New insights in the evolution of the pedestal in-between edge localized modes

Recently, it has been found at ASDEX Upgrade that a saturation of the maximum pressure gradient at the plasma edge is correlated to the onset of magnetic fluctuations in the high frequency range.

The high confinement mode of a tokamak plasma is characterized by a narrow region with steep pressure gradients at the plasma edge, named pedestal. There, instabilities occur, which are called edge localized modes (ELMs). They lead to a quasi-periodic relaxation of the pedestal and expel high particle and heat fluxes towards the wall, which would exceed the material limits in future fusion devices such as Iter.

Temporal evolution of the magnetic activity (∂Br/∂t), edge electron density gradient, max(-∇ne), and temperature gradient, max(-∇Te), and the divertor current relative to the ELM onset: The burst in the divertor current indicates the ELM crash and defines t=0. When max(-∇ne) as well as max(-∇Te) saturate, high frequency magnetic fluctuations set in, visible as yellow band at 220 kHz from 10 ms relative to ELM onset on or, equivalently, just before the ELM. — Temporal evolution of the magnetic activity (∂B_r/∂t), edge electron density gradient, max(-∇n_e), and temperature gradient, max(-∇T_e), and the divertor current relative to the ELM onset: The burst in the divertor current indicates the ELM crash and defines t=0. When max(-∇n_e) as well as max(-∇T_e) saturate, high frequency magnetic fluctuations set in, visible as yellow band at 220 kHz from 10 ms relative to ELM onset on or, equivalently, just before the ELM.

Graph: IPP

Temporal evolution of the magnetic activity (∂B_r/∂t), edge electron density gradient, max(-∇n_e), and temperature gradient, max(-∇T_e), and the divertor current relative to the ELM onset: The burst in the divertor current indicates the ELM crash and defines t=0. When max(-∇n_e) as well as max(-∇T_e) saturate, high frequency magnetic fluctuations set in, visible as yellow band at 220 kHz from 10 ms relative to ELM onset on or, equivalently, just before the ELM.

Graph: IPP

The evolution of the pedestal gives important information on the mechanisms, which set the pedestal stability and probably lead to ELMs. In a recent study on ASDEX Upgrade the evolution of the edge pressure profiles in between ELM crashes was investigated [1]. For a variety of pedestal pressure values, it has been found that the pedestal electron density gradient (max(-Ñn_e)) is established before the pedestal electron temperature gradient (max(-ÑT_e)). Already milliseconds before the ELM both quantities saturate indicating a clamping of the maximum pressure gradient (see figure). This period is accompanied by magnetic fluctuations in the frequency range of several hundreds of kilohertz, which are the signature of an instability that is present in the pedestal.

The presented work was conducted in the Pedestal and Edge Physics (PEP) group in cooperation with TU Wien. The corresponding publication [1] has been selected by IOP science as a Plasma Physics and Controlled Fusion highlight of 2016.

[1] F.M. Laggner et al., Plasma Physics and Controlled Fusion 58-6, 065005 (2016)
http://dx.doi.org/10.1088/0741-3335/58/6/065005