Highlights 2015

Experimental validation of a filament transport model in turbulent magnetized plasmas

Elongated structures also called “filaments” or “blobs” are known to be the dominant mechanism of transport between the tokamak confined plasma and the main wall, the region known as Scrape-Off Layer (SOL). In many tokamaks, as the plasma density increases, a transition to enhanced perpendicular transport and the development of a “density shoulder” in the SOL are observed leading to enhanced interaction of the plasma with the wall. Hence, understanding this phenomenon is needed to predict the particle and heat fluxes to plasma facing components in a future fusion reactor.

Analytical models predict a change of propagation regime as the electron-ion collision frequency increases, leading to larger and faster filaments and to substantially higher transport. By analyzing Langmuir probe data from the ASDEX Upgrade and JET tokamaks, the formation of larger blobs and the development of a density shoulder in the SOL could now be related to the plasma conditions in the divertor summarized in an effective collisionality Λ_div. The physical model describes this transition as a switch in the filament dynamics from a typical sheath limited regime to an inertial regime in which the filament becomes disconnected from the divertor plates and both size and velocity of filaments can increase. The figure depicts this transition in terms of the density fall-off length λ_n in the SOL which substantially increases when Λ_div increases above one reflecting the formation of the density shoulder. This work was now accepted for publication (D. Carralero et al, Phys. Rev. Letter 2015).