Flow Of Liquid Helium Research Articles

Сверхпроводниковая электромагнитная система ИТЭР. Статус изготовления и сборки на 2024 год

The electromagnetic system (EMS) is one of the most significant components of the International Thermonuclear Experimental Reactor (ITER). It establishes the machine’s ability to generate and control a fusion plasma with a current of up to 15 MA and a power of up to 500 MW within hundreds of seconds. ITER EMS is the largest superconductor magnet system that has ever been created with a stored energy of up to 50 GJ. It is a highly technological magnets using superconductors based on Nb3Sn and NbTi, which work at a temperature of 4-6 K with compulsory cooling by a flow of liquid and gaseous helium. The EMS coils are required to be operable at electrical voltages up to 20-30 kV, and mechanical stresses up to 500-600 MPa. The manufacturing of the coils and the associated superconducting current carrying system are now moving into the final stage of production. The most part of them has already been manufactured and delivered to the ITER toroidal magnetic coil chamber (Tokamak) installation site. This review briefly describes the design of ITER superconductor coils of all 4 types and gives the current status of their manufacturing and assembly.

Controlled spherical deuterium droplets as Lagrangian tracers for cryogenic turbulence experiments.

The study of the smallest scales of turbulence by (Lagrangian) particle tracking faces two major challenges: the requirement of a 2D or 3D optical imaging system with sufficiently high spatial and temporal resolution and the need for particles that behave as passive tracers when seeded into the flow. While recent advances in the past decade have led to the development of fast cameras, there is still a lack of suitable methods to seed cryogenic liquid helium flows with mono-disperse particles of sufficiently small size, of the order of a few micrometers, and a density close enough to that of helium. Taking advantage of the surface tension, we propose two different techniques to generate controlled liquid spherical droplets of deuterium over a liquid helium bath. The first technique operates in a continuous mode by fragmenting a liquid jet, thanks to the Rayleigh-Taylor instability. This results in the formation of droplets with a diameter distribution of 2 ± 0.25DN, where DN is the diameter of the jet nozzle (DN = 20μm in the present experiment). This method offers a high production rate, greater than 30kHz. The second technique operates in a drop-on-demand mode by detaching droplets from the nozzle using pressure pulses generated using a piezoelectric transducer. This approach yields a much narrower diameter distribution of 2.1 ± 0.05DN but at a smaller production rate, in the range 500 Hz-2kHz. The initial trajectories and shapes of the droplets, from the moment they are released from the nozzle until they fall 3mm below, are investigated and discussed based on back-light illumination images.

Numerical Research on the Heat-Transfer Enhancement of Printed Circuit Heat Exchanger in the Presence of Pulsating Flow

Abstract This paper presents a numerical investigation on the flow and heat-transfer characteristics of liquid lithium and helium in printed circuit heat exchanger (PCHE) channels with pulsating flow introduced. The effects of the mass flowrate of helium, frequency, and amplitude of pulsating flow were examined, respectively. The findings indicate that the introduced pulsating flow has a positive effect on heat transfer at a lower Reynolds number. With the increase of pulsating frequency, the heat-transfer enhancement may decline, and the pressure drop is reduced, with Colburn factor (j) and Fanning friction factor (f) varying marginally. In the flow state with enhanced heat transfer, keeping a lower pulsating frequency, the heat-transfer enhancement ratio E(h) exhibits an approximately linear increase with the pulsating amplitude. Meanwhile, the average pressure drop, Colburn factor (j) and Fanning friction factor (f) also increase with the amplitude, with the fluctuation amplitude of each parameter directly proportional to the amplitude.

Numerical investigation into the flow and heat transfer in corrugated and embossed heat-transfer plates for space nuclear power applications

In this article, the flow and heat transfer of liquid lithium and helium in a traditional corrugated plate and an embossed plate is numerically investigated, and on the basis of obtaining the temperature and velocity field distributions, the flow and heat transfer characteristics under different inlet parameters of helium are compared and analyzed. It is found that under the same working condition, the heat transfer performance of the corrugated plate on the helium side is higher than that of the embossed plate, while the flow performance is worse. In the range of the helium inlet temperature between 323 K and 448 K, the comprehensive performance of the embossed plate is superior to that of the corrugated plate, with Nu·f (–1/3) 10.74% higher than the corrugated plate. Also, at a relatively lower helium inlet velocity ranging from 0.8 m/s to 1.3 m/s, the embossed plate outperforms the corrugated plate by 8.83% in terms of comprehensive performance; whereas for the higher inlet velocity from 1.3 m/s to 1.8 m/s, the corrugated plate shows a better comprehensive performance, with Nu·f (–1/3) 6.51% higher than the embossed plate. In detail, the flow characteristics across a single embossed structure are examined, revealing the streamlined distribution, and the evolution process of the vortex structure.

Cryogenic processes of SuperKEKB superconducting magnet cryostat with continuous flow of saturated liquid helium

SuperKEKB is a particle accelerator to collide electron and positron beams and employs 55 superconducting (SC) magnets which are accommodated in two cryostats and cooled by subcooled liquid helium (LHe) of ∼ 20 g/s from refrigerators. One cryostat was set up in laboratory for exciting tests and cooled by continuous flow of saturated LHe (<3.0 g/s) from dewars of 1000 L. The differences of two cooling schemes of subcooled and saturated LHe were investigated and described in this paper. The required liquids to cool down the cryostat cold mass (>1700 kg) to 4.5 K were evaluated with a quantified utilization of vapor sensible heat, which fills the large gap between the maximum and minimum liquid requirement presented in most cryogenic literature. The saturated LHe flow was computed to ensure LHe accumulation in the cryostat and immerse SC magnets. The cryogenic processes were monitored by the measured cryogenic temperatures, pressures, flow rates and resistances of SC magnets, which demonstrate cryogenic stabilities with two-phase helium flow and illustrate the agreements with the thermal studies and analyses. This study provides a successful experience to cool SC magnet cryostats of large cold mass with continuous flow of saturated LHe from dewars instead of cryogenic refrigerators.

A Pressure Drop Model for Helium Flow in CIC Conductors Based on Porous Media Analogy

Pressure drop of cable-in-conduit conductors (CICCs) cooled by a flow of liquid or supercritical helium is one of the key parameters for the design of large superconducting magnet systems, which determines the heat removal capability and the thermal stability. In this article, an analytical model for predicting the pressure drop in CICCs is developed based on an analogy to porous media with the equivalent geometry configuration. This new prediction model indicates that the pressure drop in CICCs is affected by the geometry parameters of cable and the physical properties of the liquid helium, such as void fraction, tortuosity, pore-throat ratio, hydraulic diameter, density, and viscosity of the liquid helium. To verify the validity of the pressure drop model proposed in this article, the predictions are compared with the experimental data, the results show that the predictions are in good agreement with the experimental data. Then, an analytical expression for predicting the friction factor is derived, the obtained expression is in good agreement with experimental data and is superior to the Katheder correlation at large Reynolds number. In addition, the effects of the major geometric parameters on pressure drop are analyzed, the results explain why the Katheder correlation can be used to predict the pressure drop of CICCs.

A FRACTAL MODEL OF PERMEABILITY FOR THE LIQUID HELIUM FLOW IN CABLE-IN-CONDUIT CONDUCTORS

The heat removal capability and the coolant pumping costs in the design of cable-in-conduit conductors are depend on the thermo-hydraulics of the liquid helium flow. Therefore, the accurate knowledge of the thermo-hydraulics of the flow is significant for the design of the cables, especially for permeability. In this paper, the fractal method is proposed to describe the cable cross-section and an approximate expression is derived. Then, a fractal permeability model for helium flow in CICCs is presented based on a porous medium analogy. The feasibility and validity of this model is verified by the comparison of the predicted values and the experimental values. The fractal model indicates that permeability of cables is determined by the cable geometric parameters, such as the effective porosity, the average cabling angle, the average diameter of strands and the pore area fractal dimension of the cable cross-section. This model does not contain any empirical constants or fitting constants and can be used to explain the mechanism and to predict the permeability of the helium flow in CICCs. Furthermore, the effects of cable geometric characteristics on the presented fractal permeability model are also analyzed and simulated. The results imply that permeability of cables decreases with increasing the cabling angle, increases with the effective porosity, the pore fractal dimension and the average diameter of the strands increase. These results are consistent with the physical situations.

Spectral Analysis of the Growth of Translational Superfluid Flow in a Bose Liquid

Spectral analysis of the translational superfluid flow of a Bose liquid is attempted. When cooling a dissipative flow of liquid helium 4 through a capillary at the lambda temperature,a superfluid flow abruptly appears at the lambda temperature. By a thought experiment in which the pressure difference between two ends of a capillary slowly oscillates, the spectrum of fluidity (the reciprocal of kinematic viscosity) just above the lambda temperature ($2.17 K<T<2.18 K$) is examined using a sum rule that incorporates the result of linear response theory and fluid mechanics. According to Feynman's picture on the many-body wave function of bosons, as the condensate grows in the flow, the transverse motion in the flow becomes suppressed by the emergence of a gap, originating from the Bose statistics, in the excitation spectrum. This statistical gap induces a continuous change in the fluidity spectrum just above the lambda temperature, which demonstrates the growth of a superfluid flow. For this mechanism, the size distribution of the coherent wave function plays an important role. As a byproduct, a new interpretation of the critical velocity is proposed.

Conceptualization and preliminary studies on the development of a 2K refrigeration system at BARC

Conceptualization and preliminary studies on the development of a 2K helium refrigeration system at BARC are reported in this article. Two different refrigeration schemes are considered and studied. In Scheme A, a separate cold box with 2 heat exchangers, a liquid helium bath and a sub atmospheric heat exchanger is considered and 2K refrigeration capacities for different liquid helium flow rates corresponding to LHP 50 and LHP 100 (BARC helium liquefiers), are computed and reported. In Scheme B, the 2K refrigeration system is considered to work in tandem with a back-up 4.5K helium liquefier (LHP 50/LHP100), receiving a stream of cold supercritical helium from the liquefier. Different cases, with and without liquid nitrogen precooling options of the 4.5K liquefier, are considered. After preliminary studies, Scheme B is preferred over Scheme A and the process inputs for the sub atmospheric pressure heat exchanger are also arrived at and presented.

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.

Stealth Superconducting Magnet Technology for Collider IR and Injector Requirements

The intersection regions and the beam injection channel of a high-energy collider require magnets that must act strongly upon one beam yet not at all on a closely neighboring beam. Designs are presented for three examples: a septum dipole that serves as a forward spectrometer centered on an ion beam after collision, which must clear an electron beam leaving the IP; a final-focus quadrupole that must provide high-gradient focusing of electrons with large aperture but pass a close-lying ion beam; and a high-gradient quadrupole that must operate in the background field of a spectrometer solenoid. All designs use to advantage a new superconducting cable-in-conduit that provides for compact winding, robust end geometry, and in-cable flow of liquid helium.

A compact membrane-driven diamond anvil cell and cryostat system for nuclear resonant scattering at high pressure and low temperature.

A new miniature panoramic diamond anvil cell (mini-pDAC) as well as a unique gas membrane-driven mechanism is developed and implemented to measure electronic, magnetic, vibrational, and thermodynamic properties of materials using the nuclear resonant inelastic X-ray scattering (NRIXS) and the synchrotron Mössbauer spectroscopy (SMS) simultaneously at high pressure (over Mbar) and low temperature (T < 10 K). The gas membrane system allows in situ pressure tuning of the mini-pDAC at low temperature. The mini-pDAC fits into a specially designed compact liquid helium flow cryostat system to achieve low temperatures, where liquid helium flows through the holder of the mini-pDAC to cool the sample more efficiently. The system has achieved sample temperatures as low as 9 K. Using the membrane, sample pressures of up to 1.4 Mbar have been generated from this mini-pDAC. The instrument has been routinely used at 3-ID, Advanced Photon Source, for NRIXS and SMS studies. The same instrument can easily be used for other X-ray techniques, such as X-ray radial diffraction, X-ray Raman scattering, X-ray emission spectroscopy, and X-ray inelastic scattering under high pressure and low temperature. In this paper, technical details of the mini-pDAC, membrane engaging mechanism, and the cryostat system are described, and some experimental results are discussed.

Superfluid Boundary Layer.

We model the superfluid flow of liquid helium over the rough surface of a wire (used to experimentally generate turbulence) profiled by atomic force microscopy. Numerical simulations of the Gross-Pitaevskii equation reveal that the sharpest features in the surface induce vortex nucleation both intrinsically (due to the raised local fluid velocity) and extrinsically (providing pinning sites to vortex lines aligned with the flow). Vortex interactions and reconnections contribute to form a dense turbulent layer of vortices with a nonclassical average velocity profile which continually sheds small vortex rings into the bulk. We characterize this layer for various imposed flows. As boundary layers conventionally arise from viscous forces, this result opens up new insight into the nature of superflows.

Performance studies of Cryocooler based cryosorption pumps with indigenous activated carbons for fusion applications

Cryosorption pumps are the only solution for pumping helium and hydrogen in fusion systems, due to their high pumping speeds and suitability in harsh environments. Their development requires the right Activated Carbons (ACs) and suitable adhesives to bind them to metallic panels with liquid helium (LHe) flow channels. However, their performance evaluation will require large quantities of LHe. Alternatively, these pumps can be built with small size panels adhered with ACs and cooled by a cryocooler. The paper describes the development of a cryopump using a commercial cryocooler (Sumitomo RDK415D), with 1.5W@4.2 K, integrated with small size AC panel mounted on 2nd stage, with the 1st stage acting as radiation shield. Under no load, the cryopump reaches the ultimate pressure of 2.1E-7 mbar. The pump is built using panels with different indigenously developed ACs such as granules, pellets, ACF-FK2 and activated carbon of knitted IPR cloth. We present the experimental results of pumping speeds for gases such as nitrogen, argon and helium using the procedures outlined by American Vacuum Society (AVS). These studies will enable to arrive at the right ACs and adhesives for the development of large scale cryosorption pumps with liquid helium flow.

Sub-micron solid air tracers for quantum vortices and liquid helium flows.

The dynamics of quantized vortices in superfluids has received increased attention recently because of novel techniques developed to visualize them directly. One of these techniques [G. P. Bewley et al., Nature 441, 588 (2006)] visualized quantized vortices and their reconnections in superfluid flows of (4)He by using solid hydrogen tracers of micron-size or larger. The present work improves upon the previous technique by using substantially smaller particles created by injecting atmospheric air diluted in helium gas. These smaller particles are detectable thanks to the higher index of refraction of nitrogen compared to hydrogen and thanks to an improved visualization setup. The optical counting estimate, which agrees with terminal velocity estimates, suggests that the tracer diameter is typically 400 ± 200 nm and could be as small as 200 nm; being smaller, but not so small as to be influenced by thermal motion, the particles get trapped on the vortices faster, perturb the vortices less, possess smaller Stokes drag, and stay trapped on fast-moving vortices, as also on vortices generated closer to the superfluid transition temperature. Unlike the past, the ability to create particles in the superfluid state directly (instead of creating them above the λ-point and cooling the fluid subsequently), ensures greater temperature stability for longer periods, and enables the tracking of long and isolated vortices. These advantages have also led to the direct visualization of Kelvin waves. The use of other seed gases could lead to the visualization of even smaller tracers for quantized vortices. We discuss the visualization setup and provide suggestions for further improvement.

Note: control of liquid helium supply to cryopanels of Kolkata superconducting cyclotron.

The Kolkata superconducting cyclotron utilises liquid helium to cool the main magnet niobium-titanium (NbTi) coil and the cryopanels. Three liquid helium cooled cryopanels, placed inside the dees of the radio-frequency system, maintain the high vacuum in the acceleration region of the superconducting cyclotron. The small cryostat placed inside the cryogenic distribution manifold located at the basement of the superconducting cyclotron building supplies liquid helium in parallel branches to three cold heads, used for cooling their associated cryopanels. The level in the cryostat has to be maintained at an optimum value to ensure uninterrupted flow of liquid helium to these three cold heads. This paper describes the transfer function of the overall system, its tuning parameters, and discusses the actual control of cryostat level by using these parameters.

Jet Pump for Liquid Helium Circulation Through the Fast Cycling Magnets of Nuclotron

Nuclotron is the first fast cycling superconducting synchrotron intended for the acceleration of high-energy nuclei and heavy ions. Its cryogenic system includes two helium refrigerators with a total capacity of 4000 W at 4.5 K. The 251.5 m long accelerator ring consists of 144 superconducting dipole and quadruple magnets. The magnets connected in parallel are refrigerated by a two-phase flow of boiling helium. In order to increase liquid helium flow directed to the superconducting magnets, jet pumps are used. We explain theoretical and experimental results that allow one to determinate main technical specifications and optimal geometric dimensions of the jet pumps. The experience of using this device and corresponding flow diagrams are described.

Efficient cooling of superconducting fiber core via holey cladding

Superconductivity has the potential to alter the entire landscape of technological advancement and innovation. Unfortunately, its true potential has been limited, in part, by the lack of conventional geometries, adequate stability, cooling efficiencies and in turn, cost. In this study, we demonstrate an optical fiber design with a superconducting core that is cooled via the flow of liquid helium in holes disposed in the fused silica cladding. The efficiently micro cooled superconducting fiber lends itself to low current electronic applications such as ultrasensitive sensing and imaging, quantum measurement instrumentation and supercomputing. Although not presently applicable for large scale applications such as high current transmission lines or motors, the basic approach may be combined with other traditional technologies to improve cooling efficiency and reliability.

Kiloampere, Variable-Temperature, Critical-Current Measurements of High-Field Superconductors.

We review variable-temperature, transport critical-current (Ic) measurements made on commercial superconductors over a range of critical currents from less than 0.1 A to about 1 kA. We have developed and used a number of systems to make these measurements over the last 15 years. Two exemplary variable-temperature systems with coil sample geometries will be described: a probe that is only variable-temperature and a probe that is variable-temperature and variable-strain. The most significant challenge for these measurements is temperature stability, since large amounts of heat can be generated by the flow of high current through the resistive sample fixture. Therefore, a significant portion of this review is focused on the reduction of temperature errors to less than ±0.05 K in such measurements. A key feature of our system is a pre-regulator that converts a flow of liquid helium to gas and heats the gas to a temperature close to the target sample temperature. The pre-regulator is not in close proximity to the sample and it is controlled independently of the sample temperature. This allows us to independently control the total cooling power, and thereby fine tune the sample cooling power at any sample temperature. The same general temperature-control philosophy is used in all of our variable-temperature systems, but the addition of another variable, such as strain, forces compromises in design and results in some differences in operation and protocol. These aspects are analyzed to assess the extent to which the protocols for our systems might be generalized to other systems at other laboratories. Our approach to variable-temperature measurements is also placed in the general context of measurement-system design, and the perceived advantages and disadvantages of design choices are presented. To verify the accuracy of the variable-temperature measurements, we compared critical-current values obtained on a specimen immersed in liquid helium (“liquid” or Ic liq) at 5 K to those measured on the same specimen in flowing helium gas (“gas” or Ic gas) at the same temperature. These comparisons indicate the temperature control is effective over the superconducting wire length between the voltage taps, and this condition is valid for all types of sample investigated, including Nb-Ti, Nb3Sn, and MgB2 wires. The liquid/gas comparisons are used to study the variable-temperature measurement protocol that was necessary to obtain the “correct” critical current, which was assumed to be the Ic liq. We also calibrated the magnetoresistance effect of resistive thermometers for temperatures from 4 K to 35 K and magnetic fields from 0 T to 16 T. This calibration reduces systematic errors in the variable-temperature data, but it does not affect the liquid/gas comparison since the same thermometers are used in both cases.

Nucleate boiling heat transfer in a helium natural circulation loop coupled with a cryocooler

A gravity based U-shape natural circulation loop coupled with a cryocooler is proposed to serve as a self-sustaining thermal link at the temperature of liquid helium. Experiments are performed with a constant helium mass inside the system by keeping the loop closed. This thermal link is mainly composed of a condenser/phase separator cooled by the second stage of a 1.5W-class cryocooler at 4.2K. Connected to the condenser, a 23cm high testing tube is heated along its entire length. Liquid helium flows through this tube to the condenser and is recondensed there. Helium in both normal and supercritical regions is encountered in the experiment and the boiling curve is obtained with the heat flux being controlled. The different boiling regimes and hysteresis during the recovery process are observed and analyzed. Heat transfer coefficient is determined in the nucleate boiling regime and compared with existing correlations. The correlation proposed by Kandlikar reproduces the heat transfer coefficient within 30% error in the regime of two-phase nucleate boiling.