Saturated Liquid Helium Research Articles

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.

Design and operation of the in-vessel cryopump for the new upper divertor in ASDEX Upgrade

To study alternative divertor configurations in the ASDEX Upgrade (AUG) tokamak a new upper divertor, Div-IIo, with two in-vessel coils and a cryopump (CP) will be installed in 2022. For efficient density control in the new divertor configuration during plasma discharges, the CP has to guarantee an effective pumping speed for deuterium (D2) of at least Seff = 15 m3/s. The CP will be installed behind the inner divertor with a toroidal pumping gap of width 30 mm between the inner and outer divertor plates. It consists of 7 modules with a major radius of R = 1.2 m and a total active length of 5.4 m.To investigate reactor relevant D2-He gas mixtures during upper single null discharges, the liquid helium cryo-panels are coated with activated charcoal as cryosorbent.For quick regeneration of the charcoal from the accumulated gases, heating wires on the cryo-panels are installed.To reduce the pumping speed during high density discharges, an in-vessel cryo valve between two modules is installed. The pumping speed can be reduced by 35%.The cryo-panels operate with saturated liquid helium (LHe) at a temperature of 4.4 K. The thermal radiation shield consisting of reflector and chevron-baffle operate with saturated liquid nitrogen (LN2) at 80 K. To supply the new upper CP with LHe and LN2 the existing LHe-LN2-supply of the lower CP will be used. Therefore, an additional distributor box and a multi-channel transfer line to the upper CP have been installed in 2019.

The Influence of the Clearance on Performance of Compressor

Cold compressor (CC) was adopted to pump saturated liquid helium to obtain subcooled helium. In the Experimental Advanced Superconducting Tokamak (EAST) system, cold presses were studied and would replace the oil ring pump. The cold compressor processing and installation had been completed, and it had been tested by cold nitrogen gas. For cold compressors, testing at low temperature was essential, but the real working condition of low-temperature helium gas had great risks for large helium cryogenic system. Therefore, an independent cold nitrogen test system was an economical solution. In this work, numerical simulation and quantitative analysis method were used to investigate the flow field around different tip clearances and mass flow by CFD at liquid nitrogen temperature, and they were validated by experiments. When the tip clearance was increased by 0.1mm, the pressure ratio dropped 0.27% and the polytropic efficiency dropped 0.82%. To a certain extent, a larger tip clearance could improve the surge condition and had little influence on the efficiency. Therefore, in the design and operation, the tip clearance could be appropriately increased.

Robust control of active magnetic bearing systems for helium centrifugal cold compressors

Centrifugal cold compressors, now being developed at Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, are used to pump gaseous helium from saturated liquid helium tank to obtain super-fluid helium in cryogenic refrigeration systems. Active magnetic bearings (AMB) are replacing traditional oil-fed bearings as the optimal supporting assembly for cold compressors because of their many advantages. In this paper, five degrees of freedom AMBs are developed for the centrifugal cold compressor application. Considering the rotational speed is high, causing the nominal plant model to deviate materially from the precise mathematical model, robust H∞ control methods are proposed to control AMB systems. Experimental results show that H∞ controllers guarantee good dynamic and steady state performance in the closed-loop system at the rotation speed of 50 000 rpm.

Structure design and simulation research of active magnetic bearing for helium centrifugal cold compressor

Helium centrifugal cold compressors are utilized to pump gaseous helium from saturated liquid helium tank to obtain super-fluid helium in cryogenic refrigeration system, which is now being developed at TIPC, CAS. Active magnetic bearing (AMB) is replacing traditional oil-fed bearing as the optimal supporting assembly for cold compressor because of its many advantages: free of contact, high rotation speed, no lubrication and so on. In this paper, five degrees of freedom for AMB are developed for the helium centrifugal cold compressor application. The structure parameters of the axial and radial magnetic bearings as well as hardware and software of the electronic control system is discussed in detail. Based on modal analysis and critical speeds calculation, a control strategy combining PID arithmetic with other phase compensators is proposed. Simulation results demonstrate that the control method not only stables AMB system but also guarantees good performance of closed-loop behaviour. The prior research work offers important base and experience for test and application of AMB experimental platform for system centrifugal cold compressor.

Model-based minimization algorithm of a supercritical helium loop consumption subject to operational constraints

Supercritical helium loops at 4.2 K are the baseline cooling strategy of tokamaks superconducting magnets (JT-60SA, ITER, DEMO, etc.). This loops work with cryogenic circulators that force a supercritical helium flow through the superconducting magnets in order that the temperature stay below the working range all along their length. This paper shows that a supercritical helium loop associated with a saturated liquid helium bath can satisfy temperature constraints in different ways (playing on bath temperature and on the supercritical flow), but that only one is optimal from an energy point of view (every Watt consumed at 4.2 K consumes at least 220 W of electrical power). To find the optimal operational conditions, an algorithm capable of minimizing an objective function (energy consumption at 5 bar, 5 K) subject to constraints has been written. This algorithm works with a supercritical loop model realized with the Simcryogenics [2] library. This article describes the model used and the results of constrained optimization. It will be possible to see that the changes in operating point on the temperature of the magnet (e.g. in case of a change in the plasma configuration) involves large changes on the cryodistribution optimal operating point. Recommendations will be made to ensure that the energetic consumption is kept as low as possible despite the changing operating point. This work is partially supported by EUROfusion Consortium through the Euratom Research and Training Program 20142018 under Grant 633053.

Incorporating Artificial Neural Networks in the dynamic thermal–hydraulic model of a controlled cryogenic circuit

A model based on Artificial Neural Networks (ANNs) is developed for the heated line portion of a cryogenic circuit, where supercritical helium (SHe) flows and that also includes a cold circulator, valves, pipes/cryolines and heat exchangers between the main loop and a saturated liquid helium (LHe) bath. The heated line mimics the heat load coming from the superconducting magnets to their cryogenic cooling circuits during the operation of a tokamak fusion reactor. An ANN is trained, using the output from simulations of the circuit performed with the 4C thermal–hydraulic (TH) code, to reproduce the dynamic behavior of the heated line, including for the first time also scenarios where different types of controls act on the circuit. The ANN is then implemented in the 4C circuit model as a new component, which substitutes the original 4C heated line model. For different operational scenarios and control strategies, a good agreement is shown between the simplified ANN model results and the original 4C results, as well as with experimental data from the HELIOS facility confirming the suitability of this new approach which, extended to an entire magnet systems, can lead to real-time control of the cooling loops and fast assessment of control strategies for heat load smoothing to the cryoplant.

Numerical Study of Unsteady Flow in Centrifugal Cold Compressor

In helium refrigeration system, high-speed centrifugal cold compressor is utilized to pumped gaseous helium from saturated liquid helium tank at low temperature and low pressure for producing superfluid helium or sub-cooled helium. Stall and surge are common unsteady flow phenomena in centrifugal cold compressors which severely limit operation range and impact efficiency reliability. In order to obtain the installed range of cold compressor, unsteady flow in the case of low mass flow or high pressure ratio is investigated by the CFD. From the results of the numerical analysis, it can be deduced that the pressure ratio increases with the decrease in reduced mass flow. With the decrease of the reduced mass flow, backflow and vortex are intensified near the shroud of impeller. The unsteady flow will not only increase the flow loss, but also damage the compressor. It provided a numerical foundation of analyzing the effect of unsteady flow field and reducing the flow loss, and it is helpful for the further study and able to instruct the designing.

Increase of Operating Current of the LHD Helical Coils by Upgrading the Cooling System

The Large Helical Device (LHD), one of the world's largest superconducting systems, has been operated for the research of fusion plasma since 1998. The toroidal field of almost 3 T is made by a pair of pool-cooled helical coils. When they were cooled with saturated liquid helium at 4.4 K, a normal-zone had been induced several times in their innermost layer at the higher current than 11.0 kA. In order to improve the cryogenic stability by lowering temperatures, an additional cooler with two-stage cold compressors was installed at the inlet of the coil. The inlet and outlet temperatures of the coils are successfully lowered to 3.2 K and 3.8 K, respectively, with a mass flow of 50 g/s. Stable operation of the subcooling system has been established, and the operating current for the plasma experiments can be increased up to 11.4 kA.

DC breakdown voltage characteristics of saturated liquid helium in the presence of metallic particles

This paper deals with the breakdown voltage characteristics of saturated liquid helium in the presence of a needle-shaped or spherical metallic particle to obtain insulation design data for pool-cooled, low-temperature superconducting coils and to find the predominant factor affecting the breakdown voltage of liquid helium at a given state. The results show the following. (1) The generation of bubbles at the instant of collision of a particle with an electrode is caused mainly by the kinetic energy released from the moving particle to the liquid helium, while the electrostatic energy accompanying the microdischarge between the particle and the electrode has a lesser effect on it. (2) The lowest breakdown voltage at different particle conditions appears in the case of a free needle particle. The low breakdown voltage is caused by the bubble triggered by the particle collision, the high electric field at the tip of the needle particle, and the rich initial electrons supplied by the microdischarge

RRR of copper coating and low temperature electrical resistivity of material for TTF couplers

In the framework of the R&D program on the TTF III main RF coupler, IPN Orsay developed in close collaboration with LAL institute, a dedicated facility for the electrical characterization of different materials at low temperature. This apparatus was used for measuring the electrical resistivity versus temperature (4.2 K–300 K) of various samples produced in the industry. These tests were performed in order to compare the RRR of the samples, qualify and find the optimum parameters for the coating process. Seven flat samples were tested in a saturated liquid helium bath under ∼1013 mbar pressure: measurements were performed on bare 316L samples, nickel coated 316L samples, and copper coated 316L samples with a nickel under layer. We investigated, in particular, the effect of vacuum annealing at 400 °C on the RRR of the copper coating. Our experimental data are compared to previous measurements reported by other groups, and theoretical results (e.g., Gruneisen–Bloch equation) and a good agreement was found. Finally, the tested samples fulfil the TTF III coupler design parameters requirements in terms of heat loads to the refrigerator at 2 K, 4 K and 70 K.

Design and Operation of the Sub-Cooled Helium Test Facility for the LHD Helical Coils

In order to increase the cooling stability of the helical coils, an upgrade of the cooling scheme is planned from 4.4 K pool-boiling to 3.0 K sub-cooled helium of flow rate of about 50 g/sec. We have designed and constructed a R&D coil made of the same superconductor as that used for the helical coils and a sub-cooled helium test facility on the assumption of the application for actual large helical device (LHD). This facility is composed of decompression tank for the sub-cooled helium generation, the R&D coil, a current lead storage tank, etc., installed in a cryostat with a liquid nitrogen shield. Two stage cryogenic compressors are installed in the top flange, and sub-cooled helium is generated by decompressing of saturated liquid helium from 0.12 MPa to 24 kPa. Liquid helium of 4.4 K and 0.12 MPa is heat exchanged by the sub-cooled helium in the decompression tank and supplied to the R&D coil from the bottom. The sub-cooling R&D experiment has been carried out. As the results of experiment, we found that the saturated liquid helium could be cooled from 4.4 K to 3.0 K in 8 hours and maintained at 3.0 K. The rotation speed of the first and second stage cryogenic compressors were about 87,000 and 88,000 rpm, respectively. These values agreed well with the designed values of the two-stage cryogenic compressors. Finally, this facility operated stably during 8 days of the two sub-cooling R&D experiment campaigns.

Heat Load Measurements on a Large Superconducting Magnet: An Application of a Void Fraction Meter

ATLAS is one of the two major experiments of the LHC project at CERN using cryogenics. The superconducting magnet system of ATLAS is composed of the barrel toroid (BT), two end caps toroids and the Central Solenoid. The BT is formed of 8 race-track superconducting dipoles, each one 25 m long and 5 m wide. A reduced scale prototype (named B0) of one of the 8 dipoles, about one third of the length, has been constructed and tested in a dedicated cryogenic facility at CERN. To simulate the final thermal and hydraulic operating conditions, the B0 was cooled by a forced flow of 4.5 K saturated liquid helium provided by a centrifugal pump of 80 g/s nominal capacity. Both static and dynamic heat loads, generated by the induced currents on the B0 casing during a slow dump or a ramp up, have been measured to verify the expected thermal budget of the entire BT. The instrument used for the heat load measurements was a void fraction meter (VFM) installed on the magnet return line. The instrument constructed at CERN was calibrated in order to provide direct readings of heat loads. An example of application of the VFM measuring method to a large-scale apparatus cooled at liquid helium temperature.

Electrical breakdown triggered by a free conducting spherical particle in saturated liquid He I and He II under uniform DC field

This paper is concerned with the pre-breakdown phenomena and the breakdown voltage characteristics, in the presence of a free moving conducting spherical particle, of saturated normal liquid helium (He I) and saturated superfluid liquid helium (He II) under uniform dc field. Experiments show that the particle lifts off around the theoretical value of the lift-off electric field, and the particle oscillates between the electrodes at higher applied voltages. In that case, the microdischarge appears just before the charged particle collides with the oppositely charged electrode, and the bubble is generated at the moment of every collision of the particle with the electrode. It is confirmed theoretically as well as experimentally that the maximum bubble radius in He II is nearly proportional to the E/sub in//sup 1/3/, which is the released energy from the particle for the bubble generation. The insulation environment before the electrical breakdown suddenly changes at the /spl lambda/-point since the bubble behavior in He I and He II greatly differs. In the characteristics of the breakdown voltage vs. liquid pressure, a clear discontinuity appears at the /spl lambda/-point. Furthermore, the breakdown voltage in the parallel plane gap contaminated by a particle is lower than that in the rod-plane gap without a particle. It is found that the breakdown voltage characteristics are closely related to the trigger effect of the microdischarge and the bubble generation.

The CERN cryogenic test facility for the ATLAS barrel toroid magnets

The superconducting magnet system of the ATLAS detector will consist of a central solenoid, two end-cap toroidal magnets (ECT) and the barrel toroid magnet (BT) made of eight coils symmetrically placed around the central axis of the detector. The magnets will be tested individually in a 5000 m/sup 2/ experimental area prior to their final installation at an underground cavern of the LHC Collider. For the BT magnets, a dedicated cryogenic test facility has been designed which is currently under the construction and commissioning phase. A liquid nitrogen pre-cooling unit and a 1200 W@4.5K refrigerator will allow flexible operating conditions via a rather complex distribution and transfer line system. Flow of two-phase helium for cooling the coils is provided by centrifugal pumps immersed in a saturated liquid helium bath. The integration of the pumps in an existing cryostat required the adoption of novel mechanical solutions. Tests conducted permitted the validation of the technical design of the cryostat and its instrumentation. The characteristics of one pump were measured and pressure rise of 300 mbar at nominal flow of 80 g/s confirmed the specifications.

Calibration of saturated liquid helium weir flowmeter at Fermilab

The flow of saturated liquid helium at 4.2 K was studied in a unique weir flowmeter mounted inside a duct. The objective was to develop a calibration for helium flow rate in the weir, and thereby, making it a viable flowmeter. The weir flowmeter was tested in a liquid helium system at Fermi National Laboratory used for cooling the superconducting magnets in the experimental areas of the site. Unlike for open channel weir devices, the calibration of this flowmeter was influenced by both the liquid flow through the weir notch and the vapor flow over it. The results of this study and the calibration are presented taking into account effects of both liquid and vapor.

Boiling heat transfer to liquid helium from surface with pin-fins

The heat transfer characteristics of a surface covered with fine pin-fins were measured in saturated liquid helium at atmospheric pressure. The test pieces were fabricated by soldering copper filaments 60 μm in diameter and 1–1.8 mm in height perpendicularly to a copper plate so as to vary the coverage between 5 and 17%. The experimental results, which indicate excellent heat transfer characteristics, are similar to those for a frost-coated surface, known to be one of the best boiling surfaces.

Pool boiling heat transfer to saturated liquid helium from coated surface

Boiling experiments were performed at atmospheric pressure to study the influence of surface coatings on the pool boiling heat transfer to saturated liquid helium from flat surfaces. The parameters of interest were the coating material and the thickness, and the boiling surface inclination. The heat transfer characteristics were related to the surface temperature of the coating and the influences of the coating on boiling heat transfer were discussed. It was established, based on the concept of cold-end recovery of a normal zone, that an insulative coating has an optimum thickness when the coating is utilized to raise the maximum operating current of a superconductor.

Steady-State Pool Boiling Heat Transfer to Saturated Liquid Helium at Atmospheric Pressure

Steady-State Pool Boiling Heat Transfer to Saturated Liquid Helium at Atmospheric Pressure

Study of steady-state heat transfer of pool boiling in saturated liquid helium at atmospheric pressure.

Effects of surface inclination and roughness on saturated pool boiling curve were experimentally established for atmospheric liquid-helium. Based on the measured boiling curves, the following results were obtained. In nucleate boiling, heat transfer coefficients are increased as either the surface inclination or the roughness is increased. As for film boiling on vertical and inclined surfaces, the so-called two-phase boundary layer model underestimates heat transfer coefficients but it can correctly estimate the effects of surface superheat and surface inclination. This result indicates that a vapor film of laminar, smooth-interface is restricted to a small distance from the leading edge of the vapor film, but the nature of the vapor film remains laminar irrespective of the distance from the leading edge. Critical heat flux is not dependent on surface roughness and its dependency on surface inclination is predicted well by Vishnev's correlation. Finally, the minimum-heat-flux point temperature is not affected by surface roughness and inclination.