Assessing ion-electron thermal equilibration in the scrape-off layer of tokamaks using UEDGE

The ion-electron thermal equilibration along the scrape-off layer (SOL) has been assessed using an analytical, one-dimensional (1D) model; a single-flux tube model assessed with the multi-fluid code UEDGE, and an orthogonal, two-dimensional (2D) slab model in UEDGE for pure deuterium plasmas. These studies highlight the importance of thermal equipartition, radial transport, and loss processes in ion-electron thermal equilibration in the SOL and demonstrates the strong dependence of equipartition on local plasma parameters. For the 2D UEDGE cases, equipartition is predicted to cool the ions by up to a factor of 2 at the upstream location and to heat the electrons by up to a factor of 3, predominantly in front of the target. Diffusive radial transport into the private flux region and far SOL is predicted to reduce the upstream ion temperatures by up to 30% compared to the 1D UEDGE predictions, and volumetric power and momentum sinks (charge-exchange, ionization, recombination, and hydrogen radiation) reduces the target ion and electron temperatures by up to one order of magnitude compared to a 1D model considering conduction and equipartition only. The losses and radial transport are, however, insufficient to achieve T_i ≈ T_e in the absence of thermal equipartition due to losses affecting the ions and electrons asymmetrically. UEDGE simulations on a 2D slab predicts radially varying T_i/T_e ratios due to radial transport, which is not considered in any 0D or 1D models of the SOL.