Superconducting Technology Assessment - nitrd

Several issues need to be resolved for working SCE systems. Modularity and assembly sequence are the most
important issues for larger scale systems. Other major technical issues include the penetration of vacuum walls and
shields with multitude of both low- and high-frequency cable, concurrent presence of high DC current and very
high frequencies (50-100 GHz) digital signals and the thermal gradient from room temperature to 4 K and the
resulting material compatibility requirements. The packaging is also a very strong function of the IO approach used
to bring high data rate signals into and out of the SCE circuits. If optical data transfer is selected, the fibers and
other OEO components represent an additional class of materials that may complicate system integration. Due to
the lack of a previous design at large scale (e.g., a functional teraflop SCE supercomputer), other potential issues
may be hidden and can only be discovered when such an engineering exercise takes place. The past funding
(commercial and government) for the development of such systems was in disconnected increments that prevented
the accumulation of engineering now-how. Therefore, substantial learning and more development is needed to
insure a functional SCE-based supercomputer system.
Cooling
An SCE-circuit-based system needs to operate at temperatures ranging from 4 to 77 K. The cooling is performed by
using cryocoolers or by liquid cooling using refrigerants such as Liquid Helium (LHe) or Liquid Nitrogen (LN 2). Liquid cooling:
■ Often produces a smaller temperature gradient between the active devices and the heat sink than
in vacuum cooling, making it easier to bias all the chips at their optimum design temperature.
■ Also produces a more stable bias temperature due to the heat capacity of the liquid that tends
to damp any temperature swings produced by the cooler, often allowing liquid cooled devices
to perform better than in vacuum mounted parts.
For safety and practical reasons, liquid cooling is limited to 4 K (LHe) and 77 K (LN2) regions. Liquid cooling which
fails to recondense the evolved gas is not suitable for large scale devices, but closed systems with liquifiers are well
established in the particle accelerator community where total cooling loads are large and direct emersion of parts
into boiling cryogen is straightforward. Additionally, the boiled-off working fluid can be used to cool electrical leads
going into the device, although this means they must be routed thru the vacuum vessel with the gas for some
distance which may be unduly constraining. Small closed cycle coolers commonly use helium gas as a working fluid
to cool cold plates to which the circuits are mounted in vacuum. By using mixed gases and other means, the
temperatures of intermediate temperature stages can readily be adjusted. The most common type of small
cryocoolers in the marketplace are the Gifford McMahon (GM) type cryocooler, but pulse tube coolers and Stirling cycle
refrigerators which offer lower vibration and maintenance and better efficiency at a currently higher cost are also used.
In selecting the cooling approach for systems, the heat load at the lowest temperatures is a critical factor. A largescale
SCE-based computer with a single, central processing volume and cylindrically symmetric temperature
gradient was analyzed under the HTMT program 5 . The heat load at 4 K arises from both the SCE circuits and the
cabling to and from room temperature. The cable heat load (estimated to be kWs) was the larger component, due to
the large volume of data flow to the warm memory. Circuit heat load may be as small as several hundred watts. If all
this heat at 4 K extracted via LHe immersion, a heat load of 1 kW would require a 1,400 liter/hour gas throughput rate.
Several implementation approaches that would divide the total system into smaller, separately cooled modules will
be discussed in the issues section. Both an approach with a few (but still large) modules and an approach with
many smaller modules have potential virtues when it comes to the ease of wiring and minimization of cable loads.
However, there are availability differences between large scale and small scale cryo-coolers. Therefore, the status
will be presented for both types separately 6 .
5 HTMT Program Phase IIII Final Report, 2002
6 “Integration of Cryogenic Cooling with Superconducting Computers”, M. Green , Report for NSA panel.
235