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We experimentally study plasma parameters including the ion acoustic Mach number and the flow direction of expanding helium plasma jet flowing along open field lines. It is experimentally found that the ion Mach number increases. We discuss the mechanism of the ion acceleration. Since the ions do not rotate in electromagnets, the Hall and the swirl acceleration do not occur. We investigate the experimental results based on quasi-1-D flow model including the aerodynamic effect and the electric field. Our model describes the ion acceleration. Therefore, it is concluded that the helium ions are accelerated both by the electric field and by the increasing cross-sectional area of the supersonic flowing channel, which is determined by the magnetic flux conservation.