Effect of Collisionality on H-mode Pedestal Structure

Variations in the H$\,$mode pedestal pressure, p$_{\rm PED}$, by a factor of 1.5 are observed in ITER baseline scenario demonstration discharges on the DIII-D tokamak. Higher p$_{\rm PED}$ occurs when the pressure increase between ELMs is primarily due to an increase in density. The variation in p$_{\rm PED}$ can be understood through the effect of collsionality, $\nu \propto n/T^2$, under the EPED model [1]. In this model, the pedestal pressure gradient, $p^\prime$, grows until it triggers a peeling-ballooning mode, PBM, which results in the ELM. Since, in these discharges, ELMs are triggered at the current driven peeling limit, higher $p^\prime$ is achieved at higher collisionality where the associated bootstrap current is reduced. Under EPED, $p^\prime$ between ELMs is constrained by the kinetic ballooning mode, KBM, for which the critical $p^\prime$ increases with pedestal width. The KBM critical $p^\prime$ for a given pedestal width is reduced at increased collisionality, resulting in the $p^\prime$ required to trigger the PBM being reached only at larger width and so larger pressure.\par \vskip6pt \noindent [1] P.B.\ Snyder et al., Phys. Plasmas {\bf 16}, 054118 (2009).