Formation of Viscous Vortex and Anomalous Viscosity in a Plasma

在磁化圆柱形等离子体中，我们观察到一个中心有密度空腔的涡旋。空穴区的密度是外围等离子体的十分之一。空间电势在核中具有强峰值，并且由此产生的径向电场变得高达40 V/cm，其驱动以超音速的方位角旋转。在核区，离心力占主导地位的电力与洛伦兹力平衡，导致核的刚性旋转。用定向朗缪尔探针测量了流场，发现等离子体孔在垂直于磁场的平面上有向芯的下沉流，也有轴向流。由流速场确定的涡量分布在中心轴附近表现出局部化，这意味着等离子体的粘性是有限的，并导致涡量的径向输运。根据流场和涡量分布，将等离子体空洞确定为Burgers涡，这是粘性流体中的一种本征涡，也是首次在等离子体中观测到。由涡度局部化特征尺度长度估算的有效粘滞系数出现异常，比经典值高出三个数量级。

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.