Image courtesy of General Atomics and LLNL
Typical structure of a peeling – ballooning mode in the DIII-D tokamak, calculated by the MHD stability code ELITE.
A theoretical model, without any fitting parameters, has been developed which can predict the height and width of the pedestal region in tokamak plasmas. The pedestal is an insulating transport barrier in the outer few percent of the confined plasma which acts much like the wall of a thermos bottle separating the very hot plasma core from the cooler edge plasma and the material surfaces.
The physics of the pedestal region is highly important for two primary reasons: (1) predicted fusion performance scales roughly with the square of the pedestal pressure (or “pedestal height”), and hence a high pedestal is required for copious fusion energy production in ITER or a fusion power plant, and (2) the large free energy in the pedestal region can drive instabilities called Edge Localized Modes (ELMs), which eject bursts of heat and particles onto material surfaces, and can cause significant material erosion in reactor – scale devices.
High fusion performance (“H –mode”) in tokamaks is achieved via the spontaneous formation of an insulating transport barrier in the outer few percent of the confined plasma. This insulating layer is relatively thin, and is referred to as the “pedestal” because it provides an abrupt step up in the temperature and density profiles. In addition, the large free energy in the pedestal region can drive instabilities called Edge Localized Modes (ELMs), which eject bursts of heat and particles onto material surfaces. FES-supported research at General Atomics has resulted in a theoretical and computational model, known as the EPED model, based on fundamental physics constraints and without any fitting parameters which can predict the height and width of the pedestal. The instabilities responsible for ELMs are known as peeling- ballooning (PB) modes, as they balloon outward and peel off part of the insulating layer of plasma. The onset of PB modes provides a constraint on the height of the pedestal as a function of its width. An additional smaller–scale instability, the kinetic ballooning mode (KBM), constrains the pressure gradient within the insulating layer by driving heat and particle transport across it. Combining the two constraints yields the EPED model, which predicts both the height and the width of pedestal.
Philip B. Snyder
DOE Office of Science, Fusion Energy Sciences (FES) program
P.B. Snyder, T.H. Osborne, K.H. Burrell, et al., “The EPED pedestal model and edge localized-mode suppressed regimes: Studies of quiescent H-mode and development of a model for edge localized mode suppression via resonant magnetic perturbations,” Phys. Plasmas 19, 056115 (2012).
P.B. Snyder, R.J. Groebner, J.W. Hughes, et al., “A first-principles predictive model of the pedestal height and width: development, testing and ITER optimization with the EPED model,” Nucl. Fusion 51 103016 (2011).