Structure formation at the edge of hot plasmas

The Young Investigators Group "Non-linear Dynamics and Structure Formation at the Edge of High-temperature Plasmas” will clarify the complex processes at the edge of fusion plasmas.

In this zone the hot inner plasma core, which is completely magnetically confined, moves over into the colder outer scrape-off layer, which comes into contact with the vessel wall. The properties of the plasma edge therefore govern whether the vessel wall can be damaged by overheating or erosion.

In the boundary layer of the plasma little eddies transport heat and particles out into the wall, thus impairing the thermal insulation of the plasma. Non-linear effects and steep temperature and density drops induce, however, flows and burst-like events in the plasma edge, which react on the turbulence. If, for example, there is strong shear flow, a barrier can spontaneously form at the plasma edge, thus impeding heat and particle motion; this enhances the thermal insulation, which is important for the economy of a future fusion power plant. What triggers formation and disruption of this barrier is, however, not yet fully clarified.

 The group is to develop and operate a new measuring apparatus that is to observe, in particular, the plasma edge: A beam of heavy caesium atoms is injected into the plasma edge. On their way through the hot plasma the caesium atoms are ionised. The caesium ions produced are then directed by the magnetic field of the fusion device on curved paths out of the plasma to a scintillator. From the luminescent traces left there it is then possible to determine the plasma and current densities as well as the electric potential at the ionisation site in the plasma.

Testing theoretical models calls for exact measured data, such as the electrostatic potential and the edge current density. A novel heavy-ion probe should be of further help here. The group is to collaborate with the University of Seville to show the feasibility of the new diagnostic and do measurements with the highest possible temporal resolution.