Understanding the Physics of EHO Generation in DIII-D Including the Role of Rotational Shear

The key to QH-mode operation is an edge electromagnetic mode, the edge harmonic oscillation (EHO), which provides the extra transport to allow the edge plasma to reach a transport equilibrium with edge pressure gradient and current density just below the edge localized mode (ELM) limit [1]. Experimental results are consistent with the theoretical prediction that the EHO is a kink-peeling mode destabilized by edge rotational shear at edge conditions just below the ELM limit [1]. Theory suggests that the essential rotation speed is $E_r/RB_\theta$; initial analysis of experimental data is consistent with this expectation [2,3]. Recent results show that the change in shear between QH-mode and ELMing H-mode occurs in the small radius side of the edge $E_r$ well near the top of the edge pedestal. Experiments have been carried out to test the whether $E_r/RB_\theta$ is the essential shear and, if so, how that critical shear varies with $\nu^\ast$.\par \vskip6pt \noindent [1] K.H.\ Burrell, et al., Nucl.\ Fusion {\bf 49}, 085024 (2009).\par \noindent [2] A.M.\ Garofalo, et al., Nucl.\ Fusion {\bf 51}, 083018 (2011).\par \noindent [3] K.H.\ Burrell, et al., Phys.\ Plasmas {\bf 19}, 056117 (2012).