In tokamak plasmas, the quality of the energy confinement – and ultimately of the fusion performance in terms of power amplification – correlates negatively with higher plasma density at the separatrix. The separatrix is the last closed magnetic surface separating the core confined plasma with closed magnetic field lines from the so-called Scrape-Off Layer (SOL) where open field lines connect to the divertor plates or to the wall of the tokamak vessel.
The plasma density at the separatrix results from the balance between sources and transport. The transport is governed by micro turbulence which inherently develops in tokamak plasmas which are out of thermodynamical equilibrium. The source is dominated by neutrals which penetrate and ionize inside the separatrix. On the one hand, the neutral penetration is expected to be reduced in ITER and larger machines. On the other hand, in order to minimize the heat flux conducted on the divertor plates, such machines are required to operate at large density at the separatrix. It is therefore important to understand the causality chain at play behind the confinement degradation with higher density at the separatrix observed in today's tokamaks.
To explore this interplay, a reduced model for neutral penetration will be tested against WEST density profile database as well as against higher fidelity Monte Carlo simulations. Neutral particle source and turbulent transport will be integrated thanks to a plateform of codes used on all european tokamaks as well as by the ITER team to prepare ITER operation. The integrated modelling will be compared to experiments in view of identifying the causality chain at play behind the confinement degradation and the higher density at the separatrix. At first on WEST, then the causality found will be tested on the German tokamak AUG. Experiments will be proposed on WEST to maximize performances at high density at the separatrix. And ITER scanerios will be explored with the same objective exploring the impact on performances of a larger opacity to neutrals.