Development of Imaging Bolometers for Long-Pulse MFE Experiments

We have developed the concept of an imaging bolometer, capable of operation with 100’s of individual channels, while relying on optical (infrared) readout of the temperature rise in a thin foil. A thin gold foil (0.5-5 microns thick) is sandwiched between pieces of copper. The copper mask (a large thermal mass) has a hole pattern drilled into it to form many "individual pixels’’forming many separate sensors. This segmented foil/mask combination is exposed on its front side to plasma radiation through a cooled pinhole camera geometry. Simultaneously, a high-resolution infrared camera monitors any temperaturechange on the backside of the thin foil. A sensitive infrared (IR) camera views the foil through an IR telescope/periscope system, and is shielded from the magnetic and nuclear radiation fields, either by distance and/or material shielding. A simple time-dependent design algorithm, using 1-D heat transport to a cold boundary, has been written in MathCad, which allows us to select optimal material and geometries to match the expected plasma conditions. We have built a compact prototype with 149 channels, and tested it successfully both in a vacuum test stand in the laboratory, and on a plasma in the Compact Helical Stellarator (CHS) at the National Institute for Fusion Science (NIFS), subjecting it to ECH and NBI-heated conditions. A water-cooled version has been built for the new Large Helical Device (LHD). Since the IR imaging bolometer uses only metal parts near the plasma, and has no need for wiring or wiring-feedthrus, it is intrinsically radiation-hard, and has direct application to ITER, or ITER-class experiments.