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

经典流体中的湍流在理论和许多实际应用中都是重要的和具有挑战性的。该研究旨在了解经典湍流如何在超流体中被修改，其中流动受到与角动量量子化相关的量子条件的严重限制。在较高的温度下，超流体表现出两种流体行为，一种正常流体与量子效应重要的超流体成分共存。已经有强有力的证据表明，在这些温度下，大长度尺度上的湍流结构可以与经典的湍流结构非常相似，尽管在小尺度上的耗散过程是非常不同的。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，并且湍流衰减的简单机制已经消失。我们的计划旨在提供与这些低温有关的实验证据，在这些低温下，非常纯粹的量子湍流形式的基本行为应该是可以观察到的。量子湍流具有很大的内在意义，它的研究可以使我们更好地理解经典湍流。由看似随机纠缠的量子化涡线组成的乱流将通过稳定移动的网格或振荡网格在超流体中产生。振荡网格的力学行为将提供有关湍流成核的证据，量热观测或离子捕获检测将测量湍流衰减，这不仅反映了小尺度上的耗散，而且反映了整个湍流结构。离子捕获检测技术的定量应用依赖于捕获截面的测量，目前正在曼彻斯特大学进行。虽然振荡网格为在mK温度范围内产生量子湍流提供了一种经过验证的技术，但湍流并没有很好地表征，也不知道它的空间分布。从这些意义上说，尽管技术上要求更高，但稳定移动的网格是首选，因为它将产生既具有良好特征又具有空间均匀性的量子湍流。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学，初步的研究将证实这些激发确实可以被困在量子化的涡旋线上。拟议实验的成功取决于佛罗里达、伯明翰和兰开斯特之间密切互动合作的发展。到2006年6月底，兰开斯特实验组和伯明翰合作研究者已经得到了EPSRC的资助。他们与佛罗里达小组的合作是由EPSRC的访问奖学金发起的，我教授将于2005年10月至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年初提交一份继续和发展兰开斯特实验项目的研究资助申请。在目前的提案中，他们只是要求旅行/生活津贴，以支持他们在拟议的NSF MWN拨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.