Compressibility as a feature of field reversal maintenance in the reversed-field pinch

Abstract

Five different codes have been used to simulate the identical problem in reversed-field pinch (RFP) dynamics using the resistive magnetohydrodynamic (MHD) model in three dimensions with the same set of initial and boundary conditions. Three codes are compressible, while two are incompressible. The assumption of incompressibility was made in the spirit of reducing the model so that, for example, the codes would run faster while retaining the relevant physics. The results show that the three compressible codes agree quite well with each other and the two incompressible codes agree with each other also, but the compressible and incompressible models show qualitatively different behavior. Most importantly, for a certain set of boundary and initial conditions, the compressible codes predict field reversal maintenance while the incompressible codes do not. Thus compressibility is an important feature of RFP physics. This is in contrast to tokamak physics where the strong toroidal field enforces incompressibility at low poloidal beta.