RENAISSANCE FUSION Unlocking Fusion Energy with High-Temperature Superconductors May 2025 / [email protected] / ICTP-IAEA Fusion Energy School / renfusion.eu Confidential, all information reserved to Renaissance Fusion 2025 Overview ▪ Relevance of superconductivity in fusion machines ▪ Basics of superconductivity ▪ From superconductor to cables and magnets ▪ High temperature superconductivity unlocking new fusion possibilities ▪ Renaissance Fusion’s approach to HTS stellarator magnets The two main fusion machine concepts rely on magnetic confinement to maintain hot plasmas Tokamak: Field twist by current in plasma caused by dB/dt in central solenoid (transformer coil) → pulsed machine Examples: Alcator C (0.7 m, 12 T (20 peak)), JET (3 m, 3 T (6T peak)), JT60 (3.4 m, 3 T (6T peak)) Stellarator: Field twist by helical winding difficult to construct → works continuously Examples: Figure 8 (0.1 T), W-7AS (2 m, 2.5 – 3 T) Conventional (copper) magnets could only operate for few seconds requiring immense power supplies 1950-1960s: Copper wire and bars, The pioneers, Most copper machines (Stellarator A 1953, B_coil = 0.1 T) 1960-1980s: Cryogenic Bitter plate magnets, The准lcators at MIT (Alcator A 1968, B_coil = 17 T; Alcator C 0.7 m, 12 T (20 peak)) Copper TF magnets: 12 T, 4 seconds, ~225 MW LHC HTS cable at 4.2 K (12500 Amperes cables normaux vs cable supra-conducteur) → 2-3 orders of magnitude lower power requirements !! Superconductivity enables fusion machines with high fields and longer operations - 1950-1960s: Copper wire and bars (The pioneers, Most copper machines, Stellarator A 1953 B_coil = 0.1 T) - 1960-1980s: Cryogenic Bitter plate magnets (The准lcators at MIT, Alcator A 1968 B_coil = 17 T) - 1980-2000s: NbTi superconductors (First superconducting devices, Tore Supra 1988 B_coil = 9 T) - 1990s-2010s: Nb3Sn for higher field (Reactor-class devices, ITER 2015 B_coil = 13 T) - 2010-2020s: REBCO for very high field (SPARC 2021 B_coil > 20 T) Resistance and its temperature dependence Resistance: The electrons are accelerated by the electric field, bump against atoms, transfer energy to these atoms which heats the wire. What is the origin for that scattering? • Electrons are scattered by impurities (not temperature dependent) → Use of pure materials • Electrons are scattered at lattice oscillations (temperature dependent) → Cool down to limit oscillations → limited by impurity scattering (ρ_i + ρ_p ∝ T^5, ρ_p ∝ T) Discovery of superconductivity Experimentally it was found in 1911 by Kammerling-Onnes that resistance of pure mercury vanishes completely at low temperature (Tc = 4.2 K). Superconductivity: Resistance drops to zero at the critical temperature Tc. Lowering temperature unveils "hidden" interactions: - 200 K: Spin system disordered - Ordered spin system - 100 K: Free electron gas - 0-20 K: Superconductivity "paired" electrons! Superconductivity is not an exception Known Superconductive Elements (Tc values): Lead (Pb): 7.196 K Lanthanum (La): 4.88 K Tantalum (Ta): 4.47 K Mercury (Hg): 4.15 K Tin (Sn): 3.72 K Indium (In): 3.41 K Aluminum (Al): 1.175 K Gallium (Ga): 1.083 K Molybdenum (Mo): 0.915 K Zinc (Zn): 0.85 K Zirconium (Zr): 0.61 K Cadmium (Cd): 0.517 K Titanium (Ti): 0.40 K Uranium (U): 0.20 K Iridium (Ir): 0.1125 K Tungsten (W): 0.0154 K Platinum (Pt)*: 0.0019 K Lithium (Li): 0.0004 K Superconductivity – Tc evolution - Discovered in 1911: Tc increases well above 20 K for Nb3Ge – but until the 1980's everybody believes Tc can never go above 30 K. - NbTi and Nb3Sn are the two superconductors used for fusion magnets (Liquid Helium cryogen range: Hg, Pb, Nb, NbTi, Niob-Nitrid, Nb3Sn, Nb3Ge). Boundaries of superconductivity Tc depends on the applied field and current. Also, the critical current Ic and the critical field Bc depend on the other two quantities. The boundaries: • Tc = critical temperature • Ic = critical current • Bc = critical field On operation conditions the critical current is given at the chosen temperature and field, e.g.: Ic(5 K, 10 T) = 5 kA. From superconducting filaments to cables Stabilization of superconducting cable and quench: - Copper wire: 1mm cross section -> 10 A - Superconductor: 1mm cross section -> 500 A - Mechanism: Heat dissipation, Current Sharing, Thermal stability (Copper - Superconductor - Copper sandwich) - Still in case a local hot spot cannot be stabilized and starts to grow, the current has to switched off immediately. - Burning of the coil: "Quench" Main considerations in case of AC currents: - Eddy Current Losses - Type I vs Type II superconductors - Coupling losses (reduce current loops, twisting/transposition) Reducing operating costs and power requirements NbTi (multi filament) in Cu matrix. Electro-magnet (10 Tesla): - 1 MW electric power - 10 kW for refrigerator to balance He losses Operating cost: - 100 €/h conventional magnet - 1 €/h superconducting magnet Design considerations for fusion magnets SC cable Inductance considerations: - U = L * dI(t)/dt - τ = L / RL (Time constant) - U0 = I0 * RL (Initial voltage) - L ~ n^2 * A / l (Inductance dependence on winding number) → High current cables with low number of turns and low discharge resistance Structural considerations: - Radial support plates Insulation considerations: - In Paschen conditions (≈0.1-50 mbar) voltages around 100 V can cause arcing! - No failure at 30 kV when tested in good vacuum or ambient pressure. Under Paschen conditions breakdown occurs around 2 kV! Thermal considerations: - Cable in conduit (flowing cooling) Overview of LTS superconducting cables for fusion - LCT: NbTi, 11.8 kA @ 4.5 K, 16 kA @ 3.5 K, 19 kA @ 1.8 K in stainless steel jacket with forced flow cooling - POLO: NbTi, 15 kA (test: 22 kA!) in stainless steel jacket with two phase forced flow cooling - W7-X: NbTi, 19.9 kA in Aluminium jacket with forced flow cooling - TFMC: Nb3Sn, 68 kA, (test 80 kA) in stainless steel jacket with forced flow cooling ITER central solenoid example of inductance, cable current and high voltage trade-offs ITER Central Solenoid: - Six modules, 17 m tall, 4.2 m diameter, 13 Tesla, 5.5 GJ of stored energy - CS Module: Conductor Nb3Sn (CICC), Outer Diameter of 4.1 m, Height of 2.2 m, Weight 110 tonne, Inductance 0.77 H, Stored energy 1 GJ, Peak Current 45 kA - 20 MA overall current in one CS module - 45 kA cable using 549 turns per CS module - Limits the inductance of one module to 0.77 H - CS modules with a discharge voltage of 11 kV (Max coil voltages up to 26.9 kV under fault) High temperature superconductivity - 1986 Discovery of "High-Temperature Superconductivity" below 35 K in the La-Ba-Cu-O system by J. G. Bednorz and K. A. Müller - 1987 Discovery of Superconductivity in Y-Ba-Cu-O by C. W. Chu and M. K. Wu - Critical Temperatures: • REBCO (Y, Gd, Eu, Sm…): 88–96 K • BiSCCO: 85–110 K - Also shows high-pressure hydrides (e.g. H2S @ 155 GPa, LaH10 @ 170 GPa up to >250 K) Not only increased critical temperatures but also critical field and current LTS critical surface (T = 4 K, Bc up to ~20 T, Jc up to 10^7 A/cm^2). Not only increased critical temperatures but also critical field and current HTS extends critical surface boundaries significantly: - Critical temperature up to >100 K - Magnetic field exceeding 100 T–250 T - High current density maintained under strong fields Not only increased critical temperatures but also critical field and current Comparison of Critical Current Density (A/mm^2, 4.2 K) vs Applied Magnetic Field (T): - Nb-Ti drops rapidly above 8 T - Nb3Sn drops above 20 T - YBCO and Bi-2212/2223 maintain critical current densities > 10^3–10^4 A/mm^2 even beyond 30–45 T - Jc dependence on field angle, magnetic field, and temperature in REBCO coated conductors. Not only increased critical temperatures but also critical field and current - 2000s: Companies started making reproducible tapes - 2010s: Companies started making reproducible tapes in quantity and length that mattered - REBCO tape structure: Copper (~20 µm), Silver (~2 µm), REBCO (~1 µm), Buffer layers (~1 µm), Hastelloy substrate (~50 µm), total width 2-12 mm, thickness ~0.1 mm. Advantages of HTS High field: - Constraint on magnetic field is now a structural issue, not a quantum mechanical issue. - Can conceive of designs to arbitrarily high magnetic fields. High current density: - More space for structure and other components. - More compact, higher-field magnets. High Temperature: - Wider flexibility in design of cooling system (various cooling fluids), or large heat loads (nuclear, joints). - Much more stable design and much larger margins. - Reduced cryogenic system (factors of 5-20). Made from steel ribbons: - Superconductor can become part of structure and can design to higher stress and thus smaller magnets. - Must design the magnets and cables differently than LTS. Thin film process: - Can get to very low-cost parameters. - Make it and use it - no heat treatments etc. HTS is no longer the bottle neck but rather structural materials for HTS magnets - Elastic limit of material is now critical (higher temperature is detrimental). - J_crit REBCO perp. to B vs coil hoop stress σ_max vs B_max. - Combination of fatigue and elastic limit matters for fusion reactors. - Stainless steel strength limits (Yield strength σ_y vs fracture toughness K_Ic). → Design limits or R&D. New possibilities for high-field compact fusion machines Scaling laws: - Energy gain: n T τ_E ~ R^1.3 B^3 (Tokamaks) - Power density: P_fusion / S_wall ~ R B^4 Machine size comparison: - ITER: V_plasma ~ 840 m^3, B ~ 5.3 T - SPARC: V_plasma ~ 20 m^3, B ~ 12.2 T - HELIAS (B = 5 T, 22 m): V_plasma ~ 1410 m^3 - Renaissance Fusion's reactor (Simplified W7X, with 10 T on axis, 6 m): V_plasma ~ 100 m^3 Similar development as LTS from HTS tapes to cables Cable concepts developed globally: - CORCC cables (Conductor on Round Core) - Roebel cable (KIT, CERN) - VIPER cable (vacuum pressure impregnated, insulated, partially transposed, extruded, and roll-formed) - CroCo-Core (KIT, Germany) - Twisted stack conductors (EPFL, Switzerland; NIFS, Japan; ENEA, Italy) HTS allows for more efficient current leads Classical Copper Current Leads: - Large ohmic heating for tens kA - Large Cu cross section: massive heat load from 300ºC to 4K - Wiedemann-Franz Law: λ / σ = L · T (λ: thermal conductivity, σ: electrical conductivity, L: Lorenz number) HTS allows for more efficient current leads Comparison: - Classical Copper Current Leads vs HTS for Current Leads (CL) - HTS gives no ohmic losses and has small cross section with low heat conduction - Cooling of the Cu heat exchanger @ 60–300 K has a much higher efficiency - Active cooling of HTS part not necessary (conduction cooling from the cold end is sufficient) → Reduction of heat load by a factor of 10 HTS allows for more efficient current leads HTS for efficient current leads: - 70 kA HTS CL Demonstrator for ITER - W7-X current leads - JT-60SA current leads - ITER current leads (80 kA with Bi-2223 world record) Impact for ITER cost saving: - ITER HTS current leads: 9 TF CL pairs (68 kA), 6 CS CL pairs (45 kA), 6 PF CL pairs (52 kA) - Using HTS current leads will: • need higher investment costs (4 M€) • save investments for refrigerator power (10 M€) • lower operational costs (0.8 M€ per year) - The ITER feeder package is in the hands of China. HTS tape suffer from low normal zone propagation velocity preventing their use as insulated coils - LTS: Longitudinal propagation 10–40 m/s - HTS: Longitudinal propagation 2–40 cm/s → Difficult to detect quench → Fully insulated magnets challenging No-Insulation (NI) REBCO coils (~2009, Hahn, Park): → Tolerant to quench → High current densities → Low voltage drops No-insulation HTS magnets research and active field of development Institutions and companies working on NI HTS coils: - CERN (Switzerland) - Tokamak Energy (England) - OpenStar Technologies (New Zealand) - National High Field Magnet Laboratory (USA) - Seoul National University (Korea) - Renaissance Fusion (France) HTS magnets architecture and type No-Insulation (NI) Coils: - Advantages: Thermal stability, Self-protection, Higher current density, More compact magnets, Simplified fabrication - Challenges: Longer time constants, potential for complex current distribution within the coil, subject to contact resistance Alternative winding types: - Metal-as-Insulation (MI) Coils - Partial insulation (PI) Coils - Metallic Cladding Insulation (MCI) → Active field with new options, metal in transition materials… Fusion magnet development: MIT / CFS building an HTS TFMC demonstrator Key Parameters: - Number of pancakes: 16 - Total turns: 256 - Total REBCO tape: 270 km - Operating temperature: 20 K - Coolant type: Supercrit. He - Operating coolant pressure: 20 bar - Operating azimuthal current: 40 kA - Peak magnetic field: 20 T (~1.5x ITER) - Peak IxB force on REBCO: 800 kN/m - Inductance: 0.14 H - Magnetic stored energy: 110 MJ - WP mass: 5113 kg - WP + case mass: 9265 kg - WP current density: 153 A/mm^2 (~10x ITER) HTS magnets are a small part of the entire magnet system System components: - 50 kA power supply - 50 kA warm bus (297 K) - 50 kA HTS current leads (297 K to 20 K) - 50 kA HTS cold bus (feeders) (20 K) - 600 W @ 20 K cryosystem (liquid free) - TFMC coil inside Main cryostat w/ LN2 shields - 15 ton gantry and lifting fixture (Not shown: SHe and LN2 distribution systems; Vacuum systems; I&C system; Safety systems; Control Room) From superconductors to HTS magnets (Design Cycle): 1. Magnet design target 2. Winding pack space 3. Superconductor choice and operating point (I_op vs I_c(B, T, θ)) 4. Magnet architecture and type 5. Current leads, joints and connections 6. Thermo-mechanical design 7. Cryogenics and cooling 8. Quench mitigation 9. Cabling, winding operation, cycling Renaissance Fusion develops a simplified compact HTS stellarator with plasma-facing liquid walls Problems & Solutions: 1. Problem: Low-field tokamak and stellarator reactors are large and expensive. Solution: High Temperature Superconductors (HTS) generate stronger fields, unleashing the same power from smaller devices. 2. Problem: D-T fusion neutrons induce radioactivity and damage to solid parts. Solution: Liquid Metals (LM) - Plasma-facing, thick Lithium-based walls absorb neutrons without getting activated, making reactors safer. Bonus: resilient to non-uniform α particle losses. 3. Problem: Stellarator coils have complex 3D shapes. Solution: Simplified Stellarators of simpler "coil winding surfaces" are easier to build, by wide HTS deposition & laser engraving. Developing and validating key building block technologies to enable simplified stellarators - HTS manufacturing: Designed, modeled and now building the first machines ever for meter-wide HTS deposition. - Free-surface flowing liquid walls: Levitated, in 1 m diameter chamber, a 10 cm thick layer of liquid GaInSn. - Simplified stellarator design & modelling: Simplified compact stellarator configurations using corrugated copper cylindrical magnets. 3-step program for simplified stellarator magnets - Transition from traditional complex coils (W7X: 50 non-planar coils + 20 planar coils) to simplified cylindrical/grooved magnets surrounding liquid metal blanket. - Step 1: Laser patterning and winding machine (Laser engraved commercial HTS) - Step 2: Grooved magnet technology (0.5 T / 4 cm → 1 T / 20 cm) - Step 3: Robust high-field HTS magnets (6 T / 1.15 m) Ongoing R&D development programs - Stellarator magnetic configuration optimization for compact high-field reactors and simplified coils (QH-4, R = 4 m, B = 10 T, Aspect ratio A = 5.0). - Simplified compact stellarator configurations using corrugated copper cylindrical magnets: Realizing W7X magnetic configuration with a 10^-4 magnetic field accuracy. - Test rig (1.6 m): Vacuum vessel, Laser patterned copper coils, Planar coils, Electron gun manipulator for magnetic field measurements, E-beam camera viewing port. Thank you! Want to learn more? [email protected] FAQs: renfusion.eu/faq YouTube video RENAISSANCE FUSION