1. Mike Sumption, M. Majoros, C. Kovacs, and E.W. Collings
Magnetization and Stability in Coated Conductor Cables for HEP applications
Mike Sumption, M. Majoros, C. Kovacs, and E.W. Collings
Center for Superconducting and Magnetic Materials, MSE, The Ohio State University
CORC Samples:
D. Van Der Laan
Advanced Conductor Technologies and University of Colorado
Twisted Strand:
University of Houston
Nijhuis and K. Yagotyntsev,
The University of Twente
M. Takayasu, MIT, PSFC
W. Goldacker
N. Long
This work was supported by the U.S. Department of Energy, Office of Science, Division of High Energy Physics, under Grant DE-SC

2. Why the focus on Magnetization
Why the focus on Magnetization? – its b3 and its change for accelerator magnets
This is based on an estimation from Tape

3. OSU Magnetization Rigs
Magnetization Measurement Facilities: A number of options are available, some systems better suited to given samples or given test requirements
Also involved with Air Force SAM machine

4. MAG-1: 4.2 K-RT, T
Invar
Magnetization rig MAG-1: Made for magnetization of short cables
Sample up to 6 cm long
Current + field

5. Mag 2: 3 T Magnet Max Field = 3.1 T Max I = 90 A L = 1 H
Max Ramp Rate = 70 mT/s
Cable Magnetization measuring facility (Magnet arrived early Feb 2016, assembly underway);
3 T coil (Cryomagnetics design and made), NbTi wound, 150 amperes, Inductance of 10 milliHenries. Clear bore is 5.5 cm total length cm.
Cryofab dewar and hang-down assembly.
Drawing of coils, homogeneity on-axis is +/- 5% over 15 cm,
radial field homogeneity.

6. Coils and Sample Holder

7. MAG-3: Screening loss rig
M-H and AC Loss device for 77 K measurements of YBCO
External-Field M-H and AC Loss is measured 150 mT, Hz
A system of pick-up coils connected to a digital oscilloscope records the samples' M-H loops
Used in this program for screening and as a comparison/calibration measurement at 77 K
Also 77 K, SF is like 20 T, 4 K

8. Key Question 1: What is the magnetization of Cables, especially at 4 K, and different ramp rates?
tape
CORC cable from Tape
Penetration field increases for cables made from tapes– a shielding effect
Complicated helical geometries

9. Key Question 2: How are Cable losses and magnetization related to that of tapes?
SAM-CORC
The hysteretic loss and magnetization of CORC above field penetration can be approximated by Tape loss * 2/
Bmax = 0.56 T
As measured
CORC * /2

10. MagnetizationMeasurements on CORC at 77 K
Of interest:
Magnetization of striated tape
Magnetization of striated and twisted tape
Magnetization of striated and unstriated tapes on CORC
Re-estimate of error field estimates

11. Measured Loss in Striated and Twisted YBCO– University of Houston tape samples
Un-Striated
Striated (soldered ends)
Striated and Twisted
width = 12 mm
length = 16.1 cm
Thickness = 70 µm

12. YBCO, Striping, Twisting
Sample 1: Striated
Sample 2: Not Striated
Sample 3: Striated and Twisted
Measurements made on University of Houston Samples
50 Hz, Bmax = 0.1 T
Striations + Twisting needed for magnetization reduction

13. Striated measurement results 77 K
Striations do significantly reduce loss
Some factor from striation, some from Ic loss

14. New UoT Studies
Nijhuis and K. Yagotyntsev,
The University of Twente
While new OSU machine is being installed, made measurements at UoT
Measured TWST, CORC, and Roebel cables at 4 K
AC loss (10-60 mHz, 0.4 T), M-H (0-1.4 T, 10 mHz)
Extracting: hysteretic, coupling, Magnetization at injection, and field penetration

15. Twist Stack cables (TSTC) for 4 K Measurement
4 mm wide SuNAM Tape
150 m SS
Ic = 200 A, 77 K, SF
Conductor Length = 200 mm
Twist Pitch = 200 mm
M. Takayasu, MIT, PSFC

16. TSTC Samples
TSTC-1:stacked tapes twisted between Cu strips, with retaining Cu and in plexiglass Tube
TSTC-2: Tapes stacked Horizontally in a single helical groove in an OFHC Cu rod with sheath (05 “ OD)
TSTC-3: Tapes stacked vertically in a single helical groove in OFHC Cu with sheath
TSTC-4: Tapes stacked in two vertical grooves in an OFHC Cu rod with a Cu sheath
No soldering, packing only

17. CORC Conductor Specs
Roebel Cable Specs

18. TSTC-Hysteretic and Coupling Loss
30 tapes, 200 A/77 K SF -> 6 kA at 4 K, 20 T
At accelerator-relevant frequencies, non-negligible coupling loss.
Ballpark of coupling currents for Nb3Sn magnets (3 x)
Hyst loss about 3 x, but not fully penetrated
(not current normalized)
Pressure of abrasion related to the twist of the tapes making for low contact resistance
Resistivity about 1 order above Cu at Lhe temp

19. CORC Hysteretic and Coupling Loss
Early Experimental cables for striped/not striped
More flux penetration here
Coupling loss values show high interstrand resistance – not infinite, but in milli-100s milliohms
Note 1: Loss per tape volume greater for fewer layers– relevant for injection
Note 2: Striped Tape CORC loss suppressed by about x 3 (not quite 5)

20. Roebel Hysteretic and Coupling Loss
More flux penetration here given geomtry
Coupling loss values show high interstrand resistance – not infinite, but in milli-100s milliohms

21. Field Penetration into cables-TWST cables
Below penetration, Q goes as B3, above, as B
Above penetration, Q/B should be fixed, with y-intercept w*Ic
1.4 T does not penetrate the sample

22. Roebel Cable Field Penetration
Loss peaks at field penetration
Penetration earlier because of large demagnetization Effect

23. Field Penetration into cables – CORC Cables
Full penetration happens at same place
Saturation “effective width” different by x 3
Approaching full penetration
But, partial penetration is more rapid for striated samples
Do S1-3 saturate earlier?

24. Discussion
Generally, for a simple cylinder, both the field at which field penetration occurs, and loss above penetration are proportional to “d” or “w”
Here, we find that the penetration field is not affected by the striation (i.e., w reduced by 5) BUT, the loss was reduced by 3
How can we have a conductor where the loss is reduced with striation, but penetration does not occur at a lower field?
Perhaps the CORC cable excludes from the whole cable (i.e., field lines do not penetrate into the central region?
But then, how are striations effective?
The CORC transitions from field exclusion from the core (acting like a filled rod) at low fields or amplitudes to acting like a collection of tapes at higher field
This has implication for the magnetization at injection. Let’s look at that …..

25. M-H Loop Results R1, R3
R1-2 layers
R3-4 layers

26. R3 and S3-normalized to tape volume
S3-4 layer-striped
R3-4 layers

27. Roebel and TWST-4 M-H (normalized to tape volume)

28. Magnetization – but loss?
For the LHC NbTi dipoles ramping at about 7 mT/s AC loss is only a small contributor to cryogenic load
Could be larger for YBCO cables.
For a YBCO cable carrying a current of 10 kA at 20 T the loss at 7 mT/s is estimated to be 200 mW/m
For an HTS insert of, say, 70 turns the winding dissipation would be 14 W/m -- more than double the LHC ring’s 4.5 K/1.8 K refrigeration capacity
This is a handle-able problem, but not of no interest

29. FEM Modelling Initial approach
Model to start as model of one helix, then moving to multiples
Treat the SC film as distributed
Jc values are representative of 4 K, 0 T

30. Modelling Treatment – Average to composite Volume
Composite, 250 m
SC, 1 m
Substrate, 50 m
Penetration field,
Bp = CJcd
So, only OK if re-normalization is in same dimension as d
M = CJcd
Msc = msc/Vsc= CJcd
Let Je=Jc/ff
Mcomp = Msc/Vcomp = Msc/(Vsc*ff)=mscJcd/ff=mscCJed
So we can treat the current as flowing through the whole composite and get a proper M

31. Magnetization calculation for 1 strand CORC cable 4 K, Bean model

32. Comparison loss shape to model
AC loss of sample R3 at 50 Hz in liquid nitrogen bath (77 K)
FEM model prediction

33. Drift in accelerator Magnets
Just as important as the absolute value of b3 is any change with time during the injection porch
It is possible to compensate for error fields with corrector coils, but the presence of drift makes this much more difficult
At right is shown the drift of the error fields as a function of time from zero to 1000 seconds for LHC magnets, followed by a snap-back once the energy ramp begins
The underlying mechanism for drift in NbTi magnets is the decay of coupling currents, (especially inhomogeneous and long length scale coupling currents) and their influence on the strand magnetization
Need to keep both b3 and its drift below 1 unit
For NbTi and Nb3Sn based magnets, this is possible

34. Magnetization Decay Suppression with Zr doping
New YBCO conductor with Zr pinning, University of Houston
Magnetization decay for coated conductors from UoH with (a) : 0% Zr, (b) : 7.5% Zr, (c) : 25%

35. Quench Stability and Protection
Quench and quench propagation in CORC and Roebel at 77 K and 4 K
Quench and stability in small pancake coils 77 K and 4 K
Will not discuss all experiments, just a few selected ideas/observations
We may quench some strands in a cable – while others remain SC
We may develop (stable) normal zones in magnets, while the other regions are still SC
Current sharing and thermal sharing need explicit encouragement in HTS cables
Meaning of MQE in LTS and HTS

36. Observation 1: We may quench some strands of a cable and not others
CORC Cable

37. Observation 2: Stable zones may form which do not grow, shrink, at hit a stable temperature
CORC Cable
Or, of course, we may hit runaway

38. Stable normal zones may happen in a coil too
77 K small coil
50 T solenoid
Remember, Ic varies strongly in a winding as a function of local field and tape orientation!

39. LTS vs HTS quench I It seems different than LTS. Is it?
Is quench different in LTS and HTS?
In LTS, an MQE exists, above which the coil or cable quenches, below which, it does not
In HTS, there seems to be two modes of quench
(1) A HUGE energy is deposited “instantaneously” and it is enough to generate an MPZ and a direct quench
(2) A smaller amount of energy is deposited over a very much longer period of time, and if the current is left on – a normal zone will grow and eventually reach the MPZ, and a non-recovering normal zone is generated
These energies are different! Which one is MQE?
It seems different than LTS. Is it?

40. LTS vs HTS quench II Is quench different in LTS and HTS?
MQE has a well defined (unique) meaning for LTS, mostly because the time scale of energy deposition is much shorter than we are normally used to thinking about
The existence of a minimum propagating zone of course says that there are lengths shorter than that, which are associated with smaller energies
Imagine that we would look at very, very short time scales
Then, we could imagine energy depositions large enough to “instantly” generate a MPZ.
We could also imagine a lower power deposition, happening over a longer time which would “grow” a normal zone to an MPZ
These two processes are difficult to distinguish for LTS (at shorter times than many physical processes, and our typical measurements, but for HTS, the large heat capacity slows things down greatly

41. LTS vs HTS quench III
So, HTS quench is a physically identical but very much slower process than LTS quench, so it appears to have a different character
Leaves us with two kinds of quench to protect against
FAST quench (Huge depositions of energy)
SLOW quench (we have time to respond)
Which can we afford to protect against? Both?
Former is like a fault mode – “disaster” – (“hit with a hammer”)
Latter is more like a lower level fault or defect mode (cryocooler turned off, wire defect, etc).
In such a case, for lower energy coils, can just “turn off”
For larger coils, must protect against large voltages during discharge, so active heaters required

42. Key Questions/comments in Measurements and modelling (M-H, loss)
Comment: AC loss is good, magnetization is better! (loss extractable from M-H)
Comment: Magnetization in cables strongly influenced by geometry of tape assemblies
Modelling which can handle many helical stacked tapes needed
How much striation is needed? Is there another way? Can we get away without it?
How important is time decay of magnetization, and how can it be suppressed?
How does ICR affect Magnetization in various cables?

43. Key questions in stability and Protection
Current sharing and thermal sharing is needed in HTS cables. How much?
Does the presence of Slow and Fast type quenches change our protection choices and strategy?
Golden ratio (Wilson): Would it be better if Tc was lower while Bc2 remained high? (what is your favorite mix of MQE, NZP, and margin?

44. Number of tapes and width vary Increasing I increases area, M const
Cable
Msat LF, 77 K (kA/m),
cable Vol
Minj, 4.2 K (kA/m), Cable Vol
N x t (mm)
Ic, A collision field
Minj, 4.2 K, kA/m, CV, per 3900 A
Minj,4.2 K kA/m Per strand vol, per 3900 A
M inj,4.2K kA/m per strand Vol, “any current”
CORC
100
500
39 x 4
3900a
1667
Roebel
120
600
10 x 2
4000a
750
Bi:2212 -- 16
240a 20
Nb3Sn
170
10,500b
 170
NbTi 10
 10
Due to different w
YBCO Cables
Similar at 77 K
Moving to 4 K, 1 T
X 10 x 1/2
Number of tapes and width vary
Increasing
I increases area,
M const
Norm to strand vol
Mag comparison for various strands
a 20 T, b 15 T
Iybco = 100 A/4 mm at 20 K
CORC Cable Total Vol =3.14*(7.34mm/2)2 = 42.3 mm2
CORC Cable Volume of Central non-SC region = 3.14*(4.28/2)2 =14.37 mm2
Volume of winding area = 27.93mm2
Strand Vol Danko =0.08*4*39=12.48mm2
CORC Fill factor = 30%
Roebel cable Vol = 5 x 0.08 mm x 5 = 5 x 0.4 = 2 mm2
Roebel Strand Vol = 4 *0.08*5=1.6 mm2
Roebel Fil factor = 80%
Nb3Sn 1500 A/mm2 at 15 T – assume 0.8 mm OD = 0.5 mm2 * 0.5 non-Cu = 375 * 28 = A