Kein Folientitel

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High Temperature Superconductors for Future
Fusion Magnet Systems Status, Prospects and
Challenges
G. Janeschitz, R. Heller, W.H. Fietz, W.
Goldacker, G. Kotzyba, R. Lietzow, R. Nast, B.
Obst, S.I. Schlachter, C. Schmidt, K.-P.
Weiss Forschungszentrum Karlsruhe, Karlsruhe,
Germany
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Long Term Fusion Magnet RD
ITER Demo / Proto
Commercial Fusion Power Plant 2016
2035 2050

• Use of High - Tc Superconductors (HTS) allows
• higher operating temperatures of 20 K to 77 K ?
  save investment ? higher efficiency
• much lower effort for thermal shielding ? save
  investment
• Higher thermal stability ? more reliable
  operation

Need for Efficiency and Reliability
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Efficiency optimization
For commercial power plants it is essential to
reduce power consumption
ITP - refrigerator 2 kW_at_ 4.4 K
0.7 MW electric power ITER 64 kW_at_ 4.4 K
22 MW electric power DEMO
??? MW electric power
With a magnet system at 20 K a fusion machine
would be more efficient with respect to electric
power consumption for cryogenics. Great would be
a machine with a superconducting magnet system at
65 K to 77 K! Cooling with liquid nitrogen would
be possible! Above 20 K operation will be more
reliable due to higher enthalpy
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Critical Temperature of Superconductors
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HTS materials BSCCO and YBCO are promising
Boundaries indicate jc 0 gt ultimate boundary
Magnets for a Fusion-reactor
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Possible scenarios
Taking into account the intrinsic properties of
present existing HTS compounds the following
scenarios have been identified for fusion devices
beyond ITER 1) 5 K, 12 15 T Improved Nb3Sn
CICC 2) 5 K, 20 T BSCCO
conductor 3) 20 K, 12 15 T BSCCO
conductor 4) 50 K, 12 15 T YBCO coated
conductor 5) 65 K, 10 12 T YBCO coated
conductor
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Problems of High - Tc Materials

• Layered structures
• Correct orientation necessary!
• S.C. properties depend on doping
• Grain boundaries are detrimental
• Brittle materials (ceramics)
• ? Long time RD was necessary on the road to High
  - Tc cables

Perfect crystal structure
Oxygen doped
For example 90 K Superconductor YBa2Cu3O7
Oxygen binds electrons gt holes are forming
Cooper pairs
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BSCCO c - axis orientation is necessary - gt
rolled tapes
This material is industrially available in long
lengths.

• Application HTS current lead demo for ITER
  (BSCCO) In the frame of the EU Fusion
  Development Program, a 70 kA HTS current lead
  with Bi - 2223/AgAu superconductor was developed
  and tested in FZK.

Current lead consists of three parts Connection
to low Tc S.C. HTS module (Bi - 2223/AgAu)
Copper heat exchanger 4.5 K
4.5 K - 65 K 65 K - 300
K
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YBCO offers higher B(T) but 3D orientation
necessary
Deviation by gt 6 degree would already reduce jc
significantly
YBCO layer thickness 1m
The protection layer serves also as a normal
conducting shuntwhen YBCO looses
superconductivity (quenches).
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Status of the YBCO Coated Conductor

• Basic idea realized in 1996 for short length
  samples
• anyhow major difficulties exist
• Homogeneity of long substrates
• Buffer layer problem (complicated and time
  consuming)
• Slow growth of YBCO film by sputtering or
  evaporation

However, progress has been achieved by
industry up to 300 m high current coated
conductor is available263 A for a 12 mm wide
tape _at_ 77 K, self field (SuperPower)
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Potential of HTS for Fusion and Challenges

• Potential of High Temperature Superconductors
• Much higher superconducting transition
  temperatures up to 105 K
• Very high upper critical fields of the order of
  100 T
• High irreversible (operating) fields at higher
  temperatures
• Excellent critical current densities up to high
  temperatures magnetic fields

• Challenges
• Structural reinforcement is required
• High conductor (cable) current is necessary for
  technical application
• Hot spot temperature and quench are problematic
  (current extraction)
• Bundling cabling development to limit AC losses
  is mandatory

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Structural Reinforcement
HTS materials are brittle materials and thus
strain sensitive.Therefore a structural
reinforcement is necessary. As a consequence the
necessary reinforcement reduces the engineering
current density.

• Conventional reinforcement is not possible,
• HTS conductors need heat treatment at high
  temperature in oxygen atmosphere
• Embedding of conductor in stainless steel is not
  possible before heat treatment
• React-and-wind technology has to be used which
  limits the conductor size and the bending radius
  by the stress-strain behavior
• The necessary reinforcement technology including
  react wind has to be developed.

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High Conductor (Cable) Current
Which minimum conductor current is reasonable?
Starting with the parameter of ITER TF coils (NI
9.1 MA, L 0.349 H)the discharge voltage and
time constant were calculated for different
conductor currents
Conductor current Number of turns Inductance ratio L/LITER Discharge voltage (?D 12 s) Discharge time constant (UD 10 kV)
68 kA 134 1 3.5 kV 4 s
30 kA 304 5 17.5 kV 21 s
10 kA 910 45 158 kV 190 s

• As a compromise to limit both discharge voltage
  and time constant, 30 kA seems to be the
  minimum acceptable conductor current.
• 30 kA made of 40 A tapes (assuming Ic50 A at 12
  T 50 K) would need 750 tapes!

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Hotspot Temperature and Quench
If the YBCO superconductor quenches the current
has to be transferred to the protection layer
which has to takes the current until the coil is
discharged.
The maximum temperature reached during quench is
called hotspot temperature. The thickness of the
Cu layer has to be adopted to limit the maximum
temperature. gt Depends also on discharge time !!
Cu YBCO Substrate
The increase of the critical current density in
the YBCO by a factor of five results in an
increase of the overall engineering current
density of only a factor of two. gt This limits
jc,eng!
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Bundling cabling development to limit AC losses
Cable-in-conduit conductor
Nb3Sn strand (EM-LMI)
AC-loss optimized TFMC conductor Multi stage
twisted cable-in-conduit with central cooling
channel, Rated current 68 kA _at_ 11.8 T and 4.6
K
AC - loss optimization is one of the most crucial
points!
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Roebel-Assembled-Coated-Conductor (RACC)
Design and concept for low AC losses and high
transport currents

• Mechanical precision punching
• Tool optimized for material and thickness
• Sequential assembling to RACC structure

Result of transport current measurements Ic
1020 A _at_ 77 K, self field (1
µV/cm) Result agrees well with expectations for a
cable with 16 tapes.
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Conclusions
HTS allows higher temperatures and fields
compared to classical superconductors However,
today a high current HTS conductor for 30 kA is a
real challenge with following critical items

• High current conductor layout
• Conductor layout has to consider structure
  reinforcement and reactwind technology.
• Main limitation is the large number of tapes
  which have to be used for cabling.
• Hotspot temperature and quench
• The copper stabilizer has to be large to limit
  the hot spot temperature which limits the
  engineering current density.
• Bundling cabling development to limit AC
  losses
• Innovative cabling techniques have to be
  developed to limit AC losses.

• A first 1 kA class Roebel type (RACC) cable was
  successfully fabricated by FZK from commercially
  YBCO coated conductor.
• The achieved critical current agreed well with
  expectations.
• The technique is reliable, suitable for long
  lengths scalable for large currents.

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Outlook

• HTS RD has to be adopted to fusion needs.
• The minimum Goal should be a 30 kA cable at 50
  K / 12 T for future Fusion reactors.Main
  targets are
• Improvement of bundling and cabling techniques
• Fusion conductor development in collaboration
  with industry
• Design, manufacturing and test of a HTS
  ModelSolenoid (around 2012)
• Design, manufacturing and test of a TF HTS
  Demonstration Coil in collaboration with
  industry (around 2015 to 2020)

Acknowledgment This work, partly supported by
the European Communities under the contract of
Association between EURATOM and Forschungszentrum
Karlsruhe, was carried out within the framework
of the European Fusion Development Agreement.
The views and opinions expressed herein do not
necessarily reflect those of the European
Commission.