Introduction to Cryogenics T. Koettig, P. Borges de Sousa, J. Bremer Contributions from S. Claudet and Ph. Lebrun CAS@ESI: Basics of Accelerator Physics and Technology, 7 - 11 Oct. 2019, Archamps 10-Oct-19 T. Koettig TE/CRG 2 Content • Introduction to cryogenic installations • Safety aspects => handling cryogenic fluids • Motivation => reducing thermal energy in a system • Heat transfer and thermal insulation • Helium cryogenics, He I => He II • Conclusions • References 10-Oct-19 T. Koettig TE/CRG 3 Overview of cryogenics at CERN - Detectors • Superconducting coils 9803026 - of LHC detectors @ DI - 4.5 K (ATLAS, CMS) rom: CERN F • LAr Calorimeter - LN2 - cooled • Different types of cryogens (Helium, Nitrogen and Argon) ://cms.web.cern.ch/news/cms design - From: http detector 10-Oct-19 T. Koettig TE/CRG 4 Overview of cryogenics at CERN - LHC • Helium at different operating temperatures (thermal shields, beam screens, distribution and magnets,…) 0712013 - • Superconducting magnets of EX - the LHC accelerator CERN CDS CERN • Accelerating SC cavities 10-Oct-19 T. Koettig TE/CRG 5 Safety aspects in cryogenics 10-Oct-19 T. Koettig TE/CRG 6 Cryogenic fluids - Thermophysical properties 4 Fluid He N2 Ar H2 O2 Kr Ne Xe Air Water Boiling temperature (K) 4.2 77.3 87.3 20.3 90.2 119.8 27.1 165.1 78.8 373 @ 1.013 bar Latent heat of evaporation @ 20.9 199.1 163.2 448 213.1 107.7 87.2 95.6 205.2 2260 Tb in kJ/kg Volume ratio gas(273 K) / liquid 709 652 795 798 808 653 1356 527 685 ----- Volume ratio saturated vapor 7.5 177.0 244.8 53.9 258.7 277.5 127.6 297.7 194.9 1623.8 to liquid (1.013 bar) Specific mass of liquid (at Tb) – 125 804 1400 71 1140 2413 1204 2942 874 960 kg/m3 10-Oct-19 T. Koettig TE/CRG 7 Cryogenic hazardous events – Discharge of helium Safer location on the floor • Displacement of oxygen => Asphyxiation risk! • Opposite behavior for e.g. argon and nitrogen! Simulated blow out in the LHC tunnel cross section 10-Oct-19 T. Koettig TE/CRG 8 Technical risks Embrittlement • Some materials become brittle at low temperature and rupture when subjected to mechanical force (carbon steel, ceramics, plastics) Thermal contraction (293 K to 80 K) • Stainless steel: 3 mm/m • Aluminum: 4 mm/m • Polymers: 10 mm/m • Requires compensation for transfer lines, QRL … Condensation of atmospheric gases • Inappropriate insulation or discharge of cryogens • Observed at transfer lines and during filling operations (liquid air ~50 % O2 instead of 21 % in atmospheric air) 10-Oct-19 T. Koettig TE/CRG 9 Cryogenics and Superconductivity 10-Oct-19 T. Koettig TE/CRG 10 Characteristic temperatures of low-energy phenomena Phenomenon Temperature Debye temperature of metals few 100 K High-temperature superconductors ~ 100 K Low-temperature superconductors ~ 10 K Intrinsic transport properties of metals < 10 K Cryopumping few K Cosmic microwave background 2.7 K Superfluid 4He 2.17 K Bolometers for cosmic radiation < 1 K ADR stages, Bose-Einstein condensates ~ mK 10-Oct-19 T. Koettig TE/CRG 11 Superconductivity • H. Kamerlingh Onnes • Liquefied helium in 1909 at 4.2 K with 60 g He inventory • Observed in 1911 for the first time superconductivity of mercury • Nobel prize 1913 Historic graph showing the superconducting transition of mercury, measured in Leiden in 1911 by H. Kamerlingh Onnes. 10-Oct-19 T. Koettig TE/CRG 12 Cryogenic application: Dipole magnets of the LHC LHC main cryostat Alignment targets (Cross-section) All circuits are cooled by helium Multi layer insulation (MLI) Vacuum vessel Thermal shield Bottom tray (50-65 K feed line Beam screen ~ 19.5 bar) 5-20 K support post heat intercept Magnet support posts (GFRE) External supports (jacks) 10-Oct-19 T. Koettig TE/CRG 13 Heat Transfer and Thermal Insulation 10-Oct-19 T. Koettig TE/CRG 14 Heat transfer: General • Solid conduction: • Thermal radiation: (with and without MLI) • Natural convection: Negligible with insulation vacuum for p< 10-6 mbar Source: Edeskuty, Safety in the Handling of Cryogenic Fluids 10-Oct-19 T. Koettig TE/CRG 15 Heat transfer: General • Solid conduction: Choosing the right • materialsThermal radiation: in terms of (with and without MLI) mechanical strength and low-temperature • Natural convection: propertiesNegligible with isinsulation essential. vacuum for p< 10-6 mbar Source: Edeskuty, Safety in the Handling of Cryogenic Fluids 10-Oct-19 T. Koettig TE/CRG 16 Heat capacity of materials Discrete lattice vibrations => Phonon Metals have a contribution of free electron gas => dominant at very low temperature Source: Ekin, Experimental Techniques for Low-Temperature Measurements. Source: Enss, Low temperature physics. 10-Oct-19 T. Koettig TE/CRG 17 Thermal conductivity, solid conduction λ (W/m K) Heat transport in solids 퐴 Fourier's law: 푄ሶ = −λ(T) 훻푇 푙 Pure dielectric crystals: phonons Dielectrics/Insulators: phonons Pure metals: free electron gas and phonons Alloyed metals: electrons and phonons T (K) From: Cryogenie, Institut International du Froid, Paris 10-Oct-19 T. Koettig TE/CRG 18 Radiative heat transfer – Black body • Wien’s law (Maximum of black body power spectrum) Qrad1 휆푚푎푥푇 = 2898 μm K => 10 μm for 푇 = 300 K T1 T2 > T1 1 Qrad2 2 Source: https://www.researchgate.net/figure/Blackbody- spectral-emissive-power-as-a-function-of- wavelength-for-various-values-of_fig4_320298109 10-Oct-19 T. Koettig TE/CRG 19 Radiative heat transfer • Wien’s law (Maximum of black body power spectrum) 휆푚푎푥푇 = 2898 μm K Qrad1 => 10 μm for 푇 = 300 K T1 T2 > T1 1 Qrad2 2 • Stefan-Boltzmann’s law . 4 • Black body Qrad = s AT s = 5.67 x 10-8 W/(m2 K4) (Stefan-Boltzmann’s constant) . 4 • “Gray” body Qrad = s A T - emissivity of surface . 4 4 • “Gray” surfaces at T1 and T2 Qrad = E s A (T1 –T2 ) E - function of 1, 2, geometry 10-Oct-19 T. Koettig TE/CRG 20 Emissivity of technical materials at low temperatures Surface at 77 K Surface at 4.2 K Stainless steel, as found 0.34 0.12 Stainless steel, mech. polished 0.12 0.07 Stainless steel, electropolished 0.10 0.07 Stainless steel + Al foil 0.05 0.02 Aluminium, as found 0.12 0.07 Aluminium, mech. polished 0.10 0.06 Aluminium, electropolished 0.08 0.04 Copper, as found 0.12 0.06 Copper, mech. polished 0.06 0.02 From: Ph. Lebrun, CAS School on Vacuum in Accelerators, 2006 Accelerators, in Vacuum on CAS School Lebrun, Ph. From: Condensed layers from gas phase easily vary these values ! 10-Oct-19 T. Koettig TE/CRG 21 Multi-layer insulation (MLI) Complex system involving three heat transfer processes • QMLI = Qradiation + Qsolid + Qresidual • With n reflective layers of equal emissivity, Qradiation ~ 1/(n+1) • Parasitic contacts between layers, Qsolid increases with layer density • Qresidual due to residual gas trapped between layers, scales as 1/n in molecular regime • Non-linear behavior requires layer-to-layer modeling Cold surface Solutions for penetrations and support structures Large surface application 10-Oct-19 T. Koettig TE/CRG 22 Typical heat fluxes between flat plates (cold side vanishingly low) Configuration W/m2 Black-body radiation from 293 K 420 Black-body radiation from 80 K 2.3 Gas conduction (10-3 mbar He) from 290 K 19 Gas conduction (10-5 mbar He) from 290 K 0.19 Gas conduction (10-3 mbar He) from 80 K 6.8 Gas conduction (10-5 mbar He) from 80 K 0.07 MLI (30 layers) from 290 K, pressure below 10-5 mbar 1-1.5 MLI (10 layers) from 80 K, pressure below 10-5 mbar 0.05 MLI (10 layers) from 80 K, pressure 10-3 mbar 1-2 10-Oct-19 T. Koettig TE/CRG 23 Refrigeration and Liquefaction 10-Oct-19 T. Koettig TE/CRG 24 Thermodynamics of cryogenic refrigeration T0= 300 K Q 0 First principle [Joule] Q0 Qi W R W : mechanical Q Q work Second principle [Clausius] 0 i T0 Ti Qi Ti (= for reversible process) Qi Hence, W T0 Qi which can be written in different ways: Ti Qi 1 W T0 ΔSi Qi introducing entropy S as ∆Si= Ti T 2 0 W Qi 1 Ti Carnot factor 10-Oct-19 T. Koettig TE/CRG 25 Thermodynamics of cryogenic refrigeration 250 Graph for Twarm = 300 K T T For low temperatures: w -1 » w 200 T T He II c c ] 1.8 K - e =166 - c - Use of low temperatures if no alternative [ r 150 o t - Better intercept heat at higher temperatures c a f t o n 100 r a He C 4.5 K - ec=65.7 50 H2 N 20 K - ec=14 2 77 K ec=2.9 0 1 2 5 10 100 300 Cold Temperature [K] 10-Oct-19 T. Koettig TE/CRG 26 Elementary cooling processes in a T-s diagram p1 p2 > p1 10-Oct-19 T. Koettig TE/CRG 27 Maximum Joule-Thomson inversion temperatures Max. inversion Cryogen temperature [K] Helium 43 Hydrogen 202 Neon 260 Air 603 Nitrogen 623 Argon 723 Source: Refprop® p/bar Source: http://faculty.chem.queensu.ca/people/faculty/mombourquette/ Chem221/3_FirstLaw/ChangeFunctions.asp • Air can be cooled down and liquefied by J-T expansion from room temperature, • Helium and hydrogen need precooling down to below the inversion temperature by heat exchange or work-extracting expansion (e.g. in turbines) 10-Oct-19 T. Koettig TE/CRG 28 Two-stage Claude cycle 10-Oct-19 T. Koettig TE/CRG 29 Process cycle & T-s diagram of LHC 18 kW @ 4.5 K cryoplant E1 T1 E3 MPa MPa MPa E4 T2 1.9 0.4 0.1 20 K - 280 K loads 201 K (LHC current leads) T1 from LHC loads LN2 Precooler T2 75 K T3 50 K - 75 K loads E6 T3 T (LHC shields) 49 K LHC shields E7 32 K E8 T4 T4 20 K Adsorber E9a 13 K T5 T7 from LHC loads 10 K E9b 9 K T8 T6 E10 T5 T7 4.4 K 4.5 K - 20 K loads MPa 0.3 (magnets + leads + cavities) To LHC loads E11 s E12 T6 E13 T8 10-Oct-19 T.