The impact of separatrix conditions on pedestal-SOL coupling: an investigation with integrated transport solvers

The primary goal of this thesis is to connect the pedestal, the steep gradient region at the edge of the confined plasma defined by its height and width, with the scrape-off layer. The scrape-off layer is a region of unconfined plasma adjacent to the pedestal that connects the main plasma to the material targets, known as the divertor. The pedestal and scrape off layer are intimately linked regions separated by a surface known as the separatrix, which denotes the transition from confined to unconfined plasma. Understanding how these two regions interact is key for the economic viability and net power production of a tokamak power plant. Within this thesis, we first address reduced models for the electron density pedestal (𝑛𝑒,𝑝ed) and the electron separatrix temperature (𝑇𝑇eu,sep ) to enable integrated modelling simulations to converge on reasonable timescales (minutes/hours rather than days/weeks). Secondly, we utilise integrated modelling tools which use ideal MHD to limit the pedestal evolution to consider the effect of the electron separatrix temperature and density on the electron pedestal pressure, similar to the IMEP model (T. Luda et al. Nuclear Fusion, 2020, 036023). Considering a reduced model for 𝑇𝑇eu,sep , we demonstrate the importance of measuring the power decay width (𝜆𝑞) in impurity seeded and unseeded plasmas to achieve agreement between the scrape-off layer edge transport fluid code - EDGE2D-EIRENE and the analytic two-point model (TPM). These results elucidate the neutral fuelling and pedestal stability as this work considered how neutrals affect 𝑇𝑇eu,sep , which in turn can effect the pedestal stability. A reduced analytic model for the electron density pedestal, the neutral penetration model (NPM) described in reference (R.J. Groebner et al. Plasma Physics and Controlled Fusion, 2002, A265), was tested. A key prediction of the neutral penetration model is that the electron pedestal density is proportional to one over the electron pedestal density width. This relationship was confirmed in the simulations only if the electron density profile was measured at the same poloidal position as the maximum neutral source. However, the predicted scaling was not observed in the measurements performed at the low field side midplane. Combining SOL and core physics using the integrated modelling code JETTO-FRANTIC-MISHKA, which predicts the confined plasma fueling due to hydrogenic atoms, the pedestal width and height, and evolving separatrix boundary conditions (as predicted by EDGE2D-EIRENE), I find that the simulations could not reproduce the degradation of the electron pressure pedestal with electron separatrix density as is observed experimentally in JET. This discrepancy is likely due to resistive MHD effects in the pedestal.