New relaxation of merging spheromaks to a field reversed configuration

Abstract

A novel high β relaxation to a field reversed configuration (FRC) has been investigated by axially colliding two spheromaks with opposing toroidal magnetic fields. The β value of the merging toroids increases from 0.1 to 0.7-1.0 within 15 μs, indicating an equilibrium transition from the low β spheromak to the high β FRC. An important finding is that the merging spheromaks relax either to a high β FRC or to another low β spheromak, depending on whether the initial normalized magnetic helicity given to these spheromaks is smaller or larger than a threshold value. This fact suggests that the FRCs are equipped with some global stability as robust as the Taylor magnetic energy minimum state.