Global properties and stability of transonic plasma acceleration in the magnetic nozzle

It is shown that transonic plasma acceleration in the converging–diverging magnetic field (magnetic nozzle) follows the unique global solution which is fully defined by the magnetic field. Such solution, which was analytically obtained earlier in the paraxial approximation, is compared here with the results of the axisymmetric two-dimensional (r−z) magnetohydrodynamics simulations. It is shown that the analytical solution not only describes well the region near the axis but also can be applied to arbitrary magnetic surfaces. The simulations with different length of the nozzle and different boundary values for plasma velocity show that the plasma flow switches to the unique transonic acceleration profile via the shock-like transition in the velocity and pressure profiles. The simulations with arbitrary (not vacuum) initial magnetic field demonstrate the global adjustment of the magnetic field such that the transonic acceleration velocity profile follows the analytic predictions with the modified magnetic field.