Transport, Impurities and Radiation
Impurities in a fusion plasma can significantly dilute the fusion fuel and are a potential loss channel for the plasma’s energy content due to the emission of line- and continuum radiation. When used appropriately, however, they can also unveil information about their transport properties in the plasma, and they can even help to control interaction between the plasma and the surrounding walls. The investigation and development of plasma scenarios with controllable impurity behavior is therefore an important step on the way to a continuously confined hot fusion plasma.
The development of such a scenario starts with understanding the governing transport processes for the impurities – that is, all plasma constituents that are heavier than the fuel – and the resulting impurity concentration and distribution. For these investigations, tiny amounts of impurities are injected into the plasma, as a particle cloud by means of a laser blow-off system or as small solid-state pellets. While being transported in the plasma, the emitted characteristic line- and continuum radiation is measured as a function of time and, partly, with a limited spatial resolution by spectrometers covering a wide wavelength region. Tomographic systems collecting the energy integrated total radiation of the plasma, including that of the impurities, allow a spatiotemporal reconstruction of the overall radiation dynamics.
The experimentally obtained information about the impurity behavior is then compared with theoretical predictions and simulations. In close collaboration with the Stellarator Theory division, simple one-dimensional and complex three-dimensional codes are employed for this task. The connection of experimental data with simulation results allows for identifying the transport processes that determine the impurities’ behavior. These processes are typically governed by the profiles – or, more precisely, their gradients – of plasma density and temperature which, directly or via the excitation of turbulence, influence the temporal and spatial distribution of impurities.
Finally, analyzing a number of different plasma configurations, a scenario can be developed that largely keeps the impurities out of the confined plasma region, or distributes the impurities in a way that is beneficial for efficient and continuous plasma operation.