Flux-surface closure during resistive-MHD simulations of Coaxial Helicity Injection (CHI) in NSTX

CHI in STs offers considerable promise for generating startup plasmas, with NSTX experiments demonstrating coupling to Ohmic drive with magnetic flux savings.\footnote{R. Raman et al., Phys. Rev. Letters 104, 095003 (2010).} Success in these experiments depends in part on the achievement of flux closure following CHI voltage crowbarring. Flux closure is demonstrated here in whole-device, resistive MHD simulations using the NIMROD code. In axisymmetric plasmas significant closure due to resistive effects requires the injection slot to be narrow (e.g. 4 cm vs. 11 cm) in agreement with experiment. In simulations reduction of the applied injector flux following the crowbar forms an X-point close to the bottom of NSTX that significantly enlarges the closed volume; closure is not seen if the flux is held constant. The physics of closure will be discussed and applied to maximizing the volume. Effects of a background plasma in simulations of flux formation and closure will also be described.