Speaker
Dr
Alex Chankin
(Max-Placnk-Institute for Plasma Physics)
Description
Scrape-off layer (SOL) and divertor make a direct impact on the outer core region via neutral penetration and ionization. Experiments on several tokamaks show however that even when neutrals’ ionization doesn’t significantly alter plasma parameter profiles in the core, changing divertor geometry can still make a strong impact on the H-mode power threshold, PLH, (factor 4 variation in the case of JET [1]), suggesting that there may be other, more subtle mechanisms for the influence of the SOL and divertor on the core plasma.
EDGE2D-EIRENE simulations of JET plasmas revealed that the near SOL radial electric field, Er, is the parameter most affected by the divertor geometry change, suggesting that the strong local ExB shear may suppress turbulence [1,2]. It may serve as an additional mechanism for the confinement improvement, with the main one located in the outer core (2D edge fluid code simulations show the largest drop in transport coefficients occurring inside of the separatrix). Further code modelling of JET plasmas by changing hydrogen isotopes (H-D-T), toroidal field direction, and by replacing carbon with the ITER-like wall, showed that the magnitude of the localized near SOL Er anti-correlates with experimentally observed PLH variations [2]. The isotope effect on the near SOL Er, in particular, was due to the increase in the convective power flux through the separatrix for lighter neutrals, which, for the given total input power, reduced electron temperature, Te, at the separatrix, causing a strongly amplified Te reduction at the strike point on the divertor target.
Code simulations of isotope exchange (D vs. H) experiments in JET showed that a large reduction of anomalous diffusion coefficients, D_purp, in the outer core and SOL is necessary to match experimental density profiles [3]. This applies to both L- and H-mode plasmas, and it is the most obvious explanation for poorer particle confinement in H. The exact mechanism responsible for poorer particle (and energy) confinement in H is unknown. If SOL physics was to contribute to this, it would likely be via the influence of Er on turbulence suppression. Earlier code modelling didn’t contradict this, as it showed progressively larger near SOL Er in the H-D-T series of runs (see above). In that modelling, however, D_purp was assumed the same for all isotopes. Recent modelling with the reduction of D_purp, from H to T cases, showed that a simultaneous reduction in electron and ion perpendicular heat conduction coefficients, Chi_e,i, is required to achieve an amplification of the isotope influence on the near SOL Er. Whether a positive feedback loop exists, with larger SOL Er in heavier isotope plasmas causing turbulence reduction and a drop in transport coefficients, leading to sharper Te gradients and an increasing local Er, can only be established in further modelling to be carried out soon.
Primary author
Dr
Alex Chankin
(Max-Placnk-Institute for Plasma Physics)
