Analysis of energy conversion during co- and counter-helicity spheromak merging by particle-in-cell simulation

Plasma merging has been proposed as an attractive startup method for obtaining compact and high-beta configurations, utilizing the fast energy conversion of magnetic reconnection. In this paper, we investigate energy conversion during spheromak mergings using particle-in-cell computations in cylindrical coordinates. The simulations reveal differences in the heating characteristics that depend upon the polarity of the toroidal field. We find symmetry breaking of the energy conversion downstream from the reconnection point, although it does not affect the total amount of energy exchanged. The differences in the ion temperature profiles are explained by a radial shift of the reconnection point and the deformation of the current sheet due to the Hall effect. The electrostatic potential structure modified by the polarity effect causes one-sided acceleration of the ions, which leads to symmetry breaking of the energy gain E· J_i. The energy gained by the electrostatic field in the perpendicular direction is the dominant term for the ions, while the electrons gain energy inductively, mainly parallel to magnetic field. (author)