High Temperature Superconducting Magnets for Next Step Fusion

High Temperature Superconducting Magnets for Next Step Fusion Reactors Yuhu Zhai Princeton Plasma Physics Laboratory With contribution from Jon Menard, Michael Zarnstorff Princeton Plasma Physics Laboratory FESAC TEC Community Input Meeting May 30 - June 1, 2017 11/10/2020 Y. Zhai PPPL 1

Outline • Fusion Magnet Design – integrated approach needed – System requirements (complex geom. , large scale, high field etc. ) • Significantly different from high field solenoids, HEP magnets etc. • ST – high power density, improved stability, but require high J at low A – Interaction with plasma performance (shielding, radiation) • Transient AC field (Eddy current, AC losses, quench & coil protection) • HTS for next step device – compact size (1 -3 m major radius) – Optimized coil shape: minimized stress distribution/reduced JXB (FFHTR) – Radiation effects – essential criterion for fusion reactor magnets (shield) – Example: HTS ST FNSF – why ST? – high neutron wall loading & fluence – Allow high field, compact size, high J - beneficial for low aspect ratio ST – Significantly higher fusion power density Pfus / V β 2 BT 4 11/10/2020 Y. Zhai PPPL 2

Possible Development Strategy • Near term goals / needs (1 -3 yrs) – Identify critical area to enhance core engr. competence (LTS vs. HTS) • Targeted magnet R&D (HTS cable, coil fab. , quench w/i novel insulation) – s/c Mag Lab for HTS R&D (initiate multi-institutional collaboration) • Mid term strategy (5 -10 yrs) – strong Nat’l & Int’l collaboration – Conductor performance (YBCO vs. Bi-2212 for FNSF) (UNIGE-PPPL, OST) – High current density cable design & testing (NIFS-MIT-PPPL) (FNSF, pilot) – TF and CS coil design (structural optimization stress/strain) (ST-FNSF) • Long term program (10 -20 yrs) - Strong mgmt. support – Integrate magnet design with burning plasma physics for fusion energy – Close gap btw advances in applied HTS & next-step fusion magnet design 11/10/2020 Y. Zhai PPPL 3

All superconducting magnets beyond LTS The HTS high magnetic fields achieved HFLSM Maximum field [T] HTS insert HTS coil field [T] Winding technology 27. 6 Bi 2223 / REBCO 4. 5 (Bi 2223) 6 (REBCO) LW 27 REBCO 12 DP 26. 4 REBCO 26. 4 DP 25 REBCO 4 LW 24. 6 Bi 2223 10. 6 DP 24. 2 Bi 2223 3. 66 LW Prof. C. Senatore, Univ. of Geneva

Fusion Magnet Design • Complex coil shape & plasma interaction – Princeton-D coil to minimize in-plane bending may not be practical – Significant out-of-plane load; wedging /bucking to support radial force • High current cables (>60 k. A) - protection of magnets (limit inductance, terminal voltage & discharge time constant) • Toroidal field magnets much less efficient in material use than solenoids (large plasma space higher Bmax/Bt >2 and lower winding pack current density) • Material irradiation limits in next-step fusion device (high neutron flux) – FNSF smaller than ITER but higher field, 30 x higher fluence, SS plasma ITER TF r 1 = 2. 7 m , r 2 = 10. 75 m coil centering Fr = 403 MN half coil vertical Fz = 205 MN LTS mag. technol. including used by ITER scales to power plants of large size & high cost A/mm 2) for ST FNSF magnets HTS provide sufficient J (>70 -140 WP 11/10/2020 Y. Zhai PPPL 5

Magnet Design Integration Conductors HTS magnets Tokamaks Stellarators YBCO tapes, Bi 2212 round wires High Field B Plasma transient ∂t ≠ 0 Plasma transient ∂t = 0 High Current J Planar TF coils & CS/PF solenoids Non-planar coils YBCO stress tolerant, Bi-2212 weak strength High Strength to react Stress management J x B reduced (FFHTR) YBCO (better) Radiation tolerant Magnet materials including Cu stabilizer (RRR) Bi-2212 (better) Stability Quench detection & novel coil protection Jx. B (thermal/electrical) HTS tapes (Super. Power Inc. ) Bi-2212 wires Bruker Energy 11/10/2020 Less quench issue T. Brown, PPPL High field cable test: from straight High Current cables Y. Zhai PPPL sample to samples of coil winding (NIFS) HTS LM ST Exp. device 8 TF 1. 2 m R 0 & 5. 5 T B 0 6

Large TF Coil Size, High Cost but Low Performance • High current cable (50 -60 k. A) – Large amount Cu for coil protection (slow NZP an issue for HTS) – Lower coil terminal voltage & coil inductance (but high cost) – Large structure coil case – min structural materials (viral theorem) • Low current density (17 A/mm 2 in ITER TF; 40 A/mm 2 in DEMO WP) – Thousands of strain sensitive s/c wires – high cost • Need significantly improved WP current density (shield space for ST) • Consider wrap/wind CS coil around inner leg of TF coils (no joint) • Bring external divertor shaping PFs close to plasma (need HTS) High current density HTS cable motivates consideration of low-A ST pilot plants Integrated approach needed: conductor design, TF winding pack & coil structure des. 11/10/2020 Y. Zhai PPPL 7

High current density HTS cable motivates consideration of lower-A tokamak pilot plants Fix plasma major radius R 0=3 m, heating power PNNBI= 50 MW Pnet [MWe] JWP • ITER-like TF magnets: – JWP=20 MA/m 2, Bmax ≤ 12 T – Pfusion ≤ 130 MW, Pnet < -90 MW 150 • JWP ~ 30 MA/m 2, Bmax ≤ 19 T 50 – Pfusion ~ 400 MW – Small Pnet at A=2. 2 -3. 5 • JWP ≥ 70 MA/m 2, Bmax ≤ 19 T – Pfusion ~500 -600 MW – Pnet = 80 -100 MW at A=1. 9 -2. 3 A ~ 2 attractive at high JWP [MA/m 2] 100 (12 T) 0 -50 -100 -150 1. 5 2. 0 2. 5 3. 0 Aspect Ratio A 3. 5 (Menard et al. Nucl. Fusion 2016) 4. 0

BNL/NHMFL/MIT NI/SS-co wound YBCO Coils BNL-1 BNL-2 BNL-SEMS NHMFL– 32 T (17 T YBCO) MIT (NI) Field on coils (T) 16. 2 9. 2 25 32 26 Aperture (mm) 25 100 32 35 Overall J (A/mm 2) >500 180 -200 400 Operating I (A) 280 250 700 180 (70% Ic) 250 Operating T (K) 4. 2 4. 2 Hoop stress (MPa) 400 400 -440 590 Coil winding High strength HTS tape co-wound with SS tape (optimized thickness) dry wound (with SS) double pancake Robust pancake Quench detection Pre-quench (1 m. V) detec - advanced low-noise elec & noise cancellation 1 V across coil none Quench protection Fast energy extraction – inductively coupled copper discs (>k. V) Quench heaters Selfprotection What’s the size limit of the direct winding of YBCO coils? Can we extrapolate direct winding of HTS tapes (pancake coils) to larger dimension TF coils for compact reactor of >1 m R 0? Advanced methodology - Novel unconventional quench detection and coil protection 11/10/2020 Y. Zhai PPPL 9

<3 -m HTS ST – CS & TF Coils • Novel insulation – direct winding of single layer YBCO tape – HTS quench protection (thermal stability) & no radiation issue for CS – Overall improved current density, mechanical integrity & thermal stability • No cables – HTS quench protection, higher J – high inductance so limited in coil size for 2 -3 k. V terminal voltage for <10 s fast discharge • No Liquid helium – helium free - cryo-cooling (20 K or helium gas) – No direct cooling channel in WP (adiabatic-NI coils) • Compactness – size of OH coils we can make (aperture size) – < 1 m (no-insulation coils) but how to reduce inductance for larger coil? – limited by fast discharge V&L (also loading? ) T. Brown, PPPL High WP current density needed; high current HTS cable needed for TF coils 11/10/2020 Y. Zhai PPPL 10

Conductor Radiation Limits ternary Nb 3 Sn binary Nb 3 Sn alloy with Ti/Ta doping for higher Jc at high field advanced Nb 3 Sn – “independent of doping” Pure Nb 3 Sn Weber, Vienna University of Technology, 2015 binary & ternary Nb 3 Sn produced in 80 -90 s REBCO at 3 x 1022 n/m 2 radiation > 50% Ic degradation for 64 K operation (seems too high) At 2 x 1022 n/m 2 radiation ~30% Ic degradation for 40 K operation (maybe acceptable) Below 40 K operation is possible at 3 x 1022 YBCO is no better than binary Nb 3 Sn but can be better (below 40 K) than ternary Nb 3 Sn Tc is nearly constant up to 1022 n/m 2 11/10/2020 PPPL-BNL collaboration on magnet design Weber, ASC-2014 11

Summary Category Technology Description Fusion Application Broader Impact Critical variable(s) Design variables Risks, Uncertainties Maturity Technology Development 11/10/2020 Description Status & Expectation Lab scale HTS conductor test but coil ideas HTS Magnets 4 Fusion Reactors remain untested experimentally in fusion High field, high Je CS and TF coils Size of CS insert pancake coils > 20 -25 T Mg. B 2 PF coils (no treatment) Advanced manufacturing (3 D print of magnets) High field NMR and MRI Significantly reduced cost while improving performance Conductor performance (>20 T) Mechanical ; Electrical; Base unit length (>1 km) High current cable test (> 20 k. A) lab test of high current cables; coil test needed Quench detection & protection R&D - novel unconventional approach needed Coil stress / strain management Coils structure optimization AC loss, Jc an-isotropy, RRR Improving Je via pinning & reducing an-isotropy Winding pack current density Je High Je in HTS cables; low Je in WP Cost, reliability (uniformity/repeatability), acceptance TRL 3 – lab test and small size Optimize conductor and winding pack architecture integrated in coil support structure to enhance stress management quench detection & coil protection Improve performance & lower cost Y. Zhai PPPL solenoid/pancake coil test of direct wind HTS tapes/wires Integrated component test data available in next 3 -5 yrs? 12

Backup Slides 11/10/2020 Y. Zhai PPPL 13

Summary • HTS ST attractive for high Pfus with magnet innovations – Promising advances in high current density HTS cables – High neutron wall loading in small device suited for FNSF missions – Improve toroidal burning plasma physics predictability • Compact size (<1 m coil diam. large aperture coils need shape optimiz. ) – 3 m ST-FNSF and Near 1 m size HTS ST-LM device (no joint on TF) • PPPL ST-FNSF Magnets – CS & PF coils – no/SS insulation & direct winding of YBCO – High current cable for TF coils (enlarge OH flux swing OH enclosure inner TF) • Radiation Limits – High performance Nb 3 Sn wire peak Jc at 3 x 1022 n/m 2 fast neutron flux & YBCO has better radiation resistant (20 -40 K – not feasible at high Temp) 11/10/2020 Y. Zhai PPPL 14

PPPL ST-FNSF (Menard et al. Nucl. Fusion 2016) Major radius: 3 m Aspect ratio: 2 Plasma current: 12 MA Central Field: Max B on TF: Max B on CS: 4 T 16. 5 T 20 T TF coils: oval shape CS coil ID: 50 -60 mm Inboard PF (high J & T) Outboard PF (high J) CS (high B) 4 -20 K operating temp. LTS PF coils 10 -TF HTS magnet system HTS OH & PF coils Most outer PFs can be LTS but some inner ones will be HTS 11/10/2020 Y. Zhai PPPL Similar or larger sized HTS high field solenoids were developed at BNL, NHMFL and MIT with or without SS co-wound insulation 15

HTS Coil – Quench Detection & Protection Quench BNL NHMFL MIT Detection Advanced electronics (>m. V) voltage (>1 V) none Protection Inductively coupled copper plate heaters self-protection R&D Hardware test pre-quench detect. scheme complex quench analysis – BNL: Detection - advanced low-noise electronics noise cancellation scheme to detect pre-quench voltage for safe operation (1 m. V) – BNL: Protection – inductively coupled copper discs for fast energy extraction – co-winding with SS tape helps in quench protection – NHMFL: transient spike >1 V, few ms or >>ms, heaters for protection • Quench initiation study (quench scenarios) & quench protection test • Heaters protect coils even in case of zero NZP – PPPL: Quench analysis model for parametric study on engineering inductively coupled coils for HTS protection Advanced methodology - Novel unconventional quench detection and coil protection 11/10/2020 Y. Zhai PPPL 16