Apparatus to visualize flows in superfluid 4He below 1 K

We describe a versatile apparatus for optical observations of experimental processes at temperatures down to 0.1 K. The cooling is achieved by a wet cryostat with a 3He-4He dilution refrigerator on a vibrationally isolated platform, capable of continuous rotation at an angular velocity of up to 3 rad s-1. The illumination light beam from lasers on a non-rotating optical table at room temperature is introduced via an optical fiber. The images are transferred to the intensified camera at room temperature through a coherent bundle of 105 optical fibers, limiting the spatial resolution to ∼30μm, depending on the magnification used. The adjustment of the position of the illumination light, as well as of the focusing of the camera on the object under investigation, can be controlled remotely with the help of piezoelectric positioners. The apparatus was used for visualization of particles dispersed in superfluid helium at temperatures down to 0.14 K. In one version of the experiment, fluorescent light from clouds of excimer molecules He2*, generated in liquid helium by electron impact from electrons injected by field-emission tips, was recorded. In another, fluorescent particles of mean radius 3.0 μm were initially loaded onto the horizontal surface of a piezoelectric crystal of LiNbO3 and then injected into liquid helium by bursts of high-amplitude oscillations at the crystal's resonant frequency 1 MHz. The trajectories of particles in superfluid helium could be recorded at a frame rate of up to 990 fps. The effective radius of particle images is ∼60μm, while the uncertainty on the positions of their centers is ∼5μm.