Transient Heat Transfer and Stability of He-cooled SC Cables

20. Conclusions • 0-D thermal model  stability of superconducting magnets • Complete heat transfer model  considers all helium phases following a transient perturbation • Concept of “decision time”  behavior of a cable against transient disturbances • Sensitivity analysis with respect to the parameters of the model • Parametric analysis of the stability of the LHC magnets with respect to: heating time, current, magnetic field PP Granieri - Transient Heat Transfer and Stability of He-cooled SC Cables

21. Perspectives • Refinement of the model are ongoing • Modeling of heat transfer through the insulation (pressure, insulation scheme) will be soon available (measurements on the drainable heat through the cable insulation, for the LHC upgrade – phase I) • Dedicated experiments are foreseen to validate the described model, as well as the building of an instrumented LHC magnet • Increase of the complexity of the 0-D approach (e.g. a multi-strand approach with periodical boundary conditions), to better describe the internal structure of the cable and the predicted shape of a beam loss energy deposit PP Granieri - Transient Heat Transfer and Stability of He-cooled SC Cables

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23. QHeII Pi,b Text bath (18) Comparison with a steady-state experiment 18 D. Bocian, B. Dehning, A. Siemko, “Modelling of quench limit for steady state heat deposits in LHC magnets”, to be published on IEEE Trans. Appl. Sup.. PP Granieri - Transient Heat Transfer and Stability of He-cooled SC Cables