Supercritical Helium Research Articles

Cryogenic viscous pump analysis based on hemisphere model

Cryogenic viscous pump (CVP) is also known as fore pump that are mainly designed for the separation of hydrogen isotopes from the helium during the process of regeneration of cryo- pumps. This model will determine the amount of hydrogen which is extracted by supercritical helium cooled pump. It also includes flow rates and inlet conditions that are related to the hydrogen gas flow. This hemisphere model (HM) determines the amount and location which are extracted by the pump as the function of time. During the development of this model the calibration check done for its results, later it has been compared with the measurements of CFP prototype. At the result of this model, it will demonstrate the quantity of hydrogen which is very sensitive to the inlet temperature that is found outside of the Cryopump. Finally, this model will represent the efficient outcome.

The cryogenic control system of SHINE

The Shanghai High repetition rate XFEL and Extreme Light Facility (SHINE), an advanced XFEL project, is now being built at Shanghai Advanced Research Institute (SARI), Chinese Academy of Sciences. It includes a hard X-ray free electron laser and a 100 pW intense laser facilities with overall length of 3.1 km. The XFEL part including an 8 GeV LINAC and 3 undulator lines is cooled with forced superfluid and supercritical helium at 2 K/4 K. The cryogenic system of SHINE consists of test facility cryogenic system (TFCS), accelerator cryogenic system (ACCS), and undulator cryogenic system (UNCS). A dedicated control system based on Experimental Physics and Industrial Control System (EPICS) will be built to automate the cryogenic system with process control, PID control loops, real-time data acquisition and storage, alarm handler and human machine interface. It is capable of automatic recovery as well. This paper describes details of control system structure, interfaces, controllers and the integration under EPICS framework.

Sound resonances in supercritical and superfluid helium

For twenty years of research, the processes of radiation and dissipation occurring during oscillations of quartz tuning forks in superfluid helium and its mixtures have turned from an object of research into a tool for studying the properties of helium. Quartz tuning forks are used to study various properties of helium - viscosity, thermal conductivity, radiation of the first and second sounds, and also as a precision temperature sensor. Experimental observations of these phenomena were carried out in a wide range of temperatures and pressures, but the results of observations have not yet been exhaustively described theoretically. The aim of this work is to study density and pressure oscillations to determine the conditions under which oscillations of a solid wall excite the first sound in superfluid helium and sound in supercritical helium, and to calculate the contributions of these processes to the formation of resonances during oscillations of closed tuning forks. In particular, the experimentally observed excitation of standing waves of pressure oscillations by an oscillating closed tuning fork, the appearance and properties of resonances depending on the temperature and pressure of helium are considered. As a result of the work, a model was built that described the physical features of the experimentally observed resonance phenomena.

Effect of flow imbalance on the operational performance of the KSTAR PF1UL magnets

The installation of a cryogenic circuit for a large-scale superconducting device such as a tokamak is quite complex. In the KSTAR tokamak, five cryogenic circuits are utilized to cool each type of the component (e.g., toroidal field magnets, poloidal field magnets, structures current leads, thermal shield, bus lines) independently. The circuit for the poloidal field magnets branches into hundreds of cooling channels. And five cryogenic valves control the flow rates according to the length of the cooling path. Ten cooling paths are arranged in parallel for cooling of the PF1 upper and lower magnets. The pressure drop is adjusted by a cryogenic valve with maintenance by a supercritical helium circulator. Although the flow rate should be uniform among the cooling channels but the flow imbalances was observed during flow tests at an individual magnets test. The flow rate should be uniform among the cooling channels but the flow imbalances observed during flow tests at an individual magnets test. The measured imbalance was around 10% between the KSTAR PF1 upper and lower magnets. To investigate the effects of the flow imbalance on the magnet temperature, a model of the PF1 upper and lower magnets system is developed using the SUPERMAGNET code. The maximum temperature trends are investigated depending on the flow imbalance.

Numerical Investigation into the Natural Convection of Cryogenic Supercritical Helium in a Spherical Enclosure

As an ideal pressurized gas, helium, especially supercritical helium, has been widely used in the pressurization system of various launch vehicles and spacecraft. This work mainly focuses on the natural convection of cryogenic supercritical helium in a spherical enclosure. Firstly, a three-dimensional numerical model is established and verified with experimental data. Then, the effects of inflation pressure and heating power on the flow and heat transfer characteristics are simulated. At the same time, the relationship between the Rayleigh number and Nusselt number is studied in detail. Finally, an improved natural convection heat transfer correlation modified by introducing the density ratio is obtained. The results show that the increase of the inflation pressure in the cavity is helpful to enhance the natural convection heat transfer of the cryogenic supercritical helium, and the temperature distribution in the cavity tends to be more uniform when the inflation pressure in the cavity increases. As to the improved natural convection heat transfer correlation, the average error between the simulation results and the calculated values is approximately 8%, which can better describe the natural convection heat transfer of cryogenic supercritical helium in the spherical enclosure.

Supercritical helium cold circulators for nuclear fusion machines: A comparative overview of different hydrodynamic design approaches

Cold circulators play a critical role in dynamic heat load mitigation across various operational scenarios to ensure stable performance of fusion reactor cryogenic system. The hydrodynamic design is a crucial parameter that critically influences the performance of these machines especially considering the future reactors with large superconducting magnets which would be handling high mass flow rates. The present study highlights quantitatively the effect of different blading schematics on the hydrodynamic performance of the pump impeller wheel, based on detailed analytical calculation and numerical analysis. The necessity and advantage of using impeller wheel with double curvature blades over single curvature blades is analysed qualitatively and quantitatively. It is observed that double curvature blades with blade hub ingress ratio larger than one deliver favourable performance and with an increase in the number of blades, this ratio decreases. It is seen that within the same wheel design parameters, optimally designed double curvature blades result in lower effective blade loading as compared to single curvature counterparts leading to better hydraulic performance. The temperature rise across pump due to viscous dissipative effects in fluid alone is more pronounced in impellers with single curvature blades as compared to the double curvature counterparts.

Analysis of the effects of thermal anchors on the reduction of the parasitic load to the EU-DEMO TF coils

The design of the superconducting magnet system of the EU-DEMO tokamak reached the end of the pre-conceptual design phase, and a careful investigation of the parasitic heat load from the room-temperature environment is needed. A possible route for the parasitic heat to enter the toroidal field (TF) coils is by conduction through their Gravity Supports (GSs), which provide mechanical support to the TFs, the Vacuum Vessel (VV) and attached coils, acting as a thermal bridge to the TF casing. A thermal anchoring of the GS using Supercritical helium (SHe) at 4.5 K, re-routing a fraction of the coolant from the TF casing cooling loop, downstream of the casing, is addressed in this study. A 3D thermal model is developed for a plausible design of the EU-DEMO GS, coupled to fluid lines that account for the active cooling by SHe. The effect of the location of the Thermal Anchor (TA) at different heights of the GS plates is investigated parametrically and the beneficial reduction of the parasitic heat load to the TF coil is demonstrated. The reduction of the static heat load to the TF winding pack, when the TA is enabled, is computed and discussed.

Breakdown voltages of cryogenic gaseous helium and in liquid helium

As practical applications of superconductivity have been made in electric power engineering, the breakdown voltage superconductor coolants, such as liquid helium or supercritical helium, have been needed intensively. Furthermore, in our discharge experiment in liquid helium to search for a novel carbon nanotube structure, it was predicted that the breakdown voltage was much higher. So, using the metal spherical electrodes, d.c. breakdown voltages in 4.2K gaseous helium (just above the liquid helium) and in liquid helium have been measured. As a result of the experiment, it was observed that the value is several times larger. Moreover, it found that there was no difference in the discharge phenomenon before and after the superfluid transition temperature.

Development of Coax Compacted Joint Assembly Process for the ITER Central Solenoid

The Central Solenoid (CS) is the core element of the ITER magnet system, contributed as in-kind procurement by the US Domestic Agency. Made up of six modular inter-exchangeable module coils, vertically stacked, forming a 15 m high and 4 m outer diameter solenoid, to be inserted inside the central tokamak core, after assembly of the 18 D-shaped Toroidal Field (TF) coils. Each module uses a Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn conductor internally cooled by circulation of supercritical helium at 4.5 K and supplied, for each module, with a 45 kA current by the way of two vertical leads exiting from the module outer radius. The available space between the CS and TF magnets being very limited, the US DA has developed a compact - so called Coax Joint (CJ) - devoted to connect the respective twelve module leads to the feeders located at top and bottom of the CS via dedicated busbar extensions. The CS coax joint assembly procedure as developed by US DA would make use of CS assembly onsite soldering process, to be executed under supervision of the ITER Organization (IO) in the tokamak assembly hall. In order to mitigate risks related to these soldering activities needed at assembly stage, IO has proposed an alternative assembly process based on indium wires compaction - so called Coax Compacted Joint (CCJ)- and initiated its prototype development and qualification in time. The hereby presented CCJ design solution is based on four copper quadrants, each including an embedded straight Rutherford-type superconductor cable-strip to transport the current. The current is transferred from the lead to the quadrants by the way of compacted indium wires. A steel jacket welded around the joint ensures the mechanical support as well as the needed leak tightness. The paper describes developments made by CEA under IO supervision from the conceptual design to the testing of joints in relevant cryogenic and operative conditions.

Effect of SHe Temperature on Cool-Down Speed in JT-60SA CS Module

The JT-60SA central solenoid (CS), which consists of four stacked modules, is cooled to an operating temperature of 4.5 K by supercritical helium (SHe). During cool-down from room temperature to 4.5 K, in order to avoid a damage on the coil, it is required to reduce thermal stress due to temperature difference in the coil. In the cool-down operation, the CS needs to control the maximum temperature difference in the longitudinal direction of the cable-in-conduit (CIC) conductor below 40 K. However, since that could damage the coil, correctly estimating the temperature distribution in the conductor is important for ensuring the cool-down operation is performed safely. In the present study, the JT-60SA CS module was prepared and the inlet and outlet temperatures, as well as the SHe mass flow rate, were measured until the CS module was cooled to operating temperature, after which a CS module simulation was performed in order to determine the effect of the inlet temperature on the cool-down speed. From the simulation results, it was concluded that the average cool-down speed was 0.73 K/h and the CS module reached 80 K in approximately 8.3 days while maintaining the maximum temperature difference within 40 K. Furthermore, the temperature difference between the inlet and outlet was kept within 25 K in order to limit the thermal stress. Taken together, the results of our analyses provide fundamental data that can be used to evaluate the safe cool-down operation of the coil and thus protect the coil systems.

Experimental investigation of pressure drop and heat transfer in high temperature supercritical CO2 and helium in a printed-circuit heat exchanger

An ASME code compliant heat exchanger was manufactured for testing between high-temperature (up to 550 °C) helium and supercritical CO2 test facilities at Georgia Institute of Technology. The multi-layerd printed-circuit heat exchanger (PCHE) consists of 17 photo-chemically etched plates which were diffusion-bonded together. Pressure losses and heat transfer measurements have been conducted for a zig-zag flow-path geometry etched into the bonded shims. These measurements have been presented in dimensionless form to facilitate comparison with thermohydraulic correlations available in the literature. The use of two fluids with drastically different properties in the same 37° zig-zag channel geometry provides a unique method of exploring the effect of working fluid on PCHE performance. Fanning friction factor and Nusselt numbers are determined for supercritical CO2 between Reynolds number of 500–18,000, spanning laminar, transition, and turbulent regimes. The same friction and heat transfer figures of merit are presented for helium operating between Reynolds number 400–3200, spanning laminar and transition turbulent regimes. Two different exponential models were fitted to the supercritical CO2 and helium friction data showing an agreement with the measurements to within ±10% over the range of test conditions. The model fitted to the heat transfer data is in agreement with the measurements to within ±15% of the range of test conditions. To enable reproducibility of the data, the methods used to define dimensions related to thermohydraulic calculations is discussed in details, and the dimensions provided by the engineering drawing of the exchanger are validated using high resolution computed tomography of the component. Finally, a detailed analysis of the thermohydraulic performance of the headers is given with the goal of setting a benchmark performance for comparison with other designs.

Structural Analysis and Fatigue Test of Helium Inlet for EAST PF Coil

The operating temperature of the EAST PF coils is 4.5 K which are cooled by supercritical helium through a helium inlet located at the jacket. The leakage was occurred in one PF helium inlet after the steady operation of EAST for 12 years. The finite element model of PF helium inlet is established and the structural analysis is performed. The static assessment shows that thermal load and G11 enforced displacement are the main causes of weld failure. PFhelium inlet is only subjected to the cyclic electromagnetic force under normal circumstances, which is designed to withstand 2e5 cycles at 4.5K. The FEM results show that the helium inlet on PF coil can bear the electromagnetic force during the service life. The fatigue test was performed on a full-size sample with a double strain range of at 4.5 K, and the fatigue specimen was fractured at 24570 cycles at the clamp welding region. The lifetime of the helium inlet can withstand more than 2e5 cycles, which is met the test required cycles. Therefore, the good structural performance of the PF helium inlet which can meet the EAST design requirements, and it can be fabricated for prototype coil applications.

The thermal-hydraulic response of the superconducting outsert during the trip of the resistive magnet in the hybrid magnet system of the CHMFL

The hybrid magnet system has been completed the commissioning and be ready to serve users at the High Magnetic Field Laboratory of the Chinese Academy of Sciences (CHMFL) in Hefei, China. The hybrid magnet consists of a 28 MW, 34 T resistive insert coils set nested inside of a 800 mm bore 11 T superconducting outsert magnet. The outsert coils are wound with high JC Nb3Sn/Cu cable-in-conduit conductor (CICC), cooled with forced flow of supercritical helium at 4.5 K. The protection system of superconducting magnet is based on the classical approach of voltage detection with a co-wound coil. In this paper, we reviewed experimental behavior of the superconducting outsert in detail when the resistive insert magnet tripped. We also conducted a thermal-hydraulic performance analysis of the superconducting outsert for the same trip situation of the resistive insert with modified GANDALF code. The numerical simulation results of maximum inlet and outlet temperature show good agreement with the experimental data. In addition, the minimum temperature margin and maximum helium pressure in the superconducting conductor were introduced and discussed.

Role of Supercritical Nitrogen (SCN) on the hydraulic and thermal characteristics of futuristic High Temperature Superconducting (HTS) cables

Cryogenic fluids such as Liquid Helium (LHe), Liquid Nitrogen (LN2), Liquid Oxygen (LOX) and Liquid Hydrogen (LH2) played a vital role in cooling various superconducting devices. The boiling temperatures of cryogenic fluids being lower than the critical temperature of the superconductors have enabled such an advantage. Moreover, supercritical fluids such as Supercritical Helium (SHe) and Supercritical Nitrogen (SCN) are also found to be replacing the liquid coolants thereby eliminating the possibility of reduction in heat transfer due to multiphase flow of working fluid. Hence, it is essential to investigate the role of such supercritical fluids in cooling the superconducting cables. Hence, in the present work, Super Critical Nitrogen (SCN) is proposed as a coolant for cooling of High Temperature Superconducting (HTS) cables. As the thermophysical properties are strongly affected by the temperature, analytical functions of thermophysical properties of SCN such as viscosity, density, specific heat and thermal conductivity are developed and implemented in the Thermohydraulic analysis of 1 m HTS cable. A 3-Dimensional computational model of HTS cable is modeled and developed in commercial software ANSYS-FLUENT for solving governing equations of mass, momentum and energy simultaneously. Different mass flow rates ranging from 16 L/min to 20 L/min and heat loads ranging from 1 W/m to 3 W/m are considered in estimating the thermohydraulic performance of HTS Cable. Friction factor, pressure drop, pumping power and heat transfer rate are estimated and compared with the results of experiments available in the literature. It can be concluded that SCN may be used as coolant to cool the HTS cables having higher critical temperature.

Adaptation of the nuclear safety code CATHARE3 to supercritical helium flow

Several international projects on nuclear fusion are in progress and comprise cryogenic systems for cooling components such as superconducting magnets. Different thermal-hydraulic codes already exist for sizing and validating these cryogenic devices. However, there is still no qualified scientific calculation tool to perform safety thermal hydraulic analyses encountered in fusion reactors. For this purpose, we have started the adaptation of the CATHARE system code, the reference thermal-hydraulic tool for safety studies of French Pressurised Water Reactors developed by CEA, EDF, Framatome, and IRSN, to model supercritical helium flows. Properties of supercritical helium were already available in CATHARE with the fluid library REFPROP. We implemented suitable correlations for friction factors and heat transfer coefficients to account for specific hydraulic parameters of helium flow in Cable In Conduit Conductors (CICC) that are used in superconducting magnets. Then, we performed first evaluations of the CATHARE code abilities to model thermal-hydraulic transient in cryogenic devices of a tokamak. Two cases of helium flow in a CICC of a JT-60SA toroidal field coil have been studied, and comparisons were performed with experimental data and the THEA code developed by CryoSoft. Simulations focusing on the behaviour of a device protected by a safety valve against incidental pressure increase were also performed. CATHARE results were compared with available experimental data on supercritical helium discharge through the safety valve of a tank in case of failure of the insulating vacuum.

&gt;16 T Hybrid Dipole for an LHC Energy Doubler

We report the design for a hybrid block-coil dual dipole using advanced cable-in-conduit windings. The dipole is designed for use in the arcs of an energy-doubling lattice in the LHC tunnel. The block-coil design facilitates configuration of hybrid sub-windings of Bi-2212 and Nb 3 Sn, each operating to the same fraction of critical current. The cryogenics utilizes supercritical helium, operating in the window 4.3-5.5 K. A novel support structure provides robust support and stress management. The three sub-windings can be separately wound and heat-treated, then assembled and preloaded to complete the dipole. The short-sample field using commercially available Nb3Sn and Bi-2212 is 16.8 T. With a recently reported APC Nb3Sn, the short-sample field is 18.5 T.

Preparation of the ITER Central Solenoid Assembly

The Central Solenoid (CS) is a key element of the ITER Magnet system, including six identical coils, called modules, assembled together to form a 4 m outer diameter, 13 m high solenoid. It is a superconducting magnet, using a 45 kA Nb3Sn conductor internally cooled by circulation of supercritical helium at 4.5 K with a peak field up to 13 T. It is enclosed inside a structure providing vertical pre-compression and mechanical support. Procurement of the components and the special assembly tooling of the ITER CS is the responsibility of US ITER, the ITER Domestic Agency of the USA, while the ITER Organization (IO) will carry out the assembly of these components. US ITER has awarded several contracts since 2011 to supply seven modules, including a spare, structure components, and the special tooling required for the CS pre-assembly. All deliveries are scheduled with the objective to start the CS assembly at IO site early 2021. IO is now actively preparing this new phase. This paper describes the general CS pre-assembly activities from modules stacking to the pre-compression at 210 MN. The special assembly processes and related tooling are detailed. A focus is given on the module lifting operation, the extensions assembly and the pre-compression. For these last two processes, IO has investigated alternative assembly options that will be presented.

An update of dynamic thermal-hydraulic simulations of the JT-60SA Toroidal Field coil cooling loop for preparing plasma operation

The JT-60SA cryogenic system was commissioned in 2016 in closed loop, without the cryogenic users (superconducting magnets, current leads, thermal shields). The first plasma operation is expected in 2020. This paper updates the heat load profiles resulting from the cooling loop of TF magnets, and received by the refrigerator and its thermal damper. The heat load profiles are calculated through thermal-hydraulic simulations of the magnets and the associated cryo-distribution, also named as supercritical helium loops. This update was performed by taking into account new data from the TF magnets (measured pressure drops, updated heat loads coming from the plasma), as well as a more accurate thermal model of the TF magnet. Previous simulation had been performed using the Vincenta code in 2010 and were used for the cryogenic system acceptance tests. The new thermal-hydraulic model is performed by using Simcryogenics, the modeling tool dedicated to refrigeration and cryo-distribution developed by CEA (Commissariat à l’énergie atomique et aux énergies alternatives). The differences between the two simulation results are highlighted and analyzed. These simulations provide the pulsed heat load profiles smoothed by the cryo-distribution and deposited into the thermal damper.

A new model for the analysis of quench in HTS cable-in-conduit conductors based on the twisted-stacked-tape cable concept for fusion applications

Several new conductor designs for fusion applications, based on high temperature superconducting (HTS) materials, have been recently proposed worldwide. Most of them are based on the twisted-stacked-tape cable (TSTC) idea: a stack of HTS tapes is twisted, soldered and embedded in a copper tube, constituting what we identify here as a ‘strand’. Few strands are then twisted around a central core to make an HTS cable. The cable is finally inserted in a stainless-steel jacket, which provides mechanical support and confinement for the forced flow of the supercritical helium coolant through the interstitials among the strands.The thermal gradients that build up in a TSTC make the 1D thermal-hydraulic models developed for low temperature superconducting magnets unsuitable to study fast transients such as quench propagation.A new model is presented here, which allows describing each strand, and separately accounting for the respective superconducting (SC) stack and copper tube. Compressible Euler-like equations describe the He flow and thermodynamic state in each interstitial fluid region. The distribution of the transport current is computed with a distributed parameter approach, self-consistently with the temperature distribution in the different so-called thermal components of the TSTC.The model is applied to the study of quench propagation in a conductor made of CroCo strands—a recent design proposed by the Karlsruhe Institute of Technology and based on the TSTC idea. A comparison with the modelling approach adopted for quench simulations in LTS conductors is carried out, showing that, when the heating is localized, the new model predicts, for the average temperature on the cross section at the hottest location along the conductor and for the voltage along the conductor, an evolution much slower than that predicted by a standard LTS-conductor model.

Exploration of power conversion thermodynamic cycles for ARC fusion reactor

Abstract In the worldwide energy industry, nuclear fusion could be a breakthrough in the medium-long term. One promising fusion machine under design at Massachusetts Institute of Technology is ARC reactor. It is likely that the first nuclear fusion plants will rely on a traditional thermodynamic cycle for the downstream power energy conversion. In this framework, one of the design aspects is to maximize the thermal efficiency. In the present paper the thermodynamic cycles, which could be adopted in ARC rector, are explored. Three cycles have been considered: the Rankine, the Brayton and a combined cycle. For the gas adopted in the Brayton and combined cycles, two options have been investigated: supercritical Helium and supercritical CO2. A comparison among thermal efficiency and preliminary considerations on component integrity’s, plant feasibility and economics of each studied configurations has been discussed to identify the possible best option for ARC reactor. The results show that a regenerative CO2 Brayton cycle with intercooler and re-heating systems is the most promising one. Such configurations is able to reach a thermodynamic efficiency of up to 0.6.