Dimensionless pedestal identity plasmas on Alcator C-Mod and JET

Abstract

Experiments on the Alcator C-Mod and JET tokamaks with identical values of non-dimensional variables at the pedestal top are expected to have the same local plasma transport for a ratio >4 in absolute size and thus can help to clarify other effects in pedestal formation. At the high and low fields (7.9 T and 1.4 T, respectively) involved, natural-density H-modes on C-Mod were of conventional ELM-free type, while those on JET were steady but with only small, sporadic ELMs. Nevertheless, they remained close to a common regime and a good non-dimensional match at the edge was achieved for highest C-Mod densities spanned. Pedestal profiles were measured with a fine-resolution edge Thomson scattering diagnostic on C-Mod, plus a new high-resolution (HRTS) system on JET. Electron temperature widths on JET were estimated to lie between scaled C-Mod levels and somewhat broader fitted shapes. Density pedestal widths, however, were reliably found to be proportionally broader than on C-Mod, signalling an influence other than plasma transport in their formation. Edge particle sources were modelled for both devices with the 1D kinetic KN1D code and corroborated for JET using the 2D fluid-plasma/kinetic-gas EDGE2D-NIMBUS suite. Overlaying normalized profiles of ionization rates suggested density pedestal thicknesses were at least partly related to neutral-particle penetration. Such dependence implies their scaling is not Kadomtsev-like for the collisional, low-normalized-pressure conditions investigated, so precluding unambiguous scans versus dimensionless variables like normalized Larmor radius.