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

本项目的目的是实验研究场反转结构（FRC）等离子体中发生的宏观不稳定性。众所周知，旋转不稳定性和内部倾斜不稳定性的发生会对场反转结构造成严重的破坏。由于等离子体由于前者的不稳定性而产生的变形可以很容易地观察到。从FRC研究的早期开始，许多理论家和实验家就对其进行了研究。大约在十年前，人们就发现通过施加一个多极场来控制变形。另一方面，由于模具后失稳发生在分离矩阵内部，用简单的诊断方法很难观察到。根据这一障碍，对这种不稳定性的研究速度非常缓慢。我们开发了一个具有36个光通道的诊断系统来研究分离矩阵内等离子体的结构。每个通道由一个带凸透镜的准直器、一根光纤、一个光学波长滤波器和一个光电倍增管组成。它们是用标准灯校准的。这些准直器被安排在由透明石英管构成的真空容器周围。该光学系统检测给定平面上等离子体的轫致辐射。通过阿贝尔反演和计算机断层扫描方法对数据进行分析，得到以下结果。1. FRC环面辐射强度分布图与刚性转子模型的方形密度分布图相似，只是缺少模具轴对称。2. 在极向平面上观察到等离子体的轴向膨胀和压缩。在膨胀阶段，很多等离子体向FRC的两端移动。3. 模型表明，等离子体的环面截面在旋转不稳定性开始时发生椭圆变形。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.