1. Training LHC Powering : Lesson I, Superconducting Elements, K. H. Mess, AT-MEL Superconducting Elements Karl Hubert Mess

2. Training LHC Powering : Lesson I, Superconducting Elements, K. H. Mess, AT-MEL Superconducting Elements 2 Accelerator Elements based on Superconductors Elements of Superconductivity What is particular with superconducting Elements? To burn or not to burn? Attention: Mainly a “generic talk”, applying to all SC accelerators

3. Training LHC Powering : Lesson I, Superconducting Elements, K. H. Mess, AT-MEL 3 What superconducting elements do we find? Magnets Current Leads Superconducting Busbars/ Superconducting Links RF Cavities Beam Instrumentation (High Resolution BCT based on SQIDS) Auxiliaries –Quench Protection –Cold Diodes –Energy Extraction –Beam Loss Monitoring –Post Mortem System –Interlocks……..

4. Training LHC Powering : Lesson I, Superconducting Elements, K. H. Mess, AT-MEL 4 What superconducting elements do we find? Magnets Current Leads Superconducting Busbars/ Superconducting Links RF Cavities Beam Instrumentation (High Resolution BCT based on SQIDS) Auxiliaries –Quench Protection –Cold Diodes –Energy Extraction –Beam Loss Monitoring –Post Mortem System –Interlocks…….. Not treated here.

5. Training LHC Powering : Lesson I, Superconducting Elements, K. H. Mess, AT-MEL 5 Who needs superconductivity anyway? Ban on Ohm’s Law! no power consumption (although do need refrigeration power) high current density ampere turns are cheap, so we don’t need iron (often for shielding only) Consequences lower power bills higher magnetic fields mean reduced bend radius ⇒ smaller rings ⇒ reduced capital cost ⇒ new technical possibilities (e.g. LHC) higher quadrupole gradients ⇒ higher luminosity

6. Training LHC Powering : Lesson I, Superconducting Elements, K. H. Mess, AT-MEL 6 Who needs superconductivity anyway? Ban on Ohm’s Law! no power consumption (although do need refrigeration power) high current density ampere turns are cheap, so we don’t need iron (although often use it for shielding) Consequences lower power bills higher magnetic fields mean reduced bend radius ⇒ smaller rings ⇒ reduced capital cost ⇒ new technical possibilities (e.g. LHC) higher quadrupole gradients ⇒ higher luminosity

7. Training LHC Powering : Lesson I, Superconducting Elements, K. H. Mess, AT-MEL 7 Who needs superconductivity anyway? Ban on Ohm’s Law! no power consumption (although do need refrigeration power) high current density ampere turns are cheap, so we don’t need iron (although often use it for shielding) Consequences lower power bills higher magnetic fields mean reduced bend radius ⇒ smaller rings ⇒ reduced capital cost ⇒ new technical possibilities (e.g. LHC) higher quadrupole gradients ⇒ higher luminosity

8. Training LHC Powering : Lesson I, Superconducting Elements, K. H. Mess, AT-MEL 8 The discovery Kamerlingh Onnes liquifies for the first time (1908) Helium and studies the temperature dependence of the electrical resistance of metals. (1911) Below a critical temperature the resistance (voltage drop) seems to disappear. He calls the phenomenon “Superconductivity”. Nobel Price in 1913

9. Training LHC Powering : Lesson I, Superconducting Elements, K. H. Mess, AT-MEL 9 The discovery It took a long time to understand the quantum- mechanical nature of the superconductivity. Many metals are superconducting at very low temperature. Also Pb, Nb….Most superconductors in the plot are brittle crystals. The ductile NbTi is preferred today. Most superconductors are bad normal conductors, as will be explained.

10. Training LHC Powering : Lesson I, Superconducting Elements, K. H. Mess, AT-MEL 10 Critical Temperature, Meissner Ochsenfeld Critical Temperature  c Critical Field B c : Type 1 superconductors show the Meissner effect. Field is expelled when sample is cooled down to become superconducting. Low temperature superconductivity is due to a phase transition. Phase transitions happen to keep the relevant thermodynamic energy (Gibbs energy) low. Here pairs of electrons of opposite momenta and spin form a macroscopic (nm) boson, the Cooper Pair. The binding energy determines the critical temperature. where k B = 1.38 10 -23 J/K is the Boltzmann's constant and  (0) is the energy gap (binding energy of Cooper pairs) of at  = 0 Type 1 superconductors are useless for magnets! The thermodynamic energy due to superconductivity G sup increases with the magnetic energy, which is expelled i.e. with B 2 G sup reaches G normal at the maximal field B c, which is small. (~0.2 T)

11. Training LHC Powering : Lesson I, Superconducting Elements, K. H. Mess, AT-MEL 11 London Penetration depth, Coherence Length Very thin (< ) slabs do not expel the field completely. Hence less energy needed. Thick slabs should subdivide to lower the energy. But we pay in Cooper pair condensation energy to build sc boundaries of thickness energy . We gain due to the not expelled magnetic energy in the penetration depth. There is a net gain if > . MaterialInPbSnNb 24 nm32 nm30 nm32 nm  360 nm510 nm170 nm39 nm

12. Training LHC Powering : Lesson I, Superconducting Elements, K. H. Mess, AT-MEL Critical properties: temperature and field 2 Ginzburg Landau refine the argument:: If the ratio between the distance the magnetic field penetrates ( ) London penetration depth and the characteristic distance  Coherence length over which the electronic state can change from superconducting to normal is larger than 1/  2, the magnetic field can penetrate in the form of discrete fluxoids - Type 2 Forming of Fluxoids

13. Training LHC Powering : Lesson I, Superconducting Elements, K. H. Mess, AT-MEL Critical properties: temperature and field 2 : If the ratio between the distance the magnetic field penetrates ( ) London penetration depth and the characteristic distance  Coherence length over which the electronic state can change from superconducting to normal is larger than 1/  2, the magnetic field can penetrate in the form of discrete fluxoids - Type 2 Shubnikov Phase, Type 2 Superconductors The coherence length  is proportional to the mean free path of the conduction electrons.  2 is the area of a fluxoid. The flux in a fluxoid is quantised. The upper critical field is reached, when all fluxoid touch. B c2 =  0 /(2  2 ). Hence, good superconductors are always bad conductors (short free path). Type 2 Superconductors are mostly alloys. Transport current creates a gradient in the fluxoid pattern. Fluxoids must be movable to do that. However not too much, otherwise the field decays ….. Here starts the black magic.

14. Training LHC Powering : Lesson I, Superconducting Elements, K. H. Mess, AT-MEL 14 Current Density 10 8 6 4 2 2 4 6 8 12 14 16 Field T 1 2 3 4 5 6 7 Current density kAmm -2 temper ature K The current (density) depends on the field and on the temperature and is a property of the sample. (here shown for NbTi)

15. Training LHC Powering : Lesson I, Superconducting Elements, K. H. Mess, AT-MEL 15 Working Point and Temperature Margin 10 8 6 4 2 2 4 6 8 Field T 1 2 Blue plane: constant temperature, green plane: constant field Red arrow: “load line”= constant ratio field/current If the “working point” leaves the tent (is outside the phase transition) => “Quench” Too far on the load line: Magnet Limit Energy deposition increases temperature Temperature margin Deposited Energy: 2 mJ ~10 6 p/m Movement Eddy current warming Radiation (all sorts)

16. Training LHC Powering : Lesson I, Superconducting Elements, K. H. Mess, AT-MEL 16 Quench Development Heat Capacity <= small Heat Conductivity, radial<= small Heat Conductivity, longitudinal<= good Cooling<= depends The Quench expands (if the current is above the recovery limit) The Temperature at the origin (T hot-spot ) continues to rise Only material constants, can be calculated. Measurement of the max temperature (MIITS)

17. Training LHC Powering : Lesson I, Superconducting Elements, K. H. Mess, AT-MEL Material Constants, Copper Copper Resistivity Copper Thermal Conductivity low high

18. Training LHC Powering : Lesson I, Superconducting Elements, K. H. Mess, AT-MEL Material Constants, specific heat Scales differ, Specific heat of He is by far bigger than of Cu Compares with Water 4.2 J/g K 0.1 10 Cu He 4

19. Training LHC Powering : Lesson I, Superconducting Elements, K. H. Mess, AT-MEL Material Constants, specific heat Highest at the point and around the boiling point Water

20. Training LHC Powering : Lesson I, Superconducting Elements, K. H. Mess, AT-MEL Slide 20 Introduction to testing the LHC magnets - Info Sessions 2002 Magnet Quench – Quench Signal Threshold 10ms validation window PROTECTIONPROTECTION Introduction to testing the LHC magnets - Info Sessions 2002, A. Siemko

21. Training LHC Powering : Lesson I, Superconducting Elements, K. H. Mess, AT-MEL 21 How to keep the temperature down? Keep the MIITS down by Heatcapacity and Resistivity (too late now) Keep the MIITS down by shortening the current flow Increase the bulk resistivity (Heating, spread the energy) Fast, complicated, energy into He Bypass the energy of the rest of the sector (if applicable) Using Diodes Using Resistors <= Attention, introduces a time delay L/R Extract the energy (External Resistors and Switches) Slow, energy into air/water, needed to protect the diodes High temperature results in: Movement, friction Insulation damage Magnet destruction

22. Training LHC Powering : Lesson I, Superconducting Elements, K. H. Mess, AT-MEL 22 Voltage High resistance means high I*R and high L*dI/dt High voltage is dangerous for the insulation Local damage => ground short or winding short Global damage => Diodes reverse voltage Voltage taps Overvoltage can be/ can develop to be a global phenomenon. Can cause considerable damage.

23. Training LHC Powering : Lesson I, Superconducting Elements, K. H. Mess, AT-MEL 23 Conclusion Superconductivity is not really new. Superconducting elements are a bit unusual, however. It is not easy and potentially dangerous… but possible, as the Tevatron, HERA and RHIC have demonstrated since more than ¼ of a century. LHC is much more complicated, however, and much closer to the edge. Listen carefully to what will be explained by the other speakers. Always be aware:

24. Training LHC Powering : Lesson I, Superconducting Elements, K. H. Mess, AT-MEL 24 We are teasing the tiger.

25. Training LHC Powering : Lesson I, Superconducting Elements, K. H. Mess, AT-MEL References: H. Brechna, Superconducting Magnet Systems, Springer, Berlin 1973 P. Schmueser, Superconducting magnets for particle accelerators, Rep. Prog. Phys. 54 (191) 683 M. N. Wilson, Superconducting Magnets, Clarendon Press, Oxford, 1983 See also his lectures here and at CAS A.Siemko, Introduction to testing the LHC magnets - Info Sessions 2002 http://nobelprize.org/nobel_prizes/physics/laureates/1913/onnes-lecture.pdf http://www.bnl.gov/magnets/Staff/Gupta/cryogenic-data-handbook KHM et al, Superconducting Accelerator Magnets, World Scientific, Singapore, 1996 References

26. Training LHC Powering : Lesson I, Superconducting Elements, K. H. Mess, AT-MEL Slide 26 Introduction to testing the LHC magnets - Info Sessions 2002 Voltage over one aperture Spike Irreversible quench Introduction to testing the LHC magnets - Info Sessions 2002, A. Siemko

27. Training LHC Powering : Lesson I, Superconducting Elements, K. H. Mess, AT-MEL Slide 27 Introduction to testing the LHC magnets - Info Sessions 2002 Example of the mechanical activity in dipoles Circa 1 spike per 1ms