Search

• Signal intensities and noise for current ITER bolometer diagnostic design estimated. • SNR for most lines-of-sight (LOS) is good or acceptable. • Signal intensities for optimised LOS in upper ports increased by factor of 4. • Low SNR for LOS observing plasma edge; reduced absorber thickness could help. The ITER bolometer diagnostic shall provide the measurement of the total radiation emitted from the plasma, a part of the overall energy balance. Up to 550 lines-of-sight (LOS) will be installed in ITER observing the whole plasma from many different angles to enable reliable measurements and tomographic reconstructions of the spatially resolved radiation profile. The performance of the diagnostic is intimately linked to the constraints imposed by the design requirements, the sensor and data acquisition design, as well as expected noise levels. The results presented contain the estimated signal intensities for the current design and integration of bolometer cameras in ITER, based on the simulated radiation profile for the ITER standard scenario (DT-plasma at 15 MA, 500 MW with low impurity content). The corresponding power deposited onto the absorber ranges between 0.5 μW and 2 mW. The expected noise levels derived mainly from lab measurements and compared to operating bolometer systems yield estimated values for ITER in the order of 1 μW, clearly indicating a need for optimizing the diagnostic. The optimization potential is discussed. Cameras for which the light yield could be increased are identified. Also, the potential for enhancing the sensitivity by using different absorber thicknesses is explored. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Abstract: 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. [Copyright &y& Elsevier]

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. [Copyright &y& Elsevier]

• Development of the ITER bolometer camera for measurement of the radiation emitted by the plasma. • The system has to guarantee reliable measurements within an environment characterized by high neutron flux and temperatures exceeding 200 °C. • This paper describes a possible procedure for a structural analysis of the bolometer camera body. • The results of the structural analysis regarding the camera housing structural integrity are presented and discussed here. The ITER bolometer provides an absolutely calibrated measurement of the radiation emitted by the plasma, which is a part of the total energy balance. The development is especially challenging because of the extreme environmental conditions within the vacuum vessel (VV) during plasma operation. The bolometer has to guarantee reliable measurements within an environment characterized by high neutron flux as well as temperatures exceeding 200 °C. In addition to the thermal loads, the bolometer body is exposed to the mechanical loads caused by electromagnetic forces during transient events of plasma disruption. This paper describes a possible procedure for a structural analysis of the bolometer camera body. To examine all-important structural properties of the bolometer body, a multiple nonlinear finite element model based on a CAD conceptual design has been generated. Subsequently, a transient mechanical analysis has been performed using the finite element code ANSYS. A general electromagnetic model, taking into account the contribution of all structural parts and electromagnetic loading starting with the DINA code, generated the input for the analysis. From the wide range of DINA results, the worst-case load scenario has been chosen. The results of the structural analysis regarding the camera housing structural integrity are presented and discussed here. [ABSTRACT FROM AUTHOR]

• Adjustments of ITER Blanket Modules after first plasma has been investigated. • Current design of ITER-VV mounted Bolometers has been shown. • Method to align bolometers with gaps between Blanket Modules has been shown. • Method to define CMM of the bolometers has been explained. Bolometer cameras in ITER will be mounted, among others, on the Vacuum Vessel (VV) wall behind Blanket Modules (BMs). For the first assembly phase the platform (Cable fixations and the lower part of the internal signal chain) of VV cameras has to be delivered to fix the cables and protect their termination. During First Plasma, the as-built magnetic axis and magnetic flux surface will be measured and the BMs fixation will be adjusted to align them to the shape of the magnetic flux surfaces. Adjustments of the BMs of up to ±13,3 mm in toroidal and poloidal direction and up to ±20,3 mm in radial direction are foreseen. Accordingly, VV cameras need adjustments, too, to assure a proper view of the plasma. Calculations have been performed to define the impact of the possible BM movements onto the bolometer viewing cones. The toroidal and poloidal movements can be followed by shifting the collimator and the sensor along with the BMs. The radial movement of BMs was transferred to an additional poloidal shift of the sensor and collimator. The resulting concept for camera design with its complex space envelope will be presented. It provides an alignment flexibility of the cameras of ±31 mm poloidally and ±16 mm toroidally. Because of the short period between magnetic measurements and second assembly phase, the number of customizable parts is kept as low as possible. [ABSTRACT FROM AUTHOR]

• Electromagnetic analysis procedure of the ITER (upper and equatorial) port mounted bolometer cameras • Main aim of the analysis is the evaluation of the electromagnetic forces caused by changing magnetic fields. • Investigate the worst load case per bolometer camera type and position. • Lorentz-forces resulting from the interaction of these eddy currents with the main magnetic field are deduced. This paper describes the electromagnetic analysis procedure of the ITER (upper and equatorial) port mounted bolometer cameras. The main aim of the analysis beside the evaluation of the electromagnetic forces caused by changing magnetic fields is to find out the worst load case scenario for a particular bolometer camera. For this purpose, a wide range of load case scenarios has been analyzed. The output from 2D axisymmetric code DINA is used as input to a general electromagnetic model of a 20° sector of the ITER structure, taking into account the contribution of all structural parts, in order to calculate eddy currents induced inside the bolometer components. The surrounding structure of the bolometer camera housing was modeled with particular precision in order to take the electromagnetic contributions of these parts into account as precisely as possible. To investigate the worst load case per camera type and position six representative disruption scenarios are analyzed. In a second step, the Lorentz-forces resulting from the interaction of these eddy currents with the main magnetic field are deduced. They are an essential input for the structural assessment of the camera housing. This paper discus in detail EM analysis procedures used in case of port-mounted bolometer cameras. [ABSTRACT FROM AUTHOR]

As part of ITER's fusion diagnostic systems, miniaturised metal resistor type bolometer cameras are envisaged to provide the measurement of the total plasma radiation. For this kind of bolometer sensor, the temperature of a measurement and a reference absorber is realised by metallic meanders on their back side. For the reference absorber being able to reliably compensate changes in the environmental temperature, it is necessary to assure that the temperature gradient within one bolometer channel is as low as possible, preferably <0.1 °C. In order to successfully establish this condition, the entire design of the sensor mount, camera housing and wall attachment have to be considered. A particular region of interest in ITER is the divertor because of bolometer cameras in this area having an asymmetrical design w.r.t. the thermal flow path. Due to the high number of differently oriented cameras required in ITER, the main goal of this study is to map the thermal behaviour of these devices under typical loads and, identifying the underlying principles to provide universal guidelines and starting points for future designs, in order to prevent one by one optimization of each separate camera. [ABSTRACT FROM AUTHOR]

The ITER bolometer diagnostic shall provide the measurement of the total radiation emitted from the plasma, a part of the overall energy balance. 550 lines-of-sight (LOS) will be installed in ITER observing the whole plasma from many different angles to enable reliable measurements and tomographic reconstructions of the spatially resolved radiation profile. The LOS are bundled in 71 individual cameras, which will be located behind blanket modules on the vacuum vessel wall, in five divertor cassettes, in two upper port plugs and in one equatorial port plug. For all major design issues solutions have been elaborated and will be presented. The design of collimators has been developed and tested on prototypes to provide an exact definition of the required viewing cones of about 1° while reliably reducing reflections, stray light and microwave stray radiation. A 3D-shaped ceramic printed circuit board is proposed to hold the sensor, orient it as desired, and provide good thermal contact as well as the bridge for electrically connecting external signal cables to the meanders on the sensor. The design of the camera housing for vacuum vessel and divertor cameras has been optimised for improved management of the thermal heat flow, supported by tests defining material properties and verifying analysis. Additionally, methods have been developed to derive the main design parameters of cameras and decide if pinhole or collimator type is more advantageous. Recently, the system-level design phase started and uses the achieved results to define interfaces and designs for the specific locations in ITER, with a focus on the vacuum-vessel cameras to finalise the interface with blanket modules. [ABSTRACT FROM AUTHOR]

The ITER bolometer diagnostic shall provide the measurement of the total radiation emitted from the plasma, a part of the overall energy balance. Up to 550 lines-of-sight will be installed in ITER in 71 cameras of various sizes and types. The sensor holder, the component inside those cameras to provide attachment and signal connections to the sensor itself, might vary in shape within each camera. However, concepts for implementing electrical connections and handling of loads are very similar, if not identical. Taking the current design of port-mounted bolometer cameras and the corresponding space restrictions as a basis, a concept for the sensor holder is proposed in line with the requirements on high environmental temperatures of up to 350 ∘ C, mechanical loads and compatibility to operation on vacuum as well as within a nuclear environment. The sensor holder is made of a ceramic with a coefficient of thermal expansion as close as possible to the one of the sensor material to minimise thermal stresses. Wire-bonded connections are used between the sensor and contacting pads deposited on the front plate of the sensor holder. The camera's internal signal cables are welded to these contacting pads to complete the electrical circuit. The bond wires and the contacting pads may be covered by a ceramic paste for protection against mechanical loads. This concept can be implemented in every port camera and is expected to be easily adaptable to other camera locations as well. Prototypes of this concept have been built to prove the feasibility of manufacturing and assembly as well as to give valuable input for design improvements. Thermal cycling as well as electrical tests have been performed to validate the mechanical stability and functionality under ITER-relevant conditions. [ABSTRACT FROM AUTHOR]

The first detector prototypes for the ITER bolometer diagnostic featuring a 12.5 μm thick Pt-absorber have been realized and characterized in laboratory tests. The results show linear dependencies of the calibration parameters and are in line with measurements of prototypes with thinner absorbers. However, thermal cycling tests up to 450 °C of the prototypes with thick absorbers demonstrated that their reliability at these elevated operating temperatures is not yet sufficient. Profilometer measurements showed a deflection of the membrane hinting to stresses due to the deposition processes of the absorber. Finite element analysis (FEA) managed to reproduce the deflection and identified the highest stresses in the membrane in the region around the corners of the absorber. FEA was further used to identify changes in the geometry of the absorber with a positive impact on the intrinsic stresses of the membrane. However, further improvements are still necessary. [ABSTRACT FROM AUTHOR]