Infrared imaging video bolometer for the large helical device

A new type of infrared imaging bolometer, known as the infrared imaging video bolometer (IRVB) has been under development at the National Institute of Fusion Science (NIFS) over the last year. A prototype of this diagnostic has been designed and constructed at NIFS and installed on the large helical device (LHD) and successfully operated. This diagnostic utilizes a 66×90×0.001 mm gold foil mounted in a frame to detect the radiation and neutrals from the plasma which are incident on the foil through a 1 cm diameter pinhole. The resulting temperature distribution on the foil is measured using an AGEMA THV 900 LW infrared camera having 136×272 pixels with a frame rate of 15 Hz and a nominal sensitivity of 80 mK. An image of the foil and surrounding frame consisting of 120×160 pixels is resampled down to 12×16 pixels using a linear interpolation scheme. Using a numerical technique the spatial and temporal derivatives of the temperature distribution on the foil are calculated and the incident power density on the foil is determined using a calibration of the foil obtained by means of a HeNe laser. The resulting 10×14 pixel view of the plasma radiation at a 15 Hz frame rate has a noise equivalent power density of 0.5 mW/cm2. The IRVB was mounted on a LHD tangential port for the 1999 campaign. An image of the plasma radiation during a discharge using the inboard vacuum vessel wall as a limiter shows radiation localized near the limiting surface.