35 research outputs found
Vibrating quartz crystal as a tool for studying the flow of cryogenic fluids
Department of Condensed Matter PhysicsKatedra fyziky kondenzovaných látekFaculty of Mathematics and PhysicsMatematicko-fyzikální fakult
Multiple critical velocities in oscillatory flow of superfluid 4He due to quartz tuning forks
We report recent investigations into the transition to turbulence in superfluid He, realized experimentally by measuring the drag forces acting on two custom-made quartz tuning forks with fundamental resonances at 6.5 kHz and 55.5 kHz, in the temperature range 10 mK to 2.17 K. In pure superfluid in the zero temperature limit, three distinct critical velocities were observed with both tuning forks. We discuss the signicance of all critical velocities and associate the third critical velocity reported here for the first time with the development of large vortical structures in the flow, which thus starts to mimic turbulence in classical fluids. The interpretation of our results is directly linked to previous experimental work with oscillators such as tuning forks, grids and vibrating wires, focusing on the behavior of purely superfluid He at very low temperature
Very low resistance Al/Cu joints for use at cryogenic temperatures
We present two different techniques for achieving low resistance (20 n) contacts between copper and aluminium at cryogenic temperatures. The
best method is based on gold plating of the surfaces in an e-beam evaporator
immediately after Ar plasma etching in the same apparatus, yielding resistances
as low as 3 n that are stable over time. The second approach
involves inserting indium in the Al/Cu joint. For both methods, we believe key
elements are surface polishing, total removal of the aluminum oxide surface
layer, and temporary application of large (typ. 11 kN) compression forces. We
believe the values for gold plated contacts are the lowest ever reported for a
Cu/Al joint of a few . This technology could simplify the
construction of thermal links for advanced cryogenics applications, in
particular that of extremely low resistance heat switches for nuclear
demagnetization refrigerators.Comment: Accepted by Journal of Low Temperature Physic
Acoustic emission in bulk normal and superfluid 3He
We present measurements of the damping experienced by custom-made quartz tuning forks submerged in 3He covering frequencies from 20 kHz to 600 kHz. Measurements were conducted in the bulk of normal liquid 3He at temperatures from 1.5 K down to 12 mK and in superfluid 3He-B well below the critical temperature. The presented results complement earlier work on tuning fork damping in 3He, removing possible ambiguities associated with acoustic emission within partially enclosed volumes and extend the probed range of frequencies, leading to a clearly established frequency dependence of the acoustic losses. Our results validate existing models of damping and point toward the same mechanism of wave emission of first sound in normal 3He and liquid 4He and zero sound in superfluid 3He. We observe a steep frequency dependence of the damping ≈ f5.5, which starts to dominate around 100 kHz and restricts the use of tuning forks as efficient sensors in quantum fluids. The acoustic emission model can predict the limiting frequencies for various devices, including micro-electromechanical and nano-electromechanical structures developed for quantum turbulence and single vortex dynamics research
Superconducting aluminum heat switch with 3 n equivalent resistance
Superconducting heat switches with extremely low normal state resistances are
needed for constructing continuous nuclear demagnetization refrigerators with
high cooling power. Aluminum is a suitable superconductor for the heat switch
because of its high Debye temperature and its commercial availability in high
purity. We have constructed a high quality Al heat switch whose design is
significantly different than that of previous heat switches. In order to join
the Al to Cu with low contact resistance, we plasma etched the Al to remove its
oxide layer then immediately deposited Au without breaking the vacuum of the
e-beam evaporator. In the normal state of the heat switch, we measured a
thermal conductance of W/K which is equivalent to an electrical
resistance of 3 n according to the Wiedemann-Franz law. In the
superconducting state we measured a thermal conductance that is
times lower than that of the normal state at 50 mK
A low-frequency, high-amplitude, torsional oscillator for studies of quantum fluids and solids
We introduce a low-frequency torsional oscillator suitable for studies of quantum fluids and solids. It operates at frequencies of ∼100 Hz, achieves velocities of several cm s−1, and exhibits a quality factor of Q ≃ 3×10^4. In order to achieve such velocities at this relatively low frequency, the oscillator amplitude must exceed 100 μm, which would be impracticable for a conventional capacitor-driven device where the drive is applied parallel to the main motion and there are correspondingly large changes in the separation of the capacitor plates. For the different geometry of the oscillator that we now describe, however, the separations of both the drive and detect capacitor plates remain constant regardless of the amplitude of oscillation. We discuss its design, and report our initial tests of its performance
Vibrating quartz crystal as a tool for studying the flow of cryogenic fluids
Katedra fyziky nízkých teplotDepartment of Low Temperature PhysicsFaculty of Mathematics and PhysicsMatematicko-fyzikální fakult