Superconducting Technology Assessment - nitrd

In selecting the cooling approach for systems, the heat load at the lowest temperatures is a critical factor. A large-scale,
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 were extracted via LHe immersion, a heat load of 1 kW would require a 1400 liter/hour gas throughput rate.
It is possible to partition a large cryogenic system into many modules, each separately cooled. Approaches with a
small number of large modules or approaches with many, smaller modules are feasible. Coolers for large-scale or smallscale
modules are reasonable, with the larger ones being more efficient and actually more technologically mature
than those of intermediate capacities. However, multiple smaller modules allow continued processing while some
modules are taken off-line for repair and maintenance.
Since there are availability differences between large-scale and small-scale cryo-coolers, the status for each type is
presented separately 6 .
6.4.1 COOLING – STATUS
Small Cryocoolers
Among commercial small cryocoolers, the GM coolers have been around the longest; tens of thousands of them
have been sold. With the use of newer regenerator materials, two-stage GM coolers now can go down to temperatures
as low as 2.5 K. GM coolers have a moving piston in the cold head and, as a result, produce vibration accelerations
in 0.1 g range. They are quite reliable if scheduled maintenance of both the compressor units and the cold heads
is performed regularly. A typical maintenance interval is about 5,000 hours for the compressor and about 10,000
hours for the cold head.
In the hybrid cooler and 77 K communities, there are a number of companies selling Stirling cycle coolers (e.g.,
Sunpower and Thales), but there are no commercial 4 K purely Stirling coolers on the market. These units tend to
be physically smaller than GM machines of similar lift capacity due to the absence of an oil separation system. The
vibration characteristics of Stirling machines are about the same as for GM machines. At this time—cand under conditions
where maintenance is feasible—commercial Stirling machines do not appear to be more reliable than GM machines.
Pulse tube coolers are commercially available from a number of vendors including Cryomech, Sumitomo, and
Thales. Two-stage pulse tube coolers can produce over 1 W of cooling at 4.2 K on the second stage of a two-stage
cooler and simultaneously produce about 50-80 W at 50 K. A pulse tube machine will inherently produce lower
vibration accelerations (