The LN2 plunge-tests of the conventional stepper motors was useful,
but not sufficient for certifying that the motors would work in
Mimir. Based on knowledge of failure modes for other cryogenic motors
under vacuum conditions, we wanted to test the motors at low temperature
and under vacuum conditions. We built a test chamber, again with
parts from our local "Cryogenics-R-Us", which contained
our CTI 1050 helium refrigerator cold head and several cold shields.
This permitted bringing the motors to under 30K and with good vacuums.
The picture shows two test units. At left, two stepper motors are
mounted on a common stand. The aluminum disks contain embedded magnets
that trigger the magnetic Reed switches mounted in the stand (see
the twin sets of wires) once every revolution. So, from outside
the test chamber, we could verify that the motors rotated by watching
the Reed switch closures.
We also tested various surface treatments, in an attempt to maximize
radiative heat transfer between components within Mimir. The stand
at right has been bead blasted and hard anodized and is about to
be tested for radiative coupling to the inside cold surface of the