High Temperature Superconductors for Future Fusion Magnet Systems

IAEA Conference Chengdu 20. October 2006 G. Janeschitz et.al. slide # 1

Forschungszentrum Karlsruhein der Helmholtz-Gemeinschaft

FZK - EURATOM ASSOCIATION

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




Use of High - Tc Superconductors (HTS) allows higher operating temperatures of 20 K to 77 K save investment

higher efficiencymuch lower effort for thermal shielding save investmentHigher thermal stability more reliable operation

ITER Demo / Proto CommercialFusion Power Plant

≈ 2016 ≈ 2035 ≈ 2050

Long Term Fusion Magnet R&D

Need for Efficiency




Efficiency optimization

For commercial power plants it is essential to reduce power consumption

ITP - refrigerator: 2 kW@ 4.4 K = 0.7 MW electric power

ITER: 64 kW@ 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




Critical Temperature of Superconductors

1900 1920 1940 1960 1980 20000

30

60

90

120

150

Nb3Ge

Nb3GaNb

3Sn

V3Si

NbNNbCNb

Hg Pb

Tem

pera

ture

[ °C

]

Tem

pera

ture

[K]

Year

-270

-240

-210

-180

-150

-120

BiSCCOYBCO

HgBaCaCuOTlBaCaCuO

BiSrCaCuO

YBaCuO

LaSrCuOLaBaCuO

LN2




Magnets for a Fusion-reactor

HTS materials BSCCO and YBCO are promising

Boundaries indicate jc = 0 => ultimate boundary




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




Problems of High - Tc Materials

For example: 90 K Superconductor YBa2Cu3O7

CuO2 - Layers (s.c.)Spacing LayerCuO2 - Layers (s.c.)

Charge Reservoir / Doping

•Layered structures •Correct orientation necessary!•S.C. properties depend on doping•Grain boundaries are detrimental•Brittle materials (ceramics)

Long time R&D was necessary on the road to High - Tc cables

Perfect crystal structure

Oxygen doped

Oxygen binds electrons => holes are forming Cooper pairs




BSCCO: c - axis orientation is necessary - > rolled tapes

c - axis c - axis c - axis

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.

This material is industrially available in long lengths.

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




YBCO offers higher B(T) but 3D orientation necessary

c - axisc - axisc

- axisc - axisc - axis

On top a protection layer is placed

An oriented buffer layer avoids chemical YBCO / tape reaction

The YBCO layer adopts the orientation of the buffer layer

A substrate tape is used for deposition

Composition of YBCO Coated Conductor (CC)

The protection layer serves also as a normal conducting shuntwhen YBCO looses superconductivity (quenches).

Deviation by > 6 degree would already reduce jc significantly

YBCO layer thickness ~ 1




However, progress has been achieved by industry:

up to 300 m high current coated conductor is available263 A for a 12 mm wide tape @ 77 K, self field (SuperPower)

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




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

The challenges are discussed below.




Structural Reinforcement

However,

• 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.

HTS materials are brittle materials and therefore strain sensitive.Therefore a structural reinforcement is necessary.

As a consequence the necessary reinforcement reduces the engineering current density.




High Conductor (Cable) Current

Starting with the parameter of ITER TF coils (N*I = 9.1 MA, L = 0.349 H)the discharge voltage and time constant were calculated for different conductor currents

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 Ic=50 A at 12 T & 50 K) would need 750 tapes!

Which minimum conductor current is reasonable?

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




Hotspot Temperature and Quench

Cu

YBCO

Substrate

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.

• For a 4 mm wide YBCO-CC with a critical current density of 2000 A/mm2, and a thickness of the copper stabilizer of 50 m, the hot spot temperature during a discharge ( = 21 s) will be about 120 K.

• For a critical current density of 10000 A/mm2, a copper thickness of 300 m is required to limit the hot spot temperature to 130 K.

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. => This limits jc,eng!

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.=> Depends also on discharge time !!




Bundling & cabling development to limit AC losses

AC - loss optimization is one of the most crucial points!

AC-loss optimized TFMC conductor:

Multi stage twisted cable-in-conduit with central cooling channel,

Rated current: 68 kA @ 11.8 T and 4.6 K

Nb3Sn strand (EM-LMI)

Cable-in-conduit conductor Roebel bar conductor conceptsalready used in NbTi-LCT and NET subsize

Applied in BSCCO-cable (SIEMENS)




• Mechanical precision punching• Tool optimized for material and

thickness• Sequential assembling to RACC

structure

Roebel-Assembled-Coated-Conductor (RACC)

Design and concept for low AC losses and high transport currents

Result of transport current measurements:

Ic = 1020 A @ 77 K, self field (1 μV/cm)

Result agrees well with expectations for a cable with 16 tapes.

12 mm




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 react&wind 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.




Outlook

HTS R&D has to be adopted to fusion needs.

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 Model–Solenoid

•

•Design, manufacturing and test of a TF HTS Demonstration Coil in collaboration with industry

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.

High Temperature Superconductors for Future Fusion Magnet Systems

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