Apparatus to visualize flows in superfluid $^4$He below 1 K

Abstract

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 $\sim 30 μ$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 $μ$m and 6 $μ$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.

Figures (6)

• Figure 1: A schematic of the excitation and imaging optics and the experimental cell. This is the setup used for experiments with excimers and early tests with microspheres: the illuminating light sheet has horizontal profile, and the imaging is through the top window of the cell. In later experiments with microspheres, the light sheet had vertical profile, and the imaging was through a side window. (A) multimode fiber, (B) combination of spherical and cylindrical lenses, (C) movable prism for shifting the light sheet, (D) experimental cell, (E) imaged region inside the cell, (F) dump, (G) achromat lens, (H) movable objective for focusing of the illuminated region onto the fiber bundle, (I) coherent fiber bundle.
• Figure 2: Temperatures of the cell (top curve, red line and symbols) and of the mixing chamber (bottom curve, blue line and symbols) after firing a 100 $\mu$s pulse of 1.0 kV voltage at the piezo transducer at low temperature.
• Figure 3: Elements of the optical set up, grouped by their location by grey rectangles. The two groups at the top are at room temperature outside the cryostat; the left one on a stationary optical table, while the right one is mounted on top of the cryostat. The bottom group is inside the vacuum can of the dilution refrigerator.
• Figure 4: Received photon flux vs. temperature for excimers generated by pulses of electron currents of order 1 nA for 0.2 seconds.
• Figure 5: An image of particles taken at 200 fps (left panel), along with superimposed particle trajectories in red (right panel), $T=0.8$ K.