NSF World Materials Network: A Collaborative Experimental Investigation of Pure Quantum Turbulence in Superfluid He-4 at Very Low Temperatures

经典流体中的湍流无论在理论上还是在实际应用中都是非常重要和具有挑战性的。这项研究旨在了解经典湍流是如何在超流体中被修正的，在超流体中，流动受到与角动量量子化相关的量子条件的严格限制。在更高的温度下，超流体表现出双流体行为，正常流体与超流体成分共存，其中量子效应是重要的。已经有强有力的证据表明，在这些温度下，大尺度上的湍流结构可以非常类似于它们的经典对应结构，尽管在小尺度上作用的耗散过程非常不同。人们怀疑，在非常低的温度下也存在类似的情况，在那里没有正常的流体，而且湍流衰变的简单机制已经消失。我们的计划试图提供与这些低温有关的实验证据，在这种低温下，应该可以观察到非常纯粹形式的量子湍流的基本行为。量子湍流具有很大的内在兴趣，它的研究可以帮助我们更好地理解经典湍流。由看似随机的量子化涡旋线组成的湍流/将在超流中由稳定移动的网格或振荡的网格产生。振荡网格的力学行为将提供湍流成核的证据，而量热观测或离子捕获的检测将测量湍流衰减，这不仅反映了小尺度上的耗散，而且反映了整体湍流结构。离子捕获探测技术的定量应用取决于对捕获截面的测量，目前曼彻斯特大学正在进行这项工作。虽然振荡网格提供了一种在MK温度范围内产生量子湍流的成熟技术，但这种湍流并不是很好地表征，它的空间分布也是未知的。在这些意义上，尽管在技术上要求高得多，但稳定移动的网格将是首选，因为它将产生具有良好特征和空间均质性的量子湍流。一些有希望的、更灵敏的新探测方法正在出现，可能包括中性激发的光谱，初步研究将证实激发确实可以被捕获在量子化的涡旋线上。拟议的实验的成功取决于佛罗里达州、伯明翰和兰开斯特之间密切互动合作的发展。截至2006年6月底，兰开斯特实验小组和伯明翰联合调查员已经由EPSRC为这项工作提供资金。他们与佛罗里达小组的合作是由EPSRC访问奖学金发起的，IHAS教授将在2005年10月/2006年5月访问兰开斯特。佛罗里达小组建议在NSF材料世界网络基金的支持下开展这项工作；兰开斯特/伯明翰小组将在2006年初提交继续和发展兰开斯特实验方案的研究补助金申请。在目前的提案中，他们只是申请旅行/生活费，以支持他们在拟议的NSF MWN赠款期间与佛罗里达小组的合作。

Turbulence in classical fluids is important and challenging both for theory and for many practical applications. The research aims to understand how classical turbulence is modified in a superfluid, in which flow is severely restricted by quantum conditions associated with the quantization of angular momentum.At higher temperatures, superfluids exhibit two-fluid behaviour, a normal fluid coexisting with the superfluid component in which the quantum effects are important. There is already strong evidence that at these temperatures turbulent structures on large length scales can be very similar to their classical counterparts, although dissipative processes acting on small scales are very different. It is suspected that a similar situation exists at very low temperatures, where the normal fluid is absent, and where simple mechanisms for the decay of the turbulence have disappeared. Our programme seeks to provide experimental evidence relating to these low temperatures, at which the fundamental behaviour of very pure forms of quantum turbulence ought to be observable.Quantum turbulence is of great intrinsic interest, and its study could lead to a better understanding of classical turbulence.The turbulence / comprised of a seemingly random tangle of quantized vortex lines / will be generated in the superfluid either by a steadily moving grid or by an oscillating grid. The mechanical behaviour of the oscillating grid will provide evidence about the nucleation of turbulence, and calorimetric observations or the detection of ion trapping will measure the turbulent decay, which reflects not only dissipation on small scales but also the overall turbulent structure. The quantitative application of the ion trapping detection technique is dependent on measurements of the trapping cross-section, currently in progress at the University of Manchester. Although the oscillating grid provides a proven technique for creating quantum turbulence in the mK temperature range, the turbulence is not well-characterised and nor is its spatial distribution known. In these senses, although technically far more demanding, the steadily moving grid is to be preferred because it will create quantum turbulence that is both well-characterised and spatially homogeneous. Some promising new, more sensitive, detection methods are appearing on the horizon, possibly including the spectroscopy of neutral excitations, and preliminary studies will be made to confirm that the excitations can indeed be trapped on quantized vortex lines.Success of the proposed experiments is dependent on the development of close interactive collaboration between Florida, Birmingham and Lancaster. The Lancaster experimental group and the Birmingham Co-Investigator are already funded by EPSRC for this work up to the end of June 2006. Their collaboration with the Florida group is being initiated by an EPSRC Visiting Fellowship for Professor Ihas to visit Lancaster during October 2005 / May 2006. The Florida group proposes to develop the work with the support of NSF Materials World Network funding; the Lancaster/Birmingham group will submit a research grant application for continuation and development of the Lancaster experimental programme early in 2006. In the present proposal they just request travel/subsistence to support their collaboration with the Florida group during the period of the proposed NSF MWN grant.