Transcript Apr29_2_TenKate - CERN Accelerator School

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Principles of
Superconducting Accelerator
Magnet Design
Herman ten Kate
CERN Accelerator School on Superconductivity for Accelerators, Erice, 29 April 2013
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Content
1. HEF synchrotron & what is a good magnet
2. Magnetic design
3. Relevant issues, a tour from filament
to vacuum vessel
4. Few topics: Field and Field quality, Minimum Engineering Current
Density, Grading, Training, High Currents, a 20T design.
5. Conclusion
Try to do : following the Superconductor Properties (Larbalestier, Flukiger)
and Basics of Accelerator Optics (Holzer)
to introduce the Concepts and Issues of Accelerator Magnets
to prepare for the more in-depth presentations on Cables (Bruzzone),
Magnetic Design (Todesco), Mechanical Design (Toral),
Thermal design (Baudouy) and Cryostats (Parma).
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1. Superconductivity and HE Physics
Accelerators, like LHC, can not be realized without extensive use of
superconductivity and high quality magnets
Nb/Cu cavities
for acceleration
1232 dipoles magnets
for bending
386 quadrupole
magnets for focusing
~7000 Correction
magnets
ATLAS and CMS
detector magnets
Insertion and Final
Focusing magnets
No Higgs without Superconductivity!
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What is a good magnet?

It technically performs: generates magnetic field in the required space,
dipoles, quadrupoles, multi-pole correction magnets, while respecting
space uniformity and time stability requirements; all requiring a proper
magnetic design.

It’s safe: respecting mechanical and electrical design codes, negligible
magnetic stray field, no cryogenic blasts, a sound quench protection as
well as reliable instrumentation and controls.

It’s available: has to fulfill stability, training & degradation requirements.

And at minimum cost: robust and simple design suitable for series
production within a foreseeable time, easy to operate, well managed.

Often one or more of these requirements, in particular regarding
training, robustness and costs are not fulfilled, hindering accelerators
today to run far beyond 8T level………..you are challenged to fix this.
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From Material to Magnet…..
Lattice
Sn
Filament
Wire
Cable
Magnet
12 mm
15 meter
Nb
50 nm
20 µm
1 mm
For making good magnets we need to understand in depth and able
to control the entire chain of development from lattice & grains,
through wires and cabling, to coil winding, - assembly, cryostating,
protection and proper controls……
Main issue in how to make performing multi-kA conductors and make
coil windings that guarantee the magnet not to quench or degrade ?
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2. Magnetic design: stay on orbit
Need
force acting on the particles in the “beam” (e, p, heavy ions)
 a Lorentz
 
F  ev  B by a uniform B across the beam section (dipole magnets)
How?
Fill parallel walls or intersection ellipses with uniform Jeng=Jo
or put a Jeng=Jo.cosθ on a cylinder.
Parallel walls
Intersecting ellipses
Cosθ (Roma arc)
Practical 2 layer coil
I
Either way B ≈ 0.5 µo Jo w or 0.25T/mm coil width assuming 400A/mm2
8T with ~33mm
16T with ~66 mm
24T with ~100mm
(excluding effect of iron yoke which is modest, another + 0.3-0.8%)
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Quads for squeezing
Second type of magnets required are focusing magnets.
Bringing diverging particles back on track, focusing, requires Lorentz
forces linearly increasing with distance from the beam center.


 
Fx  e v  B x  eyBs  sBy   ecBy
Such quadrupole magnets can be made by arranging 4 poles on a cylinder
following in good approximation Jeng=Jo.cos2θ
The technology is similar to dipole
magnets though the effort is on
achieving maximum magnetic
field gradients
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Practical solutions
From an ideal cos(θ) dipole or cos(2θ) quadrupole to a practical solution,
arranging windings in blocks to fulfill field quality requirements.
practical dipole windings pack
practical quadrupole windings pack
Courtesy of L.Bottura
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Limits in collision energy
• Beams are crossed with low angle at points were collisions take place,
the place where the customer of “the machine”, the physicists sit with
their physics experiment.
• Collision energy is limited by bending magnetic field and radius of the
synchrotron following
E[GeV]  0.3  B[T]  R[m]
• Which immediately links physics discovery potential at higher energy to
superconducting material B(J) and footprint 2R of the machine….& cost.
• There are clear thresholds in the combinations of superconductor used
Jengc(B) and tunnel length.
• Examples: LHC: 14 TeV: NbTi at 8 - 9T, tunnel length ≈30km
•
100 TeV: Nb3Sn at 15 -16T, ≈130km
•
200 TeV: HTS at 20-25T, ≈200km
 So we need good magnets, what are the issues….
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3. A walk from inside to outside
•
•
•
•
A wire embedding superconducting material in a matrix.
A cable with many coated strands and with insulation.
Coil windings comprising a stack of cables and wedges.
A coil with windings, ground insulation and helium cooling.
•
•
•
•
A laminated collar pack surrounding the coils, transparent for Helium.
A laminated yoke in between collars and support cylinder.
A support cylinder also helium can with ends and transfer ports.
A cryostat with vacuum vessel, thermal shield, cold mass supports, bus
bars, helium lines and instrumentation ports.
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3.1 Wire with Superconductor
A wire with a matrix embedding superconducting material
(see presentations of D.Larbalestier and R.Flukiger).
•
•
•
•
NbTi wire,
Nb3Sn wire,
B2212 wire,
Re123 tape,
good for 8 - 9T at 1.9K
good for 14 -16T at 1.9K
good for 17->30T at 4.2K
good for 17->30T at 4.2K
What counts is
• Engineering current density Jeng(B,T,Ɛ) of ~400A/mm2
and Temperature margin ΔTmargin = Tcs(B,I)-Tbath of >2-5K
to arrive at target B in a cost efficient way.
• Filament diameter <50µm to warrant filament stability and
<5µm to limit field errors in NbTi and <40µm in Nb3Sn
• Twist pitch of ~20-30mm to limit ramp losses
• Wire diameter <1.3mm to avoid self-field instability
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3.2 Cables with strands
•
•
•
•
•
•
•
•
•
Highly compacted Rutherford cable with 20-50 strands, fully transposed.
Cable width 12-24mm depending on cable current, number of layers.
Key-stoned section to accommodate cos-θ type winding blocks.
Coating on strand (SnAg,Ni,Cr) to limit and control interstrand loss and
cable magnetization, or avoid sintering (Nb3Sn).
Cable pitch of ~100-200mm to limit ramp loss, cable integrity & stiffness.
Superfluid Helium in NbTi cable voids at 1.9K to enhance enthalpy.
For Nb3Sn, no He in cable but vacuum impregnated to freeze wire
movement and reduce wire-to-wire point strain.
3-4 x 0.4mm2 Re123 tapes in Roebel type cables or may be in CORC.
So far no Cable-in-conduit-Conductors used in accelerator magnets but
possible……..may be a way to get helium cooling directly on the Nb3Sn
strands and solve the training issue in these magnets.
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3.3 Coils with cables and wedges
• Blocks of cable-turns, Cu wedges inbetween, to control field errors.
• Two layers or even more, 4, 6.
• Cable insulation (NbTi: Kapton/glass tapes;
Nb3Sn: glass tapes, glass/mica, glass braid
• A dielectric film included is preferred to
avoid risk of shorts in series production.
• Ground insulation (usually thick Kapton).
• Coil layer helium cooling (fishbone).
• Cooling in-between layers if high heat load.
• Windings superfluid He transparent at 1.9K
(NbTi) or vacuum impregnated (Nb3Sn).
I
I
I
B
• Coil instrumentation: voltage taps,
temperature sensors, strain and pressure
sensors, quench heaters.
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3.4 Coils ends
• Coil ends with end-spacers: different layouts for
cosθ coils and block coils.
• In either design this is a weak area causing
premature quenching/training.
• Axial wedges “connected” to end spacers,
mechanically not uniform, different materials,
gaps by fabrication and winding tolerances.
• A continuous and ”natural” support has to be
aimed at, still lot of discussion on best solutions.
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3.5 Collars, Yoke and Cylinder
A laminated collar pack surrounding the coils
• Provides structural precision and is spacer to yoke,
transfers the Lorentz forces to yoke and cylinder.
• Plates of stamped SS or Al alloy.
• Two halves, key locked or single ring shrink-fitted.
• Also bladder techniques are uses to lock & pre-stress.
• Collar plates are spaced, so open for sf Helium.
A laminated iron yoke in between collars and
support cylinder
• Iron yoke plates, field enhancement & spacer
to support cylinder.
Support cylinder and helium can with ends
• Reacts the Lorentz forces (100-200t/m) by elastic
tensile stress.
• Shapes the helium bath.
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3.6 Cold mass in Cryostat
• A cryostat with vacuum vessel, thermal shield, cold mass supports, bus
bars, helium lines (for 1.9 or 4K cooling) and instrumentation ports.
• Normal steel vacuum vessel.
• Aluminum alloy thermal shield with cooling tubes and layers of MLI.
• Cold mass supports to take the weight, 1 fixed, others sliding.
• Instrumentation and cables.
• Bus lines and at either side, interconnections to neighboring coils.
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4.1 Field and Field Quality
Magnetic field of a “dipole” magnet, the mainly determined by:
• Current density and coil width, B ≈ 0.5 µo Jo w
• Iron contribution by the yoke, some +5-10%, which
can analytically be estimated using the mirror method.
• Full precision with the FEA codes, ANSYS, TOSCA, Roxie & more.
Magnetic Field quality across the beam is affected by:
• Shape of coils with optimized but not perfect layout of cables & wedges.
• Deflection of the coils under Lorentz force, try to limit to ~50µm.
• Presence and local shape of the iron facing the coil.
• Boundary induced coupling currents in the cables.
• Shape and thickness of the filaments in the wire.
All this is well understood and implemented in dedicated design codes.
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4.2 Efficient design: current density
Efficient = cost effective design as well!
B ≈ 0.5 µo Jo w
----> Need maximum Jeng and minimum w
J: for a target field B we need coils to operate at
Jeng: ~400 A/mm2 at this B (8, 16, 24…T) !
Given the required margin (~40-50% of Ic, 70-80% on load line) we need
Jeng_c of ~800-1000 A/mm2 in the conductor!
• tough already with NbTi and Nb3Sn at 1.9K
• not yet achieved in B2212 (~550 A/mm2@25T)
nor in and Re123 (~350 A/mm2@25T)
Today a factor 2-3 missing and no real long production lengths available yet.
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Efficient design: current density
Jeng_c > 800A/mm2 not yet achieved in Re123 and Bi-2212 at 20-30T!
Need a factor 2-3 more, at least !
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4.3 Efficient design: grading
Width of windings / number of layers in coil:
• Number of layers is cost driver as the coil winding/impregnation&stacking
process is labor intensive: minimize number of layers.
• Achievable is 2 for 8T(NbTi) -13T(Nb3Sn) level, 3-4 for 13-16T (Nb3Sn).
Grading:
• Same cable width in all layers and same Jc is not cost optimal, apply
grading to have roughly the same margin in all layers at Bpeak/layer.
Example: MSUT(1995): 1m Nb3Sn dipole, 50mm bore LHC type, [email protected]
at first cool down and first quench, showed no training.
Inner layer:
33 Nb3Sn PIT strands dia 1.26mm
cable size 1.98/2.47 x 21.8 mm2
Outer layer:
33 Nb3Sn PIT strands dia 1.00mm
cable size 1.98/2.47 x 17.4 mm2
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4.4 Safe: High Current Cables
Magnetic field and stored energy
Magnetic field B  N.I
Stored energy E  B2.Volume
Inductance
L  N2
We need safe survival from a quench.
Energy dump within short time
before conductor burns out.
Also magnet ramps up and down with acceptable low voltage.
 Thus low N, high current I
Also Isafe  J E / Vd ,
requiring <kV-range for Vd,
with usual current densities, this leads to 10-100 kA.
 Given strand currents of typically 100 to 500A, we need for large scale
magnets multi strand cables of 20-1000 strands,
No escape!
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Scaling Isafe  J x B2 x Volume
0.0001 m3 200 A
50m3
12 kA
2 m3
200-800 A
400 m3
20 kA
25 m3
8 kA
1000 m3 I 40-70 kA
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Quest for high current conductors
200 A HTS tape
Not useful when
not cabled
65000 A@5T Al-NbTi/Cu
This works!
ATLAS Barrel Toroid @ CERN
 One cannot build large scale magnets from single NbTi, Nb3Sn, Bi-2212,
or Re123 wires or tapes.
 We need superconductors that can be cabled and survive a quench!
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4.5 Example of a 20T design
A flavor of what would be possible in principle,
a hybrid design with NbTi, Nb3Sn and Bi-2212 !
• Based on ~380 A/mm2 (~200A/mm2 in low-q Nb3Sn)
one can make 20T in 90mm width,
in a twin dipole of ~800mm diameter.
• Coil stress <200MPa, looks makeable.
• But need much better Bi-2212 at lower cost.
 Actually a new R&D effort just started at CERN to
invent novel, innovative and practical solutions……
Nb3Sn
low j
60
400
200
40
HTS
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Nb3Sn
HTS
Nb3Sn
low j
Nb3Sn
high j
Nb3Sn
high j
Nb3Sn
high j
Nb-Ti
0
5
10
15
20
Field (T)
Courtesy of E.Tedesco
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20.5 T
14.5 T
8.0 T
13.0 T
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Nb3Sn Nb3Sn
low j high j
19.9 T 14.7 T 12.6 T
Nb-Ti
8.0 T
0
0
0
40
11.3 T
12.8 T
14.9 T
60
y (mm)
Bi-2212
600
Nb-Ti
Nb3Sn
HTS
Total
%
27%
57%
16%
80
80
Nb-Ti
y (mm)
Eng. current density (A/mm2)
800
N. turns
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85
24
150
0
20
40
60
x (mm)
80
100
120
0
20
40
60
x (mm)
80
100
120
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4.6 Training in Nb3Sn magnets
• Nb3Sn dipole/quadrupole coils tend to train heavily, in far too many
steps and with relative low value of first quench, why, how to solve?
• For a real accelerator magnet, a long training, de- and re-training are
unacceptable, nominal current, say 80% on load line, should be
reached in < 3 steps.
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17.0
Average Short-Sample
1.00
16.5
0.97
0.95
16.0
0.92
15.5
12
0.90
0.87
15.0
HD1
HD1b
Iss.avg
4.4 K
14.5
1.8 K
11
0.85
0.82
0.80
14.0
0.77
D20 LBNL
13.5
0.75
HD1 LBNL
10
0.72
13.0
0
10
20
30
40
I/Iss
B(T)
central field (T)
13
50
0
5
10
15
Training Ramp #
20
25
30
HD1 Training Comparison
quench number
Examples of long training in Nb3Sn magnets
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4.6 Training and Cable Stability
A possible explanation:
NbTi cables are transparent for sf He at
1.9K, helps to remove heat and increase
enthalpy. This causes collective strands
stability to be effective at I/Ic < 0.7-0.8
Nb3Sn windings are impregnated, to
suppress wire motion and create
“hydrostatic” pressure on the brittle
strands to avoid Jc degradation. Thus a
Nb3Sn cable is less stable at 1.9K and
behaves as single strand down to I/Ic=0.4.
Meaning: we need more superconductor
in Nb3Sn cables to have the same
minimum quench energy MQE, 80% on
the load line is too much !
Point-like MQE measured on
comparable Nb3Sn and NbTi cables.
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5. Conclusion
 Design principles are well known and understood, but success often
depends on the details, in particular in the coil windings.
 For NbTi based magnets the challenge is to operate stable and
predictable at high I/Ic, scaling up to wider bores and developing special
versions like for high-ramp rate magnets.
 For Nb3Sn the primary and urgent goal is to understand and control
training in order to break through the practical limit of today of ~11T and
demonstrate full length series production in the 11-16T range.
 R&D towards designs for 20-25T using HTS are in progress, thinkable
solutions in terms of coil layouts (Hybrids) are present and look
makeable, but yet to be demonstrated.
 Investments are needed in raising the engineering current density in Bi2212 and Re123 (factors 2-3), delivery of uniform km long production
units, a proven cable technology, and a fast and elegant quench
protection, and reducing cost, the kA/m price at 20T/4K, of coarse….
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