K. Tobita

DEMO plant design beyond ITER

A conceptual design of a DEMO reactor for the first integrated demonstration of generating fusion plant is made under the assumption that its design and construction would be started in 2020s and its operation in 2030s. A steady-state tokamak is minimized to have 5.8 m of major radius with 2.3 GW of fusion power with energy amplification Q exceeding 30. Modest extrapolation of improved plasma physics as well as technology development such as superconducting magnet, blanket and reduced activation material are assumed. It is suggested that demonstrating the fusion as a viable source of energy in plant scale will be possible based on the successful operation of ITER, while a number of technical issues must be solved in coherent programs paralleling ITER.

Excitation of high n toroidicity-induced Alfvén eigenmodes and associated plasma dynamical behaviour in the JT-60U ICRF experiments

Abstract High frequency MHD activities observed during second harmonic ICRF heating are identified to be toroidicity-induced Alfven eigenmodes (TAE) driven by MeV protons produced by ICRF heating. Correlation between MeV protons and TAE modes is clearly observed. TAE mode amplitude increases exponentially with increasing toroidal mode number up to more than ten. The tendency cannot be explained by present local TAE stability theories. Long suppression of TAE modes after a giant sawtooth crash can be explained by fast ion loss due to the sawtooth crash and evolving q -profile.

First wall issues related with energetic particle deposition in a tokamak fusion power reactor

Abstract Energetic particle deposition to the wall due to toroidal magnetic field (TF) ripple was assessed for a 2 GW fusion power reactor. When the present allowance for the loss is applied, the alpha particle flux to the wall can be as high as 2×10 18 m −2 s −1 in the reactor, eroding tungsten by ∼20 μm per year. The peak particle fluence over a 2-year operation cycle can reach 10 26 m −2 , probably being larger than a critical fluence for blister formation. The result suggests that, for the steady-state tokamak fusion reactor, we should introduce a new design methodology of determining an acceptable level of TF ripple on the basis of particle fluence to the wall, instead of the present one based on a tolerable heat flux.

Attenuation of high-energy neutral hydrogen beams in high-density plasmas

Beam stopping cross section and shine-through for neutral hydrogen beam injection into fusion plasmas have been calculated by using recommended cross sections presently available for atomic processes including multistep collision processes involving excited states. The shine-through thus obtained agrees well with recent experiments of JT-60U. The present calculations show that the multistep processes play a crucial role in the stopping of high-energy neutral hydrogen beams in high-density plasmas. Analytical fits to the stopping cross sections and fitting parameters are also presented for plasma impurities with nuclear charge and Z = 26.

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.

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.

Ion temperature measurements in JT-60 plasmas by active beam scattering

The active beam scattering method was applied to ion temperature measurements in JT-60 plasmas. The ion temperature found was in reasonable agreement with that obtained from Doppler broadening of the Ti XXI and Ti XXII resonance lines in the temperature range of 1.5 to 10 keV. With the help of the scattering system, the central ion temperature of JT-60 plasmas during 40 keV and 70-75 keV NBI heating was measured. Higher ion temperature was obtained during 70 to 75 keV than during 40 keV NBI heating. These data and a possible explanation are presented. A numerical simulation of this diagnostic indicates that the ion temperature as deduced from the diagnostic is almost equal to the bulk temperature when the ratio of high energy ion component to the total ion stored energy is less than 0.3.

Experimental study on beam acceleration with combined NBI heating and second-harmonic ICRF heating in JT-60

Beam acceleration by heating in the second-harmonic ion cyclotron range of frequency (ICRF) in combination with heating by hydrogen neutral beam injection (NBI) was investigated in the JT-60 tokamak. The energy spectra of the accelerated fast ions were measured by a charge exchange neutral energy analyser whose line of sight was intersected by specific beam lines in the plasma core in order to obtain the required information. The dependences of the tail ion temperature on various parameters (electron density, NBI power and the toroidal phasing of the antenna) were examined. Optimized conditions for beam acceleration were evaluated. For combined ICRF heating and NBI heating, an incremental energy confinement time of 210 ms was achieved, which was three times higher than that obtained with NBI heating alone or with ICRF heating alone. This improvement of the energy confinement during combined NBI and ICRF heating can be explained by a build-up of the fast ions accelerated by the ICRF wave. The scaling of the incremental energy confinement time during combined ICRF+NBI heating was obtained.

The JT-60 diagnostic system

Diagnostic instruments for measuring the JT-60 plasma are fully developed and ready to operate. Each instrument and its fundamental characteristics are described as well as some aspects of the plasma parameters. The new features in JT-60 diagnostics are that the diagnostics system which has its own data processing system separate from the machine control computer, and the utilities for each diagnostics instrument (vacuum, electricity, water, air, liquid nitrogen) constitute one system in the total diagnostics system.