Edge plasma emissivity profile reconstruction by forward modelling of diode bolometer signals at ASDEX Upgrade

Fusion plasmas emit light in a wide range of photon energy. Plasma temperature varies from a few eV to several KeV, leading to a complex radiation pattern. The radiation can be generated by the main fuel itself (D), impurities and electrons.

Considering the whole plasma, one of the most important radiation sources is spectral line emission. All atoms which have still bound electrons emit it. They are mainly excited by collisions with electrons, but also with other ions or neutral particles, or due to the so-called charge exchange collisions. Photons with characteristic wavelengths are emitted as the excited atom relaxes. Plasma in AUG mainly consists of deuterium, but several other elements are present as impurities. These can be light elements (helium to neon, silicon) or heavy elements (iron to tungsten). Elements possibly found are helium from fusion reactions, carbon from sputtering of the torus components, boron from the boronization process (used in AUG to coat the plasma facing components (PFCs) with a layer of boron to improve plasma operations), but also beryllium, neon, oxygen, silicon and argon. Heavy elements originate all from torus components, and are represented mainly from tungsten which covers all the PFCs in AUG.
A small concentration of iron, copper and other elements can also be found. The emission lines of each element, considering all its charge states, depend on the atomic physics, and line emission is connected to the plasma temperature. In a tokamak generally this implies that line emission from light elements concerns mainly the zone with lower temperature (as the edge), while not fully ionized high Z elements can also be found also in the core, causing line radiation in the range of UV and soft-X-ray in this region.
Another significant source of radiation is bremsstrahlung. Its continuum spectrum is emitted by accelerated charged particles. In fusion plasmas, its primary sources are Coulomb collisions, mainly the dominant electron-ion collisions. It depends directly on the local electron temperature and density distribution function, and therefore is most significant in the plasma core, the hottest and densest region of the plasma. […]