Theory framework for scrape-off layer turbulence in limited tokamaks

In the present work we address the physical mechanisms determining the SOL width of a simple inner-wall limited (IWL) configuration. This seemingly simple configuration has important implications for ITER start-up plasmas, and has triggered an important ITPA-sponsored effort. In the past few years, we have developed a thorough computational and analytical understanding of the physical processes regulating the IWL-SOL width. Our investigations are aided by 3D global, flux driven simulations of SOL turbulence carried out with the Global Braginskii Solver (GBS), a numerical implementation of the electromagnetic, drift-reduced Braginskii fluid model. GBS is capable of carrying out massively parallel simulations of SOL plasma dynamics, involving plasma profile formation in the SOL as a power balance between plasma flux from the core, the turbulent radial transport, and the losses at the plasma sheath where the magnetic field lines intersect with the vessel. Recently, GBS has been subject to a rigorous verification procedure using the manufactured solutions method, which unequivocally demonstrated the correct numerical implementation of the model equations. An extensive simulation scan has revealed the instabilities driving turbulent transport, the mechanisms that lead to turbulent saturation, the role of ion temperature fluctuations, aspect ratio effects, and the role of electromagnetic flutter, leading to an extensive framework describing the turbulent properties of the system. Moreover, we have addressed for the first time the plasma size scaling of the SOL width by means of a dedicated simulation scan, which demonstrated a widening of the SOL as plasma size increases. The non-linear dynamics revealed by the simulations are in excellent agreement with reduced analytical models, allowing for the development of a SOL width scaling that has been compared against experimental data from IWL discharges from several tokamaks, showing good agreement.