Experimental Investigation of Reconnection in a Line-tied Plasma

在这项工作中，磁重联现象将研究在一个线系夹紧实验。在周期性环面结构的实验研究中有很长的历史，这些研究基于模有理面（或模共振面）的存在而建立了对重连的理解，其中平行波数k_||消失。然而，在天体物理学中，观察或推断有界系统中的重联是很常见的，在有界系统中，没有消失的k_ll。因此，重连接是如何发生的，它与周期性重连接有何不同，以及哪些概念将用重连接取代k_ll标识，这些问题仍然是主要的，大部分是未知的。这是一个可以与周期性重连相提并论的话题，但这方面的研究却很少。在这项工作中，将构建一个诊断，用于观察在线系扭结不稳定的饱和阶段预测存在的大的局部电流和由此产生的重联。这将在两种不同的磁平衡中进行。第一种是简单的线性捏缩，实验中已经观察到不稳定性。第二个将处于零净电流平衡，类似于从太阳日冕中出现的通量管的线系平衡，并被通量管脚点的涡旋运动扭曲。实验将在旋转壁机（也有核聚变科学任务，由美国能源部支持）中进行。实验采用线性螺旋夹紧，采用静电等离子枪产生载流柱。其主要任务是研究移动金属壁对阻壁模态的稳定作用。在实验中观察到内部扭结不稳定性的增长和饱和。详细的测量表明，当等离子体内部的有效安全系数（测量场线扭转）降至1以下时，理想的线结模式开始生长；饱和状态对应于旋转的螺旋平衡。除了理想模式外，重联事件还观察到周期性地使电流剖面变平并改变磁拓扑结构。重联事件与几乎相同几何形状的数值模拟中描述的重联现象非常相似。数值模拟预测存在一个大的局部电流，引起重联。这些电流片与环面实验中预测的电流片有很大的不同：它们的特征函数在轴向和径向上都有结构。我们建议在等离子体柱轴向中点的6 × 6网格上安装一个二维磁探针阵列，以发现该电流片是否存在。我们还建议修改等离子体枪的几何形状，使中央枪以与周围7个枪相反的方向注入电流。这种几何形状更接近于太阳表面出现的通量管的电流轮廓。结果将与解析理论和NIMROD代码中实现的实验数值模型进行比较。该提案已提交给NSF-DoE等离子体科学与工程合作项目联合招标08-589。该奖项由数学和物理科学理事会的物理和天文科学部联合资助。

In this work, magnetic reconnection phenomena will be investigated in a line-tied pinch experiment. There is a long history of experimental studies in periodic, toroidal configurations which has built up an understanding of reconnection based on the existence of mode rational (or mode-resonant)surfaces where the parallel wave number k_|| vanishes. However, it is common in astrophysics to observe or infer reconnection in bounded systems with line-tying in which a vanishing k_ll is absent. Thus, there remain major, mostly uncharted questions of how reconnection occurs, how it differs from periodic reconnection, and what concepts will replace the k_ll identification with reconnection. It is a topic arguably of comparable scope to periodic reconnection, yet research in this area has been scarce.In this work, a diagnostic will be constructed for observing the large,localized current and resulting reconnection predicted to exist in the saturated phase of the line-tied kink instability. This will be carried out in two different magnetic equilibria. The first is a simple linear pinch in which instability has already been experimentally observed. The second will be in a zero net current equilibrium, one similar to the line-tied equilibria of flux tubes emerging from the solar corona and twisted by vortical motion of the flux tube footpoints. The experiments will be carried out in the Rotating Wall Machine (which also has a fusion science mission, supported by the DoE). The experiment is a linear screw pinch and uses electrostatic plasma guns for producing a current carrying column. Its primary mission is to investigate the stabilization of the resistive wall mode by moving metal walls. An internal kink instability has been observed to grow and saturate in the the experiment. Detailed measurements showed that an ideal,line-tied kink mode begins growing when the effective safety factor (which measures the field line twist) drops below 1 inside the plasma; the saturated state corresponds to a rotating helical equilibrium. In addition to the ideal mode, reconnection events were observed to periodically flatten the current profile and change the magnetic topology. The reconnection events strongly resemble the reconnection phenomena described in numerical simulations of a nearly identical geometry. Numerical simulations predict the existence of a large, localized current which gives rise to reconnection. These current sheets differ significantly from the current sheets predicted in toroidal experiments: their eigenfunctions have structure in the axial direction as well as in the radial direction. We propose to install a 2D array of magnetic probes on a 6 x 6 grid at the axial midpoint of the plasma column to discover whether this current sheet exists or not. We also propose to modify the plasma gun geometry such that the central gun injects current in the opposite direction to the surrounding 7 guns. This geometry more closely resembles the type of current profiles expected in the flux tubes emerging from the surface of the sun. The results will be compared to analytical theory and to a numerical model of the experiment implemented in the NIMROD code.This proposal was submitted to the NSF-DoE Partnership in Plasma Science and Engineering joint solicitation 08-589. This award is being funded jointly by the Divisions of Physics and Astronomical Sciences of the Mathematical and Physical Sciences Directorate.