We also have a non-technical discussion of the high Tc leads in our Introduction to Cryogenics section.
In the original XRS, that is, XRS-1, all of the high temperature superconducting leads were made of Ytterbium-Barium-Copper-Oxide (YBCO). The YBCO leads passed all their tests, and performed well every time they were used in the XRS instrument. All the evidence suggests that they would have performed well on orbit, if the satellite had not been lost during a launch malfunction.
However, operation of the XRS instrument before launch turned up a risk of overheating the copper-clad niobium-titanium (NbTi) superconducting wire that connected to the cold end of the YBCO wires. The cold end of the YBCO wires is held at 4 Kelvin by being connected to a heat sink on the helium vent line. The helium vent line is the tubing that carries the helium gas out of the spacecraft. It is cooled by the constant flow of cold helium gas that boils off from the liquid helium cooling bath.
Although the heat sink usually runs at 4 Kelvin, the temperature sometimes rises, because of thermoacoustic oscillations in the helium gas in the vent line. If the temperature rises much above 8 kelvin, the niobium-titanium (NbTi) leads could be damaged. At that temperature, they would transition from superconducting to normal. That is, they would no longer have the zero-resistance property of superconductors. The NbTi wires would then overheat and, probably, burn out. When designing the rebuild of XRS, the team needed a way to ensure that the "hot" end of the NbTi remained below 8 kelvin.
One possible way would have been to connect the vent tube heat sink to the middle of the YBCO lead assemblies, instead of to the cold end. Then, if the heat sink temperature rose above 8 kelvin, the NbTi and the cold end of the YBCO would probably remain cold enough that there would be no danger of burning out the NbTi. Unfortunately, the team could find no simple way of redesigning the YBCO lead assembly that would have good enough thermal contact to the middle of the leads, while also ensuring that the fragile leads would have enough mechanical protection that they would survive the strains of liftoff.
The answer that the team arrived at was to use the Magnesium Diboride (MgB2) leads. The MgB2 leads are physically much stronger than the YBCO leads, and thus gave the design team a wider range of design choices. The reason for the greater strength of the MgB2 leads is that they are sheathed in stainless steel. (These steel-sheathed wires were developed by W. Goldacker and colleagues.)
As with the YBCO leads, the MgB2 are packaged in a bundle of 12 wires. Unlike the YBCO filaments, the MgB2 wires are strong enough that they do not need to be supported over their entire length by a fiberglass tube. Instead, they are epoxied to two short fiberglass cylinders. The fiberglass cylinders serve two purposes: they are heat sinks, to maintain the temperature of the wires, and they help arrange the wires into a roughly cylindrical bundle. Another fiberglass piece, a thin disk, rather than a cylinder, also helps maintain the physical arrangement of the bundle. The three fiberglass pieces, in turn, are held in place by a length of Kevlar yarn that is threaded through holes that run through the center of each piece. (Kevlar is a DuPont trademark.)
D
The cylindrical fiberglass heat sink near the "warm" end of the magnesium diboride leads is thermally connected to the 11 kelvin heat station on the helium vent line. (The helium vent line is continuously cooled by the cold helium gas that boils off from the liquid helium cooling bath.) The other fiberglass heat sink is thermally connected to the 4 kelvin heat station on the helium vent line. This 4 kelvin fiberglass heat sink is at the midpoint of the magnesium diboride leads, not at the cold end, which is the location of the 4 kelvin heat sink on the YBCO leads.
By putting the 4 kelvin heat station in the middle of the magnesium diboride wires, instead of at the cold end, the design team hopes to allow the cold end to remain really cold (that is, safely below 8 kelvin) even if the helium vent line should heat up (as happened on a few occasions on XRS-1). To further encourage the cold end of the leads to remain below 8 kelvin, the team decided to install copper wires from the MgB2 cold end to the liquid helium cooling bath, at a temperature of 1.3 kelvin. These copper wires will help dump any excess heat to the liquid helium bath.
Thus, by including the MgB2 leads, the design team guards against a potential heating problem that the operations team discoverd in XRS-1. However, since magnesium diboride is a newer material, the design team wanted to guard against unforseen problems. d So they decided to also include the YBCO lead assembly that the XRS-1 launch team had designed and tested. As long as at least one of the two materials operates properly in orbit, the XRS-2 instrument will be able to operate.
There is a slight disadvantage to having both sets of leads installed. Heat flows down each lead and into the liquid helium cooling bath. The design team could have reduced the amount of heat flowing into the liquid helium by using only one of the two lead assemblies. By reducing the heat flow into the helium, they would have increased the lifetime of the bath. However, they decided that the increased reliability of having two separate sets of wiring was important enough that they accepted the slight decrease in the lifetime of the helium coolant supply.
Revised October 28, 2004.
"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, Advances in Cryogenic Engineering 49, ed. J. Waynert et al., p. 952-957.
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