Internal plasma response measurements at the pedestal top in ELM suppressed plasmas in ASDEX Upgrade

Next step fusion devices like ITER require the suppression of edge localised modes (ELMs) to avoid harm to the first wall. One promising method is the use of externally applied non-axisymmetric magnetic perturbations (MPs), but the physics of suppressing ELMs is however to large extent still unclear.

To study the presence of a possible magnetic island limiting the pedestal pressure profile [1], we investigated the response of the pedestal electron temperature (Te) to external n=2 magnetic fields in ELM suppressed plasmas and plasmas with ELMs in ASDEX Upgrade [2]. Rigidly rotating MP-fields and high-resolution electron cyclotron emission measurements are combined to measure helical perturbations of the Te profile around the 7/2 rational surface, which is the rational surface at the pedestal top in the analysed scenario.

The measured radial displacement exhibits pronounced structures in its amplitude and phase profile around the 7/2 rational surface in ELM-suppressed plasmas, which are not seen in plasmas with ELMs. These structures are not captured by ideal magnetohydrodynamic (MHD) modelling using VMEC and suggest the presence of a magnetic island at the pedestal top in addition to ideal kink modes in ELM-suppressed plasmas. In contrast, the plasma response at the q=7/2 surface in ELMy plasmas is well described by ideal MHD modelling. Resistive structures are also observed at other rational surfaces (e.g. q=6/2) away from the pedestal top but in the presented cases not in conjunction with ELM suppression.

We further analysed the 3D ExB flow velocity (as determined from the radial force balance), which is close to zero at the rational surface where resistive structures are seen, in agreement with the observations of Ref. [3].

​​​​​​​[1] R. Nazikian et al., Physical Review Letters, 2015

[2] W. Suttrop et al., Nuclear Fusion, 2018

[3] C. Paz-Soldan et al., Nuclear Fusion, 2019