Density fluctuations at high density in the ergodic divertor configurationof Tore Supra

The effect of the ergodic divertor on the plasma edge in Tore Supra is to e nhance the perpendicular transport through ergodization of the magnetic fie ld lines [Ph. Ghendrih et al., Contrib, Plasma Phys. 32 (3&4) (1992) 179], Nevertheless? the hot spots observed on the divertor plates during ergodic divertor operation indicate that the cross-field transport driven by the fl uctuations is still playing an important role, although measurements by CO2 laser scattering and reflectometry show a decrease of the turbulence level [J. Payanl X. Garbet, J.H. Chatenet et al., Nucl. Fusion 35 (1995) 1357; P . Beyer. X. Garbet, P. Ghendrih, Phys. Plasmas 5(12)(1998)4271]. In order t o gain more understanding, fluctuation level and poloidal velocity have bee n measured with a reciprocating Langmuir probe biased to collect the ion sa turation current (j(sat)) and with a CO2 laser scattering diagnostic. Thoug h the relative fluctuation level behaves as previously observed at low dens ity, a new interesting result is that this picture is gradually modified wh en the density is increased. Both diagnostics observe an increase of deltan /n with density in the ergodic region, which is not the usual behavior obse rved in limiter configuration. This increase is detected on both sides of t he E-r inversion radius and is therefore also affecting the plasma bulk. Fi nally, the confinement time is found to follow an L-mode law at all densiti es indicating that the ergodic divertor does not change the global confinem ent properties of the plasma, (C) 2001 Elsevier Science B.V. All rights res erved.