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The current status and the progress of research and development (R&D) activities for a Fusion DEMO reactor in the National Institutes for Quantum and Radiological Science and Technology (QST) Rokka...

Quiescent high-confinement mode plasmas with edge-harmonic oscillations do not exhibit the explosive instabilities associated with edge-localized modes. Instead, an additional means of enhanced transport is considered to maintain the plasma edge under conditions just below the boundary of the peeling mode instability. Although the potential of the peeling mode has been widely recognized in plasma physics, no direct evidence for this mode has been revealed previously because decisive diagnostics were lacking. Herein, we report evidence of the structure and dynamical steady state of peeling mode in quiescent high-confinement mode. Edge-harmonic oscillations are dominated by fundamental mode at both the low- and high-field sides. Edge perturbations are confirmed to have kink parity and exhibit the frozen-in-condition predicted by linear stability analysis. The envelope signal of the fundamental mode exhibits repeated cycles of growth and damping in association with minor changes in the edge gradient. Results from this study are quantitatively consistent with limit-cycle-oscillation model. Unveiling the properties of edge harmonic oscillation is one of the major pursuits to understand complex systems in plasma physics. The authors present measurements at the JT-60U tokamak that show direct observations of peeling modes in quiescent high-confinement tokamak plasmas, offering insights into the “peeling nature” of both laboratory plasmas and astronomical plasmas.

Explosive dynamical events in controlled-nuclear-fusion devices (known as edge-localized modes) display many similarities to solar-flare events on the sun, revealing a new connection between laboratory plasma physics and astronomy. However, to date there has been no direct evidence for the peeling mode structure, due to the lack of decisive diagnostics. Here we report the first evidence for the structure and dynamical steady state of a peeling mode for low-n edge-harmonic oscillations (EHOs) in the quiescent H-mode. EHOs are dominated by the fundamental mode (1fEHO) at both the low- and high-field sides. 1fEHO edge perturbations are confirmed to have kink parity and exhibit the frozen-in-condition predicted by a linear stability analysis. The envelope signal of the 1fEHO mode exhibits repeated cycles of growth and damping to the order of a few hundred Hz associated with small changes in an edge gradient, and results are quantitatively consistent with a limit-cycle-oscillation model.

Structure-preserving discretization of the Rosenbluth–Fokker–Planck equation is still an open question especially for unlike-particle collision. In this paper, a mass-energy-conserving isotropic Rosenbluth–Fokker–Planck scheme is introduced. The structure related to the energy conservation is skew-symmetry in mathematical sense, and the action–reaction law in physical sense. A thermal relaxation term is obtained by using integration-by-parts on a volume integral in the energy moment equation, so the discontinuous Galerkin method is selected to preserve the skew-symmetry. The discontinuous Galerkin method enables ones to introduce the nonlinear upwind flux without violating the conservation laws. Some experiments show that the conservative scheme maintains the mass-energy-conservation only with round-off errors, and analytic equilibria are reproduced only with truncation errors of its formal accuracy.

Magnetohydrodynamic stability at the edge pedestal in several quiescent H-mode (QH-mode) plasmas in DIII-D and JT-60U experiments was analyzed by considering plasma rotation and ion diamagnetic drift effects. It was identified that a kink/peeling mode, which is a prime candidate for a trigger of edge harmonic oscillation in QH-mode, is stabilized by plasma rotation when considering the ion diamagnetic drift simultaneously in both experiments. The stabilizing effect by rotation becomes more effective in case using the rotation profile of the main ion species evaluated by assuming radial force balance. In addition, when inverting the rotation direction, it was found that the kink/peeling mode is more stabilized when considering the rotation of the main ion species, though the mode is less stabilized by the rotation of impurity ion species. The result implies that the kink/peeling mode stability in QH-mode plasmas is sensitive to how the rotation profile is evaluated, but it is shown that a qualitative trend stabilizing the kink/peeling mode by rotation can be reproduced with the poloidal rotation profile of an impurity predicted numerically based on the neoclassical theory.

We have investigated ohmic plasma initiation for JA DEMO, a design concept of the tokamak demonstration fusion reactor examined in Japan. A calculation model describing the tokamak plasma initiation phase consisting of the two-dimensional analysis of the coil and eddy currents and the zero-dimensional plasma analysis is developed. It is revealed that the conducting shell does not necessarily hinder plasma initiation, while it provides the stabilizing effect of vertical displacement events. The available loop voltage and prefill gas pressure for initiation in JA DEMO are clarified for developing the initiation scenario. We demonstrate a possible scenario of ohmic plasma initiation, which is consistent with the design specifications of superconducting poloidal field coils.

A novel integrated model GOTRESS+ has been developed, which consists of the iterative transport solver GOTRESS as a kernel of the integrated model, the equilibrium and current profile alignment code ACCOME and the neutral beam heating/current-drive code OFMC. GOTRESS is able to robustly find out an exact solution of the stationary-state transport equations even with a stiff turbulent transport model, taking advantage of global optimization techniques such as a genetic algorithm. GOTRESS+ is then suitable for self-consistently assessing the stationary-state plasma performance of JT-60SA as well as ITER and DEMO or validating their feasibility. Recently GOTRESS+ has been extended to incorporate the in-house EPED1 model exploiting the MHD stability code MARG2D and is now able to predict the plasma profiles even with the pedestal over the entire region from the magnetic axis to the plasma boundary in a self-consistent manner. The two JT-60SA operation scenarios including the ITER-like inductive scenario and the high β fully non-inductively current driven scenario have been assessed by GOTRESS+ with the CDBM turbulent transport model and then were found to be feasible with most of the target dimensionless parameters met.

A model is proposed for the edge harmonic oscillation, in which the stationary coherent mode is sustained in the almost linear phase as has been observed in JT-60U. We study the coupled dynamics of the peeling mode amplitude and edge pressure gradient. The limit cycle oscillation is predicted. The peeling mode (which is almost in the linear phase) is in a dynamical stationary state with amplitude modulation. In this model, the time scales for the change of parameters that specify magnetic structures (such as magnetic shear and edge plasma current) are assumed to be much slower, so that are decoupled from the limit cycle dynamics. The condition that the limit cycle state appears is shown. The oscillation frequency of the modulation is given by the hybrid mean of the typical growth rate of the peeling mode and the additional loss rate of pressure gradient by the peeling mode.

Simulations with an integrated code TOPICS-IB showed that a small pellet can significantly reduce the ELM energy loss by penetrating deeply into the pedestal and triggering high-n ballooning modes localized near the pedestal top, with conditions; the injection from the low-field-side with a speed fast enough to approach the pedestal top when the pedestal pressure is about 95% of natural ELM onset. The effectiveness of the above suitable conditions of pellet injection for ELM pacing has been confirmed by JT-60U and then ITER simulations. The pellet particle content required for ELM pacing is larger for the pedestal plasma with higher density and farther from the stability boundary of ideal ballooning mode near the pedestal top. For an ITER standard scenario, the required pellet particle content is about a few % of pedestal particle content, which gives the physics background to the present design value. Simulations also showed that fueling pellets can be injected from the high-field-side just after ELM pacing pellets without disturbing the pacing. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The integrated framework consisting of TOPICS, VMEC and FORTEC-3D, has been applied to provide an estimate of the neoclassical toroidal viscosity (NTV) produced by the applied magnetic perturbations (MPs). Six picture-frame error field correction coils in each row in JT-60SA realize the various patterns of the applied magnetic field spectrum. The NTV is investigated for various toroidal phase differences. As long as the same coil current is applied, the n = 3 MPs generate the same NTV, regardless of even or odd parity, or energizing the middle coils or not. The n = 1 MPs give rise to larger NTV than the n = 3 ones. The NTV invoked by the n = 1 resonant MPs is virtually equivalent to that by the n = 1 nonresonant MPs with five-fold coil current.

A new linear MHD stability code MINERVA is developed for investigating a toroidal rotation effect on the stability of ideal MHD modes in tokamak plasmas. This code solves the Frieman–Rotenberg equation as not only the generalized eigenvalue problem but also the initial value problem. The parallel computing method used in this code realizes the stability analysis of both long and short wavelength MHD modes in short time. The results of some benchmarking tests show the validity of this MINERVA code. The numerical study with MINERVA about the toroidal rotation effect on the edge MHD stability shows that the rotation shear destabilizes the intermediate wavelength modes but stabilizes the short wavelength edge localized MHD modes, though the rotation frequency destabilizes both the long and the short wavelength MHD modes.

A new conservative global gyrokinetic toroidal full-f five-dimensional Vlasov simulation (GT5D) is developed using a novel non-dissipative conservative finite difference scheme. The scheme guarantees numerical stability by satisfying relevant first principles in the modern gyrokinetic theory, and enables robust and accurate simulations of tokamak micro-turbulence. GT5D is verified through comparisons of zonal flow damping tests, linear analyses of ion temperature gradient driven (ITG) modes, and nonlinear ITG turbulence simulations against a global gyrokinetic toroidal δf particle code. In the comparison, global solutions of the ITG turbulence are identified quantitatively by using two gyrokinetic codes based on particle and mesh approaches.

The effect of the pressure profile on the energy loss caused by edge localized modes (ELMs) has been investigated by using an integrated simulation code TOPICS-IB based on a core transport code with a stability code for the peeling-ballooning modes and a transport model for scrape-off-layer and divertor plasmas. The steep pressure gradient inside the pedestal top is found to broaden the region of the ELM enhanced transport through the broadening of eigenfunctions and enhance the ELM energy loss. The ELM energy loss in the simulation becomes larger than 15 % of the pedestal energy, as is shown in the database of multi-machine experiments. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

An integrated simulation code TOPICS-IB based on a transport code with a stability code for the peeling–ballooning modes and a scrape-off-layer (SOL) model has been developed to clarify self-consistent effects of edge localized modes (ELMs) and the SOL on the plasma performance. Experimentally observed collisionality dependence of the ELM energy loss is found to be caused by both the edge bootstrap current and the SOL transport. The bootstrap current decreases with an increase in collisionality and intensifies the magnetic shear at the pedestal region. The increase in the magnetic shear reduces the width of eigenfunctions of unstable modes, which results in the reduction of both the area of the ELM enhanced transport and the ELM enhanced transport near the separatrix. On the other hand, when an ELM crash occurs, the energy flows into the SOL and the SOL temperature rapidly increases. The increase in the SOL temperature lowers the ELM energy loss due to the flattening of the radial edge gradient. The parallel electron heat conduction determines how the SOL temperature increases. For higher collisionality, the conduction becomes lower and the SOL electron temperature increases more. By the above two mechanisms, the ELM energy loss decreases with increasing collisionality.

The stability of peeling, ballooning, and peeling–ballooning modes, which relate to edge-localized modes, is investigated numerically with the linear ideal magnetohydrodynamic stability code MARG2D. The effects of 'sharpness' on the stability of the peeling–ballooning mode are examined, where the sharpness is defined in terms of the curvature at the top or bottom of the outermost flux surface. It is found that the stability limit of the pressure gradient significantly improves as the sharpness increases even when the ellipticity and the triangularity are unchanged. The sharpness is an important parameter for high performance H-mode operations with high pedestal pressure.

Non-disruptive ? collapses with a regular intermittency have been observed after a forced turn-off of neutral beam current drive (NBCD) in JT-60U fully non-inductive, reversed shear (RS) discharges. Self-consistent transport simulations with improved core confinement have first clarified that the redistribution of 'return current' induced after the NBCD turn-off lowers the safety factor of magnetic shear reversal, qmin. Linear MHD stability analysis has also revealed that this lowering of safety factor leads to n = 1 kink-ballooning instability with the modes localized around the internal transport barrier (ITB) of higher qmin (~6) discharges. It was also pointed out that an increase in the bootstrap current under continuous NB heating can lead to ITB reconstruction and thus causes subsequent ? collapses.

In a rotating toroidal plasma surrounded by a resistive wall, it is shown that linear MHD instabilities can be excited by couplings between the resistive wall mode (RWM) and stable ideal MHD modes. In particular, it is shown that the RWM can couple not only with stable external kink modes but also with Alfvén eigenmodes that are ordinarily in the stable continuum of a toroidal plasma. The RWM growth rate is shown to peak whenever the Doppler shift caused by the plasma rotation cancels the frequency of an ideal MHD mode, so that the mode appears to have zero frequency in the laboratory frame. At these values of the rotation frequency, the RWM can overcome the stabilizing effects of plasma rotation, continuum damping, and ion Landau damping.

The formulation for solving numerically the two-dimensional Newcomb equation has been extended to calculate the vacuum energy integral by using a vector potential method. According to this extension, a stability code MARG2D has been adapted, and coded for parallel computing in order to reduce substantially the CPU time. The MARG2D code enables a fast stability analysis of ideal external MHD modes from low to high toroidal mode numbers on the basis of the single physical model, and then the code works as a powerful tool in an integrated simulation where it is combined with transport codes, and also in the analysis of tokamak edge plasma experiments.

A strategy for integrated modeling of burning plasmas at Japan Atomic Energy Agency is described. In order to simulate the burning plasma, which has the complex feature of widely different timescal...

The theory of the Newcomb equation has been applied to low-n external modes in a tokamak and a method has been developed for computing the stability matrix that gives the change of plasma potential energy due to external modes in terms of the surface values of the perturbations. By using this method, the spectral properties of the ideal external modes have been elucidated, such as the coupling between external modes and internal modes, and the difference in the stability properties between a normal shear tokamak and a reversed shear tokamak. These results will also be useful in the stability analysis of resistive wall modes.

In tokamaks, it is observed that an ablated and ionized pellet moves toward the low field side of the torus. This phenomenon can be considered as the magnetohydrodynamic (MHD) wave propagation in the 1/ R varying toroidal field because the pressure perturbation is induced by injecting a pellet, where R is the major radius of tokamaks. The dominant mechanism which causes this displacement is revealed, by solving nonlinear MHD equations numerically, as the multi-dimensional nonlinear fast magnetosonic wave propagation in a toroidal magnetic field. Linear wave propagation does not work for this displacement. The nonlinearity is clarified by successive approximation.

When the plasma beta (ratio of thermal to magnetic pressure) in the core of a tokamak is raised to values of several percent, as required for a thermonuclear fusion reactor, continuous spectra of long-wavelength slow magnetosonic waves enter the frequency band occupied by continuous spectra of shear Alfven waves. It is found that these two branches can couple strongly, so that Alfven modes that are resonantly driven by suprathermal ions transfer some of their energy to sound waves. Since sound waves are heavily damped by thermal ion Landau resonances, these results reveal a new energy channel that contributes to the damping of Alfvenic instabilities and the noncollisional heating of bulk ions, with potentially important consequences for confinement and fusion performance.

Recurring bursts of chirping Alfvén modes that were observed in JT-60U tokamak plasmas driven by negative-ion-based neutral beams (N-NB) are reproduced in first-principle simulations performed with an extended version of the hybrid code MEGA. This code simulates the interactions between gyrokinetic fast ions and magnetohydrodynamic (MHD) modes in the presence of a realistic fast ion source and collisions, so that it self-consistently captures dynamics across a wide range of time scales (0.01–100 ms). The simulation confirms that the experimentally observed phenomena known as 'fast frequency sweeping (fast FS) modes' are caused by bursts of energetic particle modes (EPM) with dominant toroidal mode number n = 1. On the long time scale (1–10 ms), the simulation reproduces the chirping range (40–60 kHz), the burst duration (few ms) and intervals (5–10 ms). On the short time scale (0.01–0.1 ms), it reproduces pulsations and phase jumps, which we interpret as the result of beating between multiple resonant wave packets. Having reproduced at multiple levels of detail the dynamics of low-amplitude long-wavelength Alfvén modes driven by N-NB ions, the next goal is to reproduce and explain abrupt large-amplitude events (ALE) that were seen in the same experiments at longer time intervals (10–100 ms).

The effect of ion diamagnetic drift on the stability of peeling–ballooning modes in rotating tokamak plasmas has been analyzed numerically. The results show that plasma toroidal rotation can not only destabilize the peeling–ballooning mode but also can reduce the ion diamagnetic drift effect on the mode stability, even though the ion diamagnetic drift effect stabilizes the mode in a static plasma. Plasma poloidal rotation can also destabilize the mode and cancel the ion diamagnetic drift effect, even when the rotation frequency is much smaller than the toroidal one. These impacts of the rotation on the stability can resolve the discrepancy between the result of the numerical stability analysis and the experimental result in type-I ELMy H-mode plasmas in JT-60U. The reduction of the ion diamagnetic drift effect on ELM stability due to plasma rotation is shown to depend on the direction of the rotation, so that the ion diamagnetic drift effect becomes negligible only when the JT-60U plasma rotates in the direction counter to the plasma current.

The sensitivity of the growth and nonlinear evolution of fast-ion-driven modes is examined with respect to the choice of particle boundary conditions, diffusion coefficients, fast ion gyroradii and bulk compressibility. The primary purpose of this work is to justify the choice of parameters to be used in the self-consistent long-time simulations of fast ion dynamics using global MHD-kinetic hybrid codes that include fast ion sources and collisions. The present study is conducted for a scenario based on the N-NB-driven JT-60U shot E039672, which is subject to abrupt large events (ALE). We use realistic geometry, a realistic fast ion distribution, and focus on experimentally observed harmonics with low toroidal mode numbers n = 1, 2, 3. The use of realistic boundary conditions and finite Larmor radii for the fast ions is shown to be essential. The usual values μ 0 η = ν = χ ∼ 10 − 6 v A 0 R 0 used for resistivity, viscosity and thermal diffusivity, and Γ = 5 / 3 used for the specific heat ratio (controlling the effect of compressibility) are shown to be reasonable choices. Our method for performing the parameter scans around the threshold for the onset of convective amplification is proposed as a strategy for nonlinear benchmark studies.

Drift magnetohydrodynamic (MHD) equations have been derived in order to investigate the ion diamagnetic drift effect on the stability to ideal MHD modes in rotating plasmas. These drift MHD equations have been simplified with the Frieman-Rotenberg formalism under the incompressible assumption, and a new code, MINERVA-DI, has been developed to solve the derived extended Frieman-Rotenberg equation. Benchmark results of the MINERVA-DI code show good agreements with the analytic theory discussing the stability to an internal kink mode and that to a ballooning mode in static plasmas. The stability analyses of the ballooning mode with respect to toroidal rotation with the ion diamagnetic drift effect have been performed using MINERVA-DI. The stabilizing effect by the ion diamagnetic drift is found to be negligible when the rotation frequency is large compared to the ion diamagnetic drift frequency. The direction of plasma rotation affects the ballooning mode stability when the ion diamagnetic drift effect is taken into account. It is identified that there are two physics mechanisms responsible for the dependence of MHD stability on the rotation direction. One is the correction of the dynamic pressure effect on MHD stability by the ion diamagnetic drift, and the other is the change of the MHD eigenmode structure by the combined effect of plasma rotation and ion diamagnetic drift.

We have observed a bursting mode in the high-$\ensuremath{\beta}$ plasmas above the ideal $\ensuremath{\beta}$ limit without a conducting wall. The mode frequency is chirping down as the mode amplitude increases, and its initial value is close to the precession frequency of the trapped energetic particle from the perpendicular neutral beams. The mode structure is radially extended with a peak around the $q=2$ surface. This mode can finally trigger the resistive wall mode (RWM) despite enough plasma rotation for RWM stabilization. It is concluded that the mode is driven by trapped energetic particles. The mode is attributed to the interaction between the trapped energetic particles and a marginally stable mode in the wall-stabilized high-${\ensuremath{\beta}}_{N}$ region.

The dependence of pedestal structure on collisionality at fixed beta has been investigated in JT-60U. In the ITER-relevant low collisionality regime, the pedestal width does not change with edge collisionality. In the high collisionality regime, the pedestal width broadens with increased edge collisionality. The pedestal pressure gradient and width are not significantly changed when the pedestal is close to an intermediate n peeling-ballooning mode boundary at low collisionality. The experimental result indicates that conventional pedestal models where the pedestal width is independent of collisionality and is determined by βp ?> at the pedestal is not a bad assumption in the ITER-relevant low collisionality regime. On the other hand, the pressure gradient decreases and the pedestal width increases at high collisionality. The pedestal broadening becomes significant when the pedestal is marginal to be unstable at the high n ballooning mode in the high collisionality regime.

In a rotating toroidal plasma surrounded by a resistive wall, it is shown that linear magnetohydrodynamic (MHD) instabilities can be excited by interplay between the resistive wall mode (RWM) and stable ideal MHD modes, where the RWM can couple with not only a stable external kink mode but also various stable Alfven eigenmodes that abound in a toroidal plasma. The RWM growth rate is shown to peak repeatedly as the rotation frequency reaches specific values for which the frequencies of the ideal MHD modes are Doppler-shifted to the small RWM frequency. Such destabilization can be observed even when the RWM in a static plasma is stable. A dispersion relation clarifies that the unstable mode changes from the RWM to the ideal MHD mode destabilized by wall resistivity when the rotation frequency passes through these specific values. The unstable mode is excited at these rotation frequencies even though plasma rotation also tends to stabilize the RWM from the combination of the continuum damping and the ion Landau damping.

The plasma rotation necessary for stabilization of resistive-wall modes (RWMs) is investigated by controlling the toroidal plasma rotation with external momentum input by injection of tangential neutral beams. The observed threshold is 0.3% of the Alfv\'en velocity and much smaller than the previous experimental results obtained with magnetic braking. This low critical rotation has a very weak $\ensuremath{\beta}$ dependence as the ideal wall limit is approached. These results indicate that for large plasmas such as in future fusion reactors with low rotation, the requirement of the additional feedback control system for stabilizing RWM is much reduced.

Impact of plasma core profiles on magnetohydrodynamics (MHD) stability at tokamak edge pedestal is investigated numerically to extend an operation regime for small amplitude grassy edge localized mode (ELM). With the hypotheses that pedestal pressure profile can be predicted with the EPED1 model and the trigger of grassy ELM is an ideal ballooning mode, the impacts of plasma poloidal beta and plasma internal inductance on edge MHD stability are investigated, the parameters of which are related to plasma core profiles and are important parameters for grassy ELMy H-modes in JET quasi-double null plasma. The numerical results indicate that a ballooning mode can be destabilized by decreasing poloidal beta and/or internal inductance. In contrast, it is confirmed that pedestal density, which is also an important parameter for realizing grassy ELMy H-mode, can stabilize a ballooning mode. In combination with these trends, it is possible to relax the necessary conditions for grassy ELMy H-mode by adjusting the parameters carefully, though this relaxation destabilizes type-I ELM more easily due to the increase in edge current density.

Dynamics of fast ions and shear Alfven waves are simulated using MEGA, a global nonlinear hybrid code. The scenario considered is based on JT-60U shot E039672, driven by strong negative-ion-based neutral beams (N-NB), just before the onset of a so-called abrupt large event (ALE). It is found that modes with toroidal mode numbers n = 2, 3, 4 can be destabilized, besides the n = 1 mode studied previously. The properties of the modes with n > 1 are sensitive to the value of the plasma beta and the form of the fast ion distribution, so simulation conditions are set up as realistically as presently possible. When the fast ion drive exceeds a certain threshold, the n = 3 mode is enhanced through a convective amplification process while following fast ions were displaced either by the n = 3 mode itself or by the n = 1 mode. The fast ion transport in several cases, simulated with single- or multiple-n modes, is analysed and implications of the results for the explanation of ALEs are discussed.

An approximate model for a single fluid three-dimensional (3D) magnetohydrodynamic (MHD) equilibrium with pure isothermal toroidal flow with imposed nested magnetic flux surfaces is proposed. It recovers the rigorous toroidal rotation equilibrium description in the axisymmetric limit. The approximation is valid under conditions of nearly rigid or vanishing toroidal rotation in regions with significant 3D deformation of the equilibrium flux surfaces. Bifurcated helical core equilibrium simulations of long-lived modes in the MAST device demonstrate that the magnetic structure is only weakly affected by the flow but that the 3D pressure distortion is important. The pressure is displaced away from the major axis and therefore is not as noticeably helically deformed as the toroidal magnetic flux under the subsonic flow conditions measured in the experiment. The model invoked fails to predict any significant screening by toroidal plasma rotation of resonant magnetic perturbations in MAST free boundary computations.

Toroidal rotation effects are self-consistently taken into account not only in the linear magnetohydrodynamic (MHD) stability analysis but also in the equilibrium calculation. The MHD equilibrium computation is affected by centrifugal force due to the toroidal rotation. To study the toroidal rotation effects on resistive wall modes (RWMs), a new code has been developed. The RWMaC modules, which solve the electromagnetic dynamics in vacuum and the resistive wall, have been implemented in the MINERVA code, which solves the Frieman?Rotenberg equation that describes the linear ideal MHD dynamics in a rotating plasma. It is shown that modification of MHD equilibrium by the centrifugal force significantly reduces growth rates of RWMs with fast rotation in the order of M2?=?0.1 where M is the Mach number. Moreover, it can open a stable window which does not exist under the assumption that the rotation affects only the linear dynamics. The rotation modifies the equilibrium pressure gradient and current density profiles, which results in the change of potential energy including rotational effects.

A local gyrokinetic Vlasov simulation code GKV is extended to incorporate realistic tokamak equilibria including up-down asymmetry, which are produced by a free-boundary 2D Grad-Shafranov equation solver MEUDAS. By using a newly developed interface code IGS, two dimensional rectangular equilibrium data from MEUDAS is converted to straight-field-line flux coordinates such as Hamada, Boozer, and axisymmetric coordinates, which are useful for gyrokinetic micro-instability and turbulent transport analyses. The developed codes have been verified by a cross-code benchmark test using Cyclone-base-case like MHD equilibrium, where good agreement in the dispersion relation of ion temperature gradient (ITG) driven mode has been confirmed. The extended GKV is applied to two types of shaped plasmas expected in JT-60SA tokamak devices, i.e., ITER-like and highly-shaped plasmas, and ITG-mode stability and residual zonal-flow level are investigated. Through the detailed comparisons, more favorable stability properties against the ITG mode are revealed for the highly-shaped case, where the lower ITG-mode growth rate and higher residual zonal-flow levels compared to the ITER-like case are identified. c © 2014 The Japan Society of Plasma Science and Nuclear Fusion Research

The energy loss caused by the edge-localized mode (ELM) needs to be reduced for ITER operations with ELMy H-mode plasmas. The reduction in ELM energy loss by pellet injection for ELM pacing is studied by an integrated core/scrape-off layer/divertor transport code TOPICS-IB with a magnetohydrodynamic stability code and a pellet model taking account of the E × B drift of the pellet plasma cloud. It is found that the energy loss can be significantly reduced by a pellet injected to the pedestal plasma equivalent to that at the middle timing in the natural ELM cycle, whose pressure height is only about 5% lower than that of the natural ELM onset. In this case, pellet injection from the low-field side enables a small pellet, with about 1–2% of pedestal particle content and a speed high enough to approach the pedestal top, to reduce the energy loss significantly. With the above suitable conditions for ELM pacing, a pellet penetrates deep into the pedestal and triggers high-n ballooning modes with localized eigenfunctions near the pedestal top, where n is the toroidal mode number. Under suitable conditions, ELM pacing with reduced energy loss is successfully demonstrated in simulations, in which the gas puff reduction and the enhancement of divertor pumping can compensate for the core density increase due to additional particle fuelling by the pacing pellet.

In the JT-60U high-β plasmas above the no-wall β limit, a triggering of an edge localized mode (ELM) by an energetic particle (EP)-driven mode has been observed. This EP-driven mode is thought to be driven by trapped EPs and it has been named EP-driven wall mode (EWM) on JT-60U (Matsunaga et al 2009 Phys. Rev. Lett. 103 045001). When the EWM appears in an ELMy H-mode phase, ELM crashes are reproducibly synchronized with the EWM bursts. The EWM-triggered ELM has a higher repetition frequency and less energy loss than those of the natural ELM. In order to trigger an ELM by the EP-driven mode, some conditions are thought to be needed, thus an EWM with large amplitude and growth rate, and marginal edge stability. In the scrape-off layer region, several measurements indicate an ion loss induced by the EWM. The ion transport is considered as the EP transport through the edge region. From these observations, the EP contributions to edge stability are discussed as one of the ELM triggering mechanisms.

Rotation effects on the linear stability of the resistive wall mode (RWM) are investigated numerically based on the ideal magnetohydrodynamics (MHD) model. One topic is impact of poloidal rotation in a toroidally rotating plasma. Since the difference between mode frequency and rotation frequency is important to change the MHD stability by rotation, modulation of the Doppler-shifted frequency by poloidal rotation can play an important role even when the poloidal rotation velocity is significantly lower than the toroidal one. In addition, when poloidal rotation is fixed, the absolute value of the Doppler-shifted frequency relies on the direction of toroidal rotation, and hence, linear MHD stability can depend on this rotation direction. Such a dependence is sometimes observed experimentally. The other topic is the re-destabilization of the RWM by toroidal rotation in a reversed shear tokamak. Several numerical results imply that a candidate of this re-destabilized mode is a stable MHD eigenmode, which becomes unstable due to coupling with the RWM when rotation frequency is similar to the frequency of this stable eigenmode in a static equilibrium.

The hybrid code MEGA is used to simulate an abrupt large-amplitude event (ALE) in JT-60U shot E039672 that was strongly driven by a pair of negative-ion-based neutral beams (N-NBs). This configuration is known to be unstable to energetic particle modes (EPMs) with toroidal mode number n = 1. The purpose of this study is to look for a threshold with respect to the on-axis fast ion beta value, βh0, beyond which the EPM undergoes convective amplification (CA) and causes significant fast ion transport. This is motivated by the hypothesis that such a threshold may work as a trigger mechanism for relaxation events, such as ALE. In order to facilitate quantitative comparisons with the experiment, a realistic geometry and bulk pressure is used. Furthermore, MEGA is initialized with a fast ion distribution computed for JT-60U by an orbit-following Monte-Carlo code, and a passive vacuum region allows particles to travel without encountering artificial loss boundaries. Consistently with the experiment, the simulation predicts a burst time of about 0.3 ms and peak magnetic fluctuation levels around δBϑ/B

Numerical stability analysis of edge-localized MHD mode is performed to identify the origin of small-amplitude ‘grassy ELMs’ on the basis of current understanding of kinetic effects on ballooning mode stability. These qualitative and quantitative analyses show that short wavelength ballooning mode can play an important role in a grassy ELM stability even when kinetic effects are taken into account. After showing the importance of kinetic effects for discussing grassy ELM physics, impacts of plasma parameters important for realizing a grassy ELM plasma experimentally are investigated numerically from the viewpoint of the edge-localized MHD stability including these kinetic effects. These analyses show that low plasma ellipticity is preferable to realize a grassy ELM plasma due to destabilizing ballooning mode by preventing access to the second stability region of the ballooning mode.

By conducting power scan experiments using perpendicular neutral beams (NBs) after the injection of tangential NBs in directions co-, balanced and counter to the plasma current, the unified effect of counter-toroidal rotation and heating power on type-I edge-localized modes (ELMs) was investigated in a JT-60U tokamak. ELM frequency f ELM increased monotonically together with the power across the separatrix P sep for all the cases of tangential NBs. However, dependence of f ELM on P sep was explicitly stronger than a simple proportionality when perpendicular NBs were injected. The toroidal rotation velocity at the H-mode pedestal shifted continuously in the counter-direction when perpendicular NBs were applied. At fixed values of , f ELM increased clearly in proportion to P sep. In addition, this proportionality between f ELM and P sep holds regardless of the mixture of datasets of the toroidal momentum inputs. The proportional coefficient decreased as shifted in the co-direction.

JT-60SA will be operated in several operational domains to pursue its objectives, to support researches on ITER and develop physics and engineering basis towards DEMO reactor. Evolution and tailoring of the current profile, or the safety factor (q) profile in other words, is important in preparation for plasmas expected in the operational domains. Especially in the high normalized pressure (βN) domain, tailoring of the q profile is indispensable to achieve high performance. In this paper, evolution of the q profile during the current ramp-up in a candidate plasma for a steady-state high βN and modification of the q profile by means of ECRF are examined by modeling simulation. Impact of the target q profile modification on the energy confinement is also discussed. c