Study of Internal Structure of a Field-Reversed-Configuration Plasma

本项目的目的是实验研究场反转组态(FRC)等离子体中发生的宏观不稳定性，众所周知，旋转不稳定性和内倾斜不稳定性的开始会对FRC造成严重的破坏，因为由于前者的不稳定性导致的等离子体变形很容易观察到。从FRC研究的早期阶段起，许多理论家和实验学家就对其进行了研究。大约十年前，人们发现这种变形是通过施加多极场来控制的。另一方面，由于后一种不稳定性发生在分界线内，用简单的诊断方法很难观察到它。针对这一障碍，研究这种不稳定性的速度非常慢，我们开发了一套拥有36个光学通道的诊断系统来研究分离线内的等离子体结构。每个通道由一个带有凸透镜的准直器、一根光纤、一个光学波长滤光片和一个光电倍增管组成。它们是使用标准灯进行校准的。这些准直器被布置在由透明石英管构成的真空室周围。该光学系统从给定平面上的等离子体中探测到韧致辐射。通过阿贝尔逆变换和计算机层析成像方法对数据进行分析，得到了以下结果。1.FRC环形面上的辐射强度分布与刚性转子模型的正方形密度分布相似，只是没有模模轴对称。2.在极向平面上观察到了等离子体的轴向膨胀和压缩。在扩张阶段，许多等离子体向FRC的两端移动。3.模拟了旋转不稳定性开始时等离子体的环形截面是椭圆形变的。然而，从目前的测量来看，它看起来像一个哑铃。

A purpose of the present project is to study experimentally macro instabilities occurred in a Field-Reversed Configuration (FRC) plasma It is well known that onsets of a rotational instability and an internal tilt instability give a severe damage to an FRC.As the deformation of the plasma due to the former instability can be easily observed. it has been investigated by many theorist and experimentarist from the early phase of the FRC research. It was found about ten years ago that the deformation is controlled by applying a multipole field. On the other hand, since die latter instability happens inside the separatrix, it is hard to observe by using simple diagnostic methods. According to this obstacle, the speed of the study about this instability is very slow.We developed a diagnostic system having 36 optical channels to investigate the structure of the plasma inside the separatrix. Each channel consists of a collimator with a convex lens, an optical fiber, an optical wavelength filter and a photomultiplier tube. They are calibrated by using a standard lamp. These collimators are arranged around a vacuum vessel which is constructed by a transparent quartz tube. This optical system detects bremsstrahlung from the plasma on a given plane.Following results are obtained from the data analyzed by an Abel inversion and a computer tomography methods. 1. Intensity profile of the radiation at a toroidal plane of the FRC is similar to a square density profile of the rigid rotor model except lack of die axial symmetry. 2. Vividly axial expansion and compression of the plasma is observed in a poloidal plane. A lot of the plasma moves toward both ends of the FRC at the expansion phase. 3. It has been modeled that the toroidal cross section of the plasma is elliptically deformed by the onset of the rotational instability. However, it looks like that of a dumbbell from the present measurement.