X-Ray Calibration Facility (XRCF)

Cryogenic Refrigerator and Cryogenic Shroud

To provide a cryogenic optical test capability, a helium cooled thermal enclosure capable of less than 20 degrees Kelvin was installed in the optical test chamber. The enclosure is a rectangular box 3.3 m (10.8 ft) wide, 2.7 m (8.9 ft) tall, and 10.0 m (32.8 ft) long.

The enclosure is made in three sections to facilitate handling and installation, and to provide for flexibility in configuring tests. The cryogenic shroud is thermally isolated from its mechanical mounts, and insulated on its outer surface with multi-layer reflective insulation.

The inside of the shroud is coated with high emissivity (> 0.9) paint for heat transfer with the test optic. To additionally increase heat transfer between the cryogenic shroud and the optical test hardware, free molecular conductivity methods are employed using helium at vacuum pressures.

While the chamber can be taken to 30 K in 2-3 days, it is typically accomplished in 4-5 days to minimize and control thermal gradients in the mirror system under test. Mirrors up to 2 meters in diameter can be tested from ambient to temperatures below 30 K. The refrigerator helium loop/shroud temperature is selectable in increments of 0.1 K with a control band of +/- 0.2 K.

The use of this closed-loop helium refrigeration system, insulated cryogenic shroud, and short cooling time minimize the expenditure of liquid nitrogen and test time, and provide for very economical operation.

The existing facility can be readily modified to accommodate larger flight mirror segments. For example, the existing chamber diameter can easily accommodate a new cryogenic shroud capable of testing 3-meter diameter mirrors for similar optical testing at cryogenic temperatures. Required modifications include the design and construction of a new helium shroud segment, which will interface to existing shroud elements and to existing thermal control hardware within the cryogenic test facility.

Modifications to the existing refrigeration system would also have to be considered in order to accommodate the new helium shroud segment and any increased thermal loads.