Quasi-coherent fluctuations limiting the pedestal growth on Alcator C-Mod: experiment and modelling

Abstract

Performance predictions for future fusion devices rely on an accurate model of the pedestal structure. The leading candidate for predictive pedestal structure is EPED, and it is imperative to test the underlying hypothesis to further gain confidence for ITER projections. Here, we present experimental work testing one of the EPED hypothesis, namely the existence of a soft limit set by microinstabilities such as the kinetic ballooning mode (KBM). This work extends recent work on Alctor C-Mod [Diallo, et al., Phys. Rev. Lett., 112, (2014), 115001], to include detailed measurements of the edge fluctuations and comparisons of edge simulation codes and experimental observations.

Intermediate-k density and magnetic field fluctuations during inter-ELM pedestal evolution in MAST

“…In the context of models like EPED, kinetic ballooning modes are predicted to set the pedestal pressure gradient. Long wavelength fluctuations, k ⊥ ρ i ∼ 0.1, consistent with the kinetic ballooning mode have been identified in DIII-D [8,9] and Alcator C-mod [10,11]. The impact of smaller scale instabilities in the pedestal has also been investigated.…”

Impact of impurity seeding and divertor conditions on transitions, pedestal structure and ELMs

“…This posits that there are two constraints which will set the pedestal height and width, or, more accurately, its gradient. After an ELM crash, the model hypothesises that the pressure gradient will increase until it reaches a transport limit, which, based on many observations, appears to be a ballooning mode [51,52]. Once this gradient limit is reached the pedestal then widens, growing further into the plasma until the combination of this critical gradient (and hence also current density) and pedestal width creates an unstable MHD mode, the peeling-ballooning mode [53], and an ELM occurs.…”

Flow shear suppression of pedestal ion temperature gradient turbulence-A first principles theoretical framework

“…We note that this ITG turbulence is not the dominant pedestal transport mechanism in most present day experiments precisely due to its suppression by shear flow. The most important fluctuations are likely electron temperature gradient turbulence [29][30][31][32][33], microtearing modes [17,33], and low-n (toroidal mode number) magnetic fluctuations [34][35][36][37][38]-all of which are expected to be much less sensitive to shear flow than ITG.…”