1 Cryogenic in Fusion Devices” Kavita Rathore MTech – NST Delhi University

2 Outline  Role of Cryogenics in Fusion devices  Properties of material at Cryo Temperatures  Cryogenic Design aspects  Helium as magic fluid  Production, Storage and Transfer of cryogens  Cooling Method used in Tokomak  Safety issues related to cryogenics

3 Role of Cryogenics in Fusion devices  Superconducting Magnets, cryo pumping for NBI, Diverter, and Pellet injection / Fueling highly dependent on the Cryo technology.  GHz high power RF system uses Gyrotrons, operates with Cryogenics.  Cryogenics technology provides variety of operations at cold temperatures starting from sub-cooled superfluid, two-phase flow to forced flow supercritical Helium.  Higher capacity demands efficient and thermodynamic Exergy system to meet the economics of plug-power.  Cryo capacity varies from 1 kW – 100 kW at 4.5 K depending upon the machine size e.g. SST-1 has 1.3 kW whereas ITER has K

4 Where efficiency optimization lies? Efficiency of commercial Fusion reactor depends on many factors: - Plasma density, temperature and confinement time product yield (The Lawson criterion: At T ≥ 10 8 K, n  ≥ m -3 s) - Efficiency of auxiliary plasma heating schemes - Advantage of recycled alpha- particles heating - Overall input electrical power One of the major power consuming device is LHe cryogenic plant SST-1 India 1.3 kW at 4.5 K consumes electrical power: 0.7 MW ITER, IT 72 kW at 4.4 K will consume electrical power: 16.5 MW Demo power reactor 100 kW at 4.5 K will consume power: 23 MW

5 Properties of material at cryo Temperature

6 Exergy analysis : Available useful energy for cryo system 100 % input power 50 % power consumed by compressors 20 % power consumed by cold-box process Maximum 30 % is available as useful Energy : Exergy

7 Different cooling options  Cooling using Sub-atmospheric Superfluid Helium (He- II) (T op < 2.17 K) Tore Supra: The first SC Tokamak at 1. 8 K  Cooling using Pool Boiling Helium (He- I) (LHD Helical Coils)  Cooling using Two-phase flow Helium (He- I) (LHD supports and sc bus lines)  Cooling using Supercritical Helium (He- I) (SST-1, EAST, KSTAR, ITER) Monolithic conductors design for TPCCICC of ITER TF Magnets

8 Supercritical Helium Vs Two-phase cooling for Magnets He- I (Normal Helium): P: 1 bar and T ~ 4.2 K He- II (Superfluid Helium): P < 1 bar and T < T Two-phase Flow: From P op of 1.0 bar – 2.3 bar and T_Saturation of Helium, the fluid exhibits Two-phase flow behavior when heat is added. In TP mode, temperature of He does not change but the quality (  ) changes through Latent heat of vaporization. Supercritical Helium: P > P c ~ 2.3 bar Exhibits as dense cloud of mist No Latent heat of vaporization No Phase change No Quality change

9 Cryo Forced flow cooled system: single phase Helium  Why supercritical Helium as coolant? After 1980’s, advanced superconductor technology for the Fusion grade machines adopted Cable – in – conduit – conductors (CICC) concept developed at MIT by Hoenig Montgomery. The CICC concept has lot more merits with respect to Monolithic design but provides very complicated path for the coolant. Looking at the safe and stable operation of such CICCs, the high pressure turbulent cooling system is desired. Using SHe as Coolant:  Flow instabilities / chocking can be avoided  Higher heat transfer rates compared to TP  T op can be optimized over a wide range  Less pressure drop  Sudden thermal runaway can be avoided Few Pros of TP cooling:  There is only little change in temperature of fluid till complete liquid is evaporated  Flow rate requirement is less compared to SHe cooling

10 Different ways to produce supercritical Helium Supercritical Helium can be generated even without using cold circulator, the plant process is worked out that setting the outlet pressure of cold turbine, cold supercritical helium can be produced with limited mass flow rates at 4 bar / 4.5 K. When the application load demands higher mass flow rates of Supercritical Helium then one has to use cold circulator with by-pass valve mechanism, Heat load of the circulator is dumped into the LHe buffer vessel and HEX is placed into the vessel to provide the stabilized temperature of the incoming stream.

11 Pre-requirement for switch over to SHe operation Definition of Supercritical Helium:  Supercritical is a state where operation pressure is beyond critical pressure (i.e. 2.3 bar).  Supercritical state is clear single phase state.  The pressure drop of Supercritical liquid Phase is always less than that of Gas phase.  Temperature of Supercritical Fluid increases subjected to Heat loads. Pre-requirement for switch over from DP mode to Supercritical Helium: [1] Plant Return Temperature as well as Load shall be less than 7.0 K [2] The pressure drop across the load shall be less than 500 mbar at T < 7.0 K [3] The load condition shall be stable in terms of (Pressure, Temperature and Flow) [4] The total liquid filled in MCD shall be 800 L. [5] The Sub-cooler Dewar level shall be filled up to 650 mm (HEX bath pool) SST-1 HRL PFD

12 Forced flow cryo system in D-IIID Tokamak (Cryo pump) Schaubel, K.M. Baxi, C.B. et al, IEEE Fusion Engineering, 1991 Simplified Cryogenic flow diagram in D-IIID tokamak for Diverter Cryo pumping  Application: Advanced Diverter cryo pump  Cryo pumping speed: 50,000 l/s  Cryo pump: Liquid Helium cooled tube at 4.3 K  Thermal shield: 80 K cooled with liquid Nitrogen  Compressed GHe is flowing through HEXs in HP side  Liquid Helium is collected in Dewar (using J-T valve expansion) LN2  LN2 forced flow cooling through the bulk storage  Cryo system Control using a dedicated PLC system Diverter Cassata

13 Current leads and current feeders for Fusion devices Power Supply 4.5 K 300 K Joint TF Coil S-bend Dump Circuit Joint Water-cooled Al bus bar Current lead part LTS feeder part A superconductor current feeder system is required for  Current supply from power supply at 300K to the SC magnets at 4.5 K. Safe extraction of the stored magnet energy during the quench.

14 Cryo Forced flow cooled system: single phase Nitrogen

15 Ref. Courtesy from Striling Cryogenics Ref. Courtesy from Nexans 80 K forced flow cooled HTS Cables

16 Outlook  Operation of the steady state fusion device demands forced flow cooling techniques for their Helium and Nitrogen Cryogenics systems.  Supercritical Helium provides better cryo stability than the Two-phase flow cooling particularly for the SC magnets system where flow channel paths are very complicated.  In any case, controlled cool-down philosophy obeys the procedure that first the load will be cooled using two-phase mode and once the desire temperature is achieved then only the single phase booster mode can be switched ON.  Future fusion machines need HTS based Engineering solutions to meet the economics of plug-power for current leads and feeders and still challenge would be large size magnets manufactured with HTS technology.  Reliable and proven booster machines are essential for cooling of the steady state forced flow cryogenic system