Calibration parameter drift compensation of metal resistive bolometers operating in a thermal varying environment

Abstract

The ITER bolometer diagnostic will have to provide accurate measurements of the plasma radiation in a varying thermal environment of up to 250• C. Current fusion experiments perform regular in-situ calibration of the sensor properties, assuming stable calibration parameters within short discharge times, e.g. 10 s on ASDEX Upgrade. For long-pulse fusion experiments, e.g. W7-X, the diagnostic is operated with water cooling for achieving a stable temperature environment. However, ITER will be equipped with about seventy bolometer cameras and is planned to have discharge times of up to 1 h. Due to space restrictions, active cooling is not available for all locations. Thus, an alternative approach is required to allow for compensation of the changing calibration values due to thermal drifts. This paper demonstrates a method using the Wheatstone bridge current of the sensor to calculate in real-time the changing calibration values, such as the heat capacity and the thermal time constant. It is shown, that the thermal calibration parameter drift can be calculated by either extrapolating from the initial values or using a previously determined look-up table. Measurements in the ITER bolometer vacuum test facility (IBOVAC), used to simulate ITER-relevant thermal and vacuum environment, show that the change of the calibration values can be predicted during repeated thermal cycles over a duration sufficient for ITER discharges and even longer. Confidence intervals for the typical in-situ calibration method are determined and compared with the proposed extrapolation and look-up method for ITER, showing that these methods provide an equivalent quality of the measurement results.

Bolometer developments in diagnostics for magnetic confinement fusion

“…Firstly, the calibration constants depend on temperature as the underlying material parameters like thermal conductivity or specific heat capacity depend on temperature. This effect can be compensated by the additional measurement of the current through the Wheatstone bridge [38,39]. The latter provides a measurement proportional to the temperature of the sensor and can be used to interpolate the correct value of the calibration parameters from previous calibration runs in the lab.…”

Optimizing ITER Bolometer Cameras in View of Internal Reflections and Crosstalk Between Channels

“…For example, stray radiation from the electron-cyclotron heating system can be minimized by design [6], and geometric uncertainties by design measures, metrology, and optical characterization [7], [8]. Additional sources of uncertainty and errors have been characterized, such as calibration factors (with an error of the order of 1%) [9], pressure effects [10], and correction for temperature dependence of calibration factors [11]. Compensation for other sources of uncertainty is still to be demonstrated, such as cable length and reflections on the first wall inside the vacuum vessel (in other diagnostics in the visible and UV range the reflections are a significant concern [12]).…”