Naoaki Miyato

Global structure of zonal flow and electromagnetic ion temperature gradient driven turbulence in tokamak plasmas

Global characteristics of the coupled system of zonal flows and electromagnetic ion temperature gradient driven turbulence in tokamak plasmas are investigated using a global electromagnetic Landau fluid code. Zonal flow behavior changes with the safety factor q. In a low q region stationary zonal flows are excited and they suppress the turbulence effectively. Coupling between zonal flows and poloidally asymmetric pressure perturbations due to a geodesic curvature makes the zonal flows oscillatory in a high q region. Energy transfer from the oscillatory zonal flows to the turbulence via the poloidally asymmetric pressure perturbations is identified. Therefore in the high q region where the zonal flows are oscillatory, the zonal flows cannot quench the turbulence and turbulent transport is not suppressed completely. As for the zonal flow behavior, it is favorable for confinement improvement to make the low q region where the stationary zonal flows are dominant in tokamak plasmas.

Dynamics of large-scale structure and electron transport in tokamak microturbulence simulations

The important issue of whether zonal flows or streamers are preferentially formed in plasma turbulence with electron gyroradius scale is studied based on a gyrofluid model of electron temperature gradient (ETG) driven turbulence. Results from three approaches are presented. It is analytically derived first that the secondary generation of different large-scale structures is determined by the spectral anisotropy of turbulent fluctuation in two-dimensional Charney–Hasegawa–Mima turbulence. This is verified subsequently using three-dimensional simulations of sheared slab ETG turbulence, which show that the magnetic shear governs the pattern selection. It is found that a weak shear favours the enhancement of zonal flows so that the electron transport is strongly suppressed. In contrast, radially elongated streamers are formed nonlinearly in stronger-shear ETG turbulence. Finally, three-dimensional toroidal ETG simulations show that streamers are excited in the linearly stable region along the field (i.e. good curvature region) through a modulation instability after initial saturation of ETG modes. Although the electron transport at the quasi-steady state becomes higher than the initial saturation level, which is dominated by fluctuations with a peaked spectrum, the averaged value is still low at around the gyro-Bohm level. Furthermore, it is shown that the enhanced zonal flows in weak shear ETG turbulence may be limited by a Kelvin–Helmholtz instability. Also, it is found that the electromagnetic effects reduce the generation of zonal flows and reverse the so-called Okawa-scaling of electron transport on the β dependence.

Nonlocal behaviour of zonal flows in tokamak plasmas

Oscillatory zonal flows called geodesic acoustic modes (GAMs) appearing in a high safety factor region of tokamak plasmas show nonlocal behaviour in their frequency. The frequency of the GAMs is unchanged over a certain radial region and the radial variation of the frequency is step-like. The effects of ρ* = ρi/a on the nonlocal behaviour of the GAMs are investigated by global ion temperature gradient driven turbulence simulations, where ρi is an ion Larmor radius and a is a minor radius of a torus. It is found that the radial width in which the GAMs have the same frequency is almost proportional to , while the radial wavelength of the GAMs is proportional to ρi. As for turbulent transport, the boundary between a high transport region where the GAMs are dominant and a low transport region becomes clearer for smaller ρ*.

Dynamics of turbulent transport dominated by the geodesic acoustic mode near the critical gradient regime

The effects of geodesic acoustic modes (GAMs) on the toroidal ion temperature gradient turbulence and associated transport near the critical gradient regime in tokamak plasma are investigated based on global Landau-fluid simulations and extended predator-prey modeling analyses. A new type of intermittent dynamics of transport accompanied with the emission and propagation of the GAMs, i.e., GAM intermittency [K. Miki et al., Phys. Rev. Lett. 99, 145003 (2007)], has been found. The intermittent bursts are triggered by the onset of spatially propagating GAMs when the turbulent energy exceeds a critical value. The GAMs suffer collisionless damping during the propagation and nonlocally transfer local turbulence energy to wide radial region. The stationary zonal flows gradually increase due to the accumulation of non-damped residual part over many periods of quasi-periodic intermittent bursts and eventually quench the turbulence, leading to a nonlinear upshift of the linear critical gradient; namely, the Dimits...

Study of a drift wave-zonal mode system based on global electromagnetic Landau-fluid ITG simulation in toroidal plasmas

Using a global Landau-fluid code in toroidal geometry, an electromagnetic ion temperature gradient (ITG) driven turbulence–zonal mode system is investigated. Two different types of zonal flows, i.e. stationary zonal flows in a low q (safety factor) region and oscillatory ones in a high q region, which are called geodesic acoustic modes, are found to be simultaneously excited in a torus. The stationary flows efficiently suppress turbulent transport, while the oscillatory ones weakly affect the turbulence due to their time varying nature. Therefore, in the low q region where the zonal flows are almost stationary, they are dominant over the turbulence. On the other hand, the turbulence is still active in the high q region where the zonal flows are oscillatory.

Effects of toroidal rotation shear and magnetic shear on thermal and particle transport in plasmas with electron cyclotron heating on JT-60U

Increases in thermal and particle transport with electron cyclotron heating (ECH) that are observed in many tokamaks can be a critical issue in establishing ITER operational scenarios with electron heating. To address the issues, conditions with no increase in the thermal and particle transport with ECH have been experimentally investigated in positive magnetic shear, weak magnetic shear (WS) and reversed magnetic shear plasmas with internal transport barrier in the ion channel. The ion heat diffusivity (χi) around the internal transport barrier in the ion temperature remains constant with ECH when a large negative toroidal rotation shear (|dV/dr| > 4 × 105 s−1) is formed before the ECH. On the other hand, χi increases on the condition that the toroidal rotation shear is small or positive. The characteristics do not depend on magnetic shear, the electron to ion temperature ratio (Te/Ti) and ECH power. The electron heat diffusivity stays constant with ECH when the magnetic shear is negative. Effective particle transport remains constant or reduces during ECH under the condition of negative magnetic shear. An observation indicates that there is no threshold of the negative magnetic shear or a very small one for the electron channel sustainment; the electron thermal and particle confinement is maintained during ECH with a small negative magnetic shear in the WS operation.

Modifications to the edge radial electric field by angular momentum injection in JT-60U and their implication for pedestal transport

The first detailed measurements of ion-impurity dynamics for NBI-heated ELMy H-modes at the edge of the JT-60U tokamak are reported. We investigated the ability of external momentum/power input to modify and control the radial electric field, Er, and pedestal structures. The relationship between Er and pedestal structures of ion-impurity density, ni, and temperature, Ti, during the ELMing H-mode phase for various momentum input directions (i.e. co-, balanced- and counter-NBI) and input powers from perpendicular NBI are compared with the ELM-free phase. The observed trend is that the edge Er-well width increases in the co-NBI discharge, while the Er value at the base of the Er-well becomes more negative in the counter-NBI discharge. The scale length for both ni and Ti in the pedestal is ?2?cm and values are ?1 for both ELM-free and ELMing phases with different magnitudes of Er (and/or Er shear). Characteristics of the turbulent density fluctuation, in addition to a uniform toroidal MHD oscillation (i.e. n?=?0), during both ELM-free and ELMing phases are also reported.

Effects of centrifugal modification of magnetohydrodynamic equilibrium on resistive wall mode stability

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.

On the gyrokinetic model in long wavelength regime

The gyrokinetic quasi-neutrality equation is considered from the point of view of the push-forward representation of particle density associated with the guiding-center transformation and the gyro-center transformation involved in the standard gyrokinetic formulation. It is clearly shown that the higher order displacement vector of the guiding-center transformation as well as the gyro-center displacement vector should be taken into account in the long wavelength regime. The higher order displacement vector of the guiding-center transformation yields additional higher order terms related to nonuniformity of magnetic field in the gyrokinetic quasi-neutrality equation. These additional terms may be important when the gradient scale length of electric field can be comparable to that of the magnetic field as in low aspect ratio tokamaks. Also the gyrokinetic Hamiltonian should be modified for consistency.

Statistical characteristics of turbulent plasmas dominated by zonal flows

In order to better understand tlie feature of turbulent transport in tokamak plasmas dominated by zonal flows and nonlinearly excited tertiary (Kelvin-Helmholtz-like) waves, the radial dependency of the turbulent structure in the back of steady-state zonal flows was analyzed based on gyrofluid simulation of the electron temperature gradient turbulence in a sheared-slab configuration. The zonal flows change the characteristics of plasma turbulence from homogeneous to inhomogeneous. Thus. it is found that turbulent vortices are regulated by a radially alternative potential pattern where the spatiotemporal characteristic scales with respect to local fluctuations change radially. From the cross spectrum analysis between tlie poioidal electric field and the pressure perturbation, it was found that. in plasmas dominated by zonal flows, the restriction of heat flux is conducted by two mechanisms, i.e. the reduction of coherence and the phase synchronization.