Separatrix ion to electron temperature ratio in the TCV and ASDEX Upgrade tokamaks

Increased focus has been recently given to the ion temperature Ti near the separatrix and in the scrape-off layer (SOL) due to its impact on plasma confinement, wall sputtering, heat fluxes, and edge turbulence. In current tokamaks, high pedestal collisionality often clamps pedestal top ion and electron temperatures together. However, thermal decoupling occurs in the SOL due to the different ion and electron masses, leading to T i / T e > 1 at the separatrix and in the SOL. Despite consistently observing elevated Ti relative to Te near the separatrix in various tokamaks, a comprehensive study remains missing due to the measurement complexity. This study, for the first time, explores the primary dependencies of T i / T e at the separatrix, focusing on the variation of electron density and connection length to scan SOL collisionalities, and the variation of input power to scan ion and electron heat fluxes, in the ASDEX Upgrade and TCV tokamaks. Surprisingly, T i / T e shows weak or no dependence on electron density and connection length, despite their expected role in the SOL coupling. However, the T i / T e ratio is strongly influenced by ion and electron heating levels in the plasma core. This suggests that the upstream T i / T e is primarily regulated by relative ion and electron heat fluxes from the core, with coupling playing a minor role.