Tore Supra divertor screening efficiency during density regime experiments

The Tore Supra ergodic divertor (ED) screening efficiency has been investigated in density regime experiments. The ED screening efficiency is analysed by using the `tightnessʹ concept, which is the ratio of the density on the ED neutraliser plates to the volume averaged plasma density. Tightness is studied as a function of different plasma edge parameters, such as Tdiv, ED magnetic perturbation (Δ), plasma composition, location of recycling source, and additional power. Tightness is shown to increase with Δ, Pdiv0.55/(1−Fr)1.22, and 1/Tdiv0.5. These trends are well explained by a simple 0-D model, where the particle confinement time in the ergodized peripheral region is very small. Finally, tightness increases with the power conducted onto the ED plates. Since ED plasmas have low Pdiv, their tightness value remains low compared to that obtained with axisymmetric divertors for which Pdiv is considerably larger. Increasing Pdiv will result in an improved tightness and a better particle control.