Localized Gas Puffing Control of Edge Rotation and Electric Field

Neutrals at the edge of tokamaks can influence confinement by altering the radial electric field and toroidal plasma flow velocity through charge exchange and ionization interactions when the high diffusivity of the neutrals results in neutral viscosity dominating over ion neoclassical and turbulent viscosities [1-5]. We find that the flow and electric field are very sensitive to the poloidal location of the neutrals and the ion collisionality [6,7]. These predictions are consistent with differences observed on MAST between measured toroidal flows for inboard and outboard gas puffing, and may also explain observations indicating that inboard puffing allows easier H mode access [8]. In particular, we find that the inward directed radial electric field and counter-current directed outboard toroidal flow velocity in a collisional or Pfirsch-Schluter edge plasma tend to be larger if the atoms are concentrated on the inboard side rather than on the outboard side [6,7] consistent with the MAST results and suggesting that the flow shear introduced by the limited penetration of the neutrals and the radial variation of the ion temperature gradient suppresses edge turbulence and plays a role in forming the edge transport barrier. The ion temperature gradient terms enter because ion heat flow modifies the neutral viscosity via charge exchange. The results are found to be relatively insensitive to neutral collisionality [4], which tends to cause up to order unity changes in amplitudes but not changes in direction. More importantly, the results are found to be sensitive to plasma collisionality [7] because of the important role played by ion temperature gradient terms as in standard neoclassical theory. In the case of banana regime ions and collisional neutrals [7], inboard puffing reduces the counter-current rotation, while the radial electric field always remains inward. Consequently, if there is a narrow collisional layer separating the separatrix from the pedestal as in Alcator C-Mod, then this effect will further shear the flow. However, for banana regime ions the largest outboard toroidal flow is obtained for outboard puffing and is in the counter-current direction suggesting that outboard puffing might be more effective in suppressing edge turbulence and generating an edge transport barrier in tokamaks which remain in the banana regime all the way to the separatrix (assuming no other source of flow shear). Finally, impurities affect the size of the flow and electric field; increasing the effect in the Pfirsch-Schluter regime and decreasing it when the ions are in the banana regime [7]. These results suggest that flexible neutral fueling is desirable since it may allow some external control over the tokamak edge and overall performance. In spite of any obvious source of momentum input, axisymmetric tokamak plasmas rotate toroidally. One mechanism causing the edge plasma to rotate can be summarized as follows. The magnetic field introduces a preferred direction. It breaks toroidal symmetry by requiring a Pfirsch-Schluter heat flux with a preferred toroidal direction in order to satisfy the energy balance equation. The heat flow is due to the combined poloidal and toroidal magnetic field there is no momentum input from magnetic fields. Because of charge exchange the neutral viscosity contains ion heat flow terms as well as the standard ion particle flow terms. These heat viscosity terms enable neutral atoms to transfer angular momentum between otherwise stationary flux surfaces. If the plasma did not interact with a wall there would be no net momentum transfer because of angular momentum. However, the plasma and neutrals do exchange momentum with the vessel wall, by ion losses and recycling, so