Formation of Viscous Vortex and Anomalous Viscosity in a Plasma

在磁化的柱状等离子体中，我们观察到了中心有一个密度腔的涡旋。空穴区的密度是周围等离子体密度的十分之一。空间电势在核心有一个很强的峰值，由此产生的径向电场高达40V/cm，从而以超音速驱动方位旋转。研究还发现，在核心区，离心力占主导地位，电动力与洛伦兹力相平衡，导致磁芯的刚性旋转。用定向朗缪尔探头测量了等离子体孔内的速度场，发现等离子体孔在垂直于磁场的平面上有一汇流到核心，也有一轴向流。由流动速度场确定的涡量分布在中心轴线附近呈现局部化，这意味着等离子体的粘性是有限的，导致涡量的径向输运。根据流动速度场和涡量分布，确定等离子体孔为Burgers涡，这是粘性流体中固有的，并首次在等离子体中观察到。由涡度局部化特征尺度长度估算的有效粘性比经典值高出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.