Design of an imaging bolometer upgrade for JT-60U

In this paper we present the design of an upgraded version of the imaging bolometer on JT-60U. The 2.5 micron gold foil is replaced with a 5 micron Ta foil for improved rigidity. The vacuum window will be enlarged from 58 mm (ZnSe) to 100 mm (sapphire). The enlarged window and a 3.8 m IR periscope using CaF₂ optics and an aluminum mirror will improve the optical throughput. The periscope will permit a more advanced IR camera (FLIR/Indigo Phoenix-mid) (3-5 microns, 256 times 360 pixels, 345 Hz, 25 mK) to be mounted at a location of lower neutron flux and lower magnetic field in a shield box consisting of 2 cm of soft iron (against magnetic field), 2 cm of lead (against secondary gammas), and 10 cm of polyethylene (against neutrons). Through the combination of improved optical throughput using the periscope and the improved sensitivity of the IR camera we expect an improvement in the sensitivity of the imaging bolometer by at least a factor of 50 compared to the previous version. This improvement in sensitivity is used to increase the number of channels from 12 (toroidal) times 16 (poloidal) to 24 times 40 by reducing the aperture size from 5 mm times 5 mm to 2 mm times 2.5 mm, respectively. This imaging bolometer represents a reactor relevant prototype for those planned for KSTAR, LHD, JT-60SA and proposed for ITER.