Research and Development of Superconducting Magnet for Fusion Power

This paper describes mainly technological achievment of superconducting magnet for fusion power for the latest 10 years in Japan. The magnet development had been devoted to tokamak Fusion Experimental Reactor (FER). The major results obtained up to now are as follows.

In toroidal coil program, 12 T field generation, which is requested in a reactor toroidal coil, was realized with 6 kA multifilamentary Nb3Sn conductor in 1 m bore. For scaling-up of toroidal coil, half size coils of FER, LCT coils were tested up to 9 T.

In poloidal coil program, Demo Poloidal Coil project is now under way and coil testing will be started in spring of 1989. The stored energy of this coil is around 40 MJ.

In cryogenic technology program, fabrication and operation of large helium refrigerator technology were well established. A supercritical helium pump of 500 g/s was tested for forced flow coil.     

Numerical analysis of the static performance of an annular aerostatic gas thrust bearing applied in the cryogenic turbo-expander of the EAST subsystem

The EAST superconducting tokamak,an advanced steady-state plasma physics experimental device,has been built at the Institute of Plasma Physics,Chinese Academy of Sciences.All the toroidal field magnets and poloidal field magnets,made of NbTi/Cu cable-in-conduit conductor,are cooled with forced flow supercritical helium at 3.8 K.The cryogenic system of EAST consists of a 2 kW/4 K helium refrigerator and a helium distribution system for the cooling of coils,structures,thermal shields,bus-lines,etc.The high-speed turbo-expander is an important refrigerating component of the EAST cryogenic system.In the turbo-expander,the axial supporting technology is critical for the smooth operation of the rotor bearing system.In this paper,hydrostatic thrust bearings are designed based on the axial load of the turbo-expander.Thereafter,a computational fluid dynamics-based numerical model of the aerostatic thrust bearing is set up to evaluate the bearing performance.Tilting effect on the pressure distribution and bearing load is analyzed for the thrust bearing.Bearing load and stiffness are compared with different static supply pressures.The net force from the thrust bearings can be calculated for different combinations of bearing clearance and supply pressure.     

Present status and one upgrading method with a cold compressor of the EAST sub-cooling helium cryogenic system

Since 2006,the superconducting toroidal field (TF) coils of the Experimental Advanced Superconducting Tokomak (EAST) have been successfully cooled by supercritical helium at a temperature of 4.5 K and a pressure of 4 bara in eleven experiments.To obtain higher operating currents and magnetic fields it is necessary to lower the operating temperature of the TF coils.The EAST sub-cooling helium cryogenic system,with a warm oil ring pump (ORP),was tested twice in cool-down experiments,which made the TF coils operate at 3.8 K.However,the long term operational stability of the sub-cooling system cannot be guaranteed because of the ORP's poor mechanical and control performance.In this paper,the present status of the EAST subcooling helium cryogenic system is described,and then several cooling methods below 4.2 K and their merits are presented and analyzed.Finally,an upgrading method with a cold compressor for an EAST sub-cooling helium cryogenic system is proposed.The new process flow and thermodynamic calculation of the sub-cooling helium system,and the main parameters of the cold compressor,are also presented in detail.This work will provide a reference for the future upgrading of the sub-cooling helium system for higher operation parameters of the EAST device.     

Poloidal Field Control for the HT—7U Superconducting Tokamak

Controlling the poloidal field(PF) in the HT-7U superconducting tokamak is critical to the realization of the mission of advanced tokamak research.Plasma start-up,plasma position,shape,current control and plasma shape reconstruction have been performed as a part of its design process.The PF coils have been designed to produce a wide range of plasmas,Plasma start-up can be achieved for multiple conditions.Fast controlling coils for plasma position inside the vacuum vessel are sued for controloling the plasma vertical position on a short timescale.The PF coils control the plasma current and shape on a slower timescale,VXI(VME bus extensions for Instrumentation)Bus system and DSP(Digital Signal Processor is a basic unit of the feedback control system),the response time of which is about(2-4)ms.The basic unit of this system ,the shape-controlling algorithms of a few critical points on plasma boundary and real-time equilibrium fitting(RTEFIT)will be described in this paper.     

Divertor detachment operation in helium plasmas with ITER-like tungsten divertor in EAST

Detachment in helium (He) discharges has been achieved in the EAST superconducting tokamak equipped with an ITER-like tungsten divertor. This paper presents the experimental observations of divertor detachment achieved by increasing the plasma density in He discharges. During density ramp-up, the particle flux shows a clear rollover, while the electron temperature around the outer strike point is decreasing simultaneously. The divertor detachment also exhibits a significant difference from that observed in comparable deuterium (D) discharges. The density threshold of detachment in the He plasma is higher than that in the D plasma for the same heating power, and increases with the heating power. Moreover, detachment assisted with neon (Ne) seeding was also performed in L- and H-mode plasmas, pointing to the direction for reducing the density threshold of detachment in He operation. However, excessive Ne seeding causes confinement degradation during the divertor detachment phase. The precise feedback control of impurity seeding will be performed in EAST to improve the compatibility of core plasma performance with divertor detachment for future high heating power operations.     

超流氦低温恒温器冷质量准直调节及低温变形分析

为满足超导射频腔及超导磁体低温下的加速器准直精度要求,对超流氦低温恒温器冷质量的准直调节方案进行分析。通过对冷质量的低温位移进行有限元模拟计算,获得其在2 K超流氦温区的位移变形量,利用拉伸丝型位移监测仪（WPM）在线测量数据来验证模拟分析方法。结果表明,模拟值与测量值具有较好的一致性,为底部支撑型式低温恒温器的准直调节方案设计提供了重要的数据基础。     

Alignment and Cryogenic Deformation Analysis of Cold Mass-in Superfluid Helium Cryomodule

To meet the alignment requirement of superconducting radio frequency (SRF) cavity and superconducting magnet at cryogenic temperature in the accelerator system, the design of adjustment and alignment of the cold mass in superfluid helium cryomodule should be analyzed. The cryogenic deformation of the cold mass at the 2 K superfluid helium temperature level was simulated and obtained by the finite element method, meanwhile the simulation analysis method was checked by the cryogenic deformation data of the cold mass online measured by the wire of position monitor (WPM). The results show that the simulation result and measurement result have a good consistency, and it provides an important data foundation for the alignment design in the type of bottom supported cryomodule.     

650 MHz双cell超导腔冷氦气自动降温方案研究

本文以先进光源技术研发与测试平台（PAPS）中的650 MHz双cell超导腔的降温过程为主要研究对象，采用准则关系式法及计算流体力学方法构建了整体换热模型并编程计算，得到了降温过程中超导腔所需的冷氦气入口腔前参数。根据腔前参数反推流程，考虑整体布局和局部阻力的损失，计算了兑温阀门开度与腔前参数的对应关系，从而可通过兑温阀门开度时刻表来实现整体降温过程的自动控制。     

The Design and Analysis of Helium Turbine Expander Impeller with a Given All-Over-Controlled Vortex Distribution

To make the large-scale helium cryogenic system of fusion device EAST （experimen- tal advanced super-conducting tokamak） run stably, as the core part, the helium turbine expander must meet the requirement of refrigeration capacity. However, previous designs were based on one dimension flow to determine the average fluid parameters and geometric parameters of impeller cross-sections, so that it could not describe real physical processes in the internal flow of the tur- bine expander. Therefore, based on the inverse proposition of streamline curvature method in the context of quasi-three-dimensional flows, the all-over-controlled vortex concept was adopted to design the impeller under specified condition. The wrap angle of the impeller blade and the whole flow distribution on the meridian plane were obtained; meanwhile the performance of the designed impeller was analyzed. Thus a new design method is proposed here for the inverse proposition of the helium turbine expander impeller.     

Higher resolution helium measuring system for deuterium plasma on EAST tokamak via normal Penning gauge

Although the deuterium and helium have almost the same mass,a Penning Optical Gas Analyzer(POGA) system on the basis of the spectroscopic method and Penning discharging has been designed on EAST,since 2014.The POGA system was developed successfully in 2015,it was the first time that EAST could detect helium partial pressure in deuterium plasma(wall conditioning and plasma operation scenario).With dedicated calibration and proper adjustment of the parameters,the minimum concentration of helium in deuterium gas can be measured as about 0.5% instead of 1% on the other tokamak devices.Moreover,the He and D_2 partial pressures are measured simultaneously.At present,the measurable range of deuterium partial pressure is 1?×?10~(-7)mbar to 1?×?10~(-5)mbar,meanwhile the range of helium is 1?×?10~(-8) mbar to 1?×?10~(-5)mbar.The measurable range can be modified by means of the adjustment of POGA system's parameters.It is possible to detect the interesting part of the gas with a time resolution of less than 5 ms(the 200 ms because of conductance of transfer pipe at present).The POGA system was routinely employed to wall conditioning and helium enrichment investigation in2015.Last but not the least,the low temperature plasma of POGA is generated by normal penning gauge Pfeiffer IKR gauge instead of Alcatel CF2 P,which has been suspended for a few years and was used for almost all the POGA systems in the world.     

Optimization of an in-vessel visible inspection system for a long-pulse operation in KSTAR

To monitor the global formation of shaped plasmas, motion, and damage to the internal structures of the vacuum vessel, an in-vessel visible inspection system has been developed and operated on the Korean superconducting tokamak advanced research (KSTAR) device. The system contributed much to research progress on KSTAR such as the plasma start-up, plasma wall interactions, edge-localized modes, and disruptions. Moreover the need to perform inspections became important with high plasma power operation because of the increased frequency of first wall damage following off-normal events. Therefore the system is being improved from its original concept, and its final goal is operation during steady-state operation of the tokamak. The system consists of three fast visible cameras and two light-emitting diode illuminators. They are designed to be controlled fully from the control room to provide inspection capability at any time during hostile operating conditions. In this paper, we describe the upgrade of the system and recent results of the visible inspection system with the images of the KSTAR discharges for the last four years. Finally, we discuss the technical issues for a long pulse steady-state operation.     

A New Method for Measurement of Helium Mass Flow Rate in the Cryogenic System of TORE SUPRA

The TORE SUPRA Tokamak was built by EURATOM-CEA association. The NbTi conductor of superconducting coils is inserted in a tight enclosure filled with pressurized superfluid helium of 0.125 MPa at 1.8 K. The thick casing is cooled to 4.5 K by 1.8 MPa in 4.5 K supercritical helium circulation. Around this thick casing, a 80 K thermal shield protects the parts at very low temperatures from the thermal radiation, which is cooled by pressurized helium at 80 K and 1.8 MPa. A new measurement method for helium mass flow rate of 80 K shield and 4.5 K casing is described in this paper. The commissioning was done on the two helium loops of the cryoplant: the supercritical 4.5 K thick casing and 80 K shields. The purpose is to improve control of the 4.5 K and 80 K refrigeration loops.     

移动式氦液化器控制系统升级改造

低温超导高频腔技术重点实验室的车载式氦液化器控制系统存在着设备老化、自动化程度不高等缺点.在实验室建设中,对该控制系统进行了一次全面的升级改造.改造后的控制系统采用了分布式的控制策略和成熟的PLC技术,能够很好的满足液化系统自动运行的需要.     

Structural design of cryogenic component support for EAST （HT-7U）

EAST is a full superconducting tokamak with an elongated plasma cross-section. It consists of superconducting poloidal field (PF) magnet system, toroidal field (TF) magnet system, vacuum vessel with inner parts, thermal shields and cryostat vessel. The mission of the project is to widely investigate both physics and technologies of advanced tokamak operations, especially the mechanism of power and particle handling for steady-state operations. The cryogenic component is mainly composed of superconducting TF and superconducting PF coils that ensure the ability of sustaining magnetic field for plasma confinement, control and shaping in steady-state. This report describes the process of the structure design of cryogenic component support for EAST.     

Commissioning the ICRF system and an ICRF assisted discharge cleaning at the KSTAR

Korean superconducting tokamak advanced research (KSTAR) is a national superconducting tokamak with the aim of a high beta operation based on advanced tokamak (AT) scenarios, and an ion cyclotron ranges of frequency (ICRF) heating is one of the essential tools to achieve this goal. The fabrication and high voltage (HV) test of the antenna and the matching system were finished in 2006 and the installation of the antenna, matching system and the transmitter at the KSTAR site was completed in 2007. Antenna conditioning was carried out to improve the HV holding condition of the antenna installed on the KSTAR and to check on the electro-magnetic (EM) interference with other equipments such as the superconducting magnet monitoring system and other machine and/or plasma diagnostic systems. The first KSTAR tokamak experimental campaign started by a vacuum pumping, a cryostat cooling and an ICRF system contributed to the successful tokamak shots through an ICRF assisted discharge cleaning of the vacuum vessel. In this paper, the installation processes of the ICRF system (with an emphasis on the quality assurance procedures of KSTAR), as well as the results from the first RF discharge experiment for the discharge cleaning and FWEH (fast wave electron heating) experiment for the KSTAR 1st experimental campaign are outlined.     

PKU-SCAF 1.3GHz超导腔超流氦冷却循环设计及运行温度优化

超导腔的BCS表面电阻是温度的指数函数,降低超导腔工作温度可以减少超导腔的射频功率损失,但同时制冷设备的效率也会急剧降低.本文以北京大学超导加速器实验装置(PKU-SCAF)1.3 GHz的9壳型超导加速腔为例,对1.3 GHz超导腔采用的压力为3.13 kPa的饱和态超流氦冷却循环方案及运行温度进行了分析研究.从提高超流氦冷却系统的制冷系数、降低超导腔的动态损耗出发,通过对5种超流氦冷却循环方案以及超流氦冷却循环的效率和超导腔低温系统的能耗进行数值模拟和分析,得出了适合PKU-SCAF主超导腔的最佳工作温度为1.95K.     

Experiment and Operation of a LHCD—35kV／2.8MW／1000s High—Voltage Power Supply on HT—7 Tokamak

A-35kV/2.8MW/1000s high-voltage power supply(HVPS) for HT-7 superconducting tokamak has been built successfully.The HVPS is scheduled to run on a 2.45 GHz/1MW lower hybrid current drive(LHCD)^[1] system of HT-7 superoonducting tokamak before the set-up of HT-7 superconducting tokamak in 2003.The HVPS has a series of advantages such as good steady and dynamic response,logical computer program controlling the HVPS without and fault,operationa panel and experimental board for data acquisition.which both are grounded distinctively in a normative way to protec the main body of HVPS along with its attached equipments from dangers.Electric power cables and other control cables are disposed reasonably,to prevent singals from magnetic interference and ensure the precision of signal transfer.This paper involves the experiment and operation of a 35kV/2.8 MW/1000s HVPS^[2] for 2.45 GHz/1 MW LHCD system.The reliability and feasibility of the HVPS has been demonstrated in comparison with experimental results of original design and simulation data.     

SUNIST Microwave Power System

Experiments on the start-up and formation of spherical tokamak plasmas by electroncy clotron heating alone without ohmic heating and electrode discharge assisted electron cyclotron wave current start-up will be carried out on the SUNIST (Sino United Spherical Tokamak) device.The 2.45 GHz/100 kW/30 ms microwave power system and 1000 V/50 A power supply for electrode discharge are ready for experiments with non-inductive current drive.     

Operation with 1 MA plasma current in EAST

Discharge with a plasma current of 1 MA at a line-averaged density of 1.8×1019m-3 was realized in EAST, a fully superconducting tokamak. The key issues to achieve the discharge with 1 MA plasma current include both early shaping and LHCD assistance during start-up phase to extend the voltage margin of PF coils for easier plasma control, an optimization of the control methodology for PF coils to avoid over-current fault and a very good wall condition. A better wall condition was achieved mainly by extensive Lithium coating. Both stationary H-mode and diverted plasma discharge of 100 s were also obtained.     

ITER中国液态锂铅实验包层模块氦气冷却系统初步设计研究

氦气冷却系统是ITER中国液态锂铅实验包层模块(DFLL-TBM)在ITER装置内进行实验的重要辅助系统.根据ITER运行时的热工条件、安全要求、空间要求,分析了DFLL-TBM氦气冷却系统的功能,确定氦气冷却系统的设计原则和要求,在此基础上给出氦气冷却系统的初步设计方案和设备布置.该氦气系统的特点体现在:双功能,即有宽的裕量满足SLL-TBM和DLL-TBM实验;两条氦气回路共享压力控制单元和氦气净化子系统;旁路设计调节TBM和热交换器氦气的出口温度.