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Cryogenics and Fluids Branch
 

High Temperature Superconducting Leads

We also have a non-technical discussion of the high Tc leads in our Introduction to Cryogenics section.

High Tc Leads: Technical Summary

Part 1: The Need for High Temperature Superconductors

The high current leads carry power to a superconducting magnet (part of the Adiabatic Demagnetization Refrigerator (ADR)) and to two valves. Maximum current to the magnet is 2 amps. Maximum current to each valve is 1 amp. In the XRS flight dewar, high temperature superconductors form the part of the high current leads between the superfluid helium tank (at 1.3 Kelvin) and the solid neon tank (at 17 Kelvin.)

Because the XRS dewar has such a small volume of liquid helium (20 liters) to meet a long lifetime requirement (2 years), heat loads to the helium tank must be held low: below 800 microwatts. The need for low heat load rules out the possibility of using copper wires for the high current leads. The heat load from copper leads was 2.7 milliwatts, when the leads were optimized for the best tradeoff between Joule heating and heat conduction.

Low temperature superconductors, such as niobium titanium (NbTi) cannot be used because the "warm" end temperature of 17 Kelvin is too high. A hybrid lead, with the "cold" part of NbTi and the "warm" part of copper is likewise unworkable. Joule heating of the copper would warm the NbTi above its transition temperature, even if the lead were cooled through a heat sink to the helium vapor vent line. Nb3Sn has a transition temperature of 18.5 K (when chemically pure and strain free), and thus would seem a logical choice. However, it is not normally available in small enough diameters to meet our low heat load requirements.

Therefore, the cryostat design team chose to use high temperature superconductors for the "warm" section of the high current leads. The combined high electrical conductivity and low thermal conductivity will hold down both Joule heating and thermal conduction. At the time of the first build of XRS, the team chose Ytterbium-Barium-Copper-Oxide, Y1Ba2Cu3O7 (YBCO). For the "cold" section of the leads (below 4 kelvin) the team chose copper-clad niobium-titanium.

For the rebuilding of XRS, the team chose also to use Magnesium Diboride, MgB2, leads. The magnesium diboride leads have the advantage that they are mechanically stronger. That allows the design team more flexibility in how they package the leads. As we'll see below, the team used this greater flexibility to rearrange the way that the leads are heat sunk (that is, connected to sources of cooling.)

The change is in response to problems discovered during ground operations of the XRS-1 instrument before the unsuccessful launch of the first ASTRO-E spacecraft. When the ground crew performed certain operations, the temperature of the leads was higher than predicted. If this problem recurs in XRS-2, the low temperature superconducting leads (connected to the low temperature end of the high temperature superconducting leads) might overheat. Overheating could cause them to be damaged or destroyed.

Given the possible problem with the old design, it might seem that the design team would have eliminated the YBCO leads altogether. However, the YBCO leads have the advantage that they have been in use longer, and that we thus have more confidence in our ability to predict their ability to survive the stress of launch. Also, the heating problem appears to be manageable. In XRS-1, no actual damage was done, even though the operations team was not expecting the heating. In XRS-2, there's an even better chance they'll be able to prevent any damage.

Therefore, the team chose to use two sets of leads high temperature superconductor leads: one of YBCO and one of magnesium diboride. Each set is connected to its own set of low temperature superconductor leads. Either set of leads, by itself, could carry the full current.

Next: Part 2: The YBCO Leads.


Publication Reference and Other Links

"A High Tc Superconducting Current Lead Assembly for the the XDS Helium Cryostat", J.G. Tuttle, T.P. Hait, R.F. Boyle, H.J. Muller, J.D. Hodge, and S.R. Breon, Advances in Cryogenic Engineering 43, ed. P. Kittel, p. 965-972 (1998). Abstract

"Astro-E2 Magnesium Diboride High Current Leads", J.S. Panek, J.G. Tuttle, V. Marrero, S. Mustafi, R. Edmonds, A. Gray, S. Riall, presented at the 2003 Cryogenic Engineering Conference, to appear in a forthcoming volume of "Advances in Cryogenic Engineering."


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Curator: Mark O. Kimball
NASA Official: Eric A. Silk
Last Updated: 09/11/2014