Formation of Viscous Vortex and Anomalous Viscosity in a Plasma

我们在磁化的圆柱形等离子体中观察到一个核心有密度腔的漩涡。空穴区的密度是外围等离子体的十分之一。空间电势在核心处有一个很强的峰值，由此产生的径向电场高达40 V/cm，驱动了超音速的方位旋转。还发现，在核心区，离心力支配电磁力，与洛伦兹力平衡，导致核心区发生刚体旋转。用定向朗缪尔探针测量了等离子体空穴的流速场，结果表明等离子体空穴在垂直于磁场的平面上有向核心的下沉流，也有轴向流。由流速场决定的涡度分布在中心轴附近呈现局域化，这意味着等离子体的黏度是有限的，导致了涡度的径向输运。根据速度场和涡度分布，将等离子体空穴识别为黏性流体所固有的Burgers涡旋，首次在等离子体中观测到。由涡度局域化特征尺度长度估算的有效黏度出现异常，比经典值高出3个数量级。

We have observed a vortex with a density cavity in its core, in a magnetized cylindrical plasma. The density of the hole region is one tenth of that in the peripheral plasma. The space potential has a strong peak in the core, and the resulting radial electric field becomes as high as 40 V/cm, which drives an azimuthal rotation at a supersonic velocity. It is also found that in the core region, centrifugal force dominates the electric force to be balanced with the Lorenz force, leading to a rigid rotation of the core. The flow velocity field has benn measured with a directional Langmuir probe, and it revealed that the plasma hole has a sink flow to the core on a plane perpendicular to the magnetic field and also an axial flow. The vorticity distribution determined by the flow velocity field shows localization near the center axis, which means that the viscosity of the plasma is finite and causes radial transport of vorticity. According to the flow velocity field and the vorticity distribution, the plasma hole is identified as a Burgers vortex, which is intrinsic in viscous fluids and is observed in a plasma for the first time. The effective viscosity estimated from the characteristic scale length of vorticity localization exhibits an anomaly, which is three orders of magnitude higher than the classical value.