superconductor stability cern accelerator school superconductivity for accelerators erice, april 25...
TRANSCRIPT
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Superconductor Stability
CERN Accelerator SchoolSuperconductivity for
Accelerators
Erice, April 25th – May 4th, 2013
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Plan of the lecture Training and degradation Perturbation spectrum overview Heat balance Stabilization strategies and criteria A summary and more complex topics
focus on LTS magnetsadiabatic, or helium cooled
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Plan of the lecture Training and degradation Perturbation spectrum overview Heat balance Stabilization strategies and criteria A summary and more complex topics
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Training…
Superconducting solenoids built from NbZr and Nb3Sn in the early 60’s quenched much below the rated current …
… the quench current increased gradually quench after quench: training
M.A.R. LeBlanc, Phys. Rev., 124, 1423, 1961.
NbZr solenoidChester, 1967
P.F. Chester, Rep. Prog. Phys., XXX, II, 561, 1967.
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… and degradationNbZr solenoid vs. wire
Ic of NbZr wireImax reached in NbZr solenoid
… but did not quite reach the expected maximum current for the superconducting wire !
This was initially explained as a local damage of the wire: degradation, a very misleading name.
All this had to do with stability
P.F. Chester, Rep. Prog. Phys., XXX, II, 561, 1967.
The prediction of the degraded current […] proved to be impossible and […] the development of coils passed through a very speculative and empirical phase
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Training today Training of an model
Nb3Sn 11 T dipole for the LHC upgrade in liquid and superfluid helium Still, training may be
necessary to reach nominal operating current
Short sample limit is not reached, even after a long training sequence
stability is (still) important !
11 T field in the dipole bore
Courtesy of G. Chlachidze, Fermilab, April 2013, unpublished
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Dealing with early instabilities
“Those tiny, primitive magnets were, of course, terribly unstable […] One had to have faith to believe that these erratic toys of the low temperature physicist would ever be of any consequence as large engineered devices” (J. Hulm, ASC 1982)
W.B. Sampson, Proc. Int. Symp. Mag. Tech., SLAC, 530-535, 1965
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A Woodstock for SC accelerator magnets
A six weeks summer study organized and hosted by BNL in 1968
The crème de la crème addressed material and engineering issues of superconducting accelerators, among them: Stability, training and degradation Flux-jumps in composite
superconductors Twisting of multi-filamentary wires and
cables
J. Hale Y. Iwasa
W. Sampson P. SmithM. MorpurgoB. Montgomery P. Lazeyras R. Wittgenstein
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An event tree for stability (and quench)
Stable operating condition
External perturbation:flux jumpconductor
motionsinsulation
cracksAC lossheat leaksnuclear…
Approach Jc(T, B, …)
Quench
yesno
Stable operating condition
Transition to normal state and Joule heat
generation in current sharing
heat generation > heat removal
stability analysis and design
Heat balance
Perturbation spectrum
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Plan of the lecture Training and degradation Perturbation spectrum overview Heat balance Stabilization strategies and criteria A summary and more complex topics
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Perturbation spectrum Mechanical events
Wire motion under Lorentz force, micro-slips Winding deformations Failures (at insulation bonding, material yeld)
Electromagnetic events Flux-jumps (important for large filaments, old story ?) AC loss (most magnet types) Current sharing in cables through
distribution/redistribution Thermal events
Current leads, instrumentation wires Heat leaks through thermal insulation, degraded cooling
Nuclear events Particle showers in particle accelerator magnets Neutron flux in fusion experiments
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Perturbation scales
Transient (stability concern) point Q [Joules] distributed Q’’’ [Joules/m3] [mJ/cm3]
Continuous (sizing of cooling system) point q [Watts] distributed q’’’ [Watts/m3]
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Perturbation overview
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Flux jump mechanismExternal field change induces screening
persistent currents (JC) in the superconductor
A drop in screening current causes the field profile to enter in the
superconductor
Energy is dissipated (flux motion)
Thermal diffusivity and heat capacity is small in the superconductor and the
temperature increases
Critical current density decreases at increasing temperature
A small perturbation induces a temperature increase
After the work of M.N. Wilson, Superconducting Magnets, Plenum Press, 1983
B
JC
B
JC
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Flux-jumps energy During a complete flux-
jump the field profile in a superconducting filament becomes flat: e.g.: field profile in a fully
penetrated superconducting slab
energy stored in the magnetic field profile:
D = 50 mm, Jc = 10000 A/mm2 Q’’’ 6 mJ/cm3
area lost during flux jump
NOTE: to decrease Q’’’, one can decrease D
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Flux jumps then and now
A.D. McInturrf, Composite Materials, Proceedings of the 1968 Summer School on Superconducting Devices and Accelerators, BNL 50155 (C-55)
B. Bordini, et al., Magnetization Measurements of High-JC Nb3Sn Strands, CERN-ATS-2013-029
Complete flux jump
Partial flux jump
Flux jumps is not an old story, we still suffer from magnetic instabilities when pushing for high conductor
JC
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Plan of the lecture Training and degradation Perturbation spectrum overview Heat balance Stabilization strategies and criteria A summary and more complex topics
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Prototype heat balance
Heat sourc
e
Joule heat Conductio
n
Heat transferHeat
capacity
cooling
generation
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Temperature transient
heat pulse Q’’’ext…
…effect of heat conduction and cooling…
Q’’’ext = Q’’’+e generation > cooling
Q’’’ext = Q’’’-e generation <
cooling
…decision time…
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Energy margin… Q’’’, energy margin
Minimum energy density that leads to a quench
Maximum energy density that can be tolerated by a superconductor, still resulting in recovery
Simple and experimentally measurable quantity (…)
Measured in [mJ/cm3] for convenience (values 1…1000)
Also called stability margin Compared to the energy spectrum to achieve
stable design
Q, quench energy Better adapted for disturbances of limited space
extension Measured in [mJ] to [mJ]
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… and other useful margins
T
Jc
B
14.5 T9 K
typical NbTi
5 T 4.2 K
3000 A/mm2
operating planeJ
B
Jop
Bop Bc
Bmax
Jc Margin in critical current
Tcs
J
T
Jop
Top Tc
Margin in temperature
Margin along the loadline
Jop/JC ≈ 0.5Bop/Bmax ≈ 0.8 (Todesco’s 80 %)
Tcs-Top ≈ 1…2 K
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A measured stability transient (LHC dipole magnet training)
Voltage increase generated by a sudden heat input
…cooling…
…decision time…
…quench
2 ms
15 mV fast !!!
What happens here ?
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Current sharing
Tcs T
Iop
Top Tc
Ic
Tcs < T < Tc
T < Tcs
T > Tc
quenched
curent sharing
stabilizer
superconductor
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Joule heating
TTcsTop Tc
Iop
current in stabilizer
current in superconductor
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Joule heating (cont’d)
Linear approximation for Ic(T):
Joule heating
TTcsTop Tc
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Cooling: many options
Indirect: (adiabatic, no cooling) Contact to a heat sink through conduction (e.g.
to a cryo-cooler) In practice, no cooling on the time scale of
interest for stability Direct: (cooling by heat transfer at the
surface) Bath cooling, to a pool of liquid helium at
atmospheric pressure and saturation temperature (4.2 K)
Force-flow cooling to a supercritical or two-phase flow
Superfluid cooling to a stagnant bath of He-II
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Plan of the lecture Training and degradation Perturbation spectrum overview Heat balance Stabilization strategies and criteria A summary and more complex topics
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Adiabatic stability Adiabatic conditions:
No cooling (dry or impregnated windings) Energy perturbation over large volume (no
conduction)
Stable only if q’’’Joule=0 (TTcs) !energy margin
volumetric enthalpy
TTcsTop Tc
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Specific enthalpy
2
30
2
3
enthalpy reserve increases at increasing T (Note: HTS !)
do not sub-cool !(if you can only avoid it…)
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Adiabatic stability re-cap Applies to:
Adiabatic, compact, high current density windings (dry or indirectly cooled)
Very fast heat perturbations (flux-jumps) The heat capacity of the conductor
absorbs the external heat perturbation Stability (at equal temperature margin)
improves as the temperature increases (HTS !)
Choose materials with high heat capacity (e.g. loading of epoxy)
Relatively small energy margin: 1…10 mJ/cm3
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Cooling in a bath of pool boiling helium Ignore conduction for large energy perturbations
volumes Request steady state stability in all conditions
Cryostability
worst possible
case
generation
cooling
A.R. Krantowitz, Z.J.J. Stekly, Appl. Phys. Lett., 6, 3, 56-57, 1965.
Z.J.J. Stekly, J.L. Zar, IEEE Trans. Nucl. Sci., 12, 367-372, 1965.
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Heat balance (ideal case)
generation
cooling stable
cryostable
not cryostable
not cryostable
constant heat transfer to the helium
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Stekly-a Stekly cryostability condition:
can be formulated as aStekly 1 :
Improve cooling
Increase the cross section of stabilizer
Increase the temperature margin
Decrease the operating current
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Cryo-stability recap Applies to:
Well-cooled, low current density windings (pool-boiling)
Any type of heat perturbations, all time and space scales
The coolant can take the Joule heating under all possible conditions
Ideally infinite energy margin
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Assume that the normal zone is long and above cryostable operating conditions
The temperature will reach an equilibrium temperature
Cold-end effects – 1
Teq
cryostable
not cryostable
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What happens if the ends are cold ?
Request steady state stability in all conditions
Cold-end effects – 2
T
x
TcTcs
Teq
Top
cold-end conduction lead to recovery
t
T
x
TcTcs
Teq
Top
cold-end conduction not sufficient to prevent quench
t
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Cold-end effects – 3 introduce a new variable S:
B.J. Maddock, G.B. James, W.T. Norris, Cryogenics, 9, 261-273, 1969.
Sop = 0 Seq = 0
wh (T-Top)
A/w q’’’J
T
x
TcTcs
Teq
Top
q’’
x
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An equal area theorem
Stable conditions are obtained when the net area between generation and cooling curves is zero
B.J. Maddock, G.B. James, W.T. Norris, Cryogenics, 9, 261-273, 1969.
Values of a nearly twice as large as from the Stekly criterion are possible !Stekly: a ≤ 1Maddock: a ≤ 2-fop
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Cold-end effects recap Applies to:
Well-cooled, low current density windings (pool-boiling)
Any type of heat perturbations, all time and space scales
The coolant and the cold ends take the Joule heating under all possible conditions
Ideally infinite energy margin Improved stability with respect to the
cryostability condition, allow operation at higher current
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Meta-stable conductors
Adiabatically stabilized conductors: High Jop (good for cost) Small Q’’’ (bad for large magnets)
Cryo-stabilized conductors (including cold-ends): Large Q’’’, ideally infinite (good for large magnets) Low Jop (bad for cost)
Is there a compromise ?
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Idea-1: helium !
The helium heat capacity is orders of magnitude larger than for metals at low temperatureAdd helium in intimate contact with the cable
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ICS’s and CICC’s
M.O. Hoenig, Y. Iwasa, D.B. Montgomery,Proc. 5th Magn. Tech. Conf., Frascati, 519,
(1975)
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Conductor temperature:
But the helium temperature evolves as well:
Under which conditions the heat capacity is effectively used ?
NOTE: at large enough h, TThe
Heat balance for CICC’s
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Stability of CICC’sD
Q’’
’
Iopllim
helium + strand heat capacity
strand heat capacity
Joule heat<
cooling
Joule heat>
cooling
well-cooled
ill-cooled
balance of Joule heat and cooling at:
equivalent to aStekly = 1
In this case however the CICC is meta-stable as a large enough energy input will cause a quench !
J.W. Lue, J.R. Miller, L. Dresner, J. Appl. Phys., 51, 1, 772, (1980)J.H. Schultz, J.V. Minervini, Proc. 9th Magn. Tech. Conf., Zurich , 643,
(1985)
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Idea-2: heat conduction !
x
T
TcTcs
T’eq
Top
x
q’’
wh (T-Top)
A/w q’’’J
S=0 at the boundaries
equal area still possiblebut implies lower T’eq < Teq
Short normal zone
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Normal zone temperature The temperature profile can
be traced by numerical integration of the equal area balance
This is an unstable equilibrium temperature profile, and defines the minimum length of superconductor that could grow and develop into a quench: Minimum Propagating Zone (MPZ)
The energy required to form the MPZ is the Minimum Quench Energy (MQE)
x
T
q’’’J
M.N. Wilson, Y. Iwasa, Cryogenics, 18, 17-25, 1978
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MPZ estimates To estimate the size of the MPZ we can
solve the heat balance approximately Steady state conditions and no cooling
After the work of M.N. Wilson, Superconducting MagnetsPlenum Press, 1983
Jop = 400 A/mm2k = 500 W/m Kh = 1 nW mTC-Top = 2 K MPZ ≈ 3.5 mm
MQE ≈ 10 mJ
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Plan of the lecture Training and degradation Perturbation spectrum overview Heat balance Stabilization strategies and criteria A summary and more complex topics
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Summary Superconductor stability is the art of
balancing large stored energies (and potential for energy release) with a little capital (aka: “leveraging” for our US colleagues)
Extremely important in LTS-based magnet to reach the desired performance (10 MJ vs 10 mJ)
Generally not an issue for HTS-based magnets Stability is not absolute, it implies comparing
the perturbation spectrum to the available margin
Several strategies can be applied to achieve stable operating conditions under the envelope of foreseeable perturbation spectrum
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A summary of strategies
Operating current density (A/mm2)
En
erg
y m
arg
in (
mJ/cm
3)
1 10 100 10001
10
100
1000 CICC
AdiabaticCry
ost
able
Cold
en
d
MPZ/MQE
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Advanced topics Current distribution and
RRL Ramp-rate quenches Holding quenches
Magneto-thermal instabilities Dynamic stability Self-field instability
There are more things in heaven and earth, Horatio, Than are dreamt of in your
philosophy.
type B
type A
A. Devred, T. Ogitsu, Frontiers of Accelerator Magnet Technology, World Scientific, 184, 1996
B. Bordini and L. Rossi, IEEE TAS, 19, 2470 (2009).
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A zoo of configurations…CICC’sStrands and tapes power
transmission cables
Rutherford
induced flow,transient heat transfer,
AC operation,current distribution,
HTS,coolants (N2, Ne, H2)…
Internally cooled
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… and models
Things can get fairly complex
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Ah, and… what if it falls ???
Then, you need to protect !
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Where to find out more Papers, reviews and proceedings:
A.R. Krantowitz, Z.J.J. Stekly, A New Principle for the Construction of Stabilized Superconducting Coils, Applied Physics Letters, 6, 3, 56-57, 1965.
P.F. Chester, Superconducting Magnets, Rep. Prog. Phys., XXX, Part II, 561-614, 1967.
B.J. Maddock, G.B. James, W.T. Norris, Superconductive Composites: Heat Transfer and Steady State Stabilization, Cryogenics, 9, 261-273, 1969.
M.N. Wilson, Y. Iwasa, Stability of Superconductors against Localized Disturbances of Limited Magnitude, Cryogenics, 18, 17-25, 1978.
L. Dresner, Superconductor Stability 1983: a Review, Cryogenics, 24, 283, 1984.
Books: M.N. Wilson, Superconducting Magnets, Plenum Press, 1983. L. Dresner, Stability of Superconductors, Plenum Press, 1995. P.J. Lee ed., Engineering Superconductivity, J. Wiley & Sons, 2001. B. Seeber ed., Handbook of Applied Superconductivity, IoP, 1998. Y. Iwasa, Case Studies in Superconducting Magnets, Plenum Press,
1994.