A. Fukuyama

Alfvén eigenmodes driven by Alfvénic beam ions in JT-60U

Instabilities with frequency chirping in the frequency range of Alfven eigenmodes have been found in the domain 0.1% < βh < 1% and vb||/vA ~1 with high energy neutral beam injection in JT-60U. One instability with a frequency inside the Alfven continuum spectrum appears and its frequency increases slowly to the toroidicity induced Alfven eigenmode (TAE) gap on the timescale of an equilibrium change ( ≈ 200 ms). Other instabilities appear with a frequency inside the TAE gap and their frequencies change very quickly by 10-20 kHz in 1-5 ms. During the period when these fast frequency sweeping (fast FS) modes occur, abrupt large amplitude events (ALEs) often appear with a drop of neutron emission rate and an increase in fast neutral particle fluxes. The loss of energetic ions increases with a peak fluctuation amplitude of θ/Bθ. An energy dependence of the loss ions is observed and suggests a resonant interaction between energetic ions and the mode.

On the bicoherence analysis of plasma turbulence

The bicoherence of fluctuations in a system of drift waves and zonal flows is discussed. In strong drift-wave turbulence, where broadband fluctuations are excited, the bicoherence is examined. A Langevin equation formalism of turbulent interactions allows us to relate the bicoherence coefficient to the projection of nonlinear force onto the test mode. The dependence of the summed bicoherence on the amplitude of zonal flows is clarified. The importance of observing biphase is also stressed. The results provide a basis for measurement of nonlinear interaction in a system of drift waves and zonal flow.

Physics of collapse events in toroidal plasmas

A review is made of the crash phenomena in toroidal plasmas. The emphasis is placed on the physics that causes the crash of global plasma parameters. Recent progress in the measurement has provided a unified view of various crash phenomena, i.e. the sudden occurrence of the crash, the sensitivity (or probabilistic dependence) of the occurrence of the crash on the global parameters, and the abrupt excitation of a symmetry-breaking perturbation (magnetic trigger). Essential observations that describe the physics of collapse are surveyed. The theoretical study of the nonlinear plasma dynamics is overviewed. Theories of the onset and explosive growth, which are based on magnetic braiding, are discussed. As an example, a picture based on a turbulence-turbulence transition is explained. A picture based on hysteresis and bifurcation, not on the linear instability criterion, emerging from advanced measurements and recent progress, describes the basic physics of the collapse.

Current ramps in tokamaks: from present experiments to ITER scenarios

In order to prepare adequate current ramp-up and ramp-down scenarios for ITER, present experiments from various tokamaks have been analysed by means of integrated modelling in view of determining relevant heat transport models for these operation phases. A set of empirical heat transport models for L-mode (namely, the Bohm–gyroBohm model and scaling based models with a specific fixed radial shape and energy confinement time factors of H96−L = 0.6 or HIPB98 = 0.4) has been validated on a multi-machine experimental dataset for predicting the li dynamics within ±0.15 accuracy during current ramp-up and ramp-down phases. Simulations using the Coppi–Tang or GLF23 models (applied up to the LCFS) overestimate or underestimate the internal inductance beyond this accuracy (more than ±0.2 discrepancy in some cases). The most accurate heat transport models are then applied to projections to ITER current ramp-up, focusing on the baseline inductive scenario (main heating plateau current of Ip = 15 MA). These projections include a sensitivity study to various assumptions of the simulation. While the heat transport model is at the heart of such simulations (because of the intrinsic dependence of the plasma resistivity on electron temperature, among other parameters), more comprehensive simulations are required to test all operational aspects of the current ramp-up and ramp-down phases of ITER scenarios. Recent examples of such simulations, involving coupled core transport codes, free-boundary equilibrium solvers and a poloidal field (PF) systems controller are also described, focusing on ITER current ramp-down.

Transport simulation on L-mode and improved confinement associated with current profile modification

A unified model of the L-mode confinement in tokamaks and the improved modes associated with current profile modification is investigated by means of a one-dimensional transport simulation. The thermal transport coefficient employed is based on the theory of self-sustained turbulence due to the current-diffusivity-driven modes. In the case of low beta p ( beta p being the ratio of the plasma pressure to the pressure of the poloidal magnetic field), the simulation results show fairly good agreement with empirical L-mode scaling laws of the thermal energy confinement time tau E, indicating favourable dependence on the plasma current. When beta p exceeds about unity, however, the transport in the core region is strongly reduced. This confinement improvement is attributed to the weak or negative magnetic shear due to the bootstrap current and the Shafranov shift of the magnetic surface. The enhancement factor of tau E scales as beta p0.76 and is consistent with experimental observation. The effect of current profile modification due to the current ramp down and the lower hybrid current drive is also studied.

Two‐dimensional modeling of electron cyclotron resonance plasma production

For the numerical simulation of electron cyclotron resonance plasma production, a two‐dimensional model that describes wave propagation and plasma transport is developed. The modeling code calculates profiles of electromagnetic wave fields, power absorption of electrons, and temporal evolution of plasma densities in a bounded, inhomogeneous, cylindrical system. The calculation of the plasma production in a mirror magnetic field shows that the plasma production property is very sensitive to the antenna location.

Energetic particle physics in JT-60U and JFT-2M

Recent energetic particle physics research in JT-60U and JFT-2M is reported. Alfven eigenmodes (AEs) are investigated in reversed-shear (RS) plasmas in JT-60U where frequency sweeping (FS) modes are observed to follow the q-profile evolution. The RS-induced AE model can explain the FS of the modes within the context of an evolving q-profile. Enhanced energetic ion transport is also investigated with the appearance of modes in the toroidicity-induced AE range of frequency in JT-60U using a multi-channel neutron profile monitor and in JFT-2M using a lost ion probe. Additionally, the ripple loss in the complex toroidal field ripple due to ferritic steel inserts in JFT-2M is shown and compared with model analysis. The simulation code developed to predict ripple loss in JFT-2M will be of use in estimating the heat flux in the complex ripple field of a future device such as ITER.

Kinetic description of propagation and absorption structures of ICRF waves

Propagation and absorption of waves in the ion cyclotron range of frequencies (ICRF) are investigated using the kinetic theory in a high-temperature plasma. The wave equation which includes kinetic effects (such as finite-gyroradius effect and wave/particle interactions) is solved as a boundary-value problem for a two-ion-component plasma (majority deuterium and minority hydrogen) with one-dimensional inhomogeneities. The global structure of the wave field and the absorption mechanism are explained. The power deposition profile for each plasma species and the coupling to the antenna are obtained. The mode conversion of the fast wave to the ion Bernstein wave is associated with the two-ion hybrid resonance, and the latter is absorbed by electrons and deuterons via Landau damping and collisional damping, respectively. Protons absorb the wave which tunnels through the cut-off layer (for strong-field-side excitations) by cyclotron damping. The existence of cavity resonances is also confirmed; these considerably influence the energy absorption.

Energetic ion transport by abrupt large-amplitude event induced by negative-ion-based neutral beam injection in the JT-60U

To investigate energetic ion transport induced by bursting modes in the frequency range of Alfven eigenmodes, which is called abrupt large-amplitude events (ALEs) driven by negative-ion-based neutral beam (N-NB) injection, neutron emission profile measurement and charge exchange (CX) neutral particle (flux) measurement using a natural diamond detector have been performed simultaneously in JT-60U. It is found from the CX neutral particle (flux) measurement that energetic neutral particles in a limited energy range (100–370 keV) are enhanced due to ALEs, and the neutron radial profile is flattened. The change in the energetic ion density profile inferred from these measurements indicates that ALEs expel energetic ions from the core region of the plasma and induce both redistribution and loss of energetic ions. It has been shown that the energy range of transported energetic ions is consistent with a resonance condition between energetic ions and ALEs, and the energetic ion transport results from the resonant interaction between energetic ions and ALEs. Further, a fraction of the energetic ion loss has been quantitatively estimated to be ~4%.

Ion cyclotron range of frequencies heating and high-energy particle production in the Large Helical Device

Significant progress has been made with ion cyclotron range of frequencies (ICRF) heating in the Large Helical Device. This is mainly due to better confinement of the helically trapped particles and less accumulation of impurities in the region of the plasma core. During the past two years, ICRF heating power has been increased from 1.35 to 2.7 MW. Various wave-mode tests were carried out using minority-ion heating, second-harmonic heating, slow-wave heating and high-density fast-wave heating at the fundamental cyclotron frequency. This fundamental heating mode extended the plasma density range of effective ICRF heating to a value of 1×1020 m−3. This use of the heating mode was its first successful application in large fusion devices. Using the minority-ion mode gave the best performance, and the stored energy reached 240 kJ using ICRF alone. This was obtained for the inward-shifted magnetic axis configuration. The improvement associated with the axis-shift was common for both bulk plasma and highly accelerated particles. For the minority-ion mode, high-energy ions up to 500 keV were observed by concentrating the heating power near the plasma axis. The confinement properties of high-energy particles were studied for different magnetic axis configurations, using the power-modulation technique. It confirmed that with the inward-shifted configuration the confinement of high-energy particles was better than with the normal configuration. By increasing the distance of the plasma to the vessel wall to about 2 cm, the impurity influx was sufficiently reduced to allow sustainment of the plasma with ICRF heating alone for more than 2 min.