Electric field in magnetized plasmas drives a rotating motion by E×B drift, giving rise to macroscopic structures. Recently, spontaneous formation of various vortex structures has been observed in rotating magnetized plasmas produced by electron cyclotron resonance (ECR) heating [1, 2]. One of the observed vortex structures arises with density depletion around its center axis, that is, a cylindrical density cavity is spontaneously formed in the plasma. In the interfacial layer between the density-cavity region and the ambient plasma, the density decreases with a steep transition boundary (several ion Larmor radii) to one-tenth of that of ambient plasma. Since the perspective image taken by a CCD camera has a nonluminous region in its center, which seems to be a hole in the plasma, this structure is referred to as "plasma hole". The plasma hole structure has so far been observed only in an ECR helium plasma. In this paper, the first experimental observation of the plasma hole structure in an ECR argon plasma (Ar plasma hole) is reported, especially pertaining to the density and the ion flow distribution.

The experiments have been performed with the High Density Plasma Experiment (Hyper-I) device at National Institute for Fusion Science. Hyper-I is a linear plasma device (30cm in diameter and 200cm in length) with ten magnetic coils. The plasma is produced by ECR heating in a magnetic beach, using a microwave of frequency 2.45GHz. In the case of relatively low Ar gas pressure, p_Ar∼0.1mmTorr, and a input microwave power of 3kW, the Ar plasma hole arises in the center of the plasma column. The Ar plasma hole is similar in external appearance to the He plasma hole observed in previous experiments, however, the ion flow distribution of the former is fairly different from that of the latter, which has been identified as a Burgers vortex in a compressible fluid. From the measurement utilizing a directional Langmuir probe (DLP), the ambient plasma shows supersonic rotation. Moreover, the axial ion flow of the Ar plasma hole also exceeds the ion sound speed. Although the existence of supersonic ion flow over large area is notable characteristic of the Ar plasma hole, the quantitative treatment requires appropriate interpretation of the DLP data in the case of supersonic flow.

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