Bubbles, Ions and Vortices in Cryogenic Fluids

这个低温物理项目由三个部分组成。(1)利用扭振仪测量超流密度，研究吸附在多孔材料中的超薄氦-4薄膜的超流相变。这样就可以测量超流膜中的涡旋核心直径。研究了He-3的加入对涡芯尺寸的影响，以及预镀2-3层氢对基底涡结构的影响。还将对一种称为MCM-41的新型多孔陶瓷材料进行测量，通过改变制造工艺，材料的孔径可以在16到100之间变化。将进行氦膜涡旋的理论研究，研究有限超电流对Kosterlitz-Thouless超流体跃迁时涡旋对的影响。(2)在氮气和氧气等低温液体中寻找声捕获气泡的声致发光现象，并利用激光散射研究气泡的振荡动力学。低温液体具有纯净和惰性的优点。在紫外区，它们也比水更透明，大部分声致发光能量都是在紫外区发射的。(3)向液氦中注入电子也会产生气泡，形成足够大的多电子气泡，从而被困在声场中。将测量气泡半径作为电子数的函数，以及气泡的振荡动力学。这个低温物理项目由三个部分组成。(1)研究液氦膜的超流特性，重点了解液体中量子化的涡旋流线如何使无摩擦的超流停止。测量液氦中超流体的比例将使旋涡结构的大小得以确定。一种新型的陶瓷材料将被用作氦膜的衬底，其孔径可以在16到100之间变化。将进行理论研究，以提供解释实验结果的框架。这项工作在高温超导体中也有应用，在超导跃迁附近，量子化的涡旋通量线被激发。这些线的作用是使电子的无摩擦流动停止。(2)在氮气和氧气等低温液体中寻找气泡的声致发光，并利用激光散射研究气泡表面的振荡运动。从水中振荡气泡中产生100皮秒的光爆发的机制仍然不清楚，找到其他液体中发生这种现象是很重要的。稀有气体低温液体具有非常纯净和惰性的重要优点，并且在光谱的紫外区域比水更透明，大部分光都是从气泡中发射出来的。(3)在液氦中也可以通过向液体中注入电子来产生气泡，形成里面有几百万个电子的小气泡。将测量气泡半径作为电子数的函数，以及气泡表面的振荡运动。＊＊＊

9731523 Williams This low temperature physics project is composed of three components. (1) The superfluid phase transition of very thin He-4 films adsorbed in porous materials will be studied, using a torsion oscillator to measure the superfluid density. This allows a measurement of the vortex core diameter in the superfluid films. The increase of in the vortex core size with added He-3 will be studied, and also the effect on the vortex structure of pre-plating the substrate with 2-3 atomic layers of hydrogen. Measurements will also be undertaken in a new porous ceramic material, known as MCM-41, where the pore size of the material can be varied between 16 and 100 by changing the fabrication process. Theoretical studies of the vortices in helium films will be carried out, investigating the effect of a finite supercurrent on the vortex pairs at the Kosterlitz-Thouless superfluid transition. (2) A search for sonoluminescence from bubbles acoustically trapped in cryogenic liquids such as nitrogen and oxygen will be undertaken, and the oscillation dynamics of the bubbles will be studied using laser scattering. The cryogenic liquids have the advantages of being pure and inert. They are also more transparent than water in the ultraviolet region, where most of the sonoluminescent energy is emitted. (3) Bubbles will also be created in liquid helium by injecting electrons into the liquid, forming multi-electron bubbles large enough to become trapped in the acoustic fields. The bubble radius as function of the electron number will be measured, and also the oscillation dynamics of the bubble. %%% This low temperature physics project is composed of three components. (1) The superfluid properties of liquid helium films will be studied, with the emphasis on understanding how quantized vortex flow line in the liquid can bring the frictionless superfluid flow to a halt. Measurem ents of the fraction of the liquid helium that is superfluid will allow the size of the vortex structures to be determined. A new type of ceramic materials will be employed as a substrate for the helium films, one whose pore size can be varied between about 16 and 100 . Theoretical studies will be carried out to provide a framework for interpreting the experimental results. This work has applications to the High-Tc superconductors, where it is just now being understood that quantized vortex flux lines are excited near the superconducting transition. These lines have the effect of bringing to a halt the frictionless flow of the electrons. (2) A search for sonoluminescence from bubbles in cryogenic liquids such as nitrogen and oxygen will be undertaken, and the oscillatory motion of the surface of the bubbles will be studied using laser light scattering. The mechanism that results in the 100 picosecond bursts of light from oscillating bubbles in water is still not understood, and it is important to find other liquids where the phenomenon occurs. The noble-gas cryogenic liquid have the important advantages that they are very pure and inert, and are more transparent than water in the ultraviolet region of the spectrum where most of the light is emitted from the bubbles. (3) Bubbles will also be created in liquid helium by injecting electrons into the liquid, forming small bubbles with up to several million electrons inside. The radius of the bubble as a function of the number of electron will be measured, and also the oscillatory motion of the bubble surface. ***