Experimental Investigation of Ion Diffusion Suppression due to Zonal Flow in a Magnetized Plasma

1）我们观察到一个离子流流入圆柱形磁化等离子体的中心轴，轴上的电压是负的，通过施加负电压到轴端的板上。该装置的优点是离子电流的清晰检测，因为流入中心趋化的离子是在圆柱形等离子体的外围产生的。随着轴电位的增加，我们发现了四个区域：（A）碰撞离子流的形成，（B）径向离子流的增加和螺旋波的激发，（C）径向离子流的减少和离轴旋转运动，（D）限制离子流和形成局部低密度环。在中心轴处的离子密度的增量从外围流动迅速地激发m=1或2的旋转螺旋模式。结果，等离子体扩展了螺旋结构，并在径向方向上扩散离子。为了阐明这种结构，我们利用Rosenbulth & Simon导出的包含离子Larmor半径效应的离子密度微扰方程和电子密度微扰方程，导出了本征函数。理论结果很好地解释了小扰动情况下的实验数据。3）进一步增加圆柱形等离子体中心轴的负电压，出现低密度环，表明存在带状流，减小了扰动和扩散离子流。在中心轴附近存在较强的速度切变。利用上述方程分析了这种情况下的碰撞漂移不稳定性，可以解释在强电场作用下，这种不稳定性的所有模式和离子向中心轴的流动同时被抑制。

1)We observe an ion current flowing into the central axis of cylindrical magnetized plasma ; the voltage on the axis is negative by applying a negative voltage to the plate at the end of the axis. The advantage of this apparatus is the clear detection of ion current because the ions flowing into the central taxis is produced at the periphery of the cylindrical plasma. As increasing the potential of the axis, we find 4 regions, (A) formation of collisional ion flow, (B) increment of radial ion current and excitation of spiral waves, (C) decrement of radial ion current and off-axis rotating motion, and (D) limitation of ion current and formation of localized low density ring.2)The increment of ion density at the central axis flowing from the periphery rapidly excites rotating spiral modes of m=1 or 2. As the results, the plasma extends the spiral structure, and diffuses ions in the radial direction. In order to clarify this structure, we use an equation of ion density perturbation including ion Larmor radius effect derived by Rosenbulth & Simon and an equation of electron density perturbation, and derive eigen functions. The theoretical results well explain the data of the experiment in case of small perturbations.3) Further increasing the negative voltage of the central axis of the cylindrical plasma, there appears a low-density ring indicating the zonal flow, and reduces the perturbations and the diffusive ion flow. Here, the strong velocity shear exists around the central axis. Analyzing collisional drift instability at this condition by using the above equations, we are able to explain that all the mode of this instability and the ion flow into the central axis are suppressed at the same time, in case of strong electric field.