Cryogenic Device Test Facility
| Rapid feedback from cryogenic test facilities
is another essential feature of a successful superconducting device development
program. Initial screening of our devices is done in a self-contained
cryopumped single-shot 3He refrigerator which fits into a two-inch diameter
glass dewar set with an integral mu-metal shield. This system requires
only about six liters of liquid helium per transfer. The base temperature
is 270 mK. Instrumentation includes a potentiometric conductance
bridge for reading out the germanium resistance thermometer and an analog
temperature controller. Tunnel junction characteristics are measured
with a battery-powered IV curve tracer. Data acquisition instrumentation
consists of a TEK 2440 500 MHz digital scope which is IEEE interfaced to
a Centris 650 and driven by LabVIEW software. An Oxford MCA is daisy-chained
onto the SCSI port of the computer. Ultralow temperature testing of our devices is done in our custom Oxford Instruments dilution refrigerator which was paid for by NSF / ARI grant # PHY-9512371 supplemented by SFSU matching funds. This versatile cryostat has two distinct modes of operation. In the standard configuration the refrigerator is equipped with a small sample stage and vacuum can which fits into a small liquid-nitrogen-jacketed dewar. This method of operation provides an economical means of acquiring rapid feedback regarding the effect of process variations on the performance of the devices which we make in the Thin Film Laboratory. The base temperature is 15 mK. In the "tower-compatible" configuration the refrigerator is outfitted with an adapter which supports a large vacuum can inside of a vapor-shielded eight-inch dewar. This arrangement enables the refrigerator to cool a "tower" assembly holding large detector modules. Although the refrigerator had serious design problems which required a year and a half of intense effort to identify and correct, the system is operational. The refrigerator is mounted on a bridge having sand-loaded instrument racks as support columns. Additional vibration isolation is achieved by use of pneumatic vibration control mounts between the bridge and the columns and Isomode absorber pads between the refrigerator support plate and the bridge. A winch is used to raise and lower the dewars, which are stored in a sealed room in the basement beneath the laboratory. When not in use the dewars rest on trolleys that can be moved on rails in the storage room. The cryostat is enclosed by an rf-shielded structure ("screen room") funded by SFSU and built by Lindgren, Incorporated. The double electrically isolated 3-ounce copper enclosure provides 120 dB attenuation with a high-frequency roll-off greater than 10 GHz. Battery-powered NIM bins have been set up for front-end electronics, and highly filtered AC power also is available. The automated gas handling system and the computers which interface with the refrigerator are, of course, located outside of the screen room. At this time a PowerMAC 7100 monitors the refrigerator, and a MAC G3 and a PC are available for pulse data acquisition. We have prepared a glove box for storage of our devices in a controlled atmosphere, and we also have set up a clean bench as a work station for installing detector modules in towers. A HEPA-filtered tent shrouds the lower part of the refrigerator so that a tower can be installed while maintaining a clean room environment. |
Updated 15 Sept 2003