ABSTRACT
Ambient pressure may influence the thermal path between the absorber and heat sink of resistive bolometers and thus impact the calibration parameters. This effect is investigated for metal resistive bolometer sensors as used in bolometer diagnostics on fusion experiments. Measurements in the test facility IBOVAC indicate that pressure has no effect up to 10-3 mbar. However, a significant change in the cooling time constant is observed for pressures above 10-2 mbar, a reduction up to a factor of three at 1 mbar. The measurements performed in N2 and He atmospheres and simulations in H2 indicate no difference between the results from different gas species up to 10-3 mbar and less than 10 % up to 0.1 mbar. A model based on the thermal conductivity of the surrounding gas combined with the geometry of the sensor holder successfully demonstrates that the additional cooling path through the gas, which may vary between the measurement and reference absorbers, can explain the measurement results. Applying the model to the geometry of a sensor holder designed for port-mounted bolometer cameras in ITER led to design modifications that should help reduce the impact of high environmental pressures on the bolometer measurements. Similarly, it can be assumed that applying the model to the geometries and sensors of operating bolometer diagnostics can help correct the measurements and improve the understanding of plasma radiation in the case of high pressures at the location of bolometer sensors.
ABSTRACT
On present-day magnetic-confinement fusion experiments, the performance of multi-channel bolometer diagnostics has typically evolved over time through experience with earlier versions of the diagnostic and experimental results obtained. For future large-scale fusion experiments and reactors, it is necessary to be able to predict the performance as a function of design decisions and constraints. A methodology has been developed to predict the accuracy with which the volume-integrated total radiated power can be estimated from the measurements by a resistive bolometer diagnostic, considering, in particular, its line-of-sight geometry, étendues of individual lines of sight, bolometer-sensor characteristics, and the expected noise level that can be obtained with its electronics and signal chain. The methodology depends on a number of assumptions in order to arrive at analytical expressions but does not restrict the final implementation of data-processing of the diagnostic measurements. The methodology allows us to predict the performance in terms of accuracy, total-radiated power level, and frequency or time resolution and to optimize bolometer-sensor characteristics for a set of performance requirements. This is illustrated for the bolometer diagnostic that is being designed for the ITER experiment. The reasonableness, consequences, and limitations of the assumptions are discussed in detail.
ABSTRACT
The ITER bolometer diagnostic is planned to have 550 lines of sight (LOS) distributed all over the vessel. 240 channels are provided by cameras mounted in two upper ports and in one equatorial port. This paper describes the current status of the system level design of the port cameras and the solutions proposed on how to implement all required camera components while meeting a multitude of competing requirements. Sensor holders, support structures, and different apertures depending on the camera type (pinhole or collimator), cable connectors, ceramic track plates, and many mineral insulated cables have to be integrated within a restricted space envelope to guarantee functionality. The design of the internal electrical interfaces and the external mechanical mountings will be described as well. Using the example of an upper port camera with 60 LOS, the assembly of the camera components is explained and two currently discussed architecture options for the remote handling maintenance scheme in the hot cell are compared.
ABSTRACT
Experiments on the JET tokamak show that the wave-induced pinch in the presence of toroidally asymmetric waves can provide a tool for controlling the profile of ion-cyclotron-resonant 3He ions. Direct evidence for the wave-induced pinch has been obtained from the measured gamma-ray emission profiles. Concurrent differences in the excitation of Alfvén eigenmodes (AEs), sawtooth stabilization, electron temperatures, and fast-ion stored energies are observed. The measured location of the AEs and gamma-ray emission profiles are consistent with the fast-ion radial gradient providing the drive for AEs.