Superconducting Elements Karl Hubert Mess Training LHC Powering : Lesson I , Superconducting Elements, K. H. Mess, AT-MEL
Superconducting Elements 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 Training LHC Powering : Lesson I , Superconducting Elements, K. H. Mess, AT-MEL 2
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……. . 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……. . Not treated here. Training LHC Powering : Lesson I , Superconducting Elements, K. H. Mess, AT-MEL 4
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 Training LHC Powering : Lesson I , Superconducting Elements, K. H. Mess, AT-MEL 5
Who needs superconductivity anyway? Ban on Ohm’s Law! d! rl y o l w tivit ea uc Consequences id he cond y t er ll na sup fi , w ed is o W ne we ll A • 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) • 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 Training LHC Powering : Lesson I , Superconducting Elements, K. H. Mess, AT-MEL 6
Who needs superconductivity anyway? M Ban on Ohm’s Law! i sc on d! rl y cep l wo Su it pe t eaio uctiv Consequences rco he id nn! d Id y tnd erco alrl eaflina e sup c u , C isno to ow eeon t d rs W n du e cto lw l A rs! • 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) • 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 Training LHC Powering : Lesson I , Superconducting Elements, K. H. Mess, AT-MEL 7
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 Training LHC Powering : Lesson I , Superconducting Elements, K. H. Mess, AT-MEL 8
The discovery It took a long time to understand the quantummechanical 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 Nb. Ti is preferred today. Most superconductors are bad normal conductors, as will be explained. Training LHC Powering : Lesson I , Superconducting Elements, K. H. Mess, AT-MEL 9
Critical Temperature, Meissner Ochsenfeld 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. Critical Field Bc: Type 1 superconductors show the Meissner effect. Field is expelled when sample is cooled down to become superconducting. Critical Temperature qc 10 -23 where k. B = 1. 38 J/K is the Boltzmann's constant and D(0) is the energy gap (binding energy of Cooper pairs) of at q = 0 The thermodynamic energy due to superconductivity Gsup increases with the magnetic energy, which is expelled i. e. with B 2 Gsup reaches Gnormal at the maximal field Bc, which is small. (~0. 2 T) Type 1 superconductors are useless for magnets! Training LHC Powering : Lesson I , Superconducting Elements, K. H. Mess, AT-MEL 10
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 > . Material In Pb Sn Nb 24 nm 32 nm 30 nm 32 nm 360 nm 510 nm 170 nm 39 nm Training LHC Powering : Lesson I , Superconducting Elements, K. H. Mess, AT-MEL 11
Forming of Fluxoids Critical properties: temperature and field 2 Ginzburg Landau refine the argument: : If the ratio between the distance the magnetic field penetrates (l ) 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 Training LHC Powering : Lesson I , Superconducting Elements, K. H. Mess, AT-MEL
Shubnikov Phase, Type 2 Superconductors Critical properties: temperature and field 2 : If the ratio between the distance the magnetic field penetrates (l ) 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 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. Bc 2= 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. Training LHC Powering : Lesson I , Superconducting Elements, K. H. Mess, AT-MEL
Current Density The current (density) depends on the field and on the temperature and is a property of the sample. (here shown for Nb. Ti) 7 Current density k. Amm-2 6 5 4 3 re atu er p tem K 2 1 2 4 6 8 10 2 Fie ld T 4 6 8 10 12 14 16 Training LHC Powering : Lesson I , Superconducting Elements, K. H. Mess, AT-MEL 14
Working Point and Temperature Margin 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 2 • Energy deposition increases temperature • Temperature margin 1 Fie 2 4 6 8 10 2 ld T Deposited Energy: 2 m. J ~106 p/m • Movement • Eddy current warming • Radiation (all sorts) 4 6 8 10 Training LHC Powering : Lesson I , Superconducting Elements, K. H. Mess, AT-MEL 15
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) Only material • The Temperature at the origin (Thot-spot) continues to rise constants, can be calculated. Measurement of the max temperature (MIITS) Training LHC Powering : Lesson I , Superconducting Elements, K. H. Mess, AT-MEL 16
Material Constants, Copper low high Copper Resistivity Copper Thermal Conductivity Training LHC Powering : Lesson I , Superconducting Elements, K. H. Mess, AT-MEL
Material Constants, specific heat 0. 1 Cu 10 He 4 Scales differ, Specific heat of He is by far bigger than of Cu Compares with Water 4. 2 J/g K 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 Training LHC Powering : Lesson I , Superconducting Elements, K. H. Mess, AT-MEL
Magnet Quench – Quench Signal Introduction to testing the LHC magnets Info Sessions 2002, A. Siemko Threshold 10 ms validation window P R O T E C T I O N Training LHC Powering : Lesson I , Superconducting Elements, K. H. Mess, AT-MEL Slide 20
How to keep the temperature down? High temperature results in: Movement, friction Insulation damage Magnet destruction • 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 Training LHC Powering : Lesson I , Superconducting Elements, K. H. Mess, AT-MEL 21
Voltage High resistance means and high I*R high L*d. I/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. Training LHC Powering : Lesson I , Superconducting Elements, K. H. Mess, AT-MEL 22
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: Training LHC Powering : Lesson I , Superconducting Elements, K. H. Mess, AT-MEL 23
We are teasing the tiger. Training LHC Powering : Lesson I , Superconducting Elements, K. H. Mess, AT-MEL 24
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 Training LHC Powering : Lesson I , Superconducting Elements, K. H. Mess, AT-MEL
Voltage over one aperture Introduction to testing the LHC magnets Info Sessions 2002, A. Siemko Irreversible quench Spike Training LHC Powering : Lesson I , Superconducting Elements, K. H. Mess, AT-MEL Slide 26
Example of the mechanical activity in dipoles Circa 1 spike per 1 ms Slide 27 Introduction to testing the LHC magnets - Info Training LHC Powering : Lesson I , Superconducting Elements, K. H. Mess, AT-MEL
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