A two-stage magnetic field with cusp and divergent shapes was applied in a magneto-plasma-dynamic (MPD) thruster discharge chamber. The thrust efficiency intensively increased with enhanced thrust and unchanged discharge voltage because of more effective ionization/heating and swirl acceleration due to the applied magnetic field. When the magnetic field became much stronger and the arc current became much higher, the thrust approached the self-magnetic thrust. As a result, an optimum magnetic field strength existed. The plasma flow was observed by a high-speed camera, and plasma density, temperature and velocity were measured with electrostatic probes. Through the magnetic nozzle the exhausted plasma rotated and spreaded radially-outward, and the plasma density decreased near the central axis. Accordingly, the plasma characteristics agreed with the measured thrust performance, considering intensive plasma heating due to Hall current and swirl acceleration, i.e. conversion from azimuthal kinetic energy to axial kinetic energy.