Extended MHD studies of reconnection physics in spherical tokamaks

Fast axisymmetric reconnection has been shown to be crucial for helicity injection current-drive techniques in spherical tokamaks.  In recent years, it has been demonstrated that the transient coaxial helicity injection could take advantage of fast plasmoid reconnection to produce startup current in NSTX/NSTX-U (Ebrahimi&Raman PRL, NF 2015,2016, Ebrahimi PoP 2019).  Although in more conventional helicity injection techniques,  generated current carrying  filaments require relaxation due the dynamo terms in 3-D,  here merging of axisymmetric plasmoids  would lead to a final stable equilibrium startup current formation. By incorporating the final vessel configuration of PEGASUS III, as well as the coil currents in the NIMROD code, we have performed new sets of extended MHD simulations for this configuration. Preliminary simulations show large-volume flux closure of about 20mWb, We present simulations with further optimizing coil currents for Pegasus III, as well as the reconnecting edge stability analysis for this configuration. This work was done in collaboration with the Pegasus III team. Work supported by DOE grants DE-AC02-09CHI1466, and DE-SC0010565.