• Energy Research

ASDEX Upgrade is preparing a hardware modification of its upper divertor in order to study alternative divertor configurations, like the X-divertor (XD) and the snowflake divertor (SF), that are discussed as a possible solution for the power exhaust problem. Experiments characterizing the current upper divertor in single-null (SN) configuration before and during detachment were carried out recently and interpreted by EMC3-EIRENE (Lunt et al., 2020) now including volumetric recombination. In continuation of these studies we here report on further simulations with this code extrapolating these conditions to the future upper divertor. For the same transport parameters, input power and upstream separatrix density for which the outer target (OT) of the SN is attached, the XD and SF show significant detachment at the OT accompanied by a reduction of the parallel heat flux by a factor of more than five. Despite the shallow field line incidence angles in the XD configuration the intrinsic 3D error fields from the current feeds only cause toroidal variations of the power fluxes of the order of 10%. With a hypothetical misalignment of the divertor coils by 3 cm substantial asymmetries in the power deposition profile are found, however, even those vanish when going to detached plasma conditions. In order to fully detach the plasma at the OT in the SF configuration impurities, here in the form of nitrogen, need to be puffed directly into the region of the secondary X-point in the simulation.

ASDEX Upgrade is preparing a hardware modification of its upper divertor in order to study alternative divertor configurations, like the X-divertor (XD) and the snowflake divertor (SF), that are discussed as a possible solution for the power exhaust problem. Experiments characterizing the current upper divertor in single-null (SN) configuration before and during detachment were carried out recently and interpreted by EMC3-EIRENE (Lunt et al., 2020) now including volumetric recombination. In continuation of these studies we here report on further simulations with this code extrapolating these conditions to the future upper divertor. For the same transport parameters, input power and upstream separatrix density for which the outer target (OT) of the SN is attached, the XD and SF show significant detachment at the OT accompanied by a reduction of the parallel heat flux by a factor of more than five. Despite the shallow field line incidence angles in the XD configuration the intrinsic 3D error fields from the current feeds only cause toroidal variations of the power fluxes of the order of 10%. With a hypothetical misalignment of the divertor coils by 3 cm substantial asymmetries in the power deposition profile are found, however, even those vanish when going to detached plasma conditions. In order to fully detach the plasma at the OT in the SF configuration impurities, here in the form of nitrogen, need to be puffed directly into the region of the secondary X-point in the simulation.

The separatrix electron density is an important parameter for core-edge scenario integration in tokamak devices, as it influences plasma confinement, divertor detachment and disruption avoidance. This quantity has been measured in H-mode discharges on JET, ASDEX Upgrade and Alcator C-Mod by applying the same fitting function to Thomson scattering measurements, and by employing the same analysis technique based on scrape-off layer power balance. To estimate the power crossing the separatrix, the inter-ELM time derivative of the plasma energy dW/dt has been experimentally evaluated and found to be approximately a constant fraction of the absorbed heating power. Correlations between ne,sep and engineering parameters have been investigated, revealing that ne,sep scales with the divertor neutral pressure p0,div in a similar manner across all devices. Additionally, when ne,sep is normalized to the obtained p0,div dependency, no clear correlation with the plasma current is found. These observations are in agreement with the 2-point model, which suggests that the upstream separatrix density is mainly set by the recycling at the divertor target.

The separatrix electron density is an important parameter for core-edge scenario integration in tokamak devices, as it influences plasma confinement, divertor detachment and disruption avoidance. This quantity has been measured in H-mode discharges on JET, ASDEX Upgrade and Alcator C-Mod by applying the same fitting function to Thomson scattering measurements, and by employing the same analysis technique based on scrape-off layer power balance. To estimate the power crossing the separatrix, the inter-ELM time derivative of the plasma energy dW/dt has been experimentally evaluated and found to be approximately a constant fraction of the absorbed heating power. Correlations between ne,sep and engineering parameters have been investigated, revealing that ne,sep scales with the divertor neutral pressure p0,div in a similar manner across all devices. Additionally, when ne,sep is normalized to the obtained p0,div dependency, no clear correlation with the plasma current is found. These observations are in agreement with the 2-point model, which suggests that the upstream separatrix density is mainly set by the recycling at the divertor target.

Pairs of ASDEX Upgrade L-mode discharges with the toroidal magnetic field, BT, in the forward and reverse directions have been used to study the impact of neoclassical drifts on the divertor plasma conditions and detachment. The evolution of the peak heat flux and the total power loads onto both the outer and the inner targets depends significantly on the toroidal field direction: increasing the core plasma density affects mainly the heat loads in the BT  0 (favourable). Ion saturation current measurements show similar trends to those of the IR heat flux data. These discrepancies are not only caused by drifts but also by different levels of radiated power in the core, thus the power across the separatrix, Psep. Tomographic reconstructions show that Psep is not constant within the entire dataset. Finally, at Ip=0.8MA, a significant reduction of the peak heat flux is observed at both targets for both field directions. On the other hand, at Ip=0.6MA, a reduction of the peak heat flux is only observed for BT < 0 at the outer target. Additionally, the onset of particle detachment is only observed at the outer target for BT < 0 with Ip=0.8MA. Keywords: ASDEX Upgrade, Upper single null, Scrape-off layer, Divertor detachment, Drifts

Pairs of ASDEX Upgrade L-mode discharges with the toroidal magnetic field, BT, in the forward and reverse directions have been used to study the impact of neoclassical drifts on the divertor plasma conditions and detachment. The evolution of the peak heat flux and the total power loads onto both the outer and the inner targets depends significantly on the toroidal field direction: increasing the core plasma density affects mainly the heat loads in the BT  0 (favourable). Ion saturation current measurements show similar trends to those of the IR heat flux data. These discrepancies are not only caused by drifts but also by different levels of radiated power in the core, thus the power across the separatrix, Psep. Tomographic reconstructions show that Psep is not constant within the entire dataset. Finally, at Ip=0.8MA, a significant reduction of the peak heat flux is observed at both targets for both field directions. On the other hand, at Ip=0.6MA, a reduction of the peak heat flux is only observed for BT < 0 at the outer target. Additionally, the onset of particle detachment is only observed at the outer target for BT < 0 with Ip=0.8MA. Keywords: ASDEX Upgrade, Upper single null, Scrape-off layer, Divertor detachment, Drifts