Poloidal flux loss and axial dynamics during the formation of a field-reversed configuration

A method has been developed that uses the observed strength of the axial implosion during field-reversed configuration (FRC) formation to infer the trapped poloidal flux level in the time interval after the radial implosion, but before the axial implosion. The method uses a 2-D magnetohydrodynamic (MHD) model to characterize the implosion strength as a function of trapped flux. An inferred flux loss time history prior to the axial implosion exhibits a rather short 9 μsec lifetime until the axial implosion reaches the midplane, which causes field gradients to relax and the flux loss rate to be reduced. By comparing the experimental flux loss time histories with numerical calculations, a plasma resistivity time history has been inferred. An additional flux loss that often occurs after the axial implosion, but before 2-D equilibrium is achieved, has also been identified. It was found necessary to include the effects of classical viscosity in the model to accurately represent axial dynamics. The effects of viscosity in current experiments has been studied, and it is found that if the ion temperature is sufficiently high, the parallel component of classical ion viscosity can significantly damp the strength of the axial contraction.