Modulation of plasma electromagnetic acceleration by magnetic nozzle divergence topology

The divergence topology of the applied magnetic field plays a critical role in plasma control and electromagnetic acceleration. To elucidate its impact on electromagnetic acceleration, we employed a dual-stage magnetic coil system for an applied field magnetoplasmadynamic thruster (AF-MPDT), and four field configurations characterized by different divergence ratios. The spatial coupling between plasma parameters and the magnetic-topology parameter B_r/B_z was subsequently analyzed under these varied divergence conditions. The result show that, under an overly divergent magnetic field configurations, insufficient radial confinement leads to a lower radial electron pressure gradient p_er, resulting in a diminished diamagnetic force. Additionally, premature electron demagnetization is induced by the divergent field, and the decelerating character of the paramagnetic E×B force is directly compromises the overall electromagnetic acceleration performance. Under overly convergent magnetic field configurations, the values of B_r/B_z in regions where p_er peaks remain low, resulting in a pronounced suppression of the diamagnetic electromagnetic force density. Therefore, there is a magnetic field configuration with an optimal divergence rate to achieve the optimal electromagnetic acceleration effect. Under the conditions of discharge current of 150A and magnetic field strength of 50mT, changing the magnetic field configuration (k =3.222) can increase the thrust of the thruster by 46% and the discharge voltage by 23%. The performance improvement is attributed to enhanced coupling between plasma parameters and the magnetic field under this configuration, which optimizes the conversion of electron pressure gradient and radial electric field into axial Lorentz force.