Development of an automated method for in situ measurement of the geometrical properties of the ITER bolometer diagnostic

doi:10.1016/j.fusengdes.2010.12.020

Fusion Engineering and Design

Volume 86, Issues 6–8, October 2011, Pages 1170-1173

https://doi.org/10.1016/j.fusengdes.2010.12.020 Get rights and content

Abstract

In order to derive the local emission profile of the plasma radiation in a fusion device using the line-integrated measurements of the bolometer diagnostic, tomographic reconstruction methods have to be applied to the measurements from many lines-of-sight. A successful reconstruction needs to take the finite sizes of detectors and apertures and the resulting non-ideal measurements into account. In ITER a method for in situ measurement of the geometrical properties of the various components of the bolometer diagnostic after installation is required as the viewing cones have to pass through narrow gaps between components. The method proposed to be used for ITER uses the beam of a laser with high intensity to illuminate the bolometer assembly from many different angles ξ and θ. A light-weight robot from Kuka Robotics is used to efficiently position the laser on many points covering the complete viewing cone of each line-of-sight and to direct the beam precisely into the entrance aperture of the bolometer. Measuring the response of the bolometer allows for the calculation of the transmission function t(ξ, θ), the angular etendue and finally the geometric function in reconstruction space, which is required for the tomography algorithms. Measuring the transmission function for a laboratory assembly demonstrates the viability of the proposed method. Results for a collimator-type camera from a prototype envisaged for ITER are presented. The implemented procedure is discussed in detail, in particular with respect to the automatisation applied which takes the achievable positioning and alignment accuracies of the robot into account. This discussion is extended towards the definition of requirements for a remote-handling tool for ITER.

Introduction

The total radiated power as well as the radiation emission profile on ITER will be determined by the bolometer diagnostic. A bolometer measures the plasma radiation over a wide spectral range (from soft-X to the infrared) by monitoring the temperature rise induced by deposition of photon energy in the absorber layer of the bolometer. The reference detector type chosen for ITER is the metal resistor bolometer [1], [2]. In order to derive the spatially resolved radiation emission profile from the line integrated measurements tomographic reconstruction methods are applied to the measurements of many lines-of-sight (e.g. [3] and references therein). However, the number of the lines-of-sight feasible in ITER is restricted due to the maximum amount of space available for diagnostic components and electrical feed-throughs. Thus, a successful reconstruction needs to take the finite sizes of detectors and apertures and the resulting non-ideal measurements into account [4]. In ITER, a method for in situ measurement of the geometrical properties of the various components of the bolometer diagnostic after installation is required as the viewing cones have to pass through narrow gaps between components (e.g. for shielding) and small production tolerances might result in large deviations from the planned line-of-sight geometry due to the large size of ITER.

The method proposed to be used in ITER and its implementation in a laboratory environment are described in Section 2. First results from laboratory measurements and a comparison to the theoretically expected values are presented in Section 3. Finally, some requirements of such a method for the application in ITER will be derived in Section 4.

Section snippets

Experimental set-up

The method proposed to be used for determining the geometrical properties of the ITER bolometer diagnostic components is based on previous work performed at JET [5]. The beam of a laser with high intensity is used to illuminate the bolometer assembly from many different angles ξ (poloidal direction) and θ (toroidal direction). Measuring the response of the bolometer allows for the calculation of the transmission function t(ξ, θ), the angular etendue and finally the geometric function in

Transmission function of the prototype collimator

The theoretically expected transmission function t_i(ξ, θ) for each channel i can be calculated analytically by considering the fraction of light from the laser reaching the detector: $\begin{matrix} I_{D, i} (ξ, θ) & = I_{0} \frac{L_{ξ, i} (ξ) \cdot L_{θ, i} (θ)}{P} \cos ξ \cos θ \\ = I_{0} t_{ξ, i} (ξ) \cdot t_{θ, i} (θ) \end{matrix}$

with the detected intensity I_D,i on each channel i, the initial intensity of the laser I₀ and the area of the collimator entrance P. The illuminated area A_i of each channel can be separated in its components L_ξ,i(ξ) and L_θ,i(θ) in ξ and θ direction because the

Conclusions for the application in ITER

The results shown in Section 3 are a proof of principle that the proposed method for an automatised in situ calibration of the ITER bolometer diagnostic is feasible. However, they do suggest several improvements and a number of conclusions can be drawn with respect to the requirements at ITER.

On the one hand it is clear that measures have to be devised to reduce the amount of stray light. Coatings like, e.g. B₄C, which also absorbs microwave radiation, have to be investigated. Also, the design,

Acknowledgments

The authors wish to explicitly thank G. Fröhlich, S. Eder and W. Zeidner from the ASDEX Upgrade Team who performed the metrology measurements with the FaroArm.

This work was supported by funds from the German Ministry for Education and Research under the Grant No. 03FUS0006. The sole responsibility for the content presented lies with the authors.

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The ITER bolometer diagnostic: status and plans
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Tomography diagnostics: Bolometry and soft-X-ray detection

There are more references available in the full text version of this article.

Cited by (5)

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This needs to be considered in particular for LOS observing the plasma edge region as their orientation usually deviates significantly from the normal to the surface of plasma facing components. The transmission function of the prototype collimator has been measured in the lab [7]. The collimator including the sub-collimators was manufactured using wire-erosion.
The total radiation emission profile of fusion experiments is usually determined using the bolometer diagnostic. In order to evaluate the spatially resolved profile, many line integrated measurements are inverted using tomographic reconstruction techniques. Their success depends on a well known and optimised definition of the viewing cones of every line-of-sight. To this aim a set of equations has been derived and put in hierarchical order to define the design parameters for bolometer cameras in fusion experiments. In particular, previous considerations, which focussed on the beam width overlap and light yield optimisation, are extended to explicitly take geometrical boundary conditions imposed by the experimental device into account, with an emphasis on small gap sizes through which viewing cones have to pass through. The equations are derived for both camera types, collimator and pin-hole versions. The results obtained can be used to design bolometer cameras for any fusion device, but in particular also for ITER. An example of such an application is given and implications for the realisation of the optimal design are discussed.
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The measurement tool ITER Bolometer Robot Test Rig (IBOROB) was used for the calibration measurements presented here. The tool was once developed for the measurement and assessment of the ITER bolometer diagnostic [7] and is based on work conducted at JET [12,13]. Experience to analyse and evaluate the results were gained during the collimator development for ITER [14].
The ITER Bolometer Robot Test Rig (IBOROB) is a robot-based diagnostic tool, which allows the measurement of the lines of sight (LOS) of the ITER bolometer prototypes. Up to now, it was only used as a LOS characterization device for the ITER collimator development. IBOROB was further developed and can now be operated in ASDEX Upgrade during a regular maintenance shutdown. At present, once a diagnostic like the bolometry is mounted inside the vessel, the actual LOS orientations are not measured, they are derived from CAD. The new procedure allows the fully automatic three-dimensional in situ measurement of bolometer LOS. The spatial distribution, the poloidal and toroidal alignment in the experiment coordinate system (CS), can be determined. The absolute accuracy, in reference to the tokamak CS, is provided by an additional calibration performed with a measurement arm by FARO Technologies Inc. Therefore, the amount of misalignment from the theoretical expectations can be quantified. In addition specific camera type dependencies such as internal camera reflections can be identified. Due to the high position accuracy of the robot, the LOS can be resolved with a spatial resolution of up to 0.1°. The method is explained in detail and results from two exemplary bolometer foil cameras obtained in a first set-up in ASDEX Upgrade are presented. The different steps and components needed to apply the measurements in the vessel are described with a focus on the constraints, e.g. geometrical, for an application of this method in a tokamak. Finally the consequences of the results are extrapolated to ITER and evaluated.
Assessment of line of sight characteristics of ITER bolometer prototype collimators
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Both are manufactured by wire erosion. The collimator has four channels which do not consist of physical hollow channels like in the version analyzed in [1]. In this design, the theoretical étendue of each channel is only characterized by the construction parameters of the top plate design and the dimensions of the integrated apertures.
This work outlines the present design status of developments on the optimization of line of sight (LOS) characteristics for the ITER bolometry collimators. The verification and measurement of the LOS of the bolometry is an important issue for a reliable operation of the tomographic reconstruction algorithms. Therefore the ITER Bolometer Robot Test Facility (IBOROB) is used as a diagnostic tool in order to analyze LOS geometry and assess the performance of different collimator prototypes. The LOS characteristics as a function of number of apertures are presented and the influence of a microwave filtering aperture was determined. The results of the improvement of stray light attenuation by application of the graphite based coating AQUADAG are evaluated as well and an overview about the current collimator design is given. This paper focuses on the most remarkable results, a way to further possible upgrades is outlined. However, some results are not in accordance with the ones predicted in theoretical calculations. Thus not all key parameters which influence the LOS could be clearly identified yet.
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Development of an automated method for in situ measurement of the geometrical properties of the ITER bolometer diagnostic

Abstract

Introduction

Section snippets

Experimental set-up

Transmission function of the prototype collimator

Conclusions for the application in ITER

Acknowledgments

A low noise highly integrated bolometer array for absolute measurement of VUV and soft-X radiation

Review of Scientific Instruments

The ITER bolometer diagnostic: status and plans

Review of Scientific Instruments

Tomography diagnostics: Bolometry and soft-X-ray detection