Superconduttivita e Criogenia

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1 Superconduttivita e Criogenia Torino, 15 Giugno 2012 European Organization for Nuclear Research (CERN), Geneva

2 Introduction Superconductivity for high energy physics Part I Cryogenics for LHC Part II Superconductivity in LHC PART III Superconductivity & Cryogenics in LHC Upgrades

3 Introduction Superconductivity for high energy physics Part I Cryogenics for LHC Part II Superconductivity in LHC PART III Superconductivity & Cryogenics in LHC Upgrades

4 Introduction What is cryogenics? Cryogenic: for Greek kryos", which means cold or freezing, and "genes" meaning born or produced. In physics, cryogenics is the study of the production of very low eye temperature (below 150 C, 238 F or 123 K) and the behavior of materials at those temperatures from Wikipedia The lowest natural temperature ever recorded on earth was in 1983 in Antartica: o C or K What is superconductivity? Superconductivity is a phenomenon occurring in certain materials at very low temperatures, characterized by exactly zero electrical resistance and the exclusion of the interior magnetic field (the Meissner effect) from Wikipedia

5 Introduction Resistance (Ohm) vs Temperature (K) of Hg K. Onnes, 1911

6 Introduction Cryogenics Particles accelerator Superconductivity

7 Introduction Accelerator: Instrument of High Energy Physics h/p 1 TeV m

Introduction Accelerator Energy and Magnetic Field

beams: E CM ~2E Energy (TeV) 100.00 10.00 1.00 0.10 0.

8 Introduction Accelerator Energy and Magnetic Field Particles are produced by converting the collision energy into mass Synchrotron: E[GeV]= B[T] R[m] High energy High field magnets Colliding beams: E CM ~2E Energy (TeV) LHC Tevatron SSC UNK LHC r=10 km HERA RHIC LEP r=3 km r=1 km r=0.3 km Dipole field (T)

9 Introduction Resistive magnets The most economical designs are iron-dominated; The upper field limit for iron-dominated magnets is ~2 T due to iron saturation; Most resistive accelerator magnet rings are operated at low field (~ 1 T), to limit power consumption ( B R). Superconducting magnets Significant reduction in power consumption (cryogenic power R); Higher current density in the magnet coils.

10 Introduction Economical savings due to superconductivity The LHC has a circumference of 27 km, out of which some 20 km of main superconducting magnets operating at 8.3 T and 1.9 K. Cryogenics will consume about 40 MW electrical power from the grid. If the LHC were not superconducting: Using resistive magnets operating at 1.8 T (limited by iron saturation), the circumference should be about 100 km, and the electrical consumption 900 MW, leading to prohibitive capital and operation costs.

11 Introduction The Energy Frontier

12 Introduction Large Superconducting Accelerators Tevatron HERA RHIC LEP LHC FermiLab DESY BNL CERN CERN Type p - p e - p Heavy ion e + - e - p - p Radius (m) Energy per beam (TeV) * 7 Op. temperature (K) RT-4.5 K* 1.9 Dipole field (T) Dipole Current (ka) Commissioning * Upgrade with sc cavities to achieve the mass of the pair of W particles

13 Introduction Superconductivity for high energy physics Part I Cryogenics for LHC Part II Superconductivity for LHC PART III Superconductivity & Cryogenics for LHC Upgrades

14 Cryo-history 1848: Determination of existence of zero absolute (Lord Kelvin) 1853: Discovery of Joule-Thomson effect (cool-down of a compressed gas during adiabatic expansion) 1877: Liquefaction of oxygen below 320 atmospheres (R. P., Pictet) 1895: First liquefaction of air (Carl von Linde) 1898: Liquefaction of hydrogen (Dewar) 1908: Liquefaction of 4.2 K (*K. Onnes, Leiden Univ., ND) 1911: Discovery of superconductivity (K. Onnes) 1938 : Discovery of superfluid helium (He II). 1978: Kapitza, Nobel Price in Physics 1971: Use of superfluid helium at atmospheric pressure (Roubeau) 1990: Use of pressurized superfluid helium in large-scale projects (Claudet and Aymar) 2008: Start-up of LHC, with the world s largest cryogenic system (superfluid helium for cooling the superconducting magnets) *1913:Noble Price in Physics for his investigations on the properties of matter at low temperatures which led, inter alia, to the production of liquid helium

15 Cryogenic fluids Standard P-T and T-v Phase diagrams Cryogen Triple point (K) Normal Boiling point (K) Critical point (K) Oxygen Argon Saturated Vapor/liquid Nitrogen Temperature Neon Hydrogen Helium 2.2 ( -point) Saturated vapor curve Saturated liquid curve

16 Helium Second lightest elemental gas, after hydrogen Smallest of all molecules Colorless, odorless, inert and non-toxic Lowest boiling point of any element ( C, 4.2 K) Helium is produced continually by the radioactive decay of uranium and other elements, gradually working its way into the atmosphere Commercial extraction from air is impractical because helium's concentration is only about five parts per billion. Commercially, helium is obtained from the small fraction of natural gas deposits that contain helium volumes of 0.3 percent or higher Unusual characteristics: Helium remains liquid to absolute zero At 2.1 K, liquid helium exhibits super fluidity or virtually zero viscosity (Helium II), defies gravity to flow up container walls and becomes nearly a perfect heat conductor

17 10000 Phase diagram of helium (He 4 ) SOLID line SUPER- CRITICAL Pressure [kpa] 100 He II PRESSURIZED He II (Subcooled liquid) He I CRITICAL POINT Tc=5.19 K SATURATED He I pc=2.2 bar 10 -point T=2.17 K p=50 mbar SATURATED He II VAPOUR Temperature [K]

18 Specific heat of saturated helium

19 Viscosity of He II Allen and Misener Keesom Two fluid model: r n, n, s n r s, s =0, s s =0

20 Y(T) ± 5% Equivalent thermal conductivity of He II 2.4 K T,q q Y T dt dx q Y(T) q in W / cm T in K X in cm Helium II T [K] Conduction in a tubular conduit OFHC copper T

21 Superconductor performance at 4.2 K and 1.9 K Critical current density of NbTi as a function of Magnetic field at 4.2 K and 1.9 K

22 10000 Saturated and pressurized He II at 1.9 K SOLID line SUPER- CRITICAL Pressure [kpa] 100 He II PRESSURIZED He II (Subcooled liquid) He I CRITICAL POINT SATURATED He I 10 -point VAPOUR SATURATED He II Temperature [K]

23 Large scale He II systems Use of saturated superfluid helium. By lowering the pressure of the vapor when pumping on LHe, the boiling temperature is reduced. Liquid helium vapor pressure equilibrium and latent heat of vaporization T (K) P (kpa) L (J g -1 )

24 Large scale He II systems Sat. He Pres. He Air inleak prevention - + Dielectric strength - + Pump-down time - + Enthalpy margin for transient - + Technical simplicity SOLID line SUPER- CRITICAL Pressure [kpa] 100 He II PRESSURIZED He II (Subcooled liquid) He I CRITICAL POINT SATURATED He I 10 -point VAPOUR SATURATED He II Temperature [K]

25 Refrigeration Refrigerator: machine that extracts heat (Qc) from a low temperature source (Tc) by adsorbing mechanical work (W); by doing so, it rejects heat (Qh) to the high temperature source (Th, usually equal to room temperature) W = Qh-Qc COP = benefit/cost = Qc/W = 1/((Qh/Qc)-1) COP Tc/(Th-Tc) COP= Cofficient of Performance

26 Carnot cycle The Carnot cycle is the most efficient refrigeration cycle operating between two temperature levels Tc (K) 77 (LN 2 ) 20 (LH 2 ) 4.2 (LHe) 1.8 (LHe) W/Qc* COP *Figure of merit COP = Tc/(Th-Tc) Tc/(Th-Tc) 0 Tc Isothermal expansion 2-3 Isentropic compression 3-4 Isothermal compression 4-1 Isentropic expansion T 4 Q H Q C s

27 Hypothetical Carnot cycle for He refrigeration

28 Claudet cycle Widely used in refrigeration technology. It combines isenthalpic and isentropic expansion. A refrigerator based on the Claudet cycle comprises a compressor at RT, a series of heat exchangers, an expansion turbine and a Joule-Thomson valve.

29 Refrigeration Cooling methods: Heat transfer counter flow heat exchangers External work engine work (usually via turbine expanders, with reduction of the gas temperature and pressure) Isenthalpic expansion (Joule-Thomson effect)

30 Joule-Thompson inversion temperature: temperature above which expansion at constant enthalpy causes the temperature to rise, and below which such expansion causes cooling Joule Thomson (Kelvin) coefficient: JT =( T/ p) h (K/Pa) JT > 0 JT < 0 T<Tin T>Tin Joule-Thomson effect: adiabatic expansion of a gas (constant enthalpy)

33 Efficiency of refrigerators and liquefiers as a function refrigeration capacity 30 (W/Qc) CARNOT,4.2K = 68.7 (Q/Qc) = 68.7/

34 The cryogenic system for the LHC machine c

35 LHC layout Sector=3.3 km SSS=528 m

36 From the LEP to the LHC machine 8000

37 Layout of LHC cryogenic system 5 Cryogenic islands 8 Refrigerators: - 2 at P4, P6 and P8-1 at P2-1 at P1.8 1 Refrigerator serves 1 Sector (3.3 km) Possibility of coupling two refrigerators via the interconnection box

38 Performance of 4.5 K refrigerators at CERN LHC: COP= 230 W/W = 29 CARNOT Electrical input power installed 4.5 MW/refrigerator

39 Gas storage tanks

40 Cooling the LHC ring Total refrigeration capacity: 144 kw at 4.5 K ( K, 600 kw liquid nitrogen pre-cooler liquid used during cool-down) and 20 kw at 1.9 K ( K) distributed around the ring.

41 Principle of LHC cooling scheme In the LHC, heat is conducted from the superfluid helium to the a heat exchanger pipe containing a mixture of saturated vapor and superfluid helium (p 16 mbar) arranged in a series of 107 m cooling loops all around the ring.

42 Schematic of cooling system SHe supply (4.6 K, 3 bar) GHe pumping (15 to 19 mbar) Bayonet heat exchanger Saturated LHeII Magnet Ps0 JT Hydraulic plug TT TT TT TT TT TT TT TT Wetted length (Lw) Dried length (Ld) Pressurised LHeII Magnet cell (107 m) Slope

43 Cryogenic flow scheme of an LHC cell (107 m) 107 m

44 Temperature levels in the LHC cryogenic system In view of the high cost of refrigeration at 1.8 K, the main heat influx in intercepted at higher temperatures. The temperature levels are: 50 K to 75 K for thermal shielding of the cold masses; 4.6 K to 20 K for cooling of the beam screens and lower temperature interception; 1.9 K quasi-isothermal superfluid helium for the magnets; 4 K helium at very low pressure for transporting the superheated helium flow coming from the 1.8 K heat exchanger tubes across the sector length to the 1.8 K refrigeration unit; 4.5 K saturated helium for some insertion magnets, RF cavities, and the bottom end of the HTS leads; 20 K to 300 K cooling for the resistive upper section of the HTS leads.

45 Cryogenic block diagram 4

46 Thermodynamic states of He in LHC cryogenic system

47 E1 T1 Process flow diagram of LHC 18 kw cryoplant E3 20 K K loads E4 T2 (LHC current leads) LN2 Precooler 1.9 MPa 0.4 MPa 0.1 MPa 50 K - 75 K loads (LHC shields) E6 T3 201 K T1 from LHC loads E7 E8 Adsorber E9a T4 32 K 49 K T4 75 K T3 LHC shields T2 4.5 K - 20 K loads (magnets + leads + cavities) E9b E10 E11 E12 T5 T7 0.3 MPa 13 K 10 K T6 20 K T5 T8 To LHC loads T7 4.4 K 9 K from LHC loads T6 E13 T8

48 1.8 K He refrigeration system 1.8 K Refrigeration Unit CCB WCS Cold compressors Adsorbers Turblne D C 4.5 K Refrigerator 0.13 MPa, K 0.3 MPa 4.6 K B LHC Sector Load LHe 1.8 K Q 1.8K Installed pumping capacity 125 g/s at 15 mbar (i.e. ~ K)

49 LHC Distribution system The refrigerating helium of the magnets and cavities have to be distributed over the 27 km of the accelerator, in the LHC tunnel, 100 m below ground level. Due to the size of the experiment, ultra high performance cryogenic lines have been specially designed for the LHC project. The achieved performance of the line is: about 0.05 W/m on each of the 4.5 K tubes of the transfer line, a performance around 10 times better than usually achieved

50 LHC Cryogenic line in the LHC tunnel

51 Connection via service module and jumper Supply and recovery of helium with 26 km long cryogenic distribution line Static bath of superfluid helium at 1.9 K in cooling loops of 110 m length

52 Transverse cross section of the LHC tunnel

53 Cool-down of LHC machine Removal of air in circuit by evacuation and He filling (1 week); Removal of dust and debris by flushing the helium circuits with high helium flow at 300 K (1 to 2 week per sector) ; Cool-down of 4625 t per sector over 3.3 km: From 300 to 80 K: 600 kw pre-cooling with LN 2, up to ~5 t/h, 6 LN 2 trailer per day during 10 days (1250 t of LN 2 in total); From 80 to 5 K: Cryoplant turbo-expander cooling; LHe filling: 15 t of LHe in total (4 trailers); LHe filling and cool-down completion by using the 1.8 K refrigeration unit (1 week/sector)

54 Cool-down of Sector K 20 K Temperature evolution during cool-down of the first LHC Sector From RT to 80 K, from 80 K to 20 K, from 20 K to 4.5 K, from 4.5 K to 1.9 K with LHe filling. Electrical test are performed at each plateau. The time for cooling one Sector is about 3 weeks (3.3 km, 1250 tons LN 2, 18 tons of He, 800 g/s of He, 4600 tons of material in the Sector)

Cryogenic instrumentation Type Quantity Location Thermometers (Cernox 1.

56 Cryogenic instrumentation Type Quantity Location Thermometers (Cernox K) Thermometers (Pt100, Thermocouple) 700* Magnet cold mass, QRL, DFB 500* Level gauges 70 Cryo-heaters 300 Cryogenic control valves 180 QRL Warm control valves (DFB) 150 Current lead Total per sector 1900 Current lead, cryo-heater, thermal shield SSS phase separator, DFB, standalone magnet Magnet cold mass, DFB, QRL, beam screen

57 Heat load and cryostat design Heat inleaks Radiation 70 K shield, MLI Residual gas conduction Vacuum < 10-4 Pa Solid conduction Non-metallic supports Heat intercepts Joule heating Superconductor splices Resistance < a few nw Beam-induced heating Synchrotron radiation (0.6 W/m) } Beam image currents (0.8 W/m) } 5-20 K beam screens Localized heat due to loss of particles } (e.g particles escaping collimations)

58 Cryostat of LHC dipole

59 Thermal shield

60 Multi-layer insulation

61 Horizontal tanks for storage of He gas (250 m 3 ) at 2 MPa Vertical dewars (100 m 3 ) for liquid nitrogen He gas

62 LHC Control Room

63 The CERN accelerator network

Supercritical Helium Cooling of the LHC Beam Screens

EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCH European Laboratory for Particle Physics Large Hadron Collider Project LHC Project Report Supercritical Helium Cooling of the LHC Beam Screens Emmanuel Hatchadourian,,