H. Kawashima

Plasma regimes and research goals of JT-60SA towards ITER and DEMO

The JT-60SA device has been designed as a highly shaped large superconducting tokamak with a variety of plasma actuators (heating, current drive, momentum input, stability control coils, resonant magnetic perturbation coils, W-shaped divertor, fuelling, pumping, etc) in order to satisfy the central research needs for ITER and DEMO. In the ITER- and DEMO-relevant plasma parameter regimes and with DEMO-equivalent plasma shapes, JT-60SA quantifies the operation limits, plasma responses and operational margins in terms of MHD stability, plasma transport and confinement, high-energy particle behaviour, pedestal structures, scrape-off layer and divertor characteristics. By integrating advanced studies in these research fields, the project proceeds simultaneous and steady-state sustainment of the key performances required for DEMO with integrated control scenario development applicable to the highly self-regulating burning high-β high bootstrap current fraction plasmas.

Kinetic modelling of impurity transport in detached plasma for integrated divertor simulation with SONIC (SOLDOR/NEUT2D/IMPMC/EDDY)

A Monte Carlo (MC) impurity code IMPMC has been coupled self-consistently with a divertor code SOLDOR/NEUT2D by overcoming the intrinsic problems of MC modelling for impurity transport. MC modelling is required for impurity transport in order to take into account the kinetic effect and the complex dissociation processes of hydrocarbons. The integrated divertor code SONIC enables us to investigate the details of impurity transport including erosion/redeposition processes on the divertor plates by further coupling of an 3D plasma–surface interaction MC code EDDY. The dynamic evolution of X-point MARFE observed in JT-60U is investigated. The simulation results indicate that the hydrocarbons sputtered from the dome contribute to the enhanced radiation near the X-point. The kinetic effect of thermal force on the He transport is investigated for JT-60SA detached plasmas. Without the recycling, the kinetic effect improves the helium compression, compared with the conventional (fluid) evaluation. This effect is, however, masked by the recycling at the divertor targets.

Simulation study for divertor design to handle huge exhaust power in the SlimCS DEMO reactor

By the SOLDOR/NEUT2D simulation for divertor design study on a compact DEMO reactor, SlimCS, we estimated the prospect of handling the huge exhaust power in the divertor. Assuming exhaust power of 500u2009MW and ion outflux of 0.5 × 1023u2009s−1 into the scrape-off-layer, the peak heat load is estimated to be 70u2009MWu2009m−2 on the outer target on the initial divertor design (vertical target) with the introduction of moderate gas puff flux and Ar fraction. This value significantly exceeds the allowable level of 10u2009MWu2009m−2 which is an initial design target. By installing the V-shaped corner in the bottom of the outer divertor target, and using strong gas puffing or Ar impurity injection, the detached condition with high particle recycling and radiation loss conditions is formed, and the peak heat load is successfully reduced below 10u2009MWu2009m−2. It can also be demonstrated properly for the dependence of the exhaust power on the divertor heat load. Peak heat load is reduced exponentially with a decrease in the exhaust power and reaches 7u2009MWu2009m−2 at Qtotal = 300u2009MW for moderate gas puff flux and Ar fraction.

Statistical analysis of fluctuation characteristics at high- and low-field sides in L-mode SOL plasmas of JT-60U

For the first time, fluctuation characteristics at the high-field side (HFS) and low-field side (LFS) scrape-off-layers (SOLs) in L-mode plasmas of the JT-60U tokamak are compared in detail; the plasma transport is studied using reciprocating Langmuir probes. A statistical analysis based on probability density functions (PDFs) is employed to describe intermittent (nondiffusive) transport in SOL plasma fluctuations. It is found that positive bursty events associated with blobby plasma transport appear frequently at the LFS midplane. Further, the PDF at the LFS midplane is strongly positively skewed, while the PDF at the HFS SOL is close to a Gaussian distribution. The conventional conditional averaging method is improved by using the variable-interval time-averaging method along with it, which enables the precise reconstruction of the burst profile exhibiting a rapid increase and a slow decay. The radial evolution of plasma blobs is analysed by using an autocorrelation function, the short-time Fourier transform and wavelet analysis.

Development of imaging bolometers for magnetic fusion reactors (invited).

Imaging bolometers utilize an infrared (IR) video camera to measure the change in temperature of a thin foil exposed to the plasma radiation, thereby avoiding the risks of conventional resistive bolometers related to electric cabling and vacuum feedthroughs in a reactor environment. A prototype of the IR imaging video bolometer (IRVB) has been installed and operated on the JT-60U tokamak demonstrating its applicability to a reactor environment and its ability to provide two-dimensional measurements of the radiation emissivity in a poloidal cross section. In this paper we review this development and present the first results of an upgraded version of this IRVB on JT-60U. This upgrade utilizes a state-of-the-art IR camera (FLIR/Indigo Phoenix-InSb) (3-5 microm, 256 x 360 pixels, 345 Hz, 11 mK) mounted in a neutron/gamma/magnetic shield behind a 3.6 m IR periscope consisting of CaF(2) optics and an aluminum mirror. The IRVB foil is 7 cm x 9 cm x 5 microm tantalum. A noise equivalent power density of 300 microW/cm(2) is achieved with 40 x 24 channels and a time response of 10 ms or 23 microW/cm(2) for 16 x 12 channels and a time response of 33 ms, which is 30 times better than the previous version of the IRVB on JT-60U.

Measurement of peripheral electron temperature by electron cyclotron emission during the H-mode transition in JFT-2M tokamak

Time evolution and profile of peripheral electron temperature during the H-mode like transition in a tokamak plasma is measured using the second and third harmonic of electron cyclotron emission (ECE). The so called “ H-mode ” state which has good particle/energy confinement is characterized by sudden decrease in the spectral line intensity of deuterium molecule. Such a sudden decrease in the line intensity of D α with good energy confinement is found not only in divertor discharges, but also in limiter dischargs in JFT-2M tokamak. It is found by the measurement of ECE that the peripheral electron temperature suddenly increases in both of such phases. The relalion between H-transition and the peripheral electron temperature or its profile is investigated.

Volume recombination of C4+ in detached divertor plasmas of JT-60U

Volume recombination of C4+ and e? into C3+ is observed for the first time in detached divertor plasmas with an X-point MARFE. The recombination zone is located around the X-point, and the electron temperature and density are evaluated to be 6.3?eV and 7.8 ? 1020?m?3, respectively. In this zone, the volume recombination flux is larger by two orders of magnitude than the ionization flux of C3+. However, the radiation power due to the recombination process is only 2% of the total radiation power, measured by a bolometer. In contrast, the radiation power due to the excitation process from the ground state of C3+ by electron collision dominates the total radiation power.

Particle control in long-pulse H-mode discharges of JT-60U

An H-mode plasma was successfully maintained for~30u2009s at a constant density of by using divertor pumping even under the outgassing condition. At a high density of , an H-mode plasma was maintained for ~25u2009s with a continuous fuel retention, which was ascribed to low outgassing, high co-deposition of deuterium with carbon and the dynamically changed fuel retention process. It was found that the third process worked with the change in edge plasma parameters, in particular particle flux, so as to maintain the electron density against the density control: the outgassing dominated when the divertor pumping was on and the fuel retention dominated when the divertor pumping was off.

Research activities on tokamaks in Japan: JT-60U, JFT-2M, and TRIAM-1M

Research activities of the Japanese tokamaks JT-60U, JFT-2M, and TRIAM-1M are described. The recent JT-60 program is focused on the establishment of a scientific basis of advanced steady-state operation. Plasma performance in transient and quasi steady states has been significantly improved, utilizing reversed shear and weak shear (high-βp) ELMy H-modes characterized by both internal and edge transport barriers and high bootstrap current fractions. Development of each key issue for advanced steady-state operation has also been advanced. Advanced and basic research of JFT-2M has been performed to develop high-performance tokamak plasma as well as the structural material for a fusion reactor. Toroidal field ripple reduction with ferritic steel plates outside the vacuum vessel is successfully demonstrated. No adverse effects to the plasma were observed with poloidal fields inside the vacuum vessel (partial covering). Preparation is in progress for full-scale testing of the compatibility of the ferritic steel wall (full covering) with plasma. A heavy ion beam probe has been installed to study H-mode plasmas. Compact toroid (CT) injection experiments are performed, showing deep CT penetration into the core region of the H-mode. The TRIAM project has investigated steady-state operation and high-performance plasma of a tokamak with the high toroidal magnetic field superconducting tokamak. Four important contributions in the fields of fusion technology of superconducting tokamaks, steady-state operation, high-performance plasma, and startup of plasma current without the assistance of center solenoid coils have been achieved on TRIAM-1M, especially regarding steady-state operation by realization of a discharge for >3 h.

Development of integrated SOL/divertor code and simulation study of the JT-60U/JT-60SA tokamaks

To predict the heat and particle controllability in the divertor of tokamak reactors and to optimize the divertor design, comprehensive simulations by integrated modeling allowing for various physical processes are indispensable. SOL/divertor codes have been developed in the Japan Atomic Energy Agency for the interpretation and the prediction of behaviour of SOL/divertor plasmas, neutrals and impurities. The code system consists of the two-dimensional fluid code SOLDOR, the neutral Monte-Carlo (MC) code NEUT2D and the impurity MC code IMPMC. Their integration code SONIC is almost completed and examined to simulate self-consistently the SOL/divertor plasmas in JT-60U. In order to establish the physics modelling used in fluid simulations, the particle simulation code PARASOL has also been developed.Simulation studies using those codes have progressed with the analysis of JT-60U experiments and the divertor designing of JT-60SA (modification program of JT-60U). The X-point multifaceted asymmetric radiation from the edge in the JT-60U experiment is simulated. It is found that the deep penetration of chemically sputtered carbon at the dome causes the large radiation peaking near the X-point. The pumping capability of JT-60SA is evaluated through the simulation. A guideline to enhance the pumping efficiency is obtained in terms of the exhaust slot width and the strike point distance. Transient behaviour of SOL/divertor plasmas after an ELM crash is characterized by the PARASOL simulation; the fast-time-scale heat transport is affected by collisions while the slow-time-scale behaviour is affected by the recycling.