Overview of physics studies on ASDEX Upgrade

Abstract

The ASDEX Upgrade (AUG) programme, jointly run with the EUROfusion MST1 task force, continues to significantly enhance the physics base of ITER and DEMO. Here, the full tungsten wall is a key asset for extrapolating to future devices. The high overall heating power, flexible heating mix and comprehensive diagnostic set allows studies ranging from mimicking the scrape-off-layer and divertor conditions of ITER and DEMO at high density to fully non-inductive operation (q 95 = 5.5, ) at low density. Higher installed electron cyclotron resonance heating power 6 MW, new diagnostics and improved analysis techniques have further enhanced the capabilities of AUG. Stable high-density H-modes with MW m−1 with fully detached strike-points have been demonstrated. The ballooning instability close to the separatrix has been identified as a potential cause leading to the H-mode density limit and is also found to play an important role for the access to small edge-localized modes (ELMs). Density limit disruptions have been successfully avoided using a path-oriented approach to disruption handling and progress has been made in understanding the dissipation and avoidance of runaway electron beams. ELM suppression with resonant magnetic perturbations is now routinely achieved reaching transiently . This gives new insight into the field penetration physics, in particular with respect to plasma flows. Modelling agrees well with plasma response measurements and a helically localised ballooning structure observed prior to the ELM is evidence for the changed edge stability due to the magnetic perturbations. The impact of 3D perturbations on heat load patterns and fast-ion losses have been further elaborated. Progress has also been made in understanding the ELM cycle itself. Here, new fast measurements of and E r allow for inter ELM transport analysis confirming that E r is dominated by the diamagnetic term even for fast timescales. New analysis techniques allow detailed comparison of the ELM crash and are in good agreement with nonlinear MHD modelling. The observation of accelerated ions during the ELM crash can be seen as evidence for the reconnection during the ELM. As type-I ELMs (even mitigated) are likely not a viable operational regime in DEMO studies of ‘natural’ no ELM regimes have been extended. Stable I-modes up to have been characterised using -feedback. Core physics has been advanced by more detailed characterisation of the turbulence with new measurements such as the eddy tilt angle—measured for the first time—or the cross-phase angle of and fluctuations. These new data put strong constraints on gyro-kinetic turbulence modelling. In addition, carefully executed studies in different main species (H, D and He) and with different heating mixes highlight the importance of the collisional energy exchange for interpreting energy confinement. A new regime with a hollow profile now gives access to regimes mimicking aspects of burning plasma conditions and lead to nonlinear interactions of energetic particle modes despite the sub-Alfvénic beam energy. This will help to validate the fast-ion codes for predicting ITER and DEMO.

The JOREK non-linear extended MHD code and applications to large-scale instabilities and their control in magnetically confined fusion plasmas

“…In the following, we briefly describe some examples and main results. First simulations of type-I ELMs in the ASDEX Upgrade tokamak [196] were shown in reference [197], where a spatial and temporal substructure of the ELM were observed leading to several bursts. This work pointed out the importance of including multiple toroidal harmonics in non-linear modelling to obtain a realistic dynamics of ELMs.…”

Edge transport and fuelling studies via gas puff modulation in ASDEX Upgrade L-mode plasmas

“…All experimental data is from L-mode discharge #37758 in the tokamak ASDEX Upgrade (AUG) [24] with a plasma current of I P = 0.8 MA and a toroidal magnetic field of B t = −2.4 T. Central electron cyclotron resonance heating of 590 kW is applied in addition to Ohmic heating of 440 kW, while the radiation losses are about 200 kW [25]. In figure 1 we show selected quantities for the discharge under investigation.…”

Spatiotemporal analysis of the runaway distribution function from synchrotron images in an ASDEX Upgrade disruption

“…ASDEX Upgrade is a medium-sized tokamak (major radius , minor radius ) located at the Max Planck Institute for Plasma Physics in Garching, Germany (Meyer et al. 2019). An overview of plasma current, electron density, electron temperature and the hard X-ray count rate in ASDEX Upgrade discharge #35628 is shown in figure 2.…”