Flowing plasma has been one of interesting topics in laboratory plasmas and fusion oriented devices as well in astrophysical plasmas. Supersonic flow, especially, attracts much attention in connection with formation of shock structure and highly developed turbulence, and accordingly flow measurement method applicable to supersonic flow is of importance. In this paper, it is reported the effect of supersonic flow on flow measurement using a directional Langmuir probe (DLP).

Experiments have been performed in the high-density plasma experiment (Hyper-I) device at National Instutute for Fusion Science, which produces cylindrical plasma (30 cm diam and 200 cm axial length) by electron cyclotron resonance heating. Many types of vortex structures have been spontaneously formed in the Hyper-I device,[1,2] one of which, referred to as "plasma hole", is characterized by central density-cavity and monopole vortex. Azimuthal rotation velocity induced by the vortex often exceeds ion sound velocity. DLP, inserted into radial direction, collects a directed ion current through a small opening (1 mm diam) made on the side of the ceramic insulator (3 mm diam).[3] The ion saturation current, I(θ), has information on the effect of flow and that of magnetic field, where the effect of the magnetic field appears with π periodicity. Taking the ratio [I(θ+π)-I(θ)]/[I(θ+π)+I(θ)], the effect of magnetic field is completely canceled with the help of π periodicity and only information on flow effect is extracted. In the case that the plasma flow velocity is slow compared to ion sound velocity, the flow effect on DLP signal is weak and approximately appears as a factor of v cos(θ-θ₀), the amplitude v and phase θ₀ of which corresponds to that of flow velocity vector. On the other hand, measurements in supersonic flow have revealed the increase of the higher-order Fourier components included in the DLP signal, cos(nθ). In such a case, the origin of the higher-order components and the relation between the flow velocity and Fourier amplitudes should be clarified.

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