Resolving the diffusive and convective contributions to main ion particle transport during H-mode pedestal formation on Alcator C-Mod

This work presents the first application of the diffusive-convective ansatz to main ion particle transport in the pedestal on Alcator C-Mod. While traditional modelling assumes purely diffusive transport in the edge, spatially resolved Ly-α emissivity measurements on Alcator C-Mod have enabled time-dependent particle fluxes, and therefore convective contributions to the particle transport, to be calculated. Taking four series of L-H transition discharges at fixed B_T (5.5T), two different input power values (2MW/2.5MW of ICRF heating) and three different plasma currents (0.65MA/0.8MA/1MA), outboard Ly-α measurements have been combined with n_e and T_e measurements from Edge Thomson Scattering (ETS) to calculate diffusion and convection coefficients in the pedestal. Values of ~0.01m² /s for the diffusion coefficient and ~5m/s for the convection coefficient in the steep-gradient region of the pedestal have been estimated in the 20ms-50ms time window following the L-H transition. These results are part of a wider effort to understand the respective roles played by particle transport and neutral fuelling in the formation of the density pedestal and improve predictive capabilities for burning plasmas.