In order to clarify the electromagnetic acceleration mechanisms of an applied-field magneto-plasma-dynamic arcjet (MPDA), magnetic fields are measured in detail near the MPDA outlet. Plasma current density is calculated using the Maxwell equation (rot
B=μ
0j), and then spatial distributions of Lorentz forces (
j×
B forces) are evaluated. A uniform external magnetic field is deformed by a strong diamagnetic effect near the MPDA outlet, and as a consequence, a converging magnetic nozzle configuration is formed. It is found that an axial Lorentz force F
z (=j
rB
θ-j
θB
r) is almost eliminated by the deceleration force (the j
θB
r force), which results from the diamagnetic effect. The deceleration force can be converted to the acceleration one in an externally-applied diverging magnetic field.
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