Neutral atom dynamics and plasma turbulence in the tokamak periphery

Understanding the physical mechanisms at play in the interaction between turbulent plasma and neutral particles in the tokamak periphery is a crucial issue that we approach by using a first-principles self-consistent model that is implemented in the GBS code. While the plasma is modeled by the drift-reduced two-fluid Braginskii equations, a kinetic model for the neutrals is used, valid in short and in long mean free path scenarios. The model includes ionization, charge-exchange, recombination, and elastic collisional processes. The neutral kinetic equation is solved by using the method of characteristics.

By studying the dynamics of the neutral-plasma interplay along the magnetic field lines in the SOL of a limited tokamak, we derive a refined two-point model from the drift-reduced Braginskii equations that balances the parallel and perpendicular transport of plasma and heat, and takes into account the plasma-neutral interaction. The model estimates the electron temperature drop along a field line, from a region far from the limiter to the limiter plates. The refined two-point model is shown to be in very good agreement with the simulation results.

Furthermore, we use the model to self-consistently simulate a diagnostic neutral gas puff, which is often used experimentally as a tool to learn about the turbulence properties in the tokamak periphery. In particular, we investigate the impact of neutral density fluctuations on the D-alpha light emission, finding that at a radial distance from the gas puff smaller than the neutral mean free path, neutral density fluctuations are anti-correlated with plasma density, electron temperature, and D-alpha fluctuations, while at distances from the gas puff larger than the neutral mean free path, a non-local shadowing effect influences the neutrals, and the D-alpha fluctuations are correlated with the neutral density fluctuations.