30. HIGH TEMPERATURE SUPERCONDUCTIVITY

Substantial worldwide advances have been achieved in recent years with respect to the development and processing of second generation, high temperature superconducting (HTS) coated conductors (also known as “2G wires”). Highly successful prototype superconducting equipment and devices are being demonstrated. Compared to first generation wires, 2G coated conductors have the potential of providing lower cost and higher performance. These wires also provide the possibility of operation at moderate magnetic fields in liquid nitrogen as well as high fields at lower temperatures. For short laboratory-scale samples, very high current-carrying capacities (over 1,000 A/cm at 77K) have been reported. In addition, pre-commercial coated conductors as long as 600 meters, with current carrying capacity over 170 A/cm, have been demonstrated. Nonetheless, further innovation and development will be needed to achieve the DOE vision for commercial availability of 2G wires that have a cost/performance ratio as low as $10-30/kA-m (dollars per kiloampere-meter) and can be fabricated in practical forms. In addition to wires, further improvements in the efficiency, reliability, and cost reduction of the enabling cryogenic system are needed.

Grant applications are sought only in the following subtopics.

a. AC Loss Reduction in Coated Conductors (ref. 1-4) —Although a number of strategies (filamentization, multi-wire stranding, twisting and transposition) have been shown to reduce AC losses in superconducting wires and cables, all of them have limitations for 2G wires. For example, because 2G wire consists of a continuous layer of HTS, filamentization would require the additional step of physically removing part of the HTS layer. Filamentization difficulties are compounded by the presence of relatively large textured grains, which are contained within certain 2G templates and can range from 30 to 60 microns in size. The presence of these grains may place a lower limit on the width of the HTS filaments; yet, narrow filaments are desirable because they are more effective in reducting AC losses. The twisting and transposition strategy also has drawbacks – the flat tape geometry of present 2G wire does not lend naturally to twisting and transposition, due to mechanical concerns.

Grant applications are sought to develop innovative and cost effective approaches to reduce the AC losses of coated conductors. Approaches of interest include filamentization and substrate modification, including single crystalline round or low-aspect-ratio textured templates.

• With respect to approaches that address the continuous filamentization of the HTS and/or stabilizer, grant applications should develop a potentially cost effective filamentation technique, and determine the influences of such critical parameters as HTS filament shape, arrangement, and geometry on loss characteristics under various operating conditions. Loss characteristics may include properties such as hysteretic, coupling, eddy current, and transport losses, projected over broad ranges of frequencies and field-sweep amplitudes.
  
• With respect to approaches that address textured substrate fabrication, the substrates either must be single crystalline or must contain well-oriented grains less than five microns in size, with grain misorientation angles less than eight degrees full-width-half-maximum. The substrates also must be suitable for continuous fabrication and be able to sustain a critical current density of more than one MA/cm² at 77K self-field. Also, textured templates that have either round or low aspect ratio cross sections are preferred.

Questions – contact Debbie Haught (debbie.haught@hq.doe.gov)

b. High Performance and Reliable 2G Wire Joints—Although superconducting equipment requires long lengths of 2G wires, practical HTS wires are of finite lengths. Consequently, wire joining is required during device manufacturing, installation, and field repair. To ensure safe and reliable operation, high performance joints must have good mechanical and electrical integrity. Due to the different architectures and manufacturing processes of various commercial 2G wires, characteristics of the joints are expected to vary with joint fabrication conditions as well as with the type of 2G wire being used. Also, the asymmetric nature of present 2G wires likely will influence the joint characteristics depending on the joining arrangement (i.e., whether the joints are fabricated in face-to-face, back-to-back, or face-to-back configurations). At present, only limited information on 2G wire joints is available; further information must be developed in order to ensure reliable 2G wire joints for HTS applications.

Grant applications are sought to develop innovative and cost-effective ways to prepare high quality, reliable, joints for superconducting 2G wires, either to one-another or to non-superconducting wires. Approaches of interest must include a detailed determination of the effects of the joining method on the properties of 2G wires –including joint resistance, ac losses, and stability strength – and on the interdependence between these properties prepared under various conditions. The determination of these effects must account for the fact that the wires may have been prepared under various conditions and may have different wire architectures.

Questions – contact Debbie Haught (debbie.haught@hq.doe.gov)

c. Cryogenic Technology for Superconductors—In order to realize the benefits of superconducting equipment, HTS wires must be maintained at temperatures well below ambient. Potential applications for these superconductors will most likely be realized if the operating temperature can be maintained economically in the range 63-83K.  To economically achieve and maintain these temperatures, further development in thermal insulation systems (cryostats) and refrigerators (cryo-coolers) is needed.

Grant applications are sought to develop flexible cryostats (ref. 5-6) that are suitable for HTS electrical cable that might be placed underground or underwater. These cryostats should offer superior performance and lower price compared to today's commercially available products.  For comparison, current flexible cryostats are manufactured in 100 m lengths, have a price of approximately $500/m, admit heat at the rate of 1-3 W/m, and suffer increased heat loads at bends in the cable.  Also the getters used in the vacuum region of these cryostats have lifetimes significantly shorter than the 20-30 year HTS cable lifetime the utilities expect and improved getters are needed as a getter reconditioning or replacement for a long, underground HTS cable is difficult. Cryostats for future HTS cables must be much longer (kilometers), have a reduced price ($200/m), a reduced rate of heat invasion (e.g., less than 1 W/m), minimize performance degradation at bends and have improved getter lifetimes. Proposed cryostats must have the potential to satisfy most or all of these requirements.

Grant applications also are sought to develop efficient, reliable and cryo-coolers (ref. 7). These cryo-coolers should have the potential for unattended, maintenance-free operation for at least 10 years, and be able to function in an underground and/or underwater environment.  Proposed approaches must offer the prospect of future price reductions to less than $50/watt at 65K.

Questions – contact Debbie Haught (debbie.haught@hq.doe.gov)

References:

1 S.P. Ashworth, F. Grilli, “A strategy for the reduction of ac losses in YBCO coated conductors,” Superconductor Science and Technology 19 (1006) 227-232. (Abstract available at: http://www.iop.org/EJ/abstract/0953-2048/19/2/013 ).

2 M.D. Sumption, E.W. Collings, and P.N. Barnes, “AC Loss in Striped (Ffilamentary) YBCO Coated Conductors Leading to Designs for High Frequencies and Field-Sweep Amplitudes,” Superconductor Science and Technology 18 (2005) 122-134. (Abstract available at: http://www.iop.org/EJ/abstract/0953-2048/18/1/020 )

3 T. Nishioka, N. Amemiya, N. Enomoto, Z.A. Jiang, Y. Yamada, T. Izumi, Y. Shiohara, T. Saitoh, Y. Iijima, and K. Kakimoto, “AC loss of YBCO Coated Conductors Fabricated by IBAD/PLD Method,” IEEE Trans. On Appl. Supercond. 15 (2005) 2843-2846. (Abstract available at : http://ieeexplore.ieee.org/xpl/freeabs_all.jsp?tp=&arnumber=1440260&isnumber=31008 )

4 R.C. Duckworth, M.J. Gouge, J.W. Lue, C.L.H. Thieme, D.T. Verebelyi, “Substrate and Stabilization Effects on the Transport AC Losses in YBCO Coated Conductors,” IEEE Trans. On Appl. Supercond. 15 (2005) 1583-1586. (Abstract available at : http://ieeexplore.ieee.org/xpl/freeabs_all.jsp?tp=&arnumber=1439949&isnumber=31007 )

5 M. J. Gouge, “Flexible Cryostats for Superconducting Cables: Reliability and Lifetime Issues,” DOE 2006 Wire Development Workshop: http://www.energetics.com/meetings/wire06/pdfs/session7/gouge.pdf

6 K. Schippl, “Very Low Loss Cryogenic Envelope for long HTS Cables,”

http://www.cryo-schippl.de/EUCAS%202003%20-%20Paper.pdf

7 M. J. Gouge, J. A. Demko and B. W. McConnell, ORNL, J. M. Pfotenhauer, University of Wisconsin, “Cryogenic Assessment Report,” http://www.ornl.gov/sci/htsc/documents/pdf/CryoAssessRpt.pdf