Pedestal and core confinement of hybrid scenario in ASDEX Upgrade and DIII-D

Abstract

Pedestal and core confinement of hybrid discharges in ASDEX Upgrade (AUG) and DIII-D are studied in dedicated power scan experiments. The H98(y,2) confinement factor increases with total βN in both tokamaks and it is higher in DIII-D with higher δ plasma shape at a given βN. The pedestal beta, , increases linearly with total beta in AUG hybrid discharges, while it is roughly constant with βN at fixed shape in the DIII-D power scans. The confinement enhancement with power observed with respect to the IPB98(y,2) scaling is due to an increase in pedestal confinement in AUG hybrid discharges and to an increase in core confinement in the DIII-D hybrid power scans. The increase in pedestal pressure with power in AUG hybrid discharges is primarily due to an increase in the width of the edge transport barrier at constant pressure gradient. In the DIII-D discharges the widths of the Te and ne pedestals, and , are consistent with a scaling. In the AUG hybrid power scans a dependence of on βpol,PED cannot be excluded, while shows no dependence on βpol,PED In both machines increases with β. The maximum pedestal pressure achieved in the experiment prior to the onset of type I ELMs is consistent with predictions from ideal MHD; however, a physics model explaining the increase in the pedestal width with β is still missing. The increase in with β in the core of DIII-D is consistent with predictions by linear gyrokinetic simulations. In the plasma core, E × B shearing rate stabilization of the ITG modes is significant in both machines as beta is increased. Inclusion of electromagnetic effects in the gyrokinetic calculations provides additional stabilization at βN values achieved in the experiment. In AUG, proximity to the kinetic ballooning threshold and/or a stronger reduction in normalized ion heat flux with increasing input power are possible explanations for the constancy of at mid-radius as beta is increased.

Confinement of ‘improved H-modes’ in the all-tungsten ASDEX Upgrade with nitrogen seeding

“…The performance improvement with nitrogen seeding is very reproducible, holds for all D 2 fuelling rates under both freshly boronized as well as unboronized conditions, and enhances the energy confinement by up to 25%. lower safety factor q 95 show similar trends [9] as presented in fig. 1.…”

“…This is caused by the reduction of the ITG mode due to deuterium dilution [9]. However, this effect, which would otherwise point to a core confinement improvement in the presence of N, is completely compensated by profile by the introduction of N suggest a flat to slightly hollow nitrogen density profile, which still needs to be confirmed by CXRS nitrogen density profile measurements.…”

“…However, this effect, which would otherwise point to a core confinement improvement in the presence of N, is completely compensated by profile by the introduction of N suggest a flat to slightly hollow nitrogen density profile, which still needs to be confirmed by CXRS nitrogen density profile measurements. With the introduction of nitrogen the change in the measured Z eff profile leads to a pronounced increase of D dilution by around 25% at the edge transport barrier (ρ pol =0.9), while in the plasma core, the D dilution increases only by 5% [9]. Assuming an average charge of 7 for the dominant impurity ion in the unseeded (average of fully stripped oxygen and carbon) and the seeded case (fully stripped nitrogen), one can calculate deuterium density profiles (see fig.…”

“…3). In combination with a diminishing rise of dilution in the core [9] caused by the N 2 puff and the well-established stiffness of T i profiles, a higher core D ion pressure of N seeded IPHMs (i.e. improved confinement) can be understood.…”

Ion temperature profile stiffness: non-linear gyrokinetic simulations and comparison with experiment

“…Non-linear electromagnetic stabilisation mechanisms From linear gyrokinetic analysis, electromagnetic stabilisation of ITG modes has been invoked as a possible factor in improved hybrid scenario confinement at ASDEX Upgrade and DIII-D, particularly at outer radii (i.e. beyond half radius) [61]. The increased non-linear electromagnetic stabilisation reported here may point to an even greater importance of this effect than previously recognised.…”