Speaker
Description
Fusion reactors will operate with a strong scrape-off layer (SOL)/edge plasma-neutral interactions to benefit from the energy and momentum losses due to atomic reactions to protect the divertor walls, the so-called (semi) detached plasma. While access and basic properties of such regimes are well known, a complete and detailed understanding of the tokamak boundary of detached plasmas is absent. Progress has been made with mean-field simulations, however, this approach does not model self-consistently the cross-field transport, known to be dominated by turbulence and strongly dependent on the divertor regime as shown by experimental evidence. A first-principles understanding of the underlying physics of those plasmas is imperative to clearly define operational limits of future machines.
In this contribution, we discuss the results of the turbulence modeling of the TCV-X23 reference case, a lower single-null plasma at Btor=0.95T with an elongated outer divertor leg in attached and semi-detached conditions and portraying an extensive dataset including average and higher-order statistical moments of several observables. In particular, the filament dynamics and the plasma-neutral reactions are assessed by Gas Puffing Imaging (GPI) and spectroscopy, offering an ideal scenario for the validation of edge turbulence simulations. The modelling is carried out with SOLEDGE3X, a multispecies, electromagnetic, drift-reduced Braginskii code depicting the kinetic neutral dynamics via coupling with EIRENE, a feature recently available for turbulence simulations.
The reported simulations cover two different density levels in the reversed field direction and are rigorously validated against the experimental dataset containing average and fluctuations observables distributed across the entire tokamak boundary. Differently from the previous SOLEDGE3X simulations, where a simplified fluid neutral model including only the density equation was adopted, the current modelling includes all the atomic and molecular reaction terms for the momentum and energy equations in a kinetic description. The new physical model leads a divertor not fully detached in the high-density case, more in line with the experiments and demonstrating the improvement of code predictive capabilities. In high density conditions, a wider SOL and a larger heat flux spreading is found due to enhanced turbulent transport as consequence of the large collisionality. In particular, the dynamics of divertor-localized and upstream-connect filaments are investigated and new insights regarding the motion and associated transport are presented and confronted with GPI data of the divertor region.