Stage 2019 : Transition between detached and ionizing phases of the plasma

TITRE: Topology effect and stability of the transition between detached and ionizing phases of the plasma.

Le sujet de stage concerne l’étude des régimes en densité dans le tokamak WEST, notamment le régime détaché en comparant les résultats expérimentaux avec les simulations SOLEDGE.

The confinement of the plasmas in tokamaks must make it possible to reconcile the plasmas in the thermonuclear combustion regime in the core region of the device and the conditions of the peripheral plasma allowing the plasma-wall interaction in the stationary regime by minimizing the aging of the plasma-facing components.
In the divertor, the region dedicated to the plasma-wall interaction, the conditions must ensure the transitions of a radiation regime of light impurities in an ionizing plasma, therefore with a very small neutral atom population, to a state detached with a strong coupling between the neutrals and the plasma and finally to a state of recombinant plasma dominated by the neutral atoms. Fronts separate these different phases of the plasma and the different regimes realized are characterized by the successive detachment of these fronts from the wall. In volume the different states coexist and the dynamics of the fronts is crucial to maintain these regions of transitions in the divertor.
In the simplest approach, only the dynamics parallel to the magnetic field is retained. The effects of geometry intervene then in the localization of these fronts and their stability. This problem has strong analogies with the physics of flame fronts. However, the most recent results from our group show that rapid changes in thermodynamic properties along field lines generate large scale flows in competition with parallel flows to field lines. They can then lead to changes in the topology of the flows and thus in the spatial organization of the interfaces between the different states of the plasma.
In contrast with this theoretical vision of the physics expected in the divertors, the modeling efforts did not allow to find stable and efficient regimes putting in synergy these states of the plasma. The use of our knowledge on large-scale flows, and the nonlinear interaction with the spatial organization of the different states of the plasma must allow to overcome this issue. Indeed, a self-regulated mechanism of plasma charge loss, in particular by amplifying the momentum exchanges via turbulence, would have generic properties that are not very dependent on the neutral atoms and consequently the geometry of the plasma-facing components and the localization of the ionization front.
In this context, the experiments in WEST, where the geometry of the neutral-component interaction facing the plasma will be original, will allow to develop a deep experimental knowledge, approached with the support of the new generation of simulation tools developed by the team, and to control the role of neutral atoms in the physics of detachment.
The proposed internship work is focused on experimental study on WEST with a strong involvement of modeling.