Stability tests on the euratom LCT conductor

The stability of the conductor against local heat pulses was measured as a function of magnetic field, transport current, temperature and cooling conditions. Comparing liquid helium cooling at 1 bar and supercritical cooling at 4 bar, the stability was found to be higher in liquid helium for transport currents above ≈ 70% /_c(B). Below that current value supercritical helium cooling yielded a higher stability.     

Operational test and field measurement of prototype bending magnets for energy saver/Doubler project

A full scale 22-foot Energy Doubler bending magnet (E22-1A) was tested in a subcooled liquid helium system. Most of the key magnetic field measurements were done on this magnet. A five-foot precision model magnet (E5-1) was tested in pool boiling helium. Much more systematic and precise measurements were done on this magnet, which was excited up to 45.7 kG at the center of the beam bore. The measurements of both magnets were done on training and quench behavior, ac loss of the magnet, and field measurements with a Hall probe. Using an NMR of a Li sample, absolute field value up to 44 kG was confirmed. Extensive harmonic analysis were done for both magnets for DC and pulsed modes. Remanent field was measured with a Hall probe and also by harmonic coil, and its variation with negative bias field was studied. The magnetically vertical plane and an effective magnetic length were measured with a stretched wire coil.     

Development of a Superconducting Magnet System with Zero Liquid Helium Boil-off

A homogeneous magnetic field superconducting magnet with a cold bore of 250 mm and a central field of 4.3 T has been designed, manufactured, and tested with zero liquid helium boil-off. As a result of magnetic field homogeneity considerations, the magnet is composed of three coaxial coils: one main coil and two compensation coils. All coils are connected in series and can be charged with a single power supply. The magnetic field homogeneity is about ±3.0 % from ?200 mm to 200 mm in axial direction with 86 mm in diameter. The magnet can be operated in persistent mode with a superconducting switch. A two-stage GM cryocooler with a capacity of 1.5 W at 4.2 K was used to cool the superconducting magnet. The cryocooler prevents the liquid helium from boiling off and leads to zero helium loss during static operation. The magnet can be operated in liquid helium circumstance by cooling the gas helium with the cryocooler without additional supply of liquid helium. Under this condition, the magnet is successfully operated up to 4 T without quench. The magnet system can be generating 0.25 L/h liquid helium with the cryocooler by supplying the gas helium without loading the magnet. In this paper, the magnet design, manufacture, mechanical behavior analysis, and the performance test results of the magnet are presented.     

Thermal-Electromagnetic Analysis for 14 kA Fast Discharge of a Model Coil

A model coil for 40-T hybrid magnet superconducting outsert magnet has been constructed and tested at the High Magnetic Field Laboratory, Chinese Academy of Sciences. The model coil was wound with Nb₃Sn cable-in-conduit conductor (CICC) cabled in a 316LN jacket cooled with supercritical helium. The model coil alone can produce about 4 T maximum magnetic field with an operating current of 14 kA. The model coil, in combination with 7.57-T NbTi background coil, can produce 11.5 T central field at 14 kA. During the test campaigns, a fast discharge was triggered by a dump resistor of 3.6 mΩ to evaluate the thermal-electromagnetic behavior of the model coil. In order to avoid a quench of the background coil, no current was exerted on the background coil through a power supply during the fast discharge of the model coil. The test results show that the central magnetic field is not scaled proportionally to the current decay of the model coil. The circuit model gives excellent results compared with the measured ones for the central magnetic field evolution as a function of time in this paper. For the thermal-hydraulic behavior during the fast discharge, the maximum temperature at the inlet simulated by the 1-D Gandalf code gives excellent agreement results compared with the measured ones with the conductor coupling time constant of 63 ms.     

Guidelines for LTS magnet design based on transient stability

Stabilities of low critical temperature superconducting (LTS) magnets and their designs are studied and discussed. There are two contradictory necessities; those are low cost and high performance, in the other words, high magnetic field and large current density. Especially, the maximum magnetic fields of the latest high performance Nb₃Sn magnets are around 20 T. Mentioned necessities result in the small stability margins. Needless to say, the superconducting magnet must produce its nominal field reliably. Therefore, maintaining adequate stability margin, the magnet design to draw out the high potential of the superconductor is required. The transient stability of the superconducting magnet is determined by the relationship between mechanical disturbance energy and stability margin. The minimum quench energy (MQE) is one of the index of stability margin and it is defined as the minimum energy to trigger quenching of a superconductor. MQE should be beyond any possible disturbance energy during the operation. It is difficult to identify the mechanical disturbance energy quantitatively. On the contrary, MQE had been evaluated precisely by means of our developed resistive carbon paste heater (CPH). At the same time, we can predict MQE by numerical simulations. Because the magnet comes to quench if the mechanical disturbance exceeds the MQE, the disturbance energies are suspected to be equivalent to MQEs during the magnet-training. When we achieved somewhat larger MQE, we may exclude numbers of training quenches.In this paper, we discuss the guidelines of LTS magnet design from the standpoint of MQE. We represent some case studies for various superconducting magnets and/or some different winding methods.     

The cryogenic system for ITER CC superconducting conductor test facility

This paper describes the cryogenic system of the International Thermonuclear Experimental Reactor (ITER) Correction Coils (CC) test facility, which consists of a 500 W/4.5 K helium refrigerator, a 50 kA superconducting transformer cryostat (STC) and a background field magnet cryostat (BFMC). The 500 W/4.5 K helium refrigerator synchronously produces both the liquid helium (LHe) and supercritical helium (SHe). The background field magnet and the primary coil of the superconducting transformer (PCST) are cooled down by immersing into 4.2 K LHe. The secondary Cable-In-Conduit Conductor (CICC) coil of the superconducting transformer (SCST), superconducting joints and the testing sample of ITER CC are cooled down by forced-flow supercritical helium. During the commissioning experiment, all the superconducting coils were successfully translated into superconducting state. The background field magnet was fully cooled by immersing it into 4.2 K LHe and generated a maximal background magnetic field of 6.96 T; the temperature of transformer coils and current leads was reduced to 4.3 K; the inlet temperature of SHe loop was 5.6 K, which can meet the cooling requirements of CIC-Conductor and joint boxes. It is noted that a novel heat cut-off device for High Temperature Superconducting (HTS) binary current leads was introduced to reduce the heat losses of transformer cryostat.     

A “permanent” high-temperature superconducting magnet operated in thermal communication with a mass of solid nitrogen

A new design for a portable “permanent” superconducting magnet system is explored. The design involves a persistent-mode high-temperature superconducting (HTS) magnet that is cooled by a solid heat capacitor. The system is an alternative to permanent low-temperature superconducting (LTS) magnet systems where the magnet is cooled by a bath of liquid helium.An apparatus was constructed to demonstrate stable operation of a permanent magnet wound with Bi2223/Ag conductor while in thermal communication with a mass of solid nitrogen. The apparatus includes a room-temperature bore and can function while it stands alone, detached from its cooling source, power supply, and vacuum pump. The magnet is operated in the 20–40 K temperature range. This apparatus is the first to demonstrate the operation of a superconducting magnet with a permissible temperature variation exceeding a few degrees kelvin. Models are developed to predict the experimental system's warming trend and magnetic field decay. The models are validated with a good agreement between simulations based on these models and experimental results. The results indicate that present HTS conductor critical current and index are not yet sufficient to provide field strengths and field decay time constants that are required for typical persistent-mode applications.     

Simulation of quench for the cable-in-conduit-conductor in HT-7U superconducting Tokamak magnets using porous medium model

A cable-in-conduit-conductor (CICC) consists of superconducting cable, copper, supercritical helium and conduit. To keep the operating temperature of superconducting cable lower than its current sharing temperature, the supercritical helium is forced flow through the CICC. The supercritical helium through the cable bundle has the complex directional changes due to the interaction between the supercritical helium and strands. The structure of CICC is characterized with the porous medium. The quench characteristics of CICC are analyzed by the model which the temperature difference between the strands and helium is assumed to be very small due to the heating induced flow to generate high heat transfer coefficient of supercritical helium. A moving mesh method is developed for the numerical solution of the problem with the steep drop for temperature and density of supercritical helium in the short front region of the normal zone. The computational mesh is obtained by equidistribution of a monitor function tailored for the functional variation of the arguments for density, temperature and velocity of supercritical helium. Existence and uniqueness of the discretised equations using a moving mesh are also established. The coupled equation for porous medium is solved using the finite element method with the artificial viscosity term. The validation of the code is tested by comparing it with the other codes with good accuracy. The converged properties of numerical solution due to quench in CICC are studied. We present preliminary estimates of the maximum conductor temperature rise and helium pressure during a quench in the inner layer of toroidal field (TF) magnet for HT-7U. The quench scenarios with different dump time constants of 6.25, 12, and 21.1 s are considered. The goal of such work is to guide the protection scheme and a detailed prediction of the quench evolution of magnet.     

Magnetic field measurements of 1.5 meter model SSC collider dipolemagnets at Fermilab

Magnetic field measurements have been performed at Fermilab on 1.5 m magnetic length model dipoles for the Superconducting Supercollider (SSC). Harmonic measurements are recorded at room temperature before and after the collared cell is assembled into the yoke and at liquid helium temperature. Measurements are made as a function of longitudinal position and excitation current. High field data are compared with room temperature measurements of both the collared coil and the completed yoked magnet and with the predicted fields of both the body of the magnet and the coil ends. The effects of yoking and cold testing agree well with predictions. One of the two cold-tested dipoles satisfies the SSC specification for sextupole/decapole     

Acoustic emission from superconducting magnets

We have shown experimentally that the acoustic emission (AE) is an effective means of monitoring the operating conditions of superconducting magnets and can be used to prevent catastrophic quenching. the signal-to-noise ratio of AE transducers at liquid helium temperature has been found to be about 18 dB higher than that at room temperature; AE originating from a superconducting magnet and those from the liquid helium shower can be clearly discriminated by observing their wave forms and frequency spectra. These results are also discussed from a theoretical view point based on an elastic medium model for the superconducting magnet.     

ITER Side Correction Coil Quench model and analysis

Previous thermohydraulic studies performed for the ITER TF, CS and PF magnet systems have brought some important information on the detection and consequences of a quench as a function of the initial conditions (deposited energy, heated length). Even if the temperature margin of the Correction Coils is high, their behavior during a quench should also be studied since a quench is likely to be triggered by potential anomalies in joints, ground fault on the instrumentation wires, etc. A model has been developed with the SuperMagnet Code (Bagnasco et al., 2010) for a Side Correction Coil (SCC2) with four pancakes cooled in parallel, each of them represented by a Thea module (with the proper Cable In Conduit Conductor characteristics). All the other coils of the PF cooling loop are hydraulically connected in parallel (top/bottom correction coils and six Poloidal Field Coils) are modeled by Flower modules with equivalent hydraulics properties. The model and the analysis results are presented for five quench initiation cases with/without fast discharge: two quenches initiated by a heat input to the innermost turn of one pancake (case 1 and case 2) and two other quenches initiated at the innermost turns of four pancakes (case 3 and case 4). In the 5th case, the quench is initiated at the middle turn of one pancake. The impact on the cooling circuit, e.g. the exceedance of the opening pressure of the quench relief valves, is detailed in case of an undetected quench (i.e. no discharge of the magnet). Particular attention is also paid to a possible secondary quench detection system based on measured thermohydraulic signals (pressure, temperature and/or helium mass flow rate). The maximum cable temperature achieved in case of a fast current discharge (primary detection by voltage) is compared to the design hot spot criterion of 150 K, which includes the contribution of helium and jacket.     

Implementation of the superfluid helium phase transition using finite element modeling: Simulation of transient heat transfer and He-I/He-II phase front movement in cooling channels of superconducting magnets

In the thermal design of high magnetic field superconducting accelerator magnets, the emphasis is on the use of superfluid helium as a coolant and stabilizing medium. The very high effective thermal conductivity of helium below the lambda transition temperature significantly helps to extract heat from the coil windings during steady state and transient heat deposition. The layout and size of the helium channels have a strong effect on the maximum amount of heat that can be extracted from the porously insulated superconducting cables. To better understand the behavior of superfluid helium penetrating the magnet structure and coil windings, simulation based on a three dimensional finite element model can give valuable insight. The 3D geometries of interest can be regarded as a complex network of coupled 1D geometries. The governing physics is thus similar for both geometries and therefore validation of several and different 1D models is performed. Numerically obtained results and published experimental data are compared. Once the viability of the applied methods is proven, they can be incorporated into the 3D geometries. Not only the transport properties in the bulk of the helium are of interest, but also the strong non-linear behavior at the interfaces between solids and superfluid helium (Kapitza conductance) is important from an engineering point of view, since relatively large temperature jumps may occur here.In this work it is shown how He-II behavior in magnet windings can be simulated using COMSOL Multiphysics. 1D models are validated by experimental results taken from literature in order to improve existing 2D and 3D models with more complete physics. The examples discussed include transient heat transfer in 1D channels, Kapitza conductance and sub-cooling of normal liquid helium to temperatures below the lambda transition in long channels (phase front movement).     

Direct Lorentz force measurement for YBa2Cu3O7-δ superconductor

This study introduced a novel method using a modified helium gas closed cycle cryogenic system to measure the force acting on a C-shaped magnet by a YBCO superconductor with bias current as a function of temperature. The results showed that the measured force agrees with Lorentz force due to the bias current for normal conductor at temperature higher than the superconducting transition temperature, as expected. However, under a temperature at the beginning of the superconducting transition region, the measured force became larger than the expected Lorentz force. As temperature further decreased, the measured force gradually dropped to zero. A qualitative argument based on the Bean’s model was given to explain the peak in the measured force at the beginning of the transition temperature.     

Development of a cryogenic test bed for high amperage critical current measurement on high temperature superconductor wire

Many areas of research have benefited from the application of conduction-cooled superconducting magnet technology. The middle and small-scale magnets immersed in the liquid helium will be replaced by the easy-operating conduction-cooled superconducting magnet due to convenient operation, lower operating cost and easy for user. For the goal of superconducting magnet applications in the advanced testing for high temperature superconducting (HTS) wire and sample coils, a wide bore conduction-cooled superconducting magnet with available warm bore of ?186 mm and center field of 5-6 T for background magnetic field applications was designed, fabricated and tested. The system allows measurements to be performed in a repeatable and reliable fashion. In order to support the high stress in magnet, the detailed finite element (FE) analysis with electro-plastic model is proposed. The sample cryostat is designed with cryofree. It includes two GM cryocoolers. The detailed design, fabrication and thermal analysis are presented in the paper.     

Electro-magnetic stress-induced degradation of insulation vacuum of a large cryo-magnetic system

In superconducting magnets, the cold mass is placed in a vacuum vessel to reduce heat load to the liquid helium system. Helium leaks into the vacuum vessel can degrade the insulation vacuum, which can, in turn, cause an increase in the heat load to the liquid helium system. These leaks are called cold leaks, as they show up when the coil is cooled with liquid helium. K500 superconducting cyclotron magnet at Variable Energy Cyclotron Centre, Kolkata has such cold leaks in the helium vessel that developed during cool down. The leak rate increases with the increase of current in the superconducting coils. This paper describes a series of experiments carried out on the superconducting cyclotron magnet to find the level of degradation of insulation vacuum and measure the increase in heat load with magnet current. The leak rate was also measured and the leak size was estimated analytically. Detail magneto-structural analysis was done using Finite Element Method (FEM) to identify highly stressed zones in the helium vessel and found out that highly stressed zones coincide with the weld zones. The magneto-structural stress was applied on an estimated size of single crack and found that crack tip stress could reach beyond elastic limit of the material. We can predict that the full design current may be unachievable in this situation. Mitigation of increased heat load was also done using an additional vacuum pump for the insulation vacuum space.     

40-T混合磁体外超导线圈迫流氦流动摩擦系数分析

40-T混合磁体外超导磁体使用4.5 K超临界氦进行迫流冷却。迫流氦在管内电缆导体(CICC)内的流动过程受到摩擦阻力的影响会造成一定的压降和热量产生,同时由外界带来的热量也会由迫流氦带走来让磁体保持在超导温区。使用了超导磁体实际运行过程中的实验数据计算不同导体结构下雷诺数与摩擦系数的变化关系,利用Katheder经验公式对实验的摩擦系数进行了重新拟合,给出了适用于该导体摩擦系数的经验公式。     

AMS-02超导磁体中超流氦加注过程研究

论述了AMS-02超导磁体中超流氦的加注过程,进行了各个环节压降和温降的计算分析,并用图表表示了在不同质量流量加注超流氦工况下管路系统中氦工质的速度、压力、温度和含气量的变化情况.计算结果表明,氦工质从液氦主杜瓦加注到磁体杜瓦的过程中,其压力和温度不断降低,而含气量不断增加.同时也表明在节流阀VVP9中实现了常流氦向超流氦的转变和质量流量的控制.     

Engineering Valley Polarization of Monolayer WS2: A Physical Doping Approach

The emerging field of valleytronics has boosted intensive interests in investigating and controlling valley polarized light emission of monolayer transition metal dichalcogenides (1L TMDs). However, so far, the effective control of valley polarization degree in monolayer TMDs semiconductors is mostly achieved at liquid helium cryogenic temperature (4.2 K), with the requirements of high magnetic field and on‐resonance laser, which are of high cost and unwelcome for applications. To overcome this obstacle, it is depicted that by electrostatic and optical doping, even at temperatures far above liquid helium cryogenic temperature (80 K) and under off‐resonance laser excitation, a competitive valley polarization degree of monolayer WS₂ can be achieved (more than threefold enhancement). The enhanced polarization is understood by a general doping dependent valley relaxation mechanism, which agrees well with the unified theory of carrier screening effects on intervalley scattering process. These results demonstrate that the tunability corresponds to an effective magnet field of ≈10 T at 4.2 K. This work not only serves as a reference to future valleytronic studies based on monolayer TMDs with various external or native carrier densities, but also provides an alternative approach toward enhanced polarization degree, which denotes an essential step toward practical valleytronic applications.     

Wide Variety of Experiments Using a Cryogen-Free 27.5 T Hybrid Magnet and a Cryogen-Free 18.1 T Superconducting Magnet

A cryogen-free hybrid magnet without liquid helium for operation, generating 27.5 T in a 32 mm room temperature bore of an 8 MW water-cooled resistive insert magnet in an 8.5 T background field of a cryogen-free superconducting outsert magnet, is being operated for basic research at low temperatures down to 17 mK in combination with a dilution refrigerator. In addition, we are developing functional materials using a differential thermal analysis DTA at high temperatures up to 1473 K in high fields up to 27 T. This cryogen-free hybrid magnet will be upgraded to generate 29 T by improving the outer superconducting magnet. A cryogen-free 18.1 T superconducting magnet with a 52 mm room temperature experimental bore, consisting of a Bi₂Sr₂Ca₂Cu₃O₁₀ (Bi2223) insert coil, has been developed using a GM-JT cryocooler. Recently, bronze-tape-laminated Bi2223 has revealed excellent irreversible stress tolerance of 250 MPa at 77 K. In addition, the critical current properties for recent Bi2223 tapes are largely improved from 200 to 400 A/cm-width at 77 K in a self-field. Therefore, the stainless steel reinforcement tape incorporated for the previous Bi2223 insert coil is no longer needed for a new Bi2223 one. A new Bi2223 insert coil with almost the same size as the existing insert coil can generate two times higher fields at the elevated operation current from 162 to 191 A. An upgraded cryogen-free superconducting magnet can offer a long-term experiment at the constant magnetic field of 20 T for an in-field heat-treatment investigation.