Laboratory Search for Magnetorotational Instability

在天体物理学中，一个比较常见但不太为人所知的现象是吸积盘中带电等离子体的轨道能量耗散。这种普遍现象发生在各种各样的情况下，如原行星盘和活动星系核。耗散被认为是由一种或两种不同的现象引起的。首先，非线性流体动力学剪切流不稳定性可以由圆盘中以不同速度运动的多层材料引起，类似于更简单的开尔文-亥姆霍兹不稳定性。第二，在等离子体的带电和磁化流动中会出现线性不稳定性。后一种现象称为磁旋不稳定性（MRI）。该团队已经进行了一系列实验，似乎排除了流体动力剪切流不稳定性是轨道能量耗散的原因。他们现在正试图利用带电液态金属进行一系列实验来探索核磁共振成像。该团队还将培养一名博士后和一名研究生。普林斯顿大学已经建立了两个实验来检验可能导致天体物理吸积盘湍流的动态不稳定性。流体动力学湍流实验（HTX）研究剪切流等不稳定性；液态金属核磁共振实验旨在研究磁旋不稳定性。到目前为止，HTX提供的结果似乎排除了剪切流不稳定性是吸积盘中轨道能量耗散的主要原因。然而，MRI假说仍未得到证实。由于液态金属是不透明的，研究小组将在液态金属核磁共振实验中增加新的诊断方法，包括超声多普勒测速仪来测量流场。电机将升级，以允许在更高的转速（Rm 4.5）下运行，团队的模拟表明MRI应该出现。实验活动将伴随着数值模拟，以更好地理解实验室结果。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

One of the more common, but less understood, phenomena in astrophysics is the dissipation of orbital energy of charged plasma in accreting disks. This general phenomenon occurs in situations as diverse as protoplanetary disks and active galactic nuclei. The dissipation is thought to arise due to one or two distinct phenomena. First, a non-linear hydrodynamic shear flow instability can arise from multiple layers of material in a disk moving at different speeds, similar to the simpler Kelvin-Helmholtz instability. Second, a linear instability can arise in the charged and magnetized flow of a plasma. This latter phenomenon is called a magnetorotational instability (MRI). The team has already conducted a series of experiments that seem to exclude hydrodynamic shear flow instability as the reason for the dissipation of orbital energy. They now seek to conduct a series of experiments using charged liquid metal to explore MRI. The team will also train a postdoc and a graduate student.Two experiments have been set up at Princeton to examine dynamic instabilities that might lead to turbulent flow in astrophysical accretion disks. The Hydrodynamics Turbulence Experiment (HTX) studies instabilities such as shear flow instability; the Liquid-Metal MRI experiment is meant to study magnetorotational instability. So far, HTX has provided results that seem to preclude shear flow instability as a major contributor to the dissipation of orbital energy in accretion disks. However, the MRI hypothesis remains unproven. The team will add new diagnostics to the Liquid-Metal MRI experiment, to include ultrasound doppler velocimetry to measure the flow field, since the liquid metal is opaque. Motors will be upgraded to allow operation at higher rotation speeds (Rm 4.5) where the team’s simulations indicate that MRI should arise. The experimental campaign will be accompanied by numerical simulations to better understand the laboratory results.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.