H. Takeuchi

Absolute calibration of the JT‐60U neutron monitors using a 252Cf neutron sourcea)

Absolutely calibrated measurements of the neutron yield are important for the evaluation of plasma performance such as the fusion gain Q in D–D operating tokamaks. The time‐resolved neutron yield is measured with 235U and 238U fission chambers and 3He proportional counters in the JT‐60U tokamak. The in situ calibration was performed by moving the 252Cf neutron source toroidally through the JT‐60 vacuum vessel. Detection efficiencies of three 235U and two 3He detectors were measured for 92 locations of the neutron point source in toroidal scans at two different major radii. The total detection efficiency for the torus neutron source was obtained by averaging the point efficiencies over the whole toroidal angle. The uncertainty of the resulting detection efficiency for the plasma neutrons is estimated to be ±11%.

Ripple induced fast ion loss and related effects in JT-60U

Experiments have been carried out in JT-60U to verify the modelling of fast ion ripple transport. The ripple induced loss was estimated from the neutron decay following neutral beam pulse injection and the loss related heat load on the first wall. Comparison of the lost fraction and the hot spot positions between measurements and orbit following Monte Carlo calculations exhibited good agreement, indicating that the ripple transport governing fast ion losses is explained within the framework of existing theory. Neutral beam heating experiments in JT-60U also indicate that H modes free of ELMs are still obtainable for ripple amplitudes of up to 2.2%

Achievement of high fusion triple product in the JT-60U high βp H mode

Improvement of an enhanced confinement state in a high poloidal beta (βp) regime without sawtooth activity has been achieved in JT-60U. A confinement mode has been demonstrated where both the edge and the core confinement are improved. The attainable βp was also extended to higher values in this improved mode, because of its broader pressure profile. As a result of the improvement in confinement and in attainable βp, the highest value of the fusion triple product has been extended by a factor of 2.5 over that achieved in the 1992 experiments; it has reached (1.1 ± 0.3) × 1021 m-3.s.keV with a central ion temperature of about 37 keV. The D-D neutron emission rate has also been doubled in these experiments and has reached (5.6 ± 0.6) × 1016 s-1

Improvement of plasma parameters by titanium gettering in the JFT-2 tokamak

Abstract Oxygen impurity has been reduced to about 1–2% of the electron density by titanium gettering onto the torus wall (~ 1 2 ) and the limiter. Radiation loss and effective ionic charge were decreased by a factor of ~2. As a result of reduced impurity influx, broader electron temperature profiles have been obtained. The energy confinement time and the scaling factor of the maximum electron density ( n e R/B t ) were improved by a factor of 1.6 with the titanium gettering. Limits on the density increase were investigated in connection with the radiation power. Mechanism limiting the density maximum is discussed.

Multichordal spectroscopy on JT‐60

In order to study impurity behavior on the JT‐60 plasma, a spectroscopic diagnostic system, which is capable of measuring many spectra of impurity ions and their spatial distributions simultaneously, has been developed. This system consists of interchangeable unit‐type spectrographs which have different lines of sight. Four types of grazing incidence spectrographs are prepared as unit‐type spectrographs. These spectrographs cover the nominal wavelength regions 0.5–5, 0.5–50, 2.0–50, and 50–122 nm, respectively, and have holographic gratings with flat focal fields. Image‐intensified 1024‐ or 512‐channel photodiode arrays are used as detectors for all spectrographs. Four unit‐type spectrographs are used for observing the main plasma, and two unit‐type spectrographs are used for observing the plasma near the null point of the diverted discharge. One of these spectrographs has been calibrated absolutely in the wavelength region 27.5–120 nm using synchrotron radiation from the Photon Factory (National Laborato...

Neutron–nuclear data benchmark for copper and tungsten by slab assembly transmission experiments with DT neutrons

Abstract Fusion integral benchmark experiments have been carried out for copper and tungsten at FNS of JAERI. Leakage neutron spectra were measured with an NE213 detector by the TOF method to validate evaluated nuclear data files of JENDL-3.2, JENDL-fusion file, ENDF/B-VI, FENDL/E-1.0 and FENDL/E-2.0. From the result of comparison between experiments and calculations by MCNP-4B, below 1 MeV disagreement was observed for copper and it became remarkable with increase of the thickness. It was therefore, pointed out that there might be a problem in the evaluated (n,xa02n) neutron spectrum for all the nuclear data libraries. For tungsten, the calculated result with JENDL-fusion file and FENDL/E-2.0 overpredicted the experimental values around 3–7 MeV. Near 10 MeV, underestimation of ENDF/B-VI was remarkably pronounced. For the whole energy range, JENDL-3.2 was preferable. However, there was a serious problem left that discrepancy between JENDL-fusion file and JENDL-3.2 was very large.

Invited paper: Interaction between RF and edge plasma during ICRF heating in JT-60

Abstract Heating experiments in the second harmonic ion cyclotron range of frequencies (ICRF) have been performed with a phased array of 2×2 loop antennas in JT-60. Properties of antenna-plasma coupling are examined by phasing antenna currents in the toroidal direction. In particular, it is first found that the antenna-plasma coupling resistance increases after the H-mode transition in out-of-phase excitation of antenna currents. This result is well explained with the cold plasma coupling theory which takes into account a change in the edge density profile at the transition. Two types of parametric decay instabilities near the plasma edge are observed. One type is decay into an ion Bernstein wave (IBW) and an ion cyclotron quasimode (IQM) and the other into an ion Bernstein wave and a cold electrostatic ion cyclotron wave (CESICW) or an electron quasimode (EQM). Intensity of IBW detected by a probe near the antenna in the decay into IBW and IQM increases with reduction of B T and I p . The decay instabilities are observed only in the case of in-phase excitation. The edge plasma is heated by the decay instability and the radiation loss during ICRF heating increases with the decay activity.

Ion temperature measurements on JT-60 using He beam scattering (invited)

In order to measure the ion temperature of the JT‐60 tokamak, we have developed a Rutherford scattering system using a helium atom beam. A positive‐ion beam generated by an ion source which has a capability of beam energy 200 keV and drain current 3.5 A is converted to a helium atom beam by collision with cold helium gas. The He atom beam, equivalent to 0.6 A, reaches the center of the vacuum chamber of the JT‐60 tokamak. The scattering angle is 7.0°. Scattered helium atoms are analyzed by an E∥B‐type neutral particle energy analyzer with a gas stripping cell. This scattering system has been applied to investigate additionally heated plasmas by the method of neutral beam injection (NBI), ion cyclotron wave (ICRH), lower hybrid wave (LHRH), and combined heating of NBI+LHRH or ICRH in a parameter range of Bt=4.0 to 4.5 T, Ip=1.0 to 3.2 MA, and ne≲1×1020 m−3. The ion temperatures obtained by the system are consistent with those measured by Doppler broadenings of Tiu2009xxi and Tiu2009xxii resonance lines. To invest...

Infrared TV measurement of fast ion loss on JT‐60U

Toroidal field (TF) ripple transport, wave‐particle interaction, and large magnetohydrodynamic modes can enhance fast ion losses and result in localized heat deposition on the first wall. Two‐dimensional (2D) thermal measurement on the first wall provides useful information concerning these fast ion behaviors. In this article, we focus on the application of the 2D measurement with an infrared TV camera to TF ripple loss study. The content is (1) the 2D heat flux profile on the wall due to ripple loss, (2) the effects of the radial electric field, and (3) the ICRF effect on TF ripple loss. These experimental data demonstrate a great potential of infrared thermography in fast ion behavior study.

Charge‐exchange neutral particle measurement in MeV energy range on JT‐60U

A charge‐exchange neutral particle analyzer for the measurement of the MeV energy range ions produced by nuclear fusion or radio frequency heating has been developed and installed in JT‐60U. Neutral particles entering the analyzer are ionized with a carbon foil of thickness 400 A. The energy and mass of the stripped ions are resolved by magnetic and electrostatic fields (E∥B type). The analyzer has eight CsI(Tl) scintillator detectors. The energy range is 0.5–4 MeV for 4He0, the dynamic range is 4.08 and the energy resolution is 6%–11%. The detection efficiency for 4He0 with energy above 1 MeV is 30%–40%. A pulse height analysis (PHA) with 16 channels was adopted to distinguish particle signals from noise arising from neutrons, γ rays and optical lights emitted by JT‐60U plasmas. The validity of the PHA was confirmed in a calibration experiment using a neutron source and in a high power heating experiment in JT‐60U.