Masayasu Sato

Nondimensional transport study of JT-60U plasmas

Nondimensional local transport analysis of JT-60U plasmas has been carried out for NBI heated L-mode phase. Plasmas with the same poloidal cross section and almost the same profiles of nondimensional parameters such as ν * , β th , q , T e / T i except for ρ * are compared. The ρ * dependence of the one fluid thermal diffusivity, χ eff , the electron thermal diffusivity, χ e , and the ion thermal diffusivity, χ i , has been examined. The property of χ eff is between the weak gyroBohm type diffusion and the Bohm type diffusion in the region a /3≤ r ≤2 a /3, which is consistent with the results of the global confinement study done before. We have also found that χ e is weak gyroBohm type diffusion while χ i tends to be Bohm like diffusion.

Relativistic Broadening Effect on Application of Electron Cyclotron Emission Measurements to High Temperature Tenuous Tokamak Plasma

The effects of relativistic frequency broadening on the ECE measurements for a high temperature tenuous tokamak plasma are studied. Calculations of the radiation transfer are carried out assuming a spherically-symmetric relativistic Maxwellian distribution function for electrons. From the relativistic point of view, observation of the fundamental ordinary mode ECE along a major radius from the low field side is best for the measurement of the electron temperature profile. It is impossible to obtain the electron temperature profile using measurements from the high field side without information on the optical thickness along the propagation path.

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.

Effects of Relativistic Frequency Down-Shift and Optical Thickness on Measurements of Electron Temperature Profile from Electron Cyclotron Emission in Medium Temperature Tokamak Plasmas

The importance of the relativistic frequency down-shift and the optical thickness is pointed out in the determination of the electron temperature profile from electron cyclotron emission (ECE) measurement in the medium electron temperature region. These effects cause a pseudo radial displacement (Δ r ) of the obtained electron temperature profile. The pseudo displacement depends on only three parameters (the electron temperature, T e , the optical depth, τ, and the major radius, R ). In order to correct the radial position in any tokamak, a scaling law of the displacement is derived as Δ r ( m)=0.0009 R (m) T e (keV) (1+40/(5τ)). This scaling law is applicable to the plasma with τ>5 where the error is less than 0.5% of the major radius.

Relativistic Down-Shift Frequency Effect on the Application of Electron Cyclotron Emission Measurements to Medium Temperature Tokamak Plasmas.

The effects of relativistic down-shift frequency on the determination of the electron temperature profile from electron cyclotron emission (ECE) for medium temperature (2 keV≤T e≤10 keV) tokamak plasmas are studied. It has been found that the radial shift due to the relativistic effect is not negligible compared with the frequency resolution of present day ECE measurement systems. Therefore for precise measurement of the electron temperature profile it is necessary even in a medium temperature tokamak plasma to correct the shift due to relativistic down-shift frequency.

Time Behavior of Heat Diffusivity during L-H-L Transitions in JT-60U

The L-H-L transitions have been analyzed mainly for high-field pulses (Bt ≈4 T) in JT-60U. The simultaneous response of electron temperature T e has been clearly observed during L-H-L transitions over a wide plasma region. This T e evolution is described as the result of the fast jump of electron heat diffusivity, δχ e, over the wide plasma region. Values of δχ e were obtained as 0.5 m2/s < | δχ e | < 1 m2/s, which usually increase with radius. The simultaneous response of ion temperature T i was also observed to be that of similar to the T e response. The jump of ion heat diffusivity, δχ i, is similar to that of δχ e and usually increases with radius, as does δχ e. In a low-field pulse (B t ≈2.5 T), | δχ i | is as large as 2 m2/s. Values of δχ are consistent with the change of the energy confinement time during transitions.

Faraday Rotation and Elliptization of Electron Cyclotron Emission on Reactor-Grade Tokamak Plasma

The effects of the polarization state evolution on two-dimensional electron cyclotron emission (ECE) measurements along horizontal lines of sight for a reactor-grade plasma are studied. In order to investigate the polarization evolution, the propagation of ECE through the plasma is calculated. The final polarization states are very different from the initial states. The exact electron temperature is obtained in a reactor-grade plasma by measuring the polarization states described by the Stokes parameters.

Maximum Entropy Estimation of Electron Cyclotron Emission Spectra from Incomplete Interferograms in ELMy H-mode Tokamak Experiment

To measure the electron temperature profile of tokamak plasma in a high confinement mode with edge localized mode (ELMy H-mode), a method of data analysis is presented on the fast-scanning Fourier transform spectrometer (FTS), and results in the JT-60U tokamak are described. In the ELMy H-mode, where the FTS interferogram of electron cyclotron emission (ECE) becomes incomplete due to nonthermal emission pulses of ELM, the spectrum of ECE is estimated by using the maximum entropy method (MEM) of nonparametric type. The optimization of the regularization parameter is studied in terms of adopting the generalized cross validation (GCV) from the linear regularization of Tikhonov–Phillips. A study is also performed on a method to remove the nonthermal emission pulses from interferogram data before the MEM analysis, and on an extension of minimum cross entropy in order to meet with the missing of interferogram values in the region of zero path difference. From the application to experimental data of many shots, it is found that the electron temperature profiles can be automatically measured using the FTS during the ELMy H-mode operation of the tokamak.

Feasibility of Electron Density Measurement Using Relativistic Downshift Frequency Effect of Electron Cyclotron Emission in Tokamak Plasmas

The feasibility of electron density measurement using the relativistic downshift frequency effect of electron cyclotron emission in tokamak plasmas has been studied by numerical calculation. It is found that electron density measurement can be performed by observing the frequency shift from the nonrelativistic electron cyclotron frequency. The frequency shift is large when the observation is carried out from the high-field-side of torus. As this large frequency shift depends mainly on electron temperature and density, electron density can be determined while electron temperature is obtained from the observation of the low-field-side.

Singular Value Decomposition Analysis of Multichannel Electron Cyclotron Emission Signals of Tokamak Plasma

Singular value decomposition (SVD) was applied to multichannel signals obtained by an electron cyclotron emission diagnostic system in the JT-60U tokamak. As a result, on grating polychromator signals, the equilibrium and perturbative terms of electron temperature profile were successfully separated, and the structure of a magnetic island produced by the tearing mode was revealed. By neglecting the components of small singular values, the noise component of the signals was removed to improve the signal-to-noise ratio while conserving well the pulse-like waveforms. The usefulness of the SVD-based spectral analysis is shown by a typical set of signal waveforms.