1. Cesar Luongo, Jefferson Lab, Engineering Division
High Magnetic Fields and Physics: Issues in the Design and Construction of Superconducting Magnets
12/07/2016
Cesar Luongo, Jefferson Lab, Engineering Division

2. Part i: research AT HIGH MAGNETIC FIELDS (Or how scientific research and magnet engineering drive each other)

3. Science and magnet technology
Partial list of Nobel prizes related to research involving magnetic fields

4. Hall Effect Hall resistance is a function of material
Edwin Hall (USA) – While grad student at Johns Hopkins in 1879 (20 years after Maxwell) discovered that “something is not right”. Proved that magnetic field affects a current flowing in a metal (voltage in direction perpendicular to current flow) – Not a Nobel Laureate
Hall resistance is a
function of material RH
High field ~ 11,000 times earth’s field T
Magnetic field

5. Integer Quantum Hall Effect
Klaus Von Klitzing (Germany)
Physics 1985 for Integer Quantum Hall Effect (1980)
Klaus Von Klitzing
Hall resistance is quantized
Original work in Silicon MOSFETs
Temperature ~1.5K, Magnetic field ~12T (high B/T)
NOW used as standard for resistance

6. Fractional Quantum Hall Effect
Robert Laughlin (USA), Horst Stormer (Germany/USA), and Daniel Tsui (China/USA)
Physics 1998 for discovery of new quantum fluid with fractional charged excitations (even higher B/T, need to reach milli-Kelvin at very high B) (1983)
Laughlin
Stormer
Tsui
Quanta in Hall resistance show fractional charge

7. Research at Extremely High (B/T)
Temperature 30mK
Magnetic Field 45T
Combine extremely strong DC magnet (45T), with He3 dilution refrigerator to obtain very low sample temperature (30 mK), to explore thermodynamic space at extremely high (B/T). Investigate properties of quantum condensates (bosons, etc.)
UNIQUE

8. NMR/MRI
A major application of high field magnets for research in the physical sciences and medicine

9. Physical Principle behind NMR
The electric charge on the surface of a proton (H nucleus) as it rotates (spin) behaves like a current loop, generating a magnetic moment μ
The rotating mass of the proton has an angular momentum J
Both are related, μ = γJ (γ being a constant that depends on the nucleus)
In the presence of a magnetic field B, a perfect dipole would align, but a rotating dipole will precess at a frequency: ν = (γ/2π)B, the Larmor frequency, 42.6 MHz/Tesla for the proton (H nucleus)
Some (more) nuclei will precess parallel to B (low energy configuration), the rest (less) will precess anti-parallel to B (high-energy configuration). The difference in energy between one and the other configuration is proportional to B μ J

10. Physical Principle behind NMR/MRI
Excite system with RF at the Larmor frequency to force redistribution of nuclei from low energy state to high energy state
Turn off RF excitation and use pick-up coils to detect relaxation back to original state
By using field gradients and observations on the different types of relaxation (energy states, phase angle of precession), it is possible to reconstruct an accurate picture of the different types of tissue (contrast)
Quality of image depends on magnetic field strength and spatial/temporal homogeneity

11. Nobel Prizes related to NMR
Felix Block (Swiss/USA) and Edward Mills Purcell (USA)
Physics 1952 for the discovery of NMR (Mills’45) and development of the theory behind NMR (Bloch’46)
Bloch
Mills

12. Nobel Prizes related to NMR (2)
Richard Ernst (Swiss/USA)
Chemistry 1991 for the development of high-resolution FFT NMR spectroscopy (1966, at Varian)
Kurt Wuthrich (Swiss)
Chemistry 2002 for the development of three-dimensional solution-NMR spectroscopy (full protein structure, 1970s)
Ernst
Wuthrich

13. 900 MHz N uclear M agnetic R esonance Imaging Why do we need high B?
For spectroscopy, to image molecules of ever higher molecular weight (very complex proteins, on the verge of decoding a full virus)
For bio-MRI to get better images faster (in-vivo studies, diffusion of drugs in tissue, etc.)
What does NMR stand for?
who would let someone do NMR on themselves - their kids?
MRI
WORLD Record & UNIQUE

14. Nobel Prizes related to NMR (Cont.)
Paul Lauterbur (USA) and Peter Mansfield (UK)
Medicine 2003 for the development of field gradient NMR for biological/medical applications (MRI) allowing for high-resolution multi-dimensional images of tissue (1970s)
Lauterbur
Mansfield
Ubiquitous medical diagnosis tool. Non-invasive, high-resolution, and only available tool for imaging certain organs (e.g., brain). First, and largest, commercial application of superconducting magnets

15. Giant MagnetoResistance
Albert Fert (France) and Peter Gruenberg (Germany)
Physics 2007, for their discovery of giant magnetoresistance (in 1988), in which small changes in magnetic field induce large changes in resistance
Fert
Gruenberg
Crucial in the development of high-density hard drives (enabling technology for the original iPod)

16. Part ii: DESIGN AND FABRICATION issues with (superconducting) magnets (including a brief review of magnets in general) (Or why are superconducting magnets so difficult and tricky)

17. Magnet technology is cross-disciplinary
A magnet involves:
Special materials
Strong structures
Cooling challenges
Electric current & insulation
An electromagnetic device subject to strong forces, with acute cooling requirements, carrying high currents and able to sustain high voltages, and storing a lot of energy (protection)
Physics
Chemistry
Materials
Science
Magnetic
Fields
Medicine
Engineering
Biology
Geology
Multiple Uses

18. What are the records, what are the challenges
100T Second-Pulse Magnet
Key Milestones from the
National Academy of Sciences
“It is in the vein of recognizing common challenges that could be better addressed by coordinated efforts that the committee identifies several targets for magnet technology…”
A 30T SC, high resolution NMR
A 60T DC Hybrid magnet
A 100T long-pulse magnet
COHMAG Report
60T Hybrid Magnet
30T Superconducting NMR Magnet
Visitor program
Objective is to lower activation barrier to make the initial trips to Tallahassee. We make partial reimbursement of student or postdoctoral travel at $500/trip.
Seminar series
Establish a seminar program to introduce new users to NHMFL and familiarize the community with the EPR program. Speakers will be encouraged to visit UF during the same trip. Current lineup of speakers includes: D. Britt, S. Eaton, P. Dinse, A. Ardavan, B. Hoffman, van Slagaren, K. Dunbar.
Conference support
Objective is to increase visibility/goodwill of the program. We have established Travel Fellowships for graduate students/postdocs for Rocky Mountain Conference and for Southeastern Magnetic Resonance Conference.
Workshops
To disseminate our expertise and aid development of likely users we establish series of hands-on workshops targeting student/postdoc audience. Suggested topics: (a) DEER; (b) high field EPR; (c) pulsed EPR. Instructors should be staff and postdocs/students. The later group will get enumerated with “extra” trips or funds. Initially, we will develop one workshop a year but hold multiple workshops in a staggered fashion. Funds are predominantly for student travel and accommodation.
Future initiatives
Conference booths: a traveling, manned booth a’la commercial vendor introducing (will contain selected publications, description of the program and available instrumentation)
All require new materials in conductor forms that are not available today

19. Resistive Magnets - Challenges
As high a magnetic field in as high a volume, with minimum power consumption (resistive!)
Material challenge: High strength/low resistivity (usually incompatible)
Engineering challenge: Design a structurally sound magnet with efficient cooling (also incompatible)
Operational challenge: Power consumption and cooling capacity

20. Resistive magnets - Schematic

21. Resistive magnets – Bitter Coils

22. Hybrid Magnets - Challenges
Combination of resistive and superconducting magnets to achieve even higher field without increasing power consumption
System complexity (two types of magnets, water and helium cooling, protection issues, etc.)
Size and cost

23. Superconducting “Outsert” portion of hybrid
Coil A
Coil B
Coil C
Cu/Nb3Sn
Cu/Nb3Sn
Cu/NbTi
15kA current in CICC
Very large bore
Cooled by HeII (superfluid)
Outsert =15T, resistive insert = 30T (45T total)
Outsert and insert connected in parallel (i.e., s/c outsert is steady state – inability to ramp field down to zero)
Power consumption of insert is high (~30MW)

24. A high-performance and compact Series-Connected Hybrid (SCH)
Objective: Achieve similar field and performance as “workhorse” resistive magnet, but with higher homogeneity and much lower power consumption (increase “throughput”)
Challenge: Low AC loss CICC to achieve series-connection
45-T Hybrid
32-mm bore
30 MW
SCH
35 T, High-homo.
40-mm bore
10 MW
33-T All-Resistive
32-mm bore
20 MW

25. NMR Magnets - Challenges
Achieve highest possible field (study as large a molecule as possible)
Extreme field uniformity/permanence requirements. Typically better than 1 ppb over sample volume ( ∆B/B ~ 10-9 )

26. 900 MHz Wide-Bore NMR Magnet
Bronze-route Nb3Sn cond.
Nb3Sn Coils
Nb3Sn coils (NbTi outer coils)
Operation at 1.9K (superfluid)
Field 21.2 T
Bore 105 mm (largest in the world)
Energy 40 MJ

27. Towards 1.25 GHz NMR (30T): HTS inserts
NbTi
Nb3Sn
HTS
black = reinforcement
Existing
21T NMR
Conceptual
30 T NMR

28. Pulsed Magnets - Challenges
Achieve even higher field than DC magnets in pulsed conditions (up to 100T for msec)
Magnetic pressure exceed capabilities of materials  need to design for fatigue
Copper + Xylon coils, capacitive power supply
Need to wind very predictable coils that will have a predictable life-span (plus accurate measurement of inductance to foresee end-of- life without having an accident)

29. What happens when a pulsed magnet fails

31. Superconducting Magnets - Challenges
A magnetic, electrical, mechanical, and thermal system pushing the envelope of what is possible on multiple fronts
Numerous design trade-offs:
Stability
AC losses
Quench protection
Cooling requirements
Space constraints
Field quality

32. 1,000,000
YBCO: Tape, || Tape-plane, SuperPower
(Used in NHMFL tested Insert Coil 2007)
At 4.2 K Unless
YBCO: Tape, |_ Tape Plane, SuperPower
YBCO B||c
Otherwise Stated
(Used in NHMFL tested Insert Coil 2007)
Bi-2212: non-Ag Jc, 427 fil. round wire,
YBCO B||ab
Ag/SC=3 (Hasegawa ASC-2000/MT )
100,000
Nb-Ti: for whole LHC NbTi
strand production (CERN, Boutboul '07)
Nb-Ti: Nb-47wt%Ti, 1.8 K, Lee, Naus and
1.9 K LHC
Larbalestier UW-ASC'96
Nb-Ti
Nb3Sn: Non-Cu Jc Internal Sn OI-ST RRP
10,000
1.3 mm, ASC'02/ICMC'03
Nb3Sn: Bronze route int. stab. -VAC-HP,
non-(Cu+Ta) Jc, Thoener et al., Erice '96.
Critical Current Density (non-Cu), A/mm² 
Nb3Sn: 1.8 K Non-Cu Jc Internal Sn OI-ST
2212
RRP 1.3 mm, ASC'02/ICMC'03
1,000
round wire
Nb3Al: RQHT+2 At.% Cu, 0.4m/s (Iijima et al
2002)
2223 Nb
Al: 3
Bi 2223: Rolled 85 Fil. Tape (AmSC) B||,
tape B|_
2223
RQHT
UW'6/96
tape B||
Bi 2223: Rolled 85 Fil. Tape (AmSC) B|_,
UW'6/96
100
MgB2: 4.2 K "high oxygen" film 2, Eom et Nb Sn
al. (UW) Nature 31 May '02
MgB 3 2
1.8 K
MgB2: Tape - Columbus (Grasso) MEM'06
film
MgB 2 Nb Sn 3 Nb Sn 3
tape
4.2K (approx)
ITER
Internal Sn 10 5 10 15 20 25 30 35
Applied Field, T
This is what superconducting materials are capable of, to engineer them into working magnets is a different story

33. Stability Cp-Copper at 4.2K ~ 103 J/m3-K
Cp-Superconductor at 4.2K ~ 3x103 J/m3-K
Cp-Saturated Helium at 4.2K ~ 700x103 J/m3-K
“Stability margin” of metal coil pack ~ 1 mJ/cc (or J/m3, compare with energy density at 6T of ~ 1.5x107 J/m3)
Any movement will drive superconductor normal
Metal has no heat capacity at 4.2K
All enthalpy margin is in the helium

34. AC losses More Cu, more stability, but also more AC loss
Need to subdivide  CICC

35. Quench protection
Current density in s/c strand 4.2K, 5T) ~ A/mm2
Current density in regular extension chord (copper) ~ 1.5 A/mm2
If all the metal in the magnet were just the superconducting strand (with its stabilizing copper), the conductor would be a “fuse”
In the event of “quench” (loss of superconductivity) the magnet needs to be protected
Adding metal (lower Joule heating, more thermal mass)
Active means to detect quench and remove energy

36. Quench protection
Protection calls for more copper, but that drops current density, defeating the purpose
Trade offs:
Add copper (or metal) to absorb Joule heating
Detect quench faster (more metal does not help)
Propagate quench faster (to include more thermal mass), more metal defeats this purpose.
Extract as much of the energy as possible (external dump circuit)

37. Quench protection
Detect (time to detect + actuate limited by maximum temperature in magnet)
Actuate switch
Propagate quench internal to magnet while extracting as much energy as possible
Trade-off: Fast and high energy extraction requires large “R”, meaning high voltage  insulation issues

38. Survey of magnets for design/achieved engineering current density
500
LHC (design) ~ 500 A/mm2
300
1.5T MRI (typical)
LHC (achieved) ~ 220 A/mm2
200
3T MRI (typical)
100 T1 T4 T2
NRIM 21T NMR
NRIM 19T NMR
Current Density in the Coil Pack (A/mm2)
NHMFL 45T Hybrid-O
JLab TORUS
NRIM 40T Hybrid-O
LLNL Fenix
ITER CS Model 10
Nominal Limit (empirical) 1 10
100
1000
Stored Energy (MJ)

39. Strand short-sample limit Caveat Emptor
Beware when a magnet design calls for engineering current density above this empirical limit
500
Stability limit
300
200
100 T1 T4
Structural limit T2
NRIM 21T NMR
NRIM 19T NMR
Current Density in the Coil Pack (A/mm2)
NHMFL 45T Hybrid-O
NRIM 40T Hybrid-O
Quench protection limit
LLNL Fenix
ITER CS Model
Nominal Limit 10 1 10
100
1000
Stored Energy (MJ)

40. Magnets for Medicine - MRI
Only truly “commercial” application of superconductivity
Ubiquitous in today’s medical practice
A US$1B+ per year business dominated by three incumbents and multiple small players

41. MRI is a series-production challenge
High magnet production throughput with high degree of repeatability and minimum cost. All steps of the chain subject to intense pressure to reduce and simplify
Supply chain (standardization vs. diversification of suppliers)
Labor reduction, part-count reduction, standardization of tests and troubleshoot (no different than auto industry)
New technology introduction (newcomers?)

42. Magnets for Research - Fusion
Plasma heating, control, and confinement to research feasibility of large-scale fusion reactor
Largest superconducting magnets ever built
Part of an international project, ITER, funded by EU, USA, Russia, China, Korea, Japan, and India being built in France

43. ITER Magnet + Cryostat System
Central Solenoid (6)
(Nb3Sn)
Feeders (6)
(Nb-Ti)
Divertor
(54 cassettes)
Blanket
(440 modules)
Cryostat
(28 m high x 29 m dia.)
Vacuum Vessel
(9 sectors)
Toroidal Field Coils (18)
(Nb3Sn)
Poloidal Field Coils (6)
(Nb-Ti)
Correction Coils (6)
(Nb-Ti)
Very large one-of-a-kind superconducting magnets. Scale?

44. Jefferson Memorial (Washington DC) ~95 ft Tall (floor to top of dome)
ITER scale comparison comparison
~91 ft Tall
Jefferson Memorial (Washington DC)
~95 ft Tall (floor to top of dome)
ITER Cryostat
~91 ft Tall x 95 ft Dia.

45. (Maximum Takeoff Weight)
TF Coil – Mass Comparison
Boeing
(Maximum Takeoff Weight)
~377 t
Mass of (1) TF Coil:
~360 t
52 ft Tall x 29 ft Wide

46. Concluding Remarks
Scientific research has been driving magnet technology towards stronger, bigger, better…
Engineering of magnets has responded by continuously achieving targets through materials and design
What came first? The chicken or the egg? Scientific discovery is not possible without the engineering of “instruments”, science determines the direction of engineering development, but ability to “discover” ultimately depends on our engineering abilities
We can only make progress through the collaboration of multiple disciplines (“Teams” + Integration !!!)