Session CI2A - Edge Transport.
INVITED session, Monday afternoon, November 15
Room Chatham C, SCC
High time resolution measurements of the edge carbon density (n_c), temperature, and rotation profiles are revealing never before observed details of the impurity and radial electric field (E_r) dynamics associated with the edge localized mode (ELM) cycle. The ELM event has three distinct phases: the ELM crash, the recovery phase, and the improved transport phase. The ELM crash is characterized by a rapid (<0.3~ms), localized (<4~cm on the outboard midplane) expulsion of impurity density, energy, and momentum from the edge region and a marked increase in n_c in the scrape-off-layer (SOL). In addition, the E_r well that is generally present in H-mode plasmas is obliterated by the ELM event. The transition between the recovery phase and the improved transport phase occurs as the E_r well begins to redevelop approximately 10~ms after each ELM and is characterized by a rapid steepening in the profiles (particularly in n_c and poloidal rotation) near the separatrix. Transport analysis indicates a 50% reduction in impurity diffusivity and a large increase in inward convection across this transition, providing further evidence of the role of the E_r well in the reduction of radial transport evident in the edge of H-mode plasmas. The observed n_c perturbation increases with the total ELM energy loss and is consistent with the envelope of the eigenfunction of the most unstable mode calculated using the ELITE code, supporting the notion that that these ELMs are medium-n peeling/ballooning instabilities. These studies were made possible by recent upgrades to the DIII-D charge-exchange recombination system to allow sub-millisecond time resolution (>0.274~ms) at excellent spatial resolution (<3~mm in the edge).