A convective divertor utilizing a 2nd-order magnetic field null*

New results motivate a detailed study of a magnetic divertor concept characterized by strong plasma convection near a poloidal magnetic field (B$_{\mathrm{p}}$) null region. The configuration is that of a near-2nd-order B$_{\mathrm{p}}$ null (B$_{\mathrm{p}}\propto \Delta $r$^{\mathrm{2}})$, as in a snowflake divertor [1,2]. The concept has 2 key features: (A) Convection spreads the heat flux between multiple divertor legs and further broadens the heat-flux profile within each leg, thereby greatly reducing target-plate heat loads [2]. (B) The heat flux is further reduced by line radiation in each leg in detachment-like ionization zones. Theory indicates that convective turbulence arises when the poloidal plasma beta, $\beta_{\mathrm{p}}=$2$\mu _{\mathrm{0}}$nT/B$_{\mathrm{p}}^{2}$ \textgreater \textgreater 1. Measurements in TCV [4] now more fully quantify earlier NSTX and TCV observations of plasma mixing [5.6], and related modeling of TCV indicates that strongly enhanced null-region transport is present [7]. Convective mixing provides a stabilizing mechanism to prevent the ionization fronts (hydrogenic and impurity) from collapsing to a highly radiating core MARFE. Also, the radiating zone maps to a very small region at the midplane owing to the very weak B$_{\mathrm{p}}$ in the convective region, thus minimizing its impact on the core plasma. Detailed calculations are reported that combine features A and B noted above. The plasma mixing mechanisms are described together with the corresponding transport model implemented in the 2D UEDGE edge transport code [2]. UEDGE calculations are presented that quantify the roles of mixing, impurity radiation, and detachment stability for a realistic snowflake configuration. Work in collaboration with D.D. Ryutov, S.I. Krasheninnikov, and M.V. Umansky.\\[4pt] [1] D.D. Ryutov et al., PPCF \textbf{54} (2012) 124050.\\[0pt] [2] T.D. Rognlien et al., J. Nucl. Mat. \textbf{438} (2013) S418.\\[0pt] [3] D.D. Ryutov et al., accepted, Physica Scripta (2014).\\[0pt][4] W. Vijvers et al., Nucl. Fusion \textbf{54} (2014) 023009.\\[0pt] [5] V.A. Soukhanovskii et al., Nucl. Fusion \textbf{51} (2011) 012001 and Phys. Plasmas \textbf{19} (2012) 082504.\\[0pt] [6] H. Reimerdes et al., PPCF \textbf{55} (2013) 124027.\\[0pt] [7] T. Lunt et al., PPCF \textbf{56} (2014) 035009.