Particle collection by the ergodic divertor of Tore Supra: high recycling and partially detached plasmas

The detached plasma regime is envisaged as an operating condition for next step tokamaks. In this regime, the edge incident ion flux decreases dramatically and goes down to nearly zero on the wall. A proposed method of pumping is to capture backscattered neutrals due to atomic processes, namely FC (Franck Condon) dissociation and CX (charge exchange), with a dedicated vented structure. This solution has been tested with the Ergodic Divertor (ED) of Tore Supra. In this configuration, the field lines connect to neutralizers located between the divertor current bars (7 neutralizers per module - 6 identical modules distributed toroidally around the vacuum chamber). These neutralizers are vented structures which are semi-transparent to neutrals and can be used for particle pumping. A comprehensive set of diagnostics has been installed: 14 Langmuir probes poloidally and toroidally distributed, pressure gauges in the modules plenum and Dα measurements along the neutralizer located in the equatorial plane. In the present study, we analyse transient experiments performed for density scan studies and concentrate on high recycling and partially detached plasmas. For known density and temperature profiles at the edge, two quantities are sufficient to describe the neutral recirculation and particle balance: the pressure in the ED plenum (which characterizes the particle collection) and the distribution of the Dα emission line in front of the neutralizers (which characterizes the ion source due to recycling). Their behaviour during the high recycling and semi-detached phases is described in the next sections. Scaling laws of the edge parameters and pressure in the pumping chamber with volume-averaged density and total power are given in the last section. (authors)