XRS-2 FET Boxes

The small rectangular JFET board hangs like a spider in a web from the inner box surrounding it, while the inner box hangs similarly from the outer one. (100K JPEG)

This is how we keep the hot stuff hot and the cold stuff cold. The first stage of amplification is provided by JFET transistors, which operate at 130 Kelvin. Sounds cold, but it's 2000 times hotter than the detectors. Even a tiny bit of that heat will overwhelm the ADR. To keep that from happening, we put the JFETs in a metal box which is thermally connected to the frozen neon (at 17 Kelvin). Then we put that box in another box, which is thermally connected to the helium bath at 1.3 Kelvin.

Here you see the JFET board suspended in the inner box, and the inner box suspended in the outer box (the lids aren't on yet). The mechanical suspension is provided by the Kevlar strings you can see on the left and right sides of the boxes. The electrical connections are made through the very thin strands going up and down. The JFET board is about 3 cm long.

National Aeronautics and Space Administration

Goddard Space Flight Center