Ion kinetic effects and instabilities in the plasma flow in the magnetic nozzle

Kinetic effects on plasma flow in a converging-diverging magnetic nozzle due to finite ion temperature and ion trapping are investigated with collisionless quasineutral hybrid simulations with drift-kinetic model for ions and isothermal Boltzmann electrons. It is shown that in the cold ions limit the ion velocity profile agrees well with the analytical theory predicting the formation of the global accelerating potential due to the maximum of the field at the magnetic mirror.The global ion velocity profile is also demonstrated for isotropic and anisotropic distributions of warm ions. Partial ion reflections are observed due to a combined effect of the magnetic mirror and time-dependent fluctuations of the potential due to the wave breaking and instabilities that occur in the regions when the fluid solutions become multi-valued. Despite some reflections, the flow of the passing ions still follows the global accelerating profile defined by the magnetic mirror configuration. In simulations with a partially reflecting source wall, which imitates plasma source and allows the transitions between trapped and passing ions, the global nature of the transonic accelerating solution is revealed as a constrain on the plasma exhaust velocity that ultimately defines plasma density in the source region.