Ideal ballooning modes in the ASDEX-Upgrade, JET and TCV pedestals

A complete understanding of pedestal structure and limits remains an open question in fusion research. Research in this direction generally makes use of some generalised version of the EPED model[1]; transport mechanisms (which can have different drives for the particles and heat, and for ions and electrons) determine, in combination with the heat and particle sources, the profile gradients. In the “standard” case, the profiles continue to extend radially further inwards away from the separatrix until there is enough free energy in the pressure gradient (and associated driven edge current density) that a large magnetohydrodynamic (MHD) instability occurs, the so-called edge localised modes (ELMs). The onset of the ELM then determines the steepest and widest pedestal pressure profile.