Three-dimensional hybrid simulations of spheromak merging

Hybrid simulations with full kinetic ions and fluid electrons of counter-helicity spheromak merging have been performed using the HYM code and compared with 3D MHD simulations. The ion heating and the effects on FRC pressure profiles have been studied. In hybrid simulations the resulting FRC has larger elongation and flatter pressure profile compared to the MHD; smaller ion flow velocities before and during the reconnection have also been observed. In both cases the resulting FRC was unstable to the n=1 tilt mode, as expected for these parameters (small FLR, MHD-like regime, and small elongation). One of the unexpected results in 3D simulations was behavior of the n=1 Fourier harmonic of the ion kinetic energy, growing faster than linear, right before the reconnection. Additional simulations with varying initial separation between spheromaks have demonstrated that this behavior is related to the tilting instability of the spheromaks prior to reconnection. At closer initial separations, the growth of the tilting mode was strongly nonlinear. It has been shown that in hybrid simulations in contrast to the MHD, the reconnection rate is also very sensitive to the initial separation of two spheromaks.