Plasma Near a Divertor Plate: Effects of Surface Roughness and Particle Drifts

Abstract

The surface of a divertor plate has usually a certain degree of ''roughness.'' Depending on the material and the exposure time, the size of surface features may range from submicrons to a fraction of a millimeter, covering a range of spatial scales from well below the electron gyroradius, {rho}{sub e} to significantly above the ion gyroradius, {rho}{sub i}. The plasma approaches the divertor plate along a magnetic field which forms a shallow angle, {alpha} << 1, with the plate surface. Under such circumstances, a significant ''shadowing'' effect takes place, with only a small part of the surface being accessible to the plasma particles. A methodology is presented that allows one to find the fraction ({var_epsilon}{sub e} and {var_epsilon}{sub i}) of the surface geometrically accessible for the electrons and the ions. At small {alpha}, {var_epsilon}{sub e,i} are typically small, meaning a strong local enhancement of heat and particle fluxes. In a broad range of parameters, {var_epsilon}{sub e}, is also much smaller than {var_epsilon}{sub i}. As the surface features are usually greater than the Debye radius, this leads to the formation of an ambipolar potential which causes reflection of part of the ions from the surface. The resulting albedo of the divertor plate for the plasma ions can be as high as 50%. Gradual diffusion of the plasma electrons into the zones reflecting the ions, reduces ambipolar fields and brings the ion albedo back to very small values. We present estimates of the time scales governing the neutralization process. This time scale shows itself up in the sheath boundary conditions for non-steady-state perturbations. The effect of surface roughness on secondary electron emission is discussed and it is shown that, depending on the surface structure, it may be smaller or greater than for a perfectly flat plate. We consider modifications of the sheath current-voltage characteristics by particle drifts. The presence of reflected ions reduces the ion diamagnetic current in the ion sub-sheath and significantly changes the ion response. This, in turn, affects sheath-controlled instabilities, which are sensitive to the tilt of the magnetic field.