Superfluid Film Flow Research Articles

The Role of Substrate Roughness in Superfluid Film Flow Velocity

It is known that the apparent film flow rate $j_0$ of superfluid $^4$He increases significantly when the container wall is contaminated by a thin layer of solid air. However, its microscopic mechanism has not yet been clarified enough. We have measured $j_0$ under largely different conditions for the container wall in terms of surface area (0.77-6.15 m$^2$) and surface morphology using silver fine powders (particle size: $0.10$ \mu m) and porous glass (pore size: 0.5, 1 \mu m). We could increase $j_0$ by more than two orders of magnitude compared to non-treated smooth glass walls, where liquid helium flows down from the bottom of container as a continuous stream rather than discrete drips. By modeling the surface morphology, we estimated the effective perimeter of container $L_{\mathrm{eff}}$ and calculated the flow rate $j~(= j_0L_0/L_{\mathrm{eff}})$, where $L_0$ is the apparent perimeter without considering the microscopic surface structures. The resultant $j$ values for the various containers are constant each other within a factor of four, suggesting that the enhancement of $L_{\mathrm{eff}}$ plays a major role to change $j_0$ to such a huge extent and that the superfluid critical velocity, $v_{\mathrm{c}}$, does not change appreciably. The measured temperature dependence of $j$ revealed that $v_{\mathrm{c}}$ values in our experiments are determined by the vortex depinning model of Schwarz (Phys. Rev. B $\textbf{31}$, 5782 (1986)) with several nm size pinning sites.

Porous plug phase separator and superfluid film flow suppression system for the soft x-ray spectrometer onboard Hitomi

When using superfluid helium in low gravity environments, porous plug phase separators are commonly used to vent boiloff gas while confining the bulk liquid to the tank. Invariably, there is a flow of superfluid film from the perimeter of the porous plug down the vent line. For the Soft X-ray Spectrometer onboard ASTRO-H (Hitomi), its approximately 30-liter helium supply has a lifetime requirement of more than 3 years. A nominal vent rate is estimated as ~ 30 ug/s, equivalent to ~ 0.7 mW heat load. It is therefore critical to suppress any film flow whose evaporation would not provide direct cooling of the remaining liquid helium. That is, the porous plug vent system must be designed to both minimize film flow and to ensure maximum extraction of latent heat from the film. The design goal for Hitomi is to reduce the film flow losses to <2 ug/s, corresponding to a loss of cooling capacity of <40 uW. The design adopts the same general design as implemented for Astro-E and E2, using a vent system composed of a porous plug, combined with an orifice, a heat exchanger, and knife-edge devices. In this paper, design, on-ground testing results and in-orbit performance are described.

Flight model measurements of the porous plug and film flow suppression system for the ASTRO-H Soft X-ray Spectrometer dewar

Flight model measurements of a porous plug phase separator and a film flow suppression system for the ASTRO-H Soft X-ray Spectrometer dewar are described. ASTRO-H is the sixth Japanese astronomy satellite and will be launched in 2016. It carries the Soft X-ray Spectrometer consisting of an X-ray optic and an X-ray microcalorimeter system operated at 50mK. Superfluid liquid He is employed as a part of the cooling system. A wide range of He flows from 28μg/s to 3.2mg/s in various operation cases must be safely vented under zero gravity. At the same time, superfluid He film flow through the vent line must be suppressed to <2μg/s in a nominal case to avoid extra loss of the liquid He. For this purpose, a porous plug phase separator together with a film flow suppression system is installed. To verify its performance, the mass flow rates and the film flow rate of the flight model system were measured at component level. The mass flow rates at various He tank temperatures (1.15, 1.30, 1.50, and 2.00K) were obtained and also the film flow rate was measured at 1.15K. Then, the mass flow rates were measured after installing the whole system into a flight model dewar at the He tank temperature of 1.16, 1.30, 1.50, and 2.00K. The dewar was tilted so that the porous plug located at the top of the dewar is immersed in the liquid He and the porous plug separates the liquid and vapor He by the thermomechanical effect as in orbit. The obtained mass flow rates and the film flow rate in these tests were confirmed to meet the requirements and to be consistent with each other. No abnormal event such as large mass flow rates was observed. All these experimental results strongly suggest that this flight model of the porous plug and the film flow suppression system will work properly in space.

Development of porous plug phase separator and superfluid film flow suppression system for the Soft X-ray Spectrometer onboard ASTRO-H

ASTRO-H is the sixth Japanese astronomy satellite scheduled for launch in 2014. The Soft X-ray Spectrometer instrument is onboard ASTRO-H. This is a 6×6 array of X-ray microcalorimeters with an energy resolution of <7eV at 0.5–10keV. Superfluid liquid helium is utilized as a part of the cooling system. To retain the liquid helium in the tank under zero-gravity, a porous plug phase separator made of sintered stainless is used. Since the vapor mass flow rate is only 29μg/s, any additional superfluid film loss influences the lifetime of the liquid helium. Therefore, a film flow suppression system consisting of an orifice, a heat exchanger, and knife edge devices is adopted based on the design used for the X-ray Spectrometer onboard Suzaku. The film flow will be suppressed to <2μg/s, sufficiently smaller than the vapor flow rate. In the present investigation, the design and ground experiments of a helium vent system composed of the porous plug and film flow suppression system are presented. The results show that the phase separation and the film flow suppression are satisfactorily achieved.

Porous plug and superfluid helium film flow suppressor for the soft X-ray spectrometer onboard Astro-H

Suppression of superfluid helium flow is critical for the Soft X-ray Spectrometer (SXS) onboard Astro-H, to achieve a life time of the liquid helium over 5 years. The superfluid film flow must be sufficiently small, compared to a nominal helium gas flow rate of the SXS ( 25 μ g / s ) . For this purpose, four devices composed of a porous plug, an orifice, a heat exchanger, and knife edge devices will be employed based on the experience of the X-ray microcalorimeter (XRS for X-Ray Spectrometer) onboard Suzaku. The porous plug is a phase separator of the liquid and gas helium. A potential film flow leaking from the porous plug is suppressed by the orifice. Almost all the remaining film flow evaporates at the heat exchanger. The knife edge devices stop the remaining film flow by using atomically sharp edges. In this paper, we describe the principle and design of these four devices.

Experimental tests and modeling of the optimal orifice size for a closed cycle 4He sorption refrigerator

Closed cycle 4He sorption refrigerators are an increasingly popular choice for remote cryogenic operations. The presence of a superfluid film on the inner walls of the evaporation pot limits their performance. We present a simple model of the gas, film, and heat flow in a 4He sorption refrigerator, taking into account the effects of an orifice to limit superfluid film flow. The model serves as a useful guideline for optimizing cryogenic performance. We also present a diagram of hold time vs. steady state temperature as a function of the radius of the orifice and compare the model to measurements.

Atomic alkali-metal gas cells at liquid-helium temperatures: Loading by light-induced atom desorption

We have studied the loading of gaseous alkali-metal atoms into glass cells held at about 2 K by light-induced atom desorption (LIAD). In LIAD loading, the cell containing dense helium gas and a small amount of alkali metal is irradiated with cw laser light. This technique has two unique features to be investigated: alkali-metal atoms are desorbed, even by weak laser irradiation, from the cell walls not covered visibly with alkali metal, and loading efficiency is much higher below the superfluid transition temperature for the helium inside the cell than above this temperature. On the basis of extensive experimental studies for Rb presented in this paper, we give a consistent picture of the loading mechanism: alkali-metal atoms are optically desorbed from alkali-metal particles on the walls, then transported effectively into the cell volume by helium-gas flow produced by the continuous evaporation of the helium film, which is supplied by the superfluid-film flow on the walls from the bulk liquid helium at the bottom of the cell.

Electrostatic charging and levitation of Helium II drops

Liquid Helium II drops, of diameter 1 mm or less, are charged with positive helium ions and subsequently levitated by static electric fields. Stable levitation was achieved for drops of order 100–150 micrometers in diameter. The suspended drops could be translated to arbitrary positions within the levitator using additional superimposed DC electric fields, and also could be made to oscillate stably about their average positions by means of an applied time-varying electric field. A weak corona discharge was used to produce the necessary ions for levitation. A novel superfluid film flow device, developed for the controlled deployment of large charged drops, is described. Also discussed is an adjustable electric fountain that requires only a field emission tip operating at modest potentials, and works in both Helium I and Helium II.

The build-up of dissipation in superfluid 4He film flow

A high-precision capacitance depth gauge technique has allowed us to make precise measurements of the meniscus levels and the build-up of dissipation in the helium film. We present measurements showing this build-up with the flow driven by helium level differences which range from 0 to 0.3 mm up to 0 to 30 mm. Data which establishes the dependence of the dissipation upon flow velocity taken at the largest level difference is compared with the predictions of the homogeneous vortex-tangle model proposed by K.W. Schwarz.

Experimental evidence for nonwetting with superfluid helium.

Wetting properties of alkali-metallic surfaces by superfluid-helium film have been tested through heat transport. Appearing as an exception among other substrates, cesium is found to interrupt superfluid-film flow, suggesting nonwetting of cesium by superfluid helium. In addition, superfluid-helium films were observed to be only marginally stable on glass having been in contact with cesium vapor.

3He Film flow: 4He substrate coating effect

Abstract Superfluid 3He film flow out of a copper beaker was measured without and then with a 4He coating on the copper surface. The effect of the 4 He was to convert the substrate from a purely diffuse to a purely specular surface for the 3He quasiparticles.

Observations on the Flow Properties of 3He Films below 1 mK

We have performed an experiment designed to observe superfluid film flow in 3He. Our apparatus measures flow rates as small as 4×10-5 mm3/s onto a horizontal surface located a distance h above a reservoir. Results indicate that when h is several millimeters no flow is detected. However when h is less than 2.5 mm from the horizontal surface, film flow is observed at temperatures well below Tc.

Thermal Convection in 3He–4He Mixtures near the Tricritical Point

Exploratory experiments are described that investigate 3He–4He mixtures near the tricritical point as convection systems. Measurements of the temperature difference between the horizontal boundaries of a convection cell heated from above exhibit features attributable to phase separation and the bulk superfluid transition but show no feature consistent with the onset of stationary convection. The presence of convection is inferred from enhanced thermal conduction and the existence of a time-dependent state showing similarities to Busse's oscillatory instability. The effect of superfluid film flow was very noticeable and measurements of the film superfluid transition temperature are presented for four concentrations of 3He in 4He.

Superfluid helium-4 film flow at 11 mK to 1.6 K

Experiments on helium-4 film flow over a stainless steel beaker rim have been carried out in the temperature region 11 mK to 1.63 K. A study of oscillation damping and low-velocity steady state flow provides the first evidence of dissipation at “subcritical” transfer rates below 1 K and suggests that a dissipation mechanism other than irreversible heat exchange between the bulk liquid reservoirs or the thermal nucleation of vortices and their subsequent transport across the stream is in operation. Flow experiments involving step-function changes in the chemical potential difference between the ends of the film confirmed the existence of a range of metastable “critical” transfer rates at all temperatures but failed to demonstrate a preferred rate structure. The variation of the mean critical rate with temperature corroborated the results of other workers above 36 mK and extended these measurements to lower temperatures.

Transport phenomena in He3-He4 mixtures near the tricritical point

We have made extensive measurements on transport phenomena in He3-He4 mixtures near the tricritical point and along the superfluid transition line at saturated vapor pressure. The He3 mole fraction X ranged from 0.51 to 0.72 and the temperature from 0.8 to 1.5 K. Our measurements were made under steady state conditions using a cell where we measured the vertical He3 concentration gradient ▽X induced by a temperature gradient ▽T produced by a vertical heat flux. The cell included two superposed capacitors and ▽X was determined by means of the dielectric constant method. In this paper, we present a comprehensive report on our results for the thermal diffusion ratio k T and the thermal conductivity κ both in the normal fluid and in the superfluid. In the tricritical region, k T was found to diverge strongly as the tricritical point was approached; no singularity in κ was found. This behavior is consistent with theoretical predictions. In the region near the lambda line, κ remains finite, as expected, but k T appears to have a stronger singularity than predicted by theory. The analysis of our experiment in the normal fluid for mixtures with X>0.51 was complicated by superfluid film flow along the walls of the sample cell. We describe this effect and analyze it with Khalatnikov's theory of superfluidity. However, for the mixture X = 0.51, where there is no such film, the behavior of k T is consistent with predictions. The k T data for the mixtures 0.6 < X < 0.7 could be cast into a tricritical scaling representation, similar to that for the concentration susceptibility. In the superfluid phase we test, for the first time and with fair success, a relation by Khalatnikov between ▽X/▽T and static properties, measured in different experiments. Finally, we discuss the relaxation times that characterize the establishment of steady state conditions. From these data it is possible, under favorable circumstances, to obtain the mass diffusivity D.

Calorimeter for the Liquid Helium

We designed a calorimeter for the liquid helium and constructed it. The calorimeter has a needle valve assembly which operates at the liquid helium temperature. The needle valve assembly enables one to seal the sample cavity and evacuate the filling tube. This excludes next troubles; the large heat leak due to the superfluid film flow, the change of the 3He mole fraction and the amount of the sample during the experiment. When this calorimeter is isolated thermally, it can be kept at extremely constant temperature by controlling the temperature of the surrounding liquid helium bath. The drift rate of temperature was less than 1.7×10-6K/hour. It corresponds to the heat leak of 0.5erg/min. This temperature stability and our temperature resolution. 1×10-7K, enable one to perform heat capacity measurements very near Tλ (superfluid transition temperature), 3×10-6K_??_|T-Tλ|<10-2K.The drift rates of resistivity at Tλ were also examined on a carbon thermometer (Allen-Bradley 1/8W 47Ω) and a germanium thermometer (Scientific Instruments model NI). The indication of a germanium thermometer was found to be constant within experimental errors, but that of a carbon thermometer varied markedly. After cooling a carbon thermometer and keeping it at Tλ±0.1K for 40 hours, the drift rate of its indication was reduced to about 6×10-6K/hour.

Dissipation in the superfluid helium film

We have measured the rate of energy dissipation in superfluid helium film flow in an attempt to test a recent theory due to Harris-Lowe, which predicts that for superfluid stream velocitiesvsthat just exceed the critical velocityvc0, the rate of dissipation is given by an equation of the form Q=C(vs−vc0)3/2. Our experiments at 1.33 K show that the exponent, predicted to be 3/2, is 1.491±0.021.

Dilution refrigerator still with 4He film-flow suppression

A still for a 3He–4He dilution refrigerator has been constructed which suppresses the 4He superfluid film flow and attains 3He/4He ratios as large as 600. This design has the novel feature that all the heaters are attached to the outside of the still.

Quasi-isothermal superfluid-film flow: Oscillations between two reservoirs

We have carried out a detailed study of the frequency and damping of the oscillations in levels of two reservoirs of HeII which are coupled only via the mobile superfluid film. Our experiments cover the temperature range $1.2lTl2.13$ K and utilize several geometries under quasi-isothermal conditions. Below 2 K our results for the damping are in quantitative agreement with predictions based on a theory first proposed by Robinson. This indicates that the damping mechanisms proposed by Allen and by Calvani, Greulich, and Maraviglia are of minor importance under our experimental conditions. It is argued that the Robinson theory can explain the previous general agreement with the Allen theory.

Experimental test of a new mechanism of dissipation in accelerated superfluid film flow

We have measured the damping, α, and the frequency, ω, of superfluid film oscillations under quasi-isothermal conditions for a variety of geometries. Experimental values for α are found to be in quantitative agreement with the Robinson theory as applied to film flow. This suggests that the damping mechanism proposed recently by Calvani, Greulich and Maraviglia does not provide a major contribution to the dissipation we observe.