Eddy current evaluation of airframes using refrigerated SQUIDs

A superconductive quantum interference device, RF SQUID, fixed to the cold pad of a Heliplex HS-4 refrigerator shows a baseline noise of 1.6/spl times/10/sup -4/ flux quanta in a one hertz bandwidth at frequencies above about 25 Hz. The refrigerator adds noise below 25 Hz. Remote pickup loops couple flux to the SQUID through a cryogenic umbilical, 500 mm long, attached to the cold pad. It enables scanning a 100/spl times/100 millimeter section of airframe without moving the expander, which connects to compressors through flexible gas lines 3 m long. Thermal conduction through copper braiding in the umbilical keeps pickup loops at 5.5 K, 3 mm from an airframe at ambient temperature. Drive currents of a few amperes, oscillating at one to three hundred Hertz enable identifying 5 to 10 percent material loss through 10 to 15 mm of aluminium.<
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