Proceedings of ITC/ISHW2007

Improvement of the Accuracy of the Imaging Bolometer Foil LaserCalibration
H. ParchamyAraghy, B.J. Peterson, N. Ashikawa, H. Hayashi, H. Kawashima1, and S. Konoshima1
National Institute for Fusion Science,322-6 Oroshi-cho, Toki 509-5292, Japan1

Japan Atomic Energy Agency, Naka-shi, Ibaraki-Ken, 311-0193, Japan

An imaging bolometer with a single graphite-coated metal foil is a diagnostic tool for diagnosing plasma
radiation from magnetic fusion plasmas. We could obtain the local foil properties (the thermal diffusivity, к, and the
product of the thermal conductivity, k, and the thickness, tf) of the metal imaging bolometer foil by analyzing thecalibration data. For improving the IRVB a Tantalum (Ta) foil is offered due to strength, low neutron cross-section,and high sensitivity, however there is a large discrepancy between the value of the foil thickness from theexperimental value and the nominal value. For calibrating of the foil the He-Ne laser beam is focused on 63 various
locations which are determined by using the marks on the frame. The parameters of the foil are determined bycomparing the measured thermal radiation data from an IR camera (FLIR/SC500) (60 Hz, 320 × 240 pixels, 7.5-13µm) with the corresponding results of a finite element model.

Keywords: Accuracy, calibration, He-Ne laser beam, improve, imaging, bolometer.

1. Introduction
The infrared imaging video (IRVB) bolometerwith a single graphite-coated metal foil is a diagnostic toolthat can act as a broadband absorber of radiation fromnearly all spectral regions from of the foil plasma andconvert its energy into a measurable temperature rise [1].
The calibration technique of the infrared imaging
bolometer foil gives confidence in the measured values of
the total radiation power from the plasma, andcompensates for non-uniformities in the foil [2, 3]. The
local foil properties of the foil such as the thermal
diffusivity, к, and the product of the thermal conductivity, k,and the thickness, tf, are obtained by the foil calibration for
one part on the foil when the foil is heated by the laser
power [2]. Tantalum (Ta) is offered for improved
sensitivity, foil strength, and lower neutron cross-section
compared to the previously used gold foil of the imaging
bolometer for the JT-60U tokamak [4].
Here, the improvement of the accuracy of the laser
calibration of the imaging bolometer foil is considered.

2. The single graphite-coated metal(Ta and Au)foil
For improving the accuracy of the foil calibrationof the imaging bolometer foil a new calibration setup inlaboratory (low noise environment) has been designed andset up [4].
A single graphite-coated gold foil with an effective
author’s e-mail: journal@jspf.or.jp

area of 9 × 7 cm2 and a nominal thickness of 2.5 µm foruse in the IRVB of LHD and a single graphite-coatedtantalum foil with an effective area of 9 × 7 cm2 and anominal thickness of 5 µm for use in the IRVB of JT-60U
is used on the calibration setup. Both sides of the foils areblackened with graphite. The blackened sides are heated bya 500 W heat lamp for drying the foils after spraying. The
foils that are supported by a copper frame are shown in Fig.
1.
Calibration of the foil was made using a He-Ne laseras a known radiation source to heat the foil. The laser beam(~27 mW) is directed by three mirrors that are positionedby z and x-axis stages, to propagate through a CaF2
window in a chamber that is evacuated by aturbomolecular pump below 10-3 Torr and is focused on thefoil. The blackbody radiation power from the foil that isheated by the laser beam is detected by an IR camera
through a ZnSe window.
The IR camera (FLIR/SC500) with a frame rate of 60
Hz and pixel number of 320 × 240 is a microbolometer
detector type that is sensitive to the far infrared wavelength
range (7.5-13.0). The IR camera is moved by using alab-jack (in z direction) and x-axis-stage (in x direction) for
providing the thermal images of the foil which is heated by
the He-Ne laser beam. In Fig.1, two contour thermal
images from the laser spot after focusing on the foil (thecenter and the edge) resulting from the blackbody
radiation from the foil are shown.

Proceedings of ITC/ISHW2007
the measured beam profile, fit to a 2-D Gaussian, as theheat source term.
A second order polynominal is fitted to the ktf (thethermal conductivity is assumed to be constant at thenominal value) and ΔT data from the FEM to find theeffective value of the tf of the foil from the experimental
value of ΔT. The variation of tf from two locations (centerand edge) of the gold foil (with nominal value of the foil
thickness, tf = 2.5 µm), and the tantalum foil (with anominal value of the foil thickness, tf = 5 µm), is shown in
Fig.2.

Fig.1 The single graphite-coated (one for Ta and one forAu separately) foil that is supported by a copper
frame and temperature contour images from thelaser spot after focusing on the foil (center andedge).

3. Improvement of Foil Calibration
The accuracy of the calibration of imaging bolometer
foil is improved by using a new calibration setup and a
FLIR/SC500 IR camera with a close-up lens at laboratory(low noise environment). This combination provides a
spatial resolution of ~ 0.09 mm which is ~ 7 times better
than the previous in situ calibration using a different
Indigo/Omega IR camera where the spatial resolution ofthis camera is ~ 0.648 mm. The laser beam is focused on
various locations which are determined by using themarks on the frame. Calibration data were taken at 63 (7× 9) points on the foil. For the foil calibration using aHeNe laser, the laser power is necessary for solving theheat diffusion equation in two dimensions analytically byusing the finite element model (FEM). A new model ismade by finite element modeling according to theresolution of the FLIR/SC500 camera and the laser beam
diameter.
The foil temperature rise was taken from the peak of a 2D
Gaussian fitted to the temperature profile from the IRcamera. The IR thermal data are averaged over 200 timeframes of the steady state data. The FEM is used forsolving of the two-dimensional heat diffusion equation
with a constant thickness and thermal parameters of thefoil and approximately 2000 spatial points assuming a
constant temperature at the foil boundary. The FEM usedFig.2 The appropriate value of the product of the ktf of the(Au and Ta) foil from the experimental value ofthe ΔT is found from fitting a second orderpolynominal to the ktf and the ΔT data from theFEM.
Acknowledgment
The authors are grateful to the support given by theMEXT Grant-in-Aid # 16082207.

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H. Parchamy, B. J. Peterson, et al., Rev. Sci. Instrum. 77,515 (2006)
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B. J. Peterson, et al., Plasma Fusion Res. 2, (2007) (inpress)
H. Parchamy, B. J. Peterson, H. Hayashi, S. Konoshima, N.
Ashikawa, and the LHD team, Submitted for publish toIEEE TPS