Highlights 2014

Research news from the division Plasma Edge and Wall

Linear stability analysis of ELMs at ASDEX Upgrade – A. Burckhart

Edge localized modes (ELMs) are magnetohydrodynamic instabilities that occur at the edge of magnetically confined fusion plasmas. They periodically expel particles and energy from the confined region. In addition to limiting the confinement, they cause high heat fluxes to the walls of the tokamak which may not be manageable in larger, next-generation devices. However, the exact nature of the instabilities that drive ELMs is still unknown. The most commonly invoked theory to explain the occurrence of ELMs is the peeling-ballooning model which posits a critical edge pressure gradient and current density.

In his PhD thesis ‘Different ELM regimes at ASDEX Upgrade and their linear stability analysis’ Andreas Burckhart investigated whether the peeling-ballooning model can explain the occurrence of ELMs. One example is shown below.

Stability analysis of a discharge with different positions of ECRH heating: Although the stability boundary is further away from the operational point for edge heating, the ELM frequency is increased.

Stability analysis of a discharge with different positions of ECRH heating: Although the stability boundary is further away from the operational point for edge heating, the ELM frequency is increased.

ELMs can be influenced by depositing localized ECRH power at the plasma edge. It was shown at TCV that the ELM frequency increases and the per-ELM losses decrease when shifting the power deposition to the edge [Rossel et al, Nucl. Fusion 2012]. At ASDEX Upgrade, however, a different behavior is observed. Rather than the ELM frequency increasing, a second, higher frequency band formes and becomes more populated as more power is moved to the edge. A peeling-ballooning analysis of the profiles shows that the core-heated reference plasma is marginally unstable, while it is stabilized when shifting the power to the edge. This is mainly due to a relaxation of the kinetic profiles while the stability boundary stays the same. One can speculate that ECRH drives turbulence at the deposition location, thereby enhancing transport and limiting the pedestal top and gradients, which prevents the plasma from reaching the peeling-ballooning stability limit. Since the relaxation of the gradients does not suppress the ELMs but, on the contrary, increases their frequency, this is an indication that peeling-ballooning theory is missing a key ingredient to explain the ELM trigger.