A review of plasma acceleration and detachment mechanisms in propulsive magnetic nozzles

The magnetic nozzle is a magnetic structure composed of a convergent-divergent (or simply divergent) coaxial magnetic field. Similar to the de Laval nozzle used in traditional chemical propulsion, this magnetic nozzle effectively confines plasma, thereby converting internal energy into axial kinetic energy. The research on propulsive magnetic nozzle (PMN), generally applied in the field of electric propulsion, has spanned several decades and is considered one of the preferred acceleration methods for future high-power electric propulsion. Within the PMN, the interaction between the magnetic nozzle and plasma is highly complex, while the magnetic field accelerates plasma, it can also constrain and decelerate plasma if the charged particles fail to detach from the closed-loop magnetic field lines timely. Therefore, understanding the particle acceleration and detachment mechanisms in PMNs is crucial for its design. Over the past fifty years, the PMN has been applied in various electric propulsion types such as magnetoplasmadynamic thruster, radio frequency thruster, and vacuum arc thruster. A substantial amount of experimental and numerical studies have been done to explore the basic principles of PMNs. In this review, we provide an overview of the state-of-the-art of the plasma acceleration and detachment mechanisms in PMN, including the breakthroughs we have achieved and the challenges that still remain. We hope this review will further enhance the understanding of the rich physical mechanisms of PMNs, shed light on future research directions, and ultimately contribute to the realization of efficient and reliable PMN designs.