Edge transport barrier characteristics of improved H-modes in ASDEX Upgrade

The improved H-mode is a candidate improved confinement scenario for ITER. It is characterised by a flat q profile with q ! 1, no or small sawteeth, 1 < H98(y,2) < 1.5, 2.5 ! ∀N ! 3 (limited by the occurrence of 2,1 NTM's) and is achieved over a broad range of #*. Recently much interest is being shown in the study of the edge pedestal in improved confinement scenarios, since there are indications that part of the confinement improvement indeed originates from the edge transport barrier (ETB) region (1,2). In order to investigate the structure of the H-mode pedestal in ASDEX Upgrade (AUG), experiments have been carried out in improved H-modes discharges in which the power was increased stepwise during the pulse. All discharges were carried out at 1 MA, 2.3 T and with the same magnetic configuration (LSN, ∃~0.25, %~1.8, q95~4.7). Discharges with different wall conditions are compared as are discharges with different timing of the additional heating. Edge electron profiles were obtained combining high-resolution measurements from the Thomson scattering system and the ECE heterodyne diagnostic. The edge profiles are then fit by a modified hyperbolic tangent function (3). As the power is increased, it is observed that: i) the width of the density barrier stays roughly constant or perhaps decreases (Fig. 1); ii) the temperature barrier broadens with power (Fig. 2); iii) the width of the electron density barrier, &ne, is narrower than that of the electron temperature: &ne~ 1 cm and 2 < &Te < 3 cm; iv) the pedestal-top density tends to increase with power (in the absence of gas fuelling), due to a combination of steepening of the density gradient in the ETB and of increasing density in the scrape off layer, which raises the base level of the density barrier (Fig. 3); and, as a result, v) the pressure at the pedestal top pePED = nePED*TePED increases with power. The observation that the density ETB is narrower than the temperature ETB is in contrast to previous analysis on (higher density) conventional H-modes (4). The present data suggest that fuelling effects may be playing a more important role in determining the edge density profile, as is observed in DIII-D (5). Detailed modelling of the kind reported in