The impact of plasma parameters, geometry and global effects on local 2^nd stable ballooning access

The EPED model had success in identifying type-I ELM and QH mode pedestals in conventional tokamaks, by combining kinetic ballooning mode (KBM) and peeling-ballooning constraints. Within EPED, the KBM constraint is usually approximated by the ideal ballooning mode stability threshold. It has been noted that differences between local ideal MHD and gyro-kinetic ballooning stability is larger at low aspect ratio. KBM critical pedestals are consistent with observation in initial studies on conventional and spherical tokamaks. In this work, we attempt to examine efficient ways to capture kinetic effects on local ballooning stability and global effects in the 2nd region of ballooning stability.

The application of an efficient model for the calculation of the local ballooning stability boundary is presented based on a newly developed Gyro-Fluid System (GFS) code. GFS is observed to capture KBMs reproducing the conventional scaling where Δ~√β_p,ped as well as the NSTX-U scaling Δ~β_p,ped. For the cases studied, it is observed that DIII-D are close to the 2^nd stable ballooning region, while NSTX-U is limited by the 1^st ballooning boundary. Variation of plasma parameters and geometry for the NSTX-U case allows the pedestal to approach the 2^nd stable region, reducing the exponent of and approaching the conventional scaling.

Finally, as the pedestal gets access to 2^nd stability the local approximation of ballooning modes cannot provide the pedestal structure and global effects need to be accounted. High-n global ballooning modes limit local 2^nd stability access and provide a transport mechanism to constrains the pedestal evolution. The high-n global stability is approximated using the ideal MHD model with ELITE at marginal stability. It is shown that nearly local high-n modes with can provide a proxy for the critical when local 2^nd stable access exists on DIII-D plasmas. Interestingly, the kink contribution is crucial even at high-n for destabilizing the pedestal which is typically absent in local gyro-kinetics.