Experimental evidence of the location of the ergodic divertor separatrix is
shown to agree with the predicted value given by codes. Variation of this
position modifies the divertor tightness, defined as the ratio of the diver
tor to core density. This effect is governed by laminar transport, i.e., tr
ansport proportional to the magnitude of the perturbation. Operation with f
eedback control of the divertor temperature allows one to optimise the choi
ce of injected impurity species. At 10 eV divertor temperature, nitrogen is
shown to lead to the largest decrease in energy flux to the divertor at lo
west contribution to Z(eff) Parallel energy fluxes as low as 2 MW m(-2) are
thus achieved on the target plates. For this impurity, radiation is locali
sed in the divertor volume thus leading to radiation compression close to 1
0. The ergodic divertor appears as a powerful tool to control plasma-wall i
nteraction with no loss of core confinement or plasma current. (C) 2001 Els
evier Science B.V. All rights reserved.