M. Sueoka

Off-axis current drive and real-time control of current profile in JT-60U

Aiming at optimization of current profile in high-β plasmas for higher confinement and stability, a real-time control system of the minimum of the safety factor (qmin) using the off-axis current drive has been developed. The off-axis current drive can raise the safety factor in the centre and help to avoid instability that limits the performance of the plasma. The system controls the injection power of lower-hybrid waves, and hence its off-axis driven current in order to control qmin. The real-time control of qmin is demonstrated in a high-β plasma, where qmin follows the temporally changing reference qmin,ref from 1.3 to 1.7. Applying the control to another high-β discharge (βN = 1.7, βp = 1.5) with m/n = 2/1 neo-classical tearing mode (NTM), qmin was raised above 2 and the NTM was suppressed. The stored energy increased by 16% with the NTM suppressed, since the resonant rational surface was eliminated. For the future use for current profile control, current density profile for off-axis neutral beam current drive (NBCD) is for the first time measured, using the motional Stark effect diagnostic. Spatially localized NBCD profile was clearly observed at the normalized minor radius ρ of about 0.6–0.8. The location was also confirmed by multi-chordal neutron emission profile measurement. The total amount of the measured beam driven current was consistent with the theoretical calculation using the ACCOME code. The CD location in the calculation was inward shifted than the measurement.

Development of advanced operation scenarios in weak magnetic-shear regime on JT-60U

Fully non-inductive discharge having a relaxed current profile and a high bootstrap current fraction fBS = 0.5 has been realized in the high-βp ELMy H-mode discharge with weak magnetic-shear having q95 = 5.8, qmin = 2.1 and q(0) = 2.4, where qmin and q(0) are the safety factor q at the minimum and the plasma centre, respectively. The rest of the plasma current is externally driven by neutral beams and lower-hybrid waves. The safety factor profile evaluated by the motional Stark effect diagnostic is kept unchanged for 0.7 s at the end of the full current drive (CD) sustainment for 2 s (1.5 times the current relaxation time τR). The loop voltage profile is spatially uniform at 0 V at the end of the sustainment. This demonstration shows, for the first time, that the steady sustainment of full-CD plasma is possible at high fBS = 0.5 and reasonably low q95 = 5.8 regime and is stably controlled by appropriate external current drivers. On the other hand, when the combination of bootstrap current and externally driven current does not match the steady current profile, a slight change in the current profile due to current relaxation resulted in the appearance of magnetohydrodynamic instability, e.g. neo-classical tearing mode, in a high-beta plasma. This discharge clarifies the importance of the demonstration of steady current profile developed here. These discharges contribute to the ITER steady-state operation scenario development.

Shape Reconstruction of RF-Driven Divertor Plasma on QUEST

In the present RF-driven plasma with a lot of high-energy electrons, there may be anisotropic plasma pressure, which makes difficult a usual equilibrium analysis, but the CCS method can reconstruct the plasma shape precisely regardless of the anisotropy. And the plasma current effect in the open magnetic surfaces outside of the closed magnetic surfaces is considered in the RF-driven plasma. Further high-energy particle guiding center orbits are calculated aiming at estimation of the open-magnetic-surface current distribution.

Response of fusion gain to density in burning plasma simulation on JT-60U

A burning plasma simulation scheme has been developed with consideration for temperature dependence of the DT fusion reaction rate in JT-60U. The heating power for the simulation of alpha particle heating was calculated using real-time measurements of density and ion temperature. The response of a simulated fusion gain to the density was investigated in this scheme where a constant heating power was used for the simulation of external heating, in order to understand burn controllability by the fuel density in a fusion reactor. When temperature dependence of the fusion reaction rate was assumed as a square of ion temperature, density dependence of the simulated fusion gain stronger than the square of density was observed. Transport analysis using a 1.5 dimension transport code indicated that the strong density dependence is induced by changes in a confinement improvement factor and changes in a pressure profile.

JT-60 Control System

The present status of the JT-60U control system is reported including its original design concept, the progress of the system, and various modifications since the JT-60 upgrade. This control system has features of a functionally distributed and hierarchical structure, using CAMAC interfaces initially, which have been replaced by versatile module Europe (VME)-bus interfaces, and a protective interlock system composed of both software and hard-wired interlock logics. Plant monitoring and control are performed by efficient data communication through CAMAC highways and Ethernet with TCP/IP protocols. Sequential control of plasma discharges is executed by a combination of a remodeled VME-bus system and a timing system. A real-time plasma control system and a human interface system have been continuously modified corresponding to the progress of JT-60U experiments.

Remodeling of JT-60 discharge control system

Remodeling of the JT-60 discharge control system (DCS) is initiated to minimize the cost and time for hardware maintenance and software development. Three principles have been adopted for remodeling. (1) The functions of the DCS must remain intact. (2) The shutdown period must be minimized to maximize JT-60 experiment time. To realize the new DCS, (3) a combination of the host workstation (WS) and target Versa Module Europe (VME)-bus system with real time OS was chosen as the fundamental structure. The prototype DCS was designed and tested to evaluate the performance of the converted discharge sequential control software modules and the message communication program. The new configuration of the DCS and procedure of smooth remodeling without shutdown is described in detail.

Development of a new discharge control system for JT-60 with a unix workstation and a VME-bus system

Abstract The old discharge control system (DCS) with HIDIC-80E mini-computer has been replaced by a control system with a unix workstation and a VME-bus system. Functionally, the workstation performs message communication with VME controller on the network, data processing using plant data and data acquisition. The commands and receiving the status data in real time. The new DCS is flexible for system extension and able to quickly respond to various requirements of software modifications. The data acquisition speed has been improved by almost a factor of two over the old one. Troubles and countermeasures experienced in the replacement work are also presented.

Shape reconstruction of RF-driven divertor plasma on QUEST

In the present RF-driven plasma with a lot of high-energy electrons, there may be anisotropic plasma pressure, which makes the usual equilibrium analysis difficult, but the Cauchy condition surface method can reconstruct the plasma shape precisely regardless of the anisotropy. In addition, the plasma current effect in the open magnetic surfaces outside of the closed magnetic surfaces is considered in the RF-driven divertor plasma. In the reconstruction process, singular value (SV) decomposition is used and optimal criterion function for generalized cross validation is estimated concerning truncation or reduction of the small-SV components.

Development of PC-Based Control System in JT-60SA

Abstract The JT-60 operations were stopped in 2008 in order to be upgraded to a superconducting tokamak device, JT-60 Super Advanced (JT-60SA). The design activity of JT-60SA is going on under the ITER-BA project, and the discussion on the operational scenarios of JT-60SA has begun. On the basis of these situations, we have been on a developmental work for JT-60SA control system. The JT-60 real time control system is composed of a workstation and a VME-based real time controller using a VxWorks. The VxWorks is one of the most commonly used real-time OSs in the embedded system markets. However, the introduction cost is higher than that of other RTOSs. From a cost-effectiveness and a long-term stable supply viewpoint, we have chosen INtime running on Windows OS-based Personal Computer. INtime is able to add a real-time performance to a PC on which Windows is installed. Therefore, in JT-60 real time control system, some of the subsystems running on VxWorks have been replaced with a PC-based control system. In future development activity, we plan to adopt the PC-based controller with INtime as the standard of JT-60SA control system. In this report, the developmental status of JT-60SA control system will be described.

Architecture Plan of the Real-Time Diagnostic Signals Acquisition System Toward JT-60SA Project

Abstract For a steady state operation of JT-60SA, a plasma feedback control using various diagnostic sensor signals plays an essential role. To realize this, Real-Time Diagnostic Signals acquisition System, RTDS, which utilizes PC-based real-time OS “INtime”, has been under consideration toward JT-60SA project. Real-Time Processor, RTP in JT-60 data processing system, which utilized a WS-based computer with real-time UNIX, was becoming increasingly difficult to maintain, manage, and improve due to the aging of the system and the limited capability of the CPU. To solve these problems and assess the feasibility of introducing clustered architecture, a prototype had been developed with the existing RTP and a VxWorks computer at hand. Its effectiveness as the architecture of the next RTDS was verified on JT-60U experiments, but the actual RTDS will employ PC-based “INtime” system, which consist of a general-purpose personal computer and a real-time operating system “INtime”. Moreover, long-pulse experiments of more than 400s will be planned in JT-60SA project. Therefore, real time monitoring will be required as an essential function that displays acquired diagnostic signals in real-time during long-pulse experiment. This function will also be realized by utilizing the RTDS.