Overview of ASDEX Upgrade results

Abstract

The ASDEX Upgrade (AUG) programme is directed towards physics input to critical elements of the ITER design and the preparation of ITER operation, as well as addressing physics issues for a future DEMO design. Since 2015, AUG is equipped with a new pair of 3-strap ICRF antennas, which were designed for a reduction of tungsten release during ICRF operation. As predicted, a factor two reduction on the ICRF-induced W plasma content could be achieved by the reduction of the sheath voltage at the antenna limiters via the compensation of the image currents of the central and side straps in the antenna frame. There are two main operational scenario lines in AUG. Experiments with low collisionality, which comprise current drive, ELM mitigation/suppression and fast ion physics, are mainly done with freshly boronized walls to reduce the tungsten influx at these high edge temperature conditions. Full ELM suppression and non-inductive operation up to a plasma current of I p = 0.8 MA could be obtained at low plasma density. Plasma exhaust is studied under conditions of high neutral divertor pressure and separatrix electron density, where a fresh boronization is not required. Substantial progress could be achieved for the understanding of the confinement degradation by strong D puffing and the improvement with nitrogen or carbon seeding. Inward/outward shifts of the electron density profile relative to the temperature profile effect the edge stability via the pressure profile changes and lead to improved/decreased pedestal performance. Seeding and D gas puffing are found to effect the core fueling via changes in a region of high density on the high field side (HFSHD). The integration of all above mentioned operational scenarios will be feasible and naturally obtained in a large device where the edge is more opaque for neutrals and higher plasma temperatures provide a lower collisionality. The combination of exhaust control with pellet fueling has been successfully demonstrated. High divertor enrichment values of nitrogen E N ⩾ 10 have been obtained during pellet injection, which is a prerequisite for the simultaneous achievement of good core plasma purity and high divertor radiation levels. Impurity accumulation observed in the all-metal AUG device caused by the strong neoclassical inward transport of tungsten in the pedestal is expected to be relieved by the higher neoclassical temperature screening in larger devices.

Approaching detachment in I-mode—response of core confinement and the edge pedestal in the ASDEX Upgrade tokamak

“…In the discharges at hand, the optical thickness in the pedestal region is too low, therefore, measurements by the correlation electron cyclotron emission diagnostic are affected by density fluctuations. To put the present studies into context, it should be pointed out that N seeding into type-I ELMy H-mode plasmas generally can lead to a confinement improvement on AUG [31,77], which is due to a reduction of the high field side high density (HFSHD) pattern through increased SOL radiation and a resulting inward shift of the density profile [78]. Since the HFSHD is a consequence of the direction of E × B drifts in the divertor in favorable configuration, it is usually not present in unfavorable configuration with reversed divertor drift directions.…”

“…The N concentration in the plasma reached only roughly 1.5%, which implies good impurity screening. Due to mostly non-stationary conditions, no estimates of divertor enrichment [31] and compression [32] can be given. L-mode parameters comparable to those before the L-I transition are obtained.…”

Targeting a Versatile Actuator for EU-DEMO: Xenon Doping of Fueling Pellets

“…The AUG is a mid-size divertor tokamak (major radius R = 1.65 m, minor radius a 0 = 0.5 m) where the plasma-facing components are mainly tungsten 14 (W) and the walls are repeatedly conditioned by boronization. In its reactor-relevant all-metal-wall configuration equipped with a powerful auxiliary heating system, the versatile set of diagnostics needed for plasma analysis and characterization are well suited for the task.…”

Synthetic conventional reflectometry probing of edge and scrape-off layer plasma turbulence

“…The AUG tokamak [5] is equipped with several reflectometry systems and was considered as experimental reference. An example of O-mode fixed frequency reflectometers in conventional set-up at AUG can be found in [6].…”