Cryogenic cantilever micro-anemometer
The experimental study of 4He "quantum turbulence" or "superfluid turbulence", ie. the hydrodynamics of strongly stirred liquid helium at very low temperature (T < 2.17 K) requires specifically designed local probes. These probes need to be both very small and highly sensitive to measure the small scale velocity fluctuations that are fast and have a low amplitude. Hot wire are fantastic probes in classical turbulence but they don't work a-priori- in superfluids due to the high effective conductivity of He-II. Pitot tubes can work in superfluid helium but it is difficult to reach a spatial resolution better than 500 microns for a number of pratical reasons. Thus, a new cryogenic probe had to be immagined.
Principe and Realisation
Inspired by a previous probe designed in Oldenburg for air and water turbulence, we designed cryogenic velocity probe based on a cantilever. The sensitive element of the probe is the cantilever tip (300 µm long, 100 µm wide, 1 to 10 µm thick), etched in a bulk silicon wafer using fluoride Deep Reactive Ion Etching. This tip is immersed in the bulk of a flow and gets deflected by the incoming fluid. The amplitude of this deflection is proportional to the square of the flow velocity in the vicinity of the cantilever tip. A precise measurement of the deflection fluctuations is achieved using a radio-frequency superconducting niobium LC resonator sputtered on the tip whose resonance frequency shifts when the cantilever is elongated.
This technique prevented us to allow the presence of any disordered dielectric material on the sample because they would lead to phase noise of the LC resonator. This precluded us from using an oxide barrier layer. Therefore we had to devise a way to properly tune the thickness. We used the etching machine refrigerating helium leak rate as an progress indicator.
The first prototype has been cooled down and tested recently and showed good performances, similar to these of the best probes known to work in superfluid helium, and we believe that we can improve them even further in a near future.
Micro-machined cantilevers (200 micron x 50 microns)
This work was done in close collaboration with Alessandro Monfardini (Institut Néel) et the PTA/Minatec clean rooms facility.
For more information
Local velocity probe for cryogenic helium
Cryogenic Hydrodynamics Team, Institut Néel, CNRS/UJF
Renatech newsletter June 2011 page 3
Ph.R. Jan, 2012