THERMAL BIFURCATION OF SCRAPE-OFF LAYER PLASMA AND DIVERTOR DETACHMENT

Models to investigate the main features of plasma-neutral interactions in the recycling region of a tokamak divertor are developed for the two opposite extremes of fluid and Knudsen neutrals. Both neutral models show that a reduction of the heat flux into the hydrogen recycling region below a critical value leads to bifurcation (or rapid change) of the plasma parameters near the target. This bifurcation causes behavior in the scrape-off layer, which is in agreement with the following main features of detached divertor regimes in current tokamak experiments: (i) strong decrease of the plasma temperature near the target, (ii) plasma pressure drop in the recycling region, and (iii) strong decrease of the target heat load and plasma flux onto the target. It is also shown that in the Knudsen limit, the neutral density in the divertor region cannot exceed a maximum density, which is of the order of 1-2×10 13 cm-3 for current experiments. © 1995 American Institute of Physics.