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Abstract. The presence of neutral atoms in the tokamak edge affects the radial electric field and plasma flow velocity through charge exchange interactions. The effect of the neutral atoms is more important than neoclassical processes if the fraction of atoms in the plasma exceeds about 10- 4, as is typical in the plasma edge region, just inside the separatrix. In the present work, it is shown that these effects depend sensitively on the poloidal location of the atoms, especially in a spherical tokamak. It is found that the radial electric field and toroidal flow velocity in a collisional edge plasma tend to be larger if the atoms are concentrated on the inboard side rather than on the outboard side. This effect may offer an explanation for recent observations on MAST and COMPASS indicating easier H-mode access when gas is puffed from the inboard side of the tokamak.
IAEA 2003
