New Theoretical Model of the Complex Edge Region of Fusion Plasmas Proves Accurate

New research advances in the modeling of the critical “pedestal” region of tokamak plasmas.

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.

The Science

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 Impact

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.

Summary

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.

Contact

Philip B. Snyder
General Atomics
[email protected]

Funding

DOE Office of Science, Fusion Energy Sciences (FES) program

Publications

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).

Highlight Categories

Program: FES

Performer: Industry