Development of 3D radiation measurement using IR imaging video bolometers for the study of radiation collapse in LHD

High density operation is preferred for a future fusion reactor since the fusion output is proportional to the density squared. Especially in magnetically confined plasmas, the energy confinement time increases with density. High density operation is also beneficial for the heat load reduction on plasma facing components called the divertor. At high density, the radiation loss is increased and the divertor heat load decreases. The maximum achievable density in a magnetically confined plasma is, however, limited by a density limit that has been expressed by semi empirical scaling, e.g. the Greenwald density limit for tokamaks and the Sudo limit for helical plasmas. The density limit of the tokamak is often accompanied by current disruption which can inflict much damage on the reactor and should be avoided. On the other hand, in helical plasmas, where large plasma current is not needed since the plasma is confined by the magnetic fields generated by external magnet coils, the radiation loss rapidly increases when the edge plasma density approaches the Sudo limit and finally, plasma collapses in a relatively benign manner due to the large radiation loss. Therefore, for the steady state operation of a fusion reactor, understanding of the physics mechanism of the density limit and the radiation collapse is quite important. In this study, the temporal evolution of the radiation collapse in helical plasmas of the Large Helical Device (LHD) is studied by using infrared (IR) imaging Video Bolometers (IRVB). Since the helical plasmas are characterized by a three dimensionally (3D) complicated shape, simple assumptions, such as e.g. axial symmetry, are not applicable and therefore a 3D approach is necessary. This thesis is composed of three parts, i.e., the development of the IRVB, the development of the 3D measurement and the investigation of the radiation collapse. The IRVB has been developed for the measurement of 2D radiation profile patterns. The IRVB provides a distribution of the incident radiation on an IRVB foil. The IRVB has the advantage of having a large number of channels in a 2D array. This advantage is also beneficial for the 3D measurement. For the 3D measurement, in this study, three improvements have been applied to the IRVB. The first improvement is the selection of the material for the IRVB foil. The performance of the IRVB depends on the foil material. In this study the thermal characteristics of foil materials have been evaluated systematically. As the candidates of the foil material, Au, Pt, Ta, W were examined. These were illuminated by a He-Ne laser and their thermal characteristics were evaluated. Among these candidates Pt shows the best characteristics in the simultaneous achievement of high sensitivity and fast time response. According to the test results Pt has been selected. The second improvement is the foil calibration. The IRVB measurement as an absolute measurement requires a knowledge of the distribution on the foil of 3 foil parameters. However because two of the three parameters, foil thickness and emissivity, had not been evaluated, the IRVB measurement mainly served as a qualitative measurement. To evaluate the distribution of these two parameters, a new calibration technique has been developed. The calibration technique evaluates the effective thickness and effective emissivity distribution on the foil with a comparison between the 2D temperature which is measured from the laser illuminated foil, and the calculated 2D temperature using a Finite Element Method (FEM). The evaluated distribution of the effective foil thickness and the effective emissivity made possible for