Gary G. Ihas

Pipe flow measurements over a wide range of Reynolds numbers using liquid helium and various gases

We demonstrate that an unusually small pipe flow apparatus using both liquid helium and room temperature gases can span an enormous range of Reynolds numbers. This paper describes the construction and operation of the apparatus in some detail. A wide range of Reynolds numbers is an advantage in any experiment seeking to establish scaling laws. This experiment also adds to evidence already in hand that the normal phase of liquid helium is a Navier–Stokes fluid. Finally, we explore recent questions concerning the influence of molecular motions on the transition to turbulence (Muriel 1998) and are unable to observe any influence.

Visualization of the normal-fluid turbulence in counterflowing superfluid He 4

We describe a new technique, using thin lines of triplet-state He2 molecular tracers created by femtosecond-laser field-ionization of helium atoms, for visualizing the flow of the normal fluid in superfluid 4He, together with its application to thermal counterflow in a channel. We show that, at relatively small velocities, where the superfluid is already turbulent, the flow of the normal fluid remains laminar, but with a distorted velocity profile, while at a higher velocity there is a transition to turbulence. The form of the structure function in this turbulent state differs significantly from that found in types of conventional turbulence. This visualization technique also promises to be applicable to other fluid dynamical problems involving cryogenic helium.

Excimers He2* as Tracers of Quantum Turbulence in 4He in the T=0 Limit

We have studied the interaction of metastable 4He2* excimer molecules with quantized vortices in superfluid 4He in the zero temperature limit. The vortices were generated by either rotation or ion injection. The trapping diameter of the molecules on quantized vortices was found to be 96±6  nm at a pressure of 0.1 bar and 27±5  nm at 5.0 bar. We have also demonstrated that a moving tangle of vortices can carry the molecules through the superfluid helium.

Low Temperature Thermometry in High Magnetic Fields

The calibration and reproducibilty of various commercial and homemade thermometers have been investigated over a temperature range from 20 mK to 4.2 K and magnetic field strength from 0 to 16 Tesla. The performance of resistance thermometers made of different materials varies widely as a function of magnetic field. One type (Scientific Instruments RO600) is found to have a relatively small magneto-resistance, which is predictable to better than 1% down to 100 mK, making calibration in a magnetic field unnecessary for many applications. This was not true of homemade ruthenium oxide and Cernox (Lakeshore Cryotronics). A new type of sensor, thin film doped germanium on a GaAs substrate, was also investigated and found to be useful at higher fields and low temperature.

Resistivity and magnetoresistance of metallic polyaniline and polypyrrole at millikelvin temperatures

Abstract The resistivity and magnetoresistance of polyaniline protonated by camphor sulfonic acid (PANI-CSA) and polypyrrole doped with PF 6 (PPy-PF 6 ) have been studied down to 25 mK in magnetic fields up to 16 T.

Nuclear demagnetization cryostat at University of Florida Microkelvin laboratory

Two nuclear-demagnetization cryostats, each containing 173 mol of copper coolant in a 8 T magnetic field and based on an Oxford Instruments model 1000 dilution refrigerator, have been under development at the University of Florida Microkelvin Laboratory for studies of nuclear magnetic ordering in metals and in solid3He, properties of superfluid3He at T≈0, and mesoscopic systems, and for development of primary thermometers based on population differences in nuclear quadrupole systems. The performance of the first cryostat is analyzed with emphasis on improvements to be made.

Producing and imaging a thin line of He2∗ molecular tracers in helium-4

Cryogenic helium-4 has long been recognized as a useful material in fluids research. The unique properties of helium-4 in the gaseous phase and the normal liquid phase allow for the generation of turbulent flows with exceptionally high Reynolds and Rayleigh numbers. In the superfluid phase, helium-4 exhibits two-fluid hydrodynamics and possesses fascinating properties due to its quantum nature. However, studying the flows in helium-4 has been very challenging largely due to the lack of effective visualization and velocimetry techniques. In this article, we discuss the development of novel instrumentation for flow-visualization in helium based on the generation and imaging of thin lines of metastable He*₂ tracer molecules. These molecular tracers are created via femtosecond-laser field-ionization of helium atoms and can be imaged using a laser-induced fluorescence technique. By observing the displacement and distortion of the tracer lines in helium, quantitative information about the flow field can be extracted. We present experimental results in the study of thermal counterflow in superfluid helium that validate the concept of this technique. We also discuss anticipated future developments of this powerful visualization technique.

Turbulent pipe flow of He I and He II

Abstract Turbulent pipe flow of liquid 4 He has been investigated for Reynolds numbers from Re=3×10 3 to Re=3×10 6 to examine He I and He II turbulence in the presence of shear. The extremely low kinematic viscosity of liquid helium allows us to achieve high Reynolds numbers in small laboratory apparatus. Our results agree with those using classical fluids, showing the utility of helium as a fluid for turbulence research.

Insert for rapidly producing temperatures between 300 and 1 K in a helium storage Dewar

A simple device is described for rapidly cycling apparatus between 300 and ∼1 K within a helium storage Dewar. The arrangement is useful for testing components or conducting experiments over this temperature range when the apparatus to be cooled is less than 5 cm in diameter and 50 cm long. Helium evaporation on inserting the device is ∼0.2 l, with consumption ∼0.1 l/h during operation.

Large negative magnetoresistance in Ge films at ultralow temperatures and low magnetic fields

Large negative magnetoresistance is found in Ge films, used as ultralow-temperature resistance thermometers, at temperatures below 0.2K and magnetic fields below 1T. This effect is very sensitive to temperature. At T<0.3K, the magnetoresistance is negative, and its magnitude increases with decreasing temperature. At 0.03K, the resistance strongly decreases (up to 100 times) when the magnetic field is increased from 0to1T and then saturates in higher fields. We discuss the mechanisms of this phenomenon and present results of calculations involving the hopping theory of conductivity with localization corrections.