Spatially resolved soft x-ray diagnostics in fusion energy research

With construction of ITER, the fusion community has progressed into a new stage of research with increased focus on reactor technologies. Corresponding development of diagnostic systems for fusion is required, including research of novel diagnostic methods, validation of radiation hard detectors, and tests of sensors for real-time operation and control, which comprise development of tools for fast data analyses. In parallel, diagnostic systems on running fusion experiments substantially contribute to better understanding of reactor-relevant plasma physics, in particular of energy confinement, plasma stability and transport of impurities. In this respect, spatially resolved Soft X-ray (SXR) diagnostic systems present an interesting case study of development towards reactor-relevant systems. In magnetic confinement fusion research, spatial distribution of SXR radiation with spectral range typically 1 keV - 15 keV is mostly measured by a photosensitive single-row semiconductor elements in a pinhole camera shielded by a beryllium foil. The SXR intensity strongly depends on plasma density, temperature and effective charge, which carry a valuable information on the plasma core physics. Data from SXR diagnostic can be also used for the operation control, among others due to their sensitivity to heavy impurity concentration or to the position of the peak temperature. In order to reconstruct the spatial distribution of SXR plasma emission from the measured line integrated signals, several tomographic methods have been developed and validated. However, the semiconductor elements cannot survive in harsh conditions of future fusion reactors due to radiation damage, which calls for development of radiation hard SXR cameras. In this contribution, role of the SXR diagnostics will be presented in experience and future plans of the Czech tokamak COMPASS (IPP Prague) and the French tokamak TORE SUPRA (CEA Cadarache). In IPP Prague, data from SXR cameras recently contributed to analyses of plasma position and stability and a tomographic code for real-time off-line tomography are available, and a new tomographic and imaging project based on three novel, radiation hard cameras with Gas Electron Multiplier (GEM) detectors has been proposed. Collaboration between the two research centres has started; the link between feasibility study of SXR real-time control on COMPASS and the foreseen real-time impurity monitoring and control in the WEST configuration of TORE SUPRA will be discussed. Acknowledgement. This work was supported by EURATOM and by the Czech MSMT Project LM2011021, and carried out within the framework of the European Fusion Development Agreement. The views and opinions expressed herein do not necessarily reflect those of the European Commission. (author)