Pedestal stability comparison and ITER pedestal prediction

Abstract

The pressure at the top of the edge transport barrier (or "pedestal height") strongly impacts fusion performance, while large Edge Localized Modes (ELMs), driven by the free energy in the pedestal region, can constrain material lifetimes. Accurately predicting the pedestal height and ELM behavior in ITER is an essential element of prediction and optimization of fusion performance. Investigation of intermediate wavelength MHD modes (or "peeling-ballooning" modes) has led to improved understanding of important constraints on the pedestal height and the mechanism for ELMs. The combination of high resolution pedestal diagnostics, including substantial recent improvements, and a suite of highly efficient stability codes, has made edge stability analysis routine on several major tokamaks, contributing both to understanding, and to experimental planning and performance optimization. Here we present extensive comparisons of observations to predicted edge stability boundaries on several tokamaks, both for the standard (Type I) ELM regime, and for small ELM and ELM-free regimes. We further use the stability constraint on pedestal height to test models of the pedestal width, and self-consistently combine a simple width model with peeling-ballooning stability calculations to develop a new predictive model (EPED1) for the pedestal height and width. This model is tested against experimental measurements, and used in initial predictions of the pedestal height for ITER.

Edge-localized mode avoidance and pedestal structure in I-mode plasmas

“…I-mode pedestal data are compared to predictions based on kineticballooning mode (KBM) turbulence (section V), which is thought to set a strong constraint on the pedestal in ELMy H-modes [26][27][28]. I-mode pedestal widths are observed to have no trend with β p,ped , in contrast to the ∆ ψ ∝ β 1/2 p,ped scaling predicted from KBM physics, and are generally wider than predicted based on KBM-limited physics.…”

“…Detailed numerical studies of the MHD instabilities associated with ELM drive are accomplished through the ELITE code [30,47]. This method is also used for the MHD stability component of the predictive EPED model [26][27][28]. MHD stability modeling has been successfully applied on C-Mod in ELMy [33] and EDA [34] H-modes, and in extensive cross-machine H-mode studies [25].…”

“…A firm understanding of the pedestal is essential for the extrapolation of any highperformance regime to ITER and reactor-scale devices: the pedestal structure sets a strong constraint on overall performance [4], as well as determining stability against large, deleterious ELMs. Recent cooperative efforts among theory, modeling, and experiment [25] have resulted in a predictive model, termed EPED [26][27][28], for ELMy H-mode based on coupled constraints from peeling-ballooning MHD stability [29][30][31] and kinetic-ballooning turbulence [32]. The EPED model has successfully predicted pedestal structure in ELMy H-mode on a number of machines [25,33,34] spanning a range of parameters, reaching ITER-relevant pedestal pressures in the case of H-modes on Alcator C-Mod.…”

Microtearing Instability In The ITER Pedestal

“…EPEDE1 [3] It is important to notice that EPEDE1's prediction of the ITER pedestal profile is steeper than the profile we use in our analysis. DIII-D experiments [4] have already demonstrated that externally imposed stochastic magnetic fields could eliminate ELMs by keeping the pedestal stable against the peeling-ballooning instabilities.…”

“…These are ideal MHD modes described by J' (current density gradient) and p' (pressure gradient). The stability boundary in this case is a line in the corresponding 2D plane [3]. Microtearing instability involves kinetic effects; the mode is primarily driven by the temperature gradient T'.…”