Neutrals modulated intrinsic rotation in tokamaks

The toroidal rotation of a tokamak plasma can stabilize instabilities and is consequently of interest. Present devices are often heated by neutral-beam injectors, which can apply a large toroidal torque, while future reactors will primarily rely on isotropic fusion reactions. Thus, understanding the rotation in low applied-torque scenarios is important. Strong transport-driven scrape-off layer (SOL) plasma flows are present even in low-torque discharges and are felt by charge-exchanged (c.x.) SOL neutrals, which have velocities characteristic of the local ions. Having high cross-field mobility, neutrals can transport this momentum into the pedestal. We extend a recent reformulation of the modulated-transport model [Stoltzfus-Dueck 2012] to include neutrals. The ion distribution function couples to the neutrals via a short c.x. step expansion, treating the neutrals as c.x. dominated. The theory retains key edge features (turbulent transport, drift orbits, ion loss cone, etc.) and allows arbitrary neutral densities. Compared to the intrinsic rotation driven by the interplay of ion drift orbits and inhomogeneous turbulence, the neutrals' effect is modest under realistic conditions. Future work will address the finite c.x. step and its role in the momentum transport of the SOL flows.