Turbulent regimes in the tokamak scrape-off layer

We identify the turbulent regimes in the tokamak scrape-off layer as a function of plasma resistivity, electron to ion mass ratio, safety factor, and magnetic shear. We find that two main instabilities drive turbulence in the SOL: Drift Waves, with their resistive and inertial branches, and the Ballooning instabilities, with their resistive, ideal, and inertial branches. First, we identify how the linear growth rate, and the properties of these instabilities depend on the system physical parameters. Then, according to the gradient removal saturation mechanism, we use the fact that the transport is dominated by the mode with the highest growth rate divided by the poloidal wave number and the non-linear saturated pressure scale length is proportional to this ratio. This allows us to evaluate the non-linear saturated pressure gradient and the poloidal wavenumber of the dominating mode as a function of the resistivity, the mass ratio, the safety factor, and the magnetic shear . We can therefore define the non-linear instability phase space, locating the regions in which each instability is influencing transport the most. In order to validate our calculations, we run non-linear SOL simulations and we compare the plasma pressure scale length and the mode characteristics to the prediction provided by the phase space description. The non-linear simulations are performed using the GBS code, which solves the drift-reduced Braginskii equations evolving self-consistently equilibrium and fluctuations in three-dimensional geometry. The non-linear simulations are interpreted in light of our non-linear analysis and confirm its validity.

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