EFFECTS OF NEUTRAL GAS COLLISIONS ON THE POWER TRANSMISSION FACTOR AT THE DIVERTOR SHEATH

We show that charge-exchange and other ion-neutral collisions can reduce the power transmission factor of the plasma sheath, thereby lowering the ion impact energy and target plate sputtering. The power transmission factor relates the heat flux reaching the divertor target to the plasma density and temperature just in front of the surface: delta = Q(surf)/J(ew)k T(e). Experimental data from the DIII-D tokamak suggests that delta could be as low as 2-3 near the region of peak divertor particle flux, instead of the 7-8 expected from usual sheath theory. Several effects combine to allow ion-neutral interactions to be important in the divertor plasma sheath. The shallow angle of incidence of the magnetic field (1-3-degrees in DIII-D) leads to the spatial extension of the sheath from approximately rho(i) approximately 1 mm normal to the plate [R. Chodura, Phys. Fluids 25 (1982) 1628] to several centimeters along the field lines. Ionization reduces the sheath potential, and collisions reduce the ion impact energy.