Does quasi-coherent mode limit pedestal pressure gradient? Rich Groebner1, Jerry Hughes2, Phil Snyder 1General Atomics, San Diego CA, USA 2MIT Plasma Science and Fusion Center, Cambridge, Massachusetts, USA Presented at the C-Mod Ideas Forum April 7, 2009

Motivation: Study pedestal limiting phenomena in C-Mod Studies in C-Mod suggest that there may be a critical pressure gradient in the pedestal The Quasi-Coherent Mode is a candidate mechanism for limiting the pressure gradient There is some evidence that this could be happening, but evidence not definitive Goal of experiment would be to obtain high quality, temporally-resolved pressure profiles to study this idea Experiment complementary to a study in DIII-D for evidence that kinetic ballooning modes limit the pressure gradient

Execution of experiment Set up discharge which develops a robust, saturated QCM The QCM should evolve for a few tens of milliseconds prior to saturation Quantify strength of QCM with fluctuation diagnostics Obtain time-resolved measurements of pedestal pressure profile during growth and saturation of QCM Thomson Scattering will be key Perform multiple repeat discharges to build up record of temporal evolution of pressure gradient Look for saturation of gradient when QCM reaches max strength