Apparatus to visualize flows in superfluid $^4$He 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 dilution refrigerator on a vibrationally-isolated platform, capable of continuous rotation at angular velocity of up to 3 rad/s. 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 $10^5$ optical fibers giving a spatial resolution of $\sim 30 \mu$m, 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 experiment, fluorescent light from clouds of excimer molecules He$_2^*$, generated in liquid helium by electron impact from electrons injected by sharp field-emission tips, was recorded and analyzed. In another, fluorescent particles of diameters between 1 $\mu$m and 6 $\mu$m were initially loaded onto the horizontal surface of a piezoelectric crystal of LiNbO$_3$ and then injected into liquid helium by short bursts of high-amplitude oscillations at the crystal's resonant frequency 1 MHz. The particle trajectories were filmed at a frame rate of up to 990 fps and analyzed.