Spatial Variation of the Foil Parameters from in Situ Calibration of the JT-60U Imaging Bolometer Foil

Abstract

We obtained the local foil properties of the JT-60U imaging bolometer foil (a single graphite-coated gold foil with an effective area of 9 × 7 cm 2 and a nominal thickness of 2.5 µm) such as the thermal diffusivity, κ, and the product of the thermal conductivity, k, and the thickness, t f , by calibrating some parts of the foil. Calibration of the foil was made in situ using a He-Ne laser (∼27 mW) as a known radiation source to heat the foil. The thermal images of the foil are provided by an infrared (IR) camera (microbolometer type). The parameters are determined by finite element modeling (FEM) of the foil temperature and comparing the solution to the experimental results. In this work we apply this calibration technique to investigate the spatial variation of the foil parameters. Significant variation in the local temperature rise of the foil due to local heating by the laser beam indicates a spatial variation of the foil parameters κ, k and t f . This variation is possibly due to nonuniformity in carbon coating and/or the thickness of the foil.

Calibration of a thin metal foil for infrared imaging video bolometer to estimate the spatial variation of thermal diffusivity using a photo-thermal technique

“…Several such calibration experiments, some of them focusing on the in situ calibration capability are reported. [4][5][6][7][8] The very initial IRVB foil calibration experiments 4 were carried out using a 500 W lamp with reflectors to collimate the light. The foil is mounted inside a vacuum chamber so as to minimize convection losses.…”

“…This can take care of the transmission and reflection losses by the folding/steering mirrors and vacuum windows of the calibration chamber and can also be used for in situ calibra-tion inside the machine. 5,6 The initial experiments with the laser were carried out by steering the laser beam with mirrors to illuminate different IRVB pixels while fixing the foil vertically inside the calibration chamber. 2,5,6 Steering the beam results in illumination non-uniformities due to changes in the path length of the beam through the vacuum window.…”

“…5,6 The initial experiments with the laser were carried out by steering the laser beam with mirrors to illuminate different IRVB pixels while fixing the foil vertically inside the calibration chamber. 2,5,6 Steering the beam results in illumination non-uniformities due to changes in the path length of the beam through the vacuum window. 7 The calibration experiments were done for each quadrant due to the limited field of view of the IR camera which needs several vacuum vents during the experiment since there was no provision to re-position the foil without breaking the vacuum.…”

“…Apart from these physical constraints; this calibration technique relies heavily on solving a large number of finite element models (FEMs) for each irradiation point. 5,6,8 This is very time consuming and often requires several days for analyzing the data. The time may increase for large foil dimensions and for finer spacing between the irradiation points, which is ultimately decided by the dimensions of the pinhole.…”

“…Some calibration methods assume the IRVB pixel area for diffusivity estimation 1 and consider the literature value for thermal conductivity. 5,6,8 The first may not be justified since the diffusivity should be invariant, irrespective of the IRVB pixel area. For the second case the thermal conductivity may not be the same as the literature value due to the fact that the foil often has many pinholes and cracks.…”