First plasma experiment on spherical tokamak device UTST

The UTST (University of Tokyo Spherical Tokamak) device was constructed for the purpose of exploring the formation of ultrahigh‐beta ST (spherical tokamak) plasma using the double null plasma merging method. When two plasmas merge together to form a single plasma, magnetic field lines reconnect, and magnetic field energy is converted to plasma kinetic energy, increasing the plasma beta. Merging start‐up has been demonstrated in the TS‐3/4, START, and MAST devices using coils inside the vacuum vessel, and the TS‐3 plasma obtained a 50% beta. In order to demonstrate start‐up in a more reactor‐relevant situation, UTST has all poloidal field (PF) coils outside the vacuum vessel. The first plasma experiment on the UTST was begun in December 2007. In the results, the plasma obtained 10 kA by using only the outer PF coils and a single ST was generated in the lower area (z = –0.3 to –1.0 m) close to a washer gun. This result suggests that another washer gun on the top of the UTST can allow the generation of ST in the upper area and merging start‐up by using outer PF coils. © 2012 Wiley Periodicals, Inc. Electr Eng Jpn, 179(2): 20–26, 2012; Published online in Wiley Online Library ( wileyonlinelibrary.com ). DOI 10.1002/eej.21216     

Application of a force‐balanced coil to a tokamak device

High field is very favorable for magnetically confined fusion devices such as a tokamak, but electromagnetic force derived from the field becomes a fatal problem. In particular, the largest in‐plane centering force hinders toroidal field coils from operating at higher fields. Variable pitch multihelical coils, which we term force‐balanced coils (FBCs), by which this force is drastically reduced, are proposed for the coil system of tokamaks. FBCs can also provide poloidal flux swing for plasma breakdown and current induction during the ramp‐up phase of the coil current. In this paper, we indicate how to design FBCs for tokamaks. The fusion reactor‐size FBCs are designed and compared with that of the conventional toroidal field coil system. © 1999 Scripta Technica, Electr Eng Jpn, 130(3): 39–48, 2000     

Study of the Counterclockwise/Clockwise (CCW/CW) Rotational Losses Measured with a Two‐dimensional Vector Magnetic Property Measurement System

In the development of high‐efficiency actuators, high‐magnetic flux density conditions are applied to achieve higher torque, lower weight and smaller size. It is therefore very important to understand the magnetic properties under high‐magnetic flux conditions in the electrical steel sheets. However, it is very difficult to measure the two‐dimensional vector magnetic properties under high‐flux density conditions due to nonlinearity and anisotropy of the magnetic materials. The measured magnetic power loss under clockwise rotating fields differs from that of the counter‐clockwise direction, and the value in counter‐clockwise rotating fields sometimes becomes negative. It can be considered that angle errors of H‐coils and B‐coils are the main causes of the measurement errors. In this paper, the angle errors of the double B‐coil (search coils) and the double H‐coil (cross‐type H‐coil) are evaluated with a projector‐type magnifying scope and a calibration device. We have compensated the measured magnetic field vector components caused by the angle errors and made clear their influence on the two‐dimensional vector magnetic property measurements. Furthermore, we proposed a method to cancel out small angle errors that remained in the saturated region. Copyright © 2008 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.     

Comparative studies on magnetic flux relaxations of varied spherical toroids produced by co‐helicity merging

The co‐helicity merging operations of compact toroid (CT) and spherical tokamak (ST) have been performed with external toroidal fields in the CT/ST merging device TS‐4. The low‐q (safety factor) CT merging as the compact RFP merging and the spheromak merging show the flux conversion from toroidal to poloidal in the course of the reconstruction of the Taylor force free state. The relaxation to the Taylor state proceeds through the following three states: (1) axisymmetric merging with increasing toroidal flux; (2) increase in the poloidal flux Ψ; and (3) relaxation to the Taylor state. The high‐q ST merging shows different relaxation process from those of the compact RFP and the spheromak mergings. Increases in Ψ were not clearly observed in the ST merging. The measured eigenvalues λ show that ST's, especially high‐q ST's, approach a unique intrinsic equilibrium state that has a λ proportional to Ψ with a longer lifetime than that of CT's. When external toroidal field is set in a certain range between the low‐q operation and the high‐q operation for ST's, an abnormal phenomenon was found in the ST formation, namely, a drastic decrease in the plasma lifetime. This phenomenon is characterized by very weak poloidal flux generations during the initial plasma production phase and the subsequent plasma separation phase when the plasma starts detaching from the flux core. © 2005 Wiley Periodicals, Inc. Electr Eng Jpn, 151(4): 7–15, 2005; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/eej.20069     

Levitation coil length optimization of a magnetically levitated train

An experimental line of a magnetically levitated train (MAGLEV) using superconducting magnets (SCM) is now under construction in Yamanashi Prefecture. In the MAGLEV system, the levitation coils are mounted in the side walls. The induced current of the levitation coils generates magnetic flux that includes higher order harmonics. These harmonics generate eddy currents at the surface of the vacuum vessels of the SCM. Forces produced by interaction between the eddy current and the static magnetic field of the SCM cause evaporation of liquid helium, or SCM quenching in the worst case. The number of levitation coil layer is now restricted to one, but by removing the restriction, it is possible to reduce the amplitude of the harmonics of the magnetic flux. In this paper, the relationship of the flux amplitude to the levitation coil length is investigated; it is found that there is an optimum length at which the amplitude is minimized.     

五种纵向磁场真空灭弧室触头磁场特性分析比较

用三维有限元法研究了线圈、杯状,两极和四极及双线圈五种纵磁真空灭弧室触头的纵向磁感应强度分布、触头片上涡流分布和纵向磁场滞后时间.研究表明:(1)电流峰值时纵向磁场由强到弱依次排列为:线圈式触头、两极式结构、双线圈式触头、杯状和四极式触头;(2) 电流过零时剩余磁场由弱到强依次为:四极式触头、两极式触头、杯状触头、双线圈式触头和线圈式触头; (3)纵向磁场较强处滞后时间由小到大依次为:两极式触头、四极式触头、线圈式触头、双线圈式触头和杯状纵磁触头.     

Merging formation and current amplification of field‐reversed configuration

The merging formation of the field‐reversed configuration (FRC) has been developed in the TS‐3 merging experiment, leading us to a new scenario of FRC slow formation, heating and current amplification. Two force‐free spheromaks with opposing toroidal fields were merged together in the axial direction to form a high‐β FRC with higher efficiency than the conventional field‐reversed theta‐pinch method. This unique relaxation from the force‐free (β ∼ 0.05 − 0.1) spheromaks to the high‐β (β ∼ 0.7 − 1) FRC is attributed to the conversion of toroidal magnetic energy into ion thermal energy through the reconnection outflow. A central ohmic heating (OH) coil worked successfully to amplify the FRC plasma current by a factor of 2. Toroidal mode analysis of magnetic structure indicated that the tilting stability of the oblate FRC was provided by an ion kinetic effect. This oblate FRC is also useful as an initial equilibrium to produce a high‐β_p spherical tokamak (ST) with diamagnetic toroidal magnetic field, suggesting the close relationship between FRCs and high‐β_p STs in the second stable region of the ballooning mode. © 2007 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.     

Persistent Current Analysis of Superconducting Coils in an Electrodynamic Suspension System

In a superconductive electrodynamic suspension system, superconducting coils experience a space harmonic magnetic field generated by induced current in the levitation coils installed along the guideway. Current in a superconducting coil varies to keep the total magnetic flux constant when the coil is exposed in an external magnetic field. However, superconducting coil current was treated as constant in the conventional analysis because the electromagnetic forces can be estimated easily. The purpose of this paper is to determine the behavior of the persistent current in making reliable and tough superconducting magnets. A simple method to analyze fluctuating persistent current in a superconducting coil is proposed first. Calculated results for the normal running condition showed that the fluctuations of persistent current are less than 1 percent of the initially excited current for the sidewall levitation system, and less than 2 percent for the ground levitation coil system. Even when the null flux guidance cable is short-circuited, the fluctuation is less than 5 percent. Considering that the magnetic shield effects by the conductive cryostat vessels were neglected in this analysis, the fluctuating magnetic field by levitation coils will never affect the stability for a superconducting magnet.     

Comparative study of electron density profiles in compact torus plasma merging experiments

Following two previous papers on comparative studies of the electron density distributions for a single compact torus (CT) and a spherical tokamak (ST), and for a single ST and a merged ST, a comparative study on the dynamics of the electron density profile during and after the CT and ST plasma merging process was performed. The sharpness of the peak in the electron density profile around the midplane just after the merging of CT with a low safety factor (q value) such as RFP or spheromak is found to be related to the speed of the magnetic axis during the plasma merging process. It is also found that the electron density gradient near the plasma edge in a high‐q ST is larger than that of a low‐q CT. High‐q ST is found to have a magnetic structure which is able to sustain a large thermal pressure by a strong j × B force. Despite these differences in the electron density profile between CT and ST during merging, the confinement characteristics evaluated from the number of electrons confined within the magnetic separatrix after the completion of the merging are almost similar between the merging CT and the merging ST. For all configurations, the electron density profiles after the completion of the merging are analogous to those of the corresponding single configuration produced without the merging process. © 2004 Wiley Periodicals, Inc. Electr Eng Jpn, 148(2): 14–21, 2004; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/eej.10312     

无线电能传输的平面螺旋线圈优化

磁耦合线圈是无线电能传输中至关重要的一个环节,线圈品质因数往往是衡量线圈优劣的重要指标。本文基于一维线圈损耗模型分析了宽度、高度、磁场强度等重要参数对损耗的影响。以此为基础,借助有限元仿真软件得到线圈磁场分布规律。首先对线圈采用渐变线宽的方案来优化线圈Q值。进一步采用将内三匝及最外匝线圈进行铜箔垂直置换的方案,该方法能够优化内三匝及最外匝线圈的损耗。通过仿真与实验验证了方案的正确性。     

Experiments on a tokamak device with force‐balanced coils

High field is very favorable for magnetically confined fusion devices such as a tokamak, but the electromagnetic force derived from the field becomes a fatal problem. In particular, the largest in‐plane centering force hinders toroidal field coils from operating at higher fields. Although variable‐pitch multihelical coils, which we term force‐balanced coils (FBCs), by which this force is drastically reduced, were proposed for the coil system of tokamaks, there had been no actual tokamak device with FBCs. Therefore, Todoroki‐1, a small tokamak device with FBCs has been manufactured and experiments started. In this paper, the structure of Todoroki‐1 is indicated, and the stray magnetic field and centering force on FBCs are investigated. The plasmas have been generated in the device, and attempts at better plasmas are beginning. © 1999 Scripta Technica, Electr Eng Jpn, 130(3): 19–29, 2000     

Accurate evaluation of the magnetic field strength of largesubstation air-core reactor coils

A circular cylindrical conductor segment is a basic modeling element of substation air-core reactor coils. The analytical expressions for evaluating the stationary and quasi-stationary magnetic field strength of a segment are derived. In the derivation, a surface source model is developed to substitute the given volume current distribution by equivalent surface sources. The magnetic flux density of sample coils is calculated by using the derived equations. The results are compared with those obtained by the finite element method. Good agreement is observed. The derived equations are very useful for solving problems that involve the accurate calculation of the magnetic field distribution around a coil, the magnitude of forces on a coil, the current distribution in the conductors of a coil and the inductive properties of a coil     

New Hybrid‐Excited Motor Using Armature Windings as Field Flux Source

In this paper, a new hybrid‐excited motor is proposed. Its unique feature is that it has no field exciting coil, despite being a type of bypass yoke core (BYC) hybrid‐excited synchronous motor. Based on finite element analysis (FEA), the following facts are clarified. Since the stator coil magnetomotive force (MMF) and claw poles rotate at the same speed and in the same direction, they are motionless relative to each other. Since the claw poles are located at one of its ends near the BYC surface with a gap, it receives immovable magnetization by the stator coil MMF through the BYC; then the claw poles can supply a magnetic field flux via the BYC. This flux via the BYC increases the total torque, because its torque is added the two other torque components: the permanent magnet torque and claw poles reluctance torque. In addition, the magnetic polarity and the amount of the flux of the claw poles can be varied by controlling the armature current phase angle.     

多级重接式电磁发射的电磁分析与有限元仿真

为了对多级发射达到高速时的电磁问题进行研究,以使用板状发射体和箱形线圈的多级重接式电磁发射为研究对象,推导了多级线圈电磁场和发射体涡流场的综合作用方程,利用ANSOFT电磁场有限元分析软件建立了三维有限元仿真模型并进行仿真计算。结果表明,多级线圈电流的同时存在,既改变了单级线圈的磁场分布和磁感应强度大小,也改变了发射体中局部涡流密度的大小和发射体受力的大小。这种改变与线圈电流的频率和相位差异相关,并且线圈间距越小,这种综合作用的影响越大。     

12kV真空断路器双线圈单稳态永磁机构的设计

针对应用于12 kV真空断路器的单线圈单稳态永磁机构的分闸电流较大及分闸速度不易提高的问题,笔者提出一种双线圈单稳态永磁机构,其合、分闸操作各使用一个线圈,且设置有一个磁短路环,达到降低分闸电流及提高分闸速度的目的。应用磁场有限元分析方法对永磁机构进行优化设计,并给出了设计方案及样机测试结果。试验结果证明,该永磁机构各项技术参数达到了设计要求,同时也验证了该方法的可行性。     

Design of guidance system for superconducting maglev vehicle using propulsion coils

Superconducting magnetically levitated vehicles have guidance coils that produce vehicle guidance forces using a null flux coil connection. However, the resulting forces do not have sufficient damping forces to suppress lateral vibration of the vehicle. Even though the experimental system has some magnitude of damping, it is still unsatisfactory. Moreover, since the guidance forces are very weak at low speeds, the minimum levitating speed is limited. The paper proposes a guidance method using LSM armature coil currents. When the currents are decomposed into the dq frame variables so that the d‐axis is assigned to the field axis, the d‐axis component of the currents generates lateral forces. The lateral forces of the vehicle are formulated, and the guidance force coefficient is defined. In addition, the author presents a design formula for a guidance regulator system that employs a dynamic compensator using a feedback signal of lateral displacement. The calculated results show that the guidance regulator system works as intended. © 2001 Scripta Technica, Electr Eng Jpn, 137(2): 44–51, 2001     

Development of new conductors of the cable‐in‐conduit type for HTS coils with high performance

A new type of cable‐in‐conduit conductor composed of HTS tapes was proposed as a winding for high‐performance HTS coils with high current capacities and low AC losses. In the fabrication of the conductor, the twist of stacked tapes around their axes was made before inserting them inside conduits. The twist angle should be changed continuously along the axis to reduce face‐on oriented magnetic fields applied to the tape in the winding conductor during coil operation. In order to confirm the high current capacity of this type conductor, two single‐layered solenoidal coils wound with sample conductors composed of five stacked Bi‐2223 tapes with bias angles of 20 and 0° were fabricated and tested in liquid nitrogen. A copper magnet system was used in this experiment to generate the spread magnetic field with a spread angle of 20° from the coil axis to the radial direction, which simulates the profile of magnetic fields near the edge windings of practical coils. A large improvement on critical‐current degradation affected by spread magnetic fields was successfully observed for the test coil wound with the 20° bias conductor. © 2005 Wiley Periodicals, Inc. Electr Eng Jpn, 153(4): 12–19, 2005; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/eej.20218     

Numerical analyses of high‐beta spherical tokamaks with varied elongation

The effect of plasma elongation on the second‐stable spherical tokamak (ST) was numerically studied using the experimentally measured pressure and current profiles of ultrahigh‐beta STs. The maximum beta of ST over 50% was obtained in the TS‐3 ST/CT experiment by applying an external toroidal field to an FRC. It was found that the marginal beta for the ballooning instability increased with the plasma elongation κ of ST. The elongated STs with κ > 2 have the magnetic shear (S)–pressure gradient (α) profiles located in the second‐stable regime for the ballooning mode and the stability margin increased with κ. The close relation between the absolute minimum‐B profile and the second stability was documented. The effect of elongation on maximum beta was observed to saturate when κ exceed 3, indicating that the optimized elongation for high‐beta STs is located around 2 < κ < 3. © 2006 Wiley Periodicals, Inc. Electr Eng Jpn, 155(4): 1–6, 2006; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/eej.20132     

多级电磁感应线圈炮的级间耦合特性

多级电磁感应线圈炮在发射过程中,前面各级驱动线圈放电产生的磁场和电枢自身感应电流产生的磁场都会耦合进入下一级驱动线圈,对驱动线圈的加速性能产生影响。基于多级感应线圈炮等效电路模型,建立了带有续流二极管的多级感应线圈炮数学模型,得到了其推力方程和运动方程。基于电磁感应定律,分析了2种耦合因素作用时下一级驱动线圈中感应电动势和感应电流的变化。基于场路耦合,对多级感应线圈炮进行了仿真,得到了不同情况下的耦合特性。研究结果表明:多级线圈发射中存在耦合效应;根据耦合磁通的不同,后面各级驱动线圈上会产生正向或反向串联的电动势;正向串联电动势有利于线圈炮的加速。     

Critical Radius of Axial Magnetic-Field Contacts in Vacuum Interrupters

Vacuum circuit breakers (VCBs) are widely used to protect medium-voltage (MV) power distribution circuits. Since SF$_{6}$ gas is specified as a global warming gas, VCBs are stepping into a higher voltage sector to protect power transmission circuits. Axial magnetic field (AMF) contacts are widely used in vacuum interrupters. In this paper, we propose a concept of critical radius of AMF contacts. When coil width and coil height are fixed, the axial magnetic flux density increases first with increasing contact diameter. Then, it reaches a maximum value. Thereafter, the axial magnetic flux density decreases. The contact radius corresponding to the maximum axial magnetic flux density is critical radius. The concept of critical radius is validated by coil-type AMF contacts and slot-type AMF contacts in MV vacuum interrupters with finite-element analysis. Critical radius is only related to contact geometry parameters and the current has no influence on critical radius. Critical radius increases with increasing contact gap, coil width, coil height, and thickness of the contact plate. In high-voltage AMF vacuum interrupters, axial magnetic flux density per kiloampere increases with increasing contact diameter since the critical radius is typically high with a high contact gap and it is in the rising branch. In an MV AMF vacuum interrupter, interrupting capacity could increase with an increase of the contact diameter with a different rising rate before and after the critical radius. This is expected to be experimentally validated.