Non-Maxwellian ion distributions in the ionization zone of the Tokamak boundary

Steady-state ion guiding-centre distributions are computed for the ionization zone of the tokamak boundary by numerically solving the drift-kinetic equation with Coulomb ion-ion and electron-ion collision terms and with sources and sinks due to ionization of and charge exchange with neutrals from the walls. For typical temperatures Tsub(e) = Tsub(i) > or approx. 50 to 100 eV and n0/n > or approx. 2 x 10-4 to 10-3 one obtains marked deviations from Maxwellian distributions that grow with increasing n0/n and Tsub(i)/T0. Both ionization and charge exchange can produce large distortions which possibly cause microinstabilities in the boundary. For Tsub(e) approx.= Tsub(i) the contributions of electron-ion collisions to the ion collision term are in the range of a few per cent. The relaxation time to steady-state exceeds the classical ion-ion collision time by about a factor of 4. (orig.)