SC Coils Research Articles

Research on Interturn Short-Circuit Characteristic of the High-Speed Permanent-Magnet Machine With Gramme-Ring Windings

The conventional lap-winding high-speed permanent-magnet synchronous machine (HSPMSM) usually has a large interturn short-circuit (ITSC) current, seriously threatening the reliability. This article founds that the SC coil of the gramme-ring-winding HSPMSM is wound around the stator yoke, so the inductance of SC coil is considerable large and thus will cause a great reduction of ITSC current. The types of ITSC faults were first analyzed for a multistrand-gramme-ring-winding HSPMSM, and then, the ITSC characteristics and influencing factors were investigated based on finite-element (FE) method. The distribution of flux density is quite different from normal state, the SC current is far lower than that of the lap-winding HSPMSM, and the inductance of SC coil is fluctuant with rotation of rotor. By theoretical analysis of the inductance, the methods of increasing the height of stator yoke and using high-permeability CoFe alloy were proposed to further reduce the SC current. In addition, decreasing the strands number is beneficial to reduce the SC current density.

Specific features of eddy current analysis with ANSYS® for fast plasma current decay event in W7-X

During the second operation phase (OP1.2) of the most advanced stellarator Wendelstein 7-X (W7-X), some events of fast plasma current decay with a time constant of about 1 ms were observed (for both toroidal and diamagnetic plasma currents). The events were triggered by Electron Cyclotron Current Drive (ECCD) and were much faster than the assumptions considered during the design phase of W7-X components: 100 kA and 2.7 MA plasma current discharge with the time constants of 140 and 50 ms for toroidal and diamagnetic currents respectively. During the observed 1 ms discharge of maximum expected plasma currents, significant eddy currents are expected to be induced in some components with resulting electro-magnetic (EM) forces. In order to avoid unnecessary changes on as-built components, evaluation of the eddy currents / EM forces with high accuracy is required, if the conservative simplified estimation shows criticality. As a result, a sophisticated EM model with plasma currents, superconducting (sc) coils, plasma vessel (PV) and multiple ports is developed in ANSYS® using the new element type SOLID236 / 237 (edge-based formulation), which is much more accurate than the legacy elements SOLID97 (vector potential formulation). The specific features of using SOLID236 elements for eddy current analysis are studied and presented in this paper. This includes the application of boundary conditions on sector EM model, modelling of excitation currents, field and eddy current accuracy, skin effect, shielding effect, voltage in conducting components, etc. Most of the issues are studied in comparison with SOLID97 elements. Finally, the pros and cons of SOLID236 and SOLID97 elements are summarized, and the experiences during the W7-X EM analyses are presented.

Design and Implementation of the Instrumentation and Sensor System for testing of a sub modelled 10 MW offshore superconducting wind turbine.

The large head mass of an offshore wind turbine can be reduced by using superconducting technology. As a proof of concept, a superconducting 10 MW wind turbine for offshore application is investigated under the SUPRAPOWER project funded by EU-FP7. SC coils were developed using MgB2 tapes and cooled to 20 K by a two stage Gifford McMahon cryocooler in a modular rotating cryostat. To study the coil performance and to measure and control the sub model, a LABVIEW based instrumentation system is developed. To measure the temperature distribution in a rotating cryostat, an optical based sensing system is designed and introduced. The effect of rotation on an optical based sensing system is studied. In this paper, the instrumentation along with the thermal characteristics and validation of the optical sensing system for rotating machines and its installation techniques will be reported.

Design and Construction of the Cryogenic Cooling System for the Rotating Magnetic Validator of the 10 MW SUPRAPOWER Offshore Superconducting Wind Turbine

The SUPRAPOWER, an EU FP7 funded research project, is developing an innovative 10 MW class superconducting generator to provide an important breakthrough in the offshore wind industry. The 10 MW generators adopts modular concept to the superconducting coil and associated cryogenic systems. And the modular superconducting coil and cryostat of this novel generator will be validated through a scale-down rotating magnetic validator (RMV). The cryogenic cooling system for the RMV consists of two modular cryostats: a distributing cryostat and the corresponding rotating cryocooler system with a self-developed rotary joint. In order to achieve the SC coils' working temperature of 20 K, a two-stage G–M cryocooler will be applied and linked to the two modular cryostats by way of a conductive copper connection. The distributing cryostat was developed to envelop the cold head of the cryocooler, thermal link, and three binary current leads to electrically feed the coils. A rotary joint with Ferrofluid sealing was developed to transfer the helium gas between the rotating cold head and stationary oil-lubricated compressor. In this paper, the design, manufacture, and assembly of this cryogenic system for RMV will be presented in detail.

Study on Electromagnetic Force Characteristics Acting on Levitation/Guidance Coils of a Superconducting Maglev Vehicle System

Superconducting Maglev has been developed as a high-speed transport system. Superconducting coils (SC coils) are installed on the vehicle, and levitation/guidance coils (LG coils) are installed on the ground. A null-flux-type electrodynamic suspension system provided by electromagnetic interaction between SC coils and LG coils serves as the levitation and guidance functions of the vehicle system. Electromagnetic forces acting on SC coils are related to levitation and guidance functions of the vehicle. These forces were studied in previous research, and characteristics of levitation and guidance forces were revealed with respect to parameters of the design of the vehicle. On the other hand, electromagnetic forces acting on LG coils also exist as action–reaction pairs with the forces acting on SC coils. The forces act on LG coils in every levitated run of the vehicle, and stress is applied on the conductor and molded resin of the LG coils. In conventional studies, the vehicle design has been decided mainly by the electromagnetic forces acting on SC coils, and not by the forces acting on LG coils. The aim of the study is to establish an improved method to design the vehicle system with the objective of not only optimization of characteristics of the vehicle, but also consideration of stress on LG coils. As a preliminary study to establish the improved method, a study by computer simulation on characteristics of electromagnetic forces acting on LG coils with respect to the parameters of the vehicle is described in this paper.

Conceptual Design of Superconducting Linear Synchronous Motor for 600-km/h Wheel-Type Railway

This study introduces 600-km/h wheel-type railway concept propelled by linear synchronous motor (LSM) and deals with the conceptual design for LSM system. The LSM is designed to be of a coreless type, which uses high- T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> superconducting electromagnet (SC-EM) as on-board field magnet. From tentative train parameters, we suggest a conceptual LSM model and its design parameters to achieve the train performances. As high- T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> SC material, we use currently available YBCO tape in designing the SC coil. We produce three SC coil models and estimate tentative design values of each model. The feasibility study described herein provides informative design ranges of LSM for 600-km wheel-type railway.

Experience with helium leak and thermal shocks test of SST-1 cryo components

A steady state superconducting Tokamak SST-1 is presently under its assembly stage at the Institute for Plasma Research. The SST-1 machine is a family of Superconducting SC coils for both Toroidal field and Poloidal Field. An ultra high vacuum compatible vacuum vessel, placed in the bore of the TF coils, houses the plasma facing components. A high vacuum cryostat encloses all the SC coils and the vacuum vessel. Liquid Nitrogen (LN2) cooled thermal shield between the vacuum vessel & SC coils as well as between cryostat and the SC coils. There are number of crucial cryogenic components as Electrical isolators, 80 K thermal shield, Cryogenic flexible hose etc., which have to be passed the performance validation tests as part of fulfillment of the stringent QA/QC before incorporated in the main assembly. The individual leak tests of components at RT as well as after thermal cycle from 300 K to 77 K ensure us to make final overall leak proof system. These components include, Large numbers of Electrical Isolators for Helium as well as LN2 services, Flexible Bellows and Hoses for Helium as well as LN2 services, Thermal shock tests of large numbers of 80 K Bubble shields In order to validate the helium leak tightness of these components, we have used the calibrated mass spectrometer leak detector (MSLD) at 300 K, 77 K and 4.2. Since it is very difficult to locate the leaks, which are appearing at rather lower temperatures e.g. less than 20 K, We have invented different approaches to resolve the issue of such leaks. This paper, in general describes the design of cryogenic flexible hose, assembly, couplings for leak testing, test method and techniques of thermal cycles test at 77 K inflow conditions and leak testing aspects of different cryogenic components. The test results, the problems encountered and its solutions techniques are discussed.

Characteristics of persistent-current mode of HTS coil on superconducting electromagnet

This paper investigates the way in which the levitation gap of an electromagnetic suspension (EMS) system affects the current decay rate of superconducting electromagnet (SC-EM) operated in persistence-current mode (PCM). Using inductance analyzed from the magnetic circuit of an EMS model, the current decay rate caused by the variation in the levitation gap was simulated. In order to experimentally verify the simulation results, we fabricated a small-scale EMS model with SC coil operated in PCM and measured the current decay rates at different levitation gaps. The result showed that the presence of iron core provides a significant benefit in the PCM performance of SC coil, but the benefit decreased as the levitation gap increases. This study revealed that the increased levitation gap of the EMS model with the SC-EM could negatively affect the design of SC-EM operated in PCM.

Dynamic response analysis of superconducting coils in Wendelstein 7-X and mechanical quench test

The Wendelstein 7-X (W7-X) modular stellarator is under construction at the Max-Planck-Institut für Plasmaphysik in Greifswald, Germany. The W7-X magnet system contains 70 superconducting (sc) coils which are supported by a massive central steel ring structure via bolted connections and by numerous welded as well as gliding inter-coil elements. Load tests on prototypes of bolted coil to support ring connections whose flanges partially open during high field operation, and of sliding support structures between the coils showed that stick–slip events at their contact surfaces cause shock waves. In order to assess the risk of a shock-induced quench of the superconductor, one of the coils was subjected to impact loads – the so-called mechanical quench (MQ) test – in the course of the cryogenic acceptance tests at CEA in Saclay, France. No quench could be triggered even with the lowest coil stability margin. The paper demonstrates that the amplitudes of the shock waves in the W7-X magnet system are comparable to those generated by the impacts in the MQ tests. For that purpose, finite element (FE) models of both W7-X and the MQ test simulating the dynamic responses were developed and successfully validated against measurements.

OVERVIEW OF SUPERCONDUCTING MAGNET POWER SUPPLY SYSTEM FOR THE KSTAR 1STPLASMA EXPERIMENT

The KSTAR Magnet Power Supply (MPS) was dedicated to the SC coil commissioning and plasma experiment as a part of the system commissioning. Although many efforts to develop large-current power supplies that are useful for high power electronic devices have been made in various application fields, such as for large metal-plating devices, there were clear discrepancies between conventional power supply technologies and that for the SC coils due to the special SC coil load conditions. Therefore, most of the power supply technologies for the SC coils were a challenge in the domestic research area due to their limited application. However, the MPS commissioning result showed that all of the hardware and controlling software operated well, and this result finally led to the success of SC coil commissioning and the KSTAR plasma experiment. This paper will describe key features of KSTAR MPS for the plasma experiment, and will also report the commissioning results of the magnet power supplies.

Harmonic characteristics of SC coil configuration for EDS maglev to reduce leakage flux

AbstractAn improved configuration of SC coils, whose end coils are small, is studied to reduce their leakage flux. This paper examines the harmonic characteristics when the improved configuration of SC coils is applied to the 120‐degree coil pitch ground coil. An adequate combination of length and mmf in the end coils can provide proper performance of harmonics and basis. © 2009 Wiley Periodicals, Inc. Electron Comm Jpn, 91(10): 20– 27, 2008; Published online in Wiley InterScience (www.interscience. wiley.com). DOI 10.1002/ecj.10163

漏れ磁界低減超電導磁石配置における高調波特性

An improved configuration of SC coils, whose end coils are small, is studied to reduce their leakage flux. This paper examines the harmonic characteristics when the improved configuration of SC coils is applied to the 120-degree coil pitch ground coil. An adequate combination of length and mmf in the end coils can provide proper performance of harmonics and basis. © 2009 Wiley Periodicals, Inc. Electron Comm Jpn, 91(10): 20– 27, 2008; Published online in Wiley InterScience (www.interscience. wiley.com). DOI 10.1002/ecj.10163

Temperature distribution in aluminum billets heated by rotation in static magnetic field produced by superconducting magnets

PurposeTo investigate the feasibility of a novel scheme of high‐efficiency induction heater for nonmagnetic metal billets which use superconducting coils.Design/methodology/approachThe idea is to force the billet to rotate in a static magnetic field produced by a DC superconducting magnet. Since a static superconducting magnet has no losses, the efficiency of the system is the efficiency of the motor used. In order to evaluate the temperature distribution arising from the field profile produced by a given SC coil configuration, a numerical model, based on an equivalent electric network with temperature‐dependent parameters, is developed.FindingsA substantial independence of the shape of the temperature profile on the angular velocity and the value of the uniform magnetic field applied, is observed. A strong temperature gradient is observed in the radial direction in the proximity of the penetration front and in the axial direction at the top and bottom surface of the billet. Small temperature gradient was observed in the central part of the billet.Research limitations/implicationsThe reported temperature profile is inadequate for an actual extrusion process which is desired to happen at a constant temperature. The appropriate profile along the billet length can be achieved by a suitable axial shaping of the magnetic field, through the optimization of the coil layout, whereas the undesired radial gradient can be reduced by interspacing the rotation with temperature smoothing intervals.Practical implicationsThe investigation of the profile of applied magnetic field and the heating procedure which allow to achieve the distribution of temperature suitable for the extrusion process can be carried out by using the present model.Originality/valueA high‐efficiency induction heater for nonmagnetic metal billets using superconducting coils in a novel scheme is investigated.

Progress in the Design and Study of a Superferric Dipole Magnet for the GSI Fast-Pulsed Synchrotron SIS100

New experimental results from the investigation of a superferric window frame type dipole magnet with the operating parameters: B=2T, dB/dt=4T/s and pulse repetition rate f=1Hz are presented. The magnet SC coil is made from a hollow multifilamentary NbTi cable, cooled with two-phase helium flow. Special attention is devoted to minimization of AC power losses in the magnet to the specified value of 13W/m. The value of 9W/m has been obtained for a model magnet with the yoke at T=50K. Different possibilities were investigated to reduce the losses in the case of a cold iron yoke (T=4.5K). Other problems, connected with the magnetic field quality, mechanical and cryogenic stability of the magnet under SIS100 operating conditions are also discussed.

Construction and operation performance of the 80 kA current leads used for the test of the ITER toroidal field model coil in the TOSKA facility

For the test of the ITER TFMC in the TOSKA facility of the Forschungszentrum Karlsruhe, two 80 kA current leads were designed and manufactured. Based on the experience coming from the performance of the 30 kA forced-flow current leads, the 80 kA leads were designed in a continuous manner. During the TFMC experiment, various optimization runs were performed at 0, 40 and 80 kA. It could be demonstrated that the leads were operated with the designed mass flow rates. Especially, the Nb/sub 3/Sn inserts used in the lower part of the heat exchanger behave as expected. The slightly different mass flow rate of both terminals can be explained by different RRR of the copper of the heat exchanger. The 80 kA current leads display the highest operating sc coil current reported up to now.

Suitability of MgB/sub 2/ tapes with iron sheaths for multiturn superconducting coils

This paper investigates the suitability of MgB/sub 2/ tapes clad in a high magnetic permeability iron sheath for use in practical multi-turn superconducting coils. The flux-density in the MgB/sub 2/ portion of a single filamentary tape under various configurations is analyzed and its effect on the critical current capability is evaluated. The magnetic field is computed numerically using the finite element (FE) method. The FE model used for the analysis is described along with the assumptions made. The results indicate that the iron-clad MgB/sub 2/ tapes are undesirable for SC coils in many practical applications from a steady state performance stand point.

Removal of SiC inclusions in molten aluminium using a 12 T static magnetic field

The analysis of particles trajectories in a vertical cylindrical coil shows that the magnetization force acting on paramagnetic particles has predominantly axial component which aims upwards and opposite to the gravitational force. In case of superconducting coil and depending on the value of the particle susceptibility, the axial force component can exceed several times the force of gravity. As a result, a motion of the particles in vertical upward direction appears. This effect was utilized to realize a magnetic separator, the main advantage of which is the high effectivity of separation process. A NbTi SC coil generates the magnetization force with a flux density field up to 12 T. Experiments with SiC inclusions in molten Aluminium have been performed to confirm the feasibility of this segregation concept.

Study of the relation between evaluation of strain distribution on superconducting coil and mechanical heat generation

In the superconducting Maglev system, on-board superconducting magnets (SCMs) are vibrated at various frequencies according to the train speed by the electromagnetic disturbance which is caused when the train passes over ground coils. Then a mechanical loss is generated inside the inner vessel in the SCM. This phenomenon increases the heat load on the cryogenic equipment in the SCM. It has been surmised that the mechanical heat inside the inner vessel is generated by the frictional heat caused by the relative microscopic slips between fasteners and superconducting coil (SC coil). Nevertheless, heat generation mechanisms inside the inner vessel have not been studied sufficiently. In this study, we suggest a hypothesis that the frictional heat generated by the relative microscopic slips between fasteners and a SC coil will be indicated if the calculated strain distribution on the SC coil is evaluated. The results of this study supported this hypothesis.

Stabilization of power system for large-scaled experimental fusion facility

In the early experiments of the LHD, the harmful third harmonic currents had been observed in the power line of the NIFS substation, when the fly-wheel motor generator (MG) for the neutral beam injection (NBI) reaccelerates between the plasma productions. The variable harmonic currents, which were estimated to be 1±6 and 1±12 ns, were induced through a Scherbius device. They extremely resonated to the circuit impedance of the power line. To stabilize electrical power in the site, following modifications were conducted: (1) reduction of the harmonic current from the Scherbius device; (2) reconnection of the power supplies of the SC coils and heating devices to suitable power feeders; (3) compensation of lagging reactive power of the MG. As a result, harmful harmonic currents were suppressed.

Testing of a low-resistance joint of CICC for Indian tokamak SST-1

In the frame of CICC testing for sc coils of Indian tokamak SST-1 the soldered joint design was developed and tested up to 50 kA in a loop comprising the secondary winding of a sc transformer. The CICC (14.8/spl times/14.8 mm/sup 2/) was manufactured by Hitachi. It consists of 135 (3/spl times/3/spl times/3/spl times/5) Nb-Ti strands each 0.86 mm dia. In the current range from 10 kA to 45 kA the joint resistance remained almost constant (from 3.6 n/spl Omega/ to 4.0 n/spl Omega/) in the bias field of about 2 T. The temperature dependence of CICC quench current during its charging was measured and showed an excellent (98%) coincidence with critical current values of a single strand. The measured temperature increase of CICC was used to evaluate its self-field AC losses at different charging rates (1.3-1.8 kA/s) and then to estimate the transverse resistivity of the cable. Its low value (approximately 8/spl middot/10/sup -10/ /spl Omega//spl middot/m) most likely reflects the fact that the clean surfaces of the strands in the cable are too tightly pressed. This can negatively influence the CICC stability towards pulse electromagnetic disturbances.