Edge Radial Electric Field and Ion Orbit Loss in DIII-D

The edge radial electric field, $E_r$, may be largely determined by the value necessary to supply the neoclassical return current to balance the loss current due to ion orbit loss. This is the indication from a phenomenological model, motivated by recent Mach Probe measurements of the edge co-$I_p$ flow layer in DIII-D [1,2], based on a simple empty loss cone orbit loss model [3,4]. Probe and charge exchange recombination measurements also show a relatively large positive edge $E_r$ just inside the LCFS in Ohmic conditions, $\sim10$ kV/m, which is explained in this model by the propensity of the flow layer to drive return current. The Er level is also dependent on $Z_{eff}$ in the edge lower $Z_{eff}$ promotes greater negative $E_r$ for current balance. The model will be compared with measurements in Ohmic, L- and H-mode conditions.\par \vskip6pt \noindent [1] J.A. Boedo et al., Phys. Plasmas 18, 035510 (2011).\par \noindent[2] S.H. M\"uller et al., Phys. Rev. Lett. 106, 115001 (2011).\par \noindent[3] J.S. deGrassie et al., Nucl. Fusion 49, 085020 (2009).\par \noindent[4] J.S. deGrassie et al., Nucl. Fusion 52, 013010 (2012).