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绑箍缩实验进行研究。 在周期性环形结构中的实验研究有着悠久的历史，它建立了对基于模式有理（或模式谐振）表面存在的重联的理解，其中平行波数k_||消失了然而，在天体物理学中，经常观察到或推断有界系统中的重联，其中没有消失的k_ll。因此，仍然存在着一些主要的，大多数是未知的问题，即重连是如何发生的，它与周期性重连有何不同，以及什么概念将取代重连的k_ll识别。 这是一个主题可以说是可比范围内的周期性reconnection，但在这方面的研究一直scarless.In这项工作中，诊断将被构造为观察大，本地化的电流和由此产生的reconnection预测存在于饱和阶段的线捆绑扭结不稳定性。这将在两种不同的磁平衡中进行。第一种是简单的线性箍缩，其中不稳定性已经在实验中观察到。 第二个将是零净电流平衡，一个类似的线绑平衡的通量管出现在太阳日冕和扭曲的旋涡运动的通量管的脚点。 这些实验将在旋转壁机器中进行（该机器也有一个聚变科学使命，由能源部支持）。实验是一个线性螺旋箍缩，并使用静电等离子体枪产生载流柱。它的主要使命是研究通过移动金属壁来稳定电阻壁模式。在实验中观察到了内部扭结不稳定性的增长和饱和。 详细的测量结果表明，一个理想的，线绑扭结模式开始增长时，有效的安全系数（测量场线扭曲）下降到1以下的等离子体内部;饱和状态对应于旋转螺旋平衡。 除了理想模式，重联事件被观察到周期性地平坦化电流分布和改变磁拓扑结构。 重联事件非常类似于一个几乎相同的几何形状的数值模拟中描述的重联现象。数值模拟预测存在一个大的，本地化的电流，从而引起重联。 这些电流片显着不同的电流片在环形实验中预测：其本征函数在轴向方向以及在径向方向上的结构。我们建议在等离子体柱的轴向中点处的6 × 6网格上安装一个2D阵列的磁探针，以发现该电流片是否存在。我们还建议修改等离子枪的几何形状，使中央枪注入电流在相反的方向周围的7枪。这种几何形状更接近于从太阳表面出现的通量管中预期的电流分布类型。结果将进行比较，分析理论和数值模型的实验中实施的NIMROD codes.This建议被提交给NSF能源部合作伙伴在等离子体科学和工程联合招标08 - 589。该奖项由数学和物理科学理事会物理和天文科学司共同资助。

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.