Experimental Investigation of Reconnection in a Line-tied Plasma

在这项工作中，磁重联现象将在一个线捆绑的箍缩实验中被研究。周期环形组态的实验研究已经有很长的历史了，它建立了一种基于模式有理(或模式共振)表面存在的重连的理解，在那里平行波数k||为零。然而，在天体物理学中，观测或推断有界系中的重联是很常见的，在有界系系中没有消失的k_1。因此，重新连接如何发生，它与周期性重新连接有何不同，以及哪些概念将用重新连接取代k_ll标识，仍然是主要的、大多是未知的问题。这是一个可以与周期性重联相媲美的课题，但在这一领域的研究一直很少。在这项工作中，将构建一个诊断方法，用于观察大的局域电流以及由此产生的重联，预计将存在于线路连接扭结不稳定的饱和阶段。这将在两种不同的磁平衡下进行。第一种是简单的线性箍缩，已经在实验中观察到了不稳定性。第二个将处于零净电流平衡，类似于从日冕中出现的通量管的线系平衡，并因通量管足点的涡旋运动而扭曲。这些实验将在旋转墙机(它也有一项聚变科学任务，由能源部支持)中进行。该实验是一种线性螺旋夹持，使用静电等离子体枪来产生载流柱。它的主要任务是研究通过移动金属墙来稳定阻性墙模式。在实验中观察到了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(测量场线扭曲)降到1以下时，理想的线束缚扭结模式开始增长；饱和状态对应于旋转的螺旋平衡。除了理想模式外，还观察到了重新连接事件，以定期平坦电流分布并改变磁拓扑结构。重新连接事件与在几乎相同的几何图形的数值模拟中描述的重新连接现象非常相似。数值模拟预测存在引起重联的大的局域电流。这些电流片与环形实验中预测的电流片有很大的不同：它们的本征函数在轴向和径向都有结构。我们建议在等离子体柱轴向中点的6×6栅格上安装一个二维磁性探头阵列，以发现这个电流片是否存在。我们还建议修改等离子喷枪的几何形状，使中央喷枪向与周围7支喷枪相反的方向注入电流。这种几何形状更接近于从太阳表面涌出的通量管中预期的电流分布类型。结果将与分析理论和在Nimrod代码中实现的实验的数值模型进行比较。这项建议被提交给NSF-DOE在等离子体科学和工程方面的合作伙伴关系08-589。该奖项由数学和物理科学局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.