Electrical principles, magnet components and schematics, risks to and from magnets, protection

1. Electrical principles, magnet components and schematics, risks to and from magnets, protection MOPS Training Session 1 21.8.2008 KHM The nice ideas and pictures are stolen from M. Wilson , A. Siemko., R. Denz and P. Schmueser. The mistakes and the rest of it are mine. Apologies for the quality of pictures and talk. It had to be prepared in a hurry, parallel to HC.

2. Electrical principles, magnet components and schematics, risks to and from magnets, protection MOPS Training Session 1 21.8.2008 KHM

3. Outline Components in a typical circuit Energies Risks Energy Management (Protection) Quench Detection Reminder

4. The basic components: Consider a superconductor, already immersed in LHe:

5. The basic components: Consider a superconductor, already immersed in LHe: As such pretty useless, but the picture is incomplete, anyhow:

6. The basic components: Consider a superconductor, already immersed in LHe: We need: Current leads and all the warm parts We will have in addition: Inductance, resistance and capacitance

7. A single wire in details C R R L C C R

8. A single wire in detail Frequency dependence Stored magnetic energy C R R L C C R Stored electrical energy

9. Stored Magnetic Energy LHC dipole magnet (twin apertures) E = ½ LI2 L = 0.12H I = 11.5kA E = 7.8 x 106 Joules the magnet weighs 26 tonnes so the magnetic stored energy is equivalent to the kinetic energy of: 26 tonnes travelling at 88km/hr

10. Stored Magnetic Energy LHC dipole magnet (twin apertures) E = ½ LI2 L = 0.12H I = 11.5kA E = 7.8 x 106 Joules the magnet weighs 26 tonnes so the magnetic stored energy is equivalent to the kinetic energy of: 26 tonnes travelling at 88km/hr

11. Stored Magnetic Energy In a sector we have 154 magnets…in LHC we have 154*8 magnets with a total stored energy of E=9.6 GJ

12. Stored Magnetic Energy In a sector we have 154 magnets…in LHC we have 154*8 magnets with a total stored energy of E=9.6 GJ This corresponds a 100 000 to ship running at 27 knots.

13. Stored Magnetic Energy In a sector we have 154 magnets…in LHC we have 154*8 magnets with a total stored energy of E=9.6 GJ This corresponds a 100 000 to ship running at 27 knots.

14. Stored Magnetic Energy Magnetic energy can be converted to electrical energy by a fast change of the current (break of busbar, opening of a switch….). U=L dI/dt

15. K H Mess, LHC days 2003

16. In 2003: About 15…20% of all cold tested magnets have isolation problems. They can (with some exceptions) not be used in the tunnel. Why are these faults not detected earlier in the manufacturing? Reason 1: The faults are produced during cool down. (heater, omega) Reason 2: It is difficult, because we use Helium or measure lousy transmission lines. In 2008: Not all were found during the tests!!! K H Mess, LHC days 2003

17. Back to the basics Consider a superconductor, already immersed in LHe:

18. 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

19. 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 FieldBc: Type 1 superconductors show the Meissner effect. Field is expelled when sample is cooled down to become superconducting. Critical Temperatureqc The thermodynamic energy due to superconductivity Gsup increases with the magnetic energy, which is expelled i.e. with B2 Gsup reaches Gnormal at the maximal field Bc, which is small. (~0.2 T) where kB = 1.38 10-23 J/K is the Boltzmann's constant and D(0) is the energy gap (binding energy of Cooper pairs) of at q = 0 Type 1 superconductors are useless for magnets!

20. 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  > .

21. 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

22. 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 The coherence length  is proportional to the mean free path of the conduction electrons. 2is the area of a fluxoid. The flux in a fluxoid is quantised. The upper critical field is reached, when all fluxoid touch. Bc2=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.

23. Current Density 7 6 5 4 Current density kAmm-2 3 temperature K 2 1 Field T 2 2 4 4 6 6 8 8 10 10 12 14 16 The current (density) depends on the field and on the temperature and is a property of the sample. (here shown for NbTi)

24. Working Point and Temperature Margin 2 1 Field T 2 2 4 4 6 6 8 8 10 10 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 ~106 p/m • ~1 A4 sheet falling 4 cm • Movement • Eddy current warming • Radiation (all sorts)

25. Material Constants, Copper Low ρ High λ Copper Thermal Conductivity Copper Resistivity

26. Material Constants, specific heat 0.1 10 Cu He 4 Scales differ, Specific heat of He is by far bigger than of Cu Compares with Water 4.2 J/g K

27. 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 (Thot-spot) continues to rise Only material constants, can be calculated. Measurement of the max temperature (MIITS)

28. Material Constants, specific heat Highest at the  point and around the boiling point Water

29. Magnet Quench – Quench Signal 10ms validation window Introduction to testing the LHC magnets - Info Sessions 2002, A. Siemko P R O T E C T I O N Threshold

30. 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 or Resistors • Using Resistors <= Attention, introduces a time delay L/R and Quench back • Extract the energy (External Resistors and Switches) • Slow, energy into air/water, needed to protect the diodes

31. 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.

32. Voltage breakdown Current I - U + K H Mess, LHC days 2003

33. Voltage breakdown K H Mess, LHC days 2003

34. U.V. light Electron avalancheNe(x)=Ne(0)* eax Ion Bombardment Per electron (ead-1) ions hit the Cathode In total ead/(1-g(ead-1)) Breakdown for (1-g(ead-1)) = 0 , ead>> 1 => g e ad ~ 1 K H Mess, LHC days 2003

35. U.V. light Electron avalancheNe(x)=Ne(0)* eax Ion Bombardment Per electron (ead-1) ions hit the Cathode In total ead/(1-g(ead-1)) • is proportional to the density n. It varies with the • field E (geometry!) • and depends on the gas Breakdown for (1-g(ead-1)) = 0 , ead>> 1 => g e ad ~ 1 K H Mess, LHC days 2003

36. Combine it to obtain: In uniform gaps E=V/d Paschens law K H Mess, LHC days 2003

37. Combine it Paschens law Approx. in LHC-PM-ES-1, in kg/l and mm K H Mess, LHC days 2003

38. In air at this density Vb=6.6kV !!! K H Mess, LHC days 2003

39. Values differ, because of different Cathodes and geometries K H Mess, LHC days 2003

40. A Data Compilation

41. Minimal detectable distance for various scenarios in He 1 bar 2 bar 6 bar 4.2 K gas Liquid He

42. The break down voltage of air is 6 * bigger than that of He. • Tests at elevated voltages run into problems at other spots. • Magnets that have seen Helium, may not be tested again at “air voltages”. • Voltages during operation (quench) may be locally higher than can be applied globally. Interturn shorts are particularly difficult. • We have observed problems with the heater strips. K H Mess, LHC days 2003

43. Evidence of the insulation deficiency K H Mess, LHC days 2003

44. The break down voltage of air is 6 * bigger than that of He. • Tests at elevated voltages run into problems at other spots. • Magnets that have seen Helium, may not be tested again at “air voltages”. • Voltages during operation (quench) may be locally higher than can be applied globally. Interturn shorts are particularly difficult. • We have observed problems with the heater strips. And continue to do so K H Mess, LHC days 2003

45. Energy Management • Divide et impera! • Treat sectors separately! • Detect resistive the transistion asap • Divide the energy in a magnet over many windings, using heaters (if necessary). • Guide the energy of all other 153 (or so) magnets around using a diode or resistor. • Protect the diode by a fast extraction of the energy.

46. Voltage over one aperture Introduction to testing the LHC magnets - Info Sessions 2002, A. Siemko Irreversible quench Spike

47. Example of the mechanical activity in dipoles Circa 1 spike per 1ms

48. Quench - What Went Wrong? • Abnormal voltage signals recorded during the provoked quench Courtesy: A. Siemko

49. How does it look at LHC?

50. Symbolic Circuit