Study of particle and impurity fluxes with the filter-camera system on Wendelstein 7-X stellarator

Full text: Wendelstein 7-X stellarator is currently being prepared for the OP1.2 operation phase which is aimed for a first experiments with an island divertor configuration. One of the important diagnostic tools for a divertor region is the endoscope system, which consists of two sets of endoscopes. Once two endoscopes for each set are installed they will serve for a tomography of impurity distribution and measurements of 2D temperature and density profiles at the island divertor region. These measurements are important for a density control and impurity accumulation avoidance. Each endoscope guides light to a detector box where a filter-camera system is located. The initial light beam is split into visible and ultraviolet. In order to record light a CCD cameras are used. In the case of ultraviolet light cameras are equipped with the image intensifier. To select emission lines of interest narrow band interference filters are used. All measurements are supported with an overview spectrometer. Five cameras for visible and ultraviolet range will simultaneously observe divertor with the same field of view. For impurity transport studies, temperature and density determination five nozzles are located in the divertor target plane. One of the possibilities is to use thermal neutral helium injection to determine electron temperature and density. Such a technic was already used for the TEXTOR tokamak. Corresponding collisional-radiative model which includes related physical processes, such as electron excitation and deexcitation, spontaneous emission and recombination processes, collision with heavy particles and other processes is needed. Moreover, related up-todate cross sections are demanded. When two endoscopes are available, it makes it possible two perform tomographic reconstruction of the 2D temperature and density profiles. During the operational phase OP 1.2a only one endoscope will be available and only line integrated results will be obtained. A mock-up observation system with the filter-camera was built for the PSI-2 linear plasma device. Line integrated electron temperature and density profiles were obtained. Due to low values of these parameters available model gives poor agreement with the reference measurements made by a single Langmuir probe. Moreover, high reflectance of the inner part of plasma vessel also complicates the spatial reconstruction process. (author)