Turbulence in a Pure Superfluid

湍流，流体的混沌旋转运动，通常被描述为经典物理学中最重要的未解决问题。然而，湍流是每个人都熟悉的东西，从乘坐飞机时的颠簸到洗澡时水龙头里的水喷涌而出。它存在于从微观到银河系的所有尺度的流体流动中，人们对它进行了一个多世纪的深入研究，但仍难以完全理解。部分问题是由于构成经典流体（如水）湍流的漩涡或漩涡的性质不断变化。纯超流体中的湍流，如4He，冷却到绝对零度半度以内，是研究湍流现象的理想模型系统。量子力学表明，这种流体显示旋转运动的唯一方式是通过制造非常细（直径0.1纳米）相同的细丝，所有的涡量都集中在细丝周围，流体循环必须取一个特定的固定值。这些物体是量子化的漩涡，湍流完全可以用这些线的混乱混乱来描述。我的建议试图回答这个基本问题，即这种特殊类型的湍流是如何衰减的。不存在像经典流体那样由于黏性而产生的摩擦机制，因此认为耗散是由于声波从高频波发射到单个涡线上造成的，但这需要实验验证。能量是如何从大尺度（厘米）转移到这些小尺度（纳米）的，这一点还不太清楚，有几个相互竞争的理论观点。因此，尽管湍流在这种流体中以概念上简单的形式存在，但仍有大量新的物理学有待探索，这使得新的实验研究已经成熟。为了显著推进我们的理解，我将进行几种不同类型的实验，这些实验将在跨越六个数量级的巨大长度范围内探测湍流。这将包括通过观察一束微米大小的漩涡环如何被湍流缠结散射来监测缠结中漩涡线的密度如何随时间衰减。当一个旋转的容器突然停止时，会产生均匀的湍流，这将引起超流体流过网格。此外，灵敏量热法将用于测量微观尺度上由于耗散而释放的热量，从而深入了解衰变机制。我还将开发一些技术来发现在单个线的尺度上发生了什么，比如通过观察最初笔直的涡旋线在其末端被震动时的表现。这将允许一种波湍流，其中能量被转移到较短波长的波由于非线性相互作用，被探测。最后一种实验将检验一种奇特的探测粒子——亚稳氦分子能否被困在涡旋线的核心上。如果是这样的话，那么在未来，这些粒子可以用来可视化湍流涡旋缠结。所有这些实验的共同特点是需要旋转仪器来产生直线涡线（在稳定旋转期间）和产生湍流（通过突然停止）。因此，通过对旧的旋转低温恒温器进行大规模翻新，将建造一个新的旋转毫开尔文低温恒温器。这台最先进的新仪器将对量子湍流的实验至关重要，但在未来，它也将有能力探测我们对液态和固态氦的理解中仍然存在的许多谜团。

Turbulence, the chaotic swirling motion of a fluid, is often described as the most important unsolved problem of classical physics. Yet turbulence is something that everyone has some familiarity with, from getting bumped around while flying on an aircraft to the water gushing out of the taps while running a bath. It occurs in fluid flows on all scales, from the microscopic to the galactic and has been studied intensively for well over a century but a full understanding remains elusive. Part of the problem is due to the constantly changing nature of the vortices, or eddies, that make up the turbulence in a classical fluid (such as water).Turbulent flow in a pure superfluid, such as 4He cooled to within half a degree of absolute zero is an ideal model system for investigating turbulent phenomena. Quantum mechanics dictates that the only way such a fluid can display rotational motion is through the creation of very fine (0.1 nm diameter) identical filaments where all the vorticity is concentrated and around which the fluid circulation has to take a particular fixed value. These objects are quantized vortices and the turbulent flow is completely described by a messy tangle of these lines.My proposal seeks to answer the fundamental question as to how this special type turbulence decays. There is no frictional mechanism due to viscosity, such as occurs in a classical fluid, so it is believed that dissipation occurs due to sound being emitted from high frequency waves on individual vortex lines but this needs to be checked experimentally. How energy can be transferred from the large scales (centimetres) where the fluid is stirred down to these small length (nanometres) scales is even less well understood and there are several competing theoretical ideas. Thus, even though turbulence takes a conceptually simple form in this fluid, there is plenty of new physics to explore, making it ripe for new experimental research.To significantly advance our understanding, I will perform several different types of experiment that will probe the turbulence over a huge range of length scales, spanning six orders of magnitude. This will involve monitoring how the density of vortex lines in the tangle decays with time by observing how a beam of micron-sized vortex rings is scattered by the turbulent tangle. Homogeneous turbulence will be generated by suddenly stopping a rotating container which will induce superfluid flow through a grid. In addition, sensitive calorimetry will be used to measure the heat released due to dissipation at microscopic scales, providing insight into the decay mechanism. I will also develop techniques to discover what happens on the scale of individual lines, such as by looking how initially straight vortex lines behave when their ends are shaken. This will allow a type of wave turbulence, where energy is transferred to shorter wavelength waves due to non-linear interactions, to be probed. The final type of experiment will check whether an exotic type of probe particle, metastable helium molecules, can be trapped on the cores of the vortex lines. If so, then in the future these particles could be used to visualize the turbulent vortex tangles. The common feature of all these experiments is the need to rotate the apparatus to create rectilinear vortex lines (during steady rotation) and generate turbulence (by suddenly stopping). Thus, a new rotating millikelvin cryostat will be constructed through the extensive refurbishment of an old rotating cryostat. The new state-of-the-art instrument will be vital for the proposed experiments on quantum turbulence, but it will also be capable, in the future, of probing many of the mysteries that still remain in our understanding of liquid and solid helium.