S-I Itoh

Zonal flows in plasma?a review

A comprehensive review of zonal flow phenomena in plasmas is presented. While the emphasis is on zonal flows in laboratory plasmas, planetary zonal flows are discussed as well. The review presents the status of theory, numerical simulation and experiments relevant to zonal flows. The emphasis is on developing an integrated understanding of the dynamics of drift wave–zonal flow turbulence by combining detailed studies of the generation of zonal flows by drift waves, the back-interaction of zonal flows on the drift waves, and the various feedback loops by which the system regulates and organizes itself. The implications of zonal flow phenomena for confinement in, and the phenomena of fusion devices are discussed. Special attention is given to the comparison of experiment with theory and to identifying directions for progress in future research.

Turbulence spreading into the linearly stable zone and transport scaling

We study the simplest problem of turbulence spreading corresponding to the spatio-temporal propagation of a patch of turbulence from a region where it is locally excited to a region of weaker excitation or even local damping. A single model equation for the local turbulence intensity, I(x, t), includes the effects of local linear growth and damping, spatially local nonlinear coupling to dissipation and spatial scattering of turbulence energy induced by nonlinear coupling. In the absence of dissipation, front propagation into the linearly stable zone occurs with the property of rapid progression at small t, followed by slower sub-diffusive progression at late times. The turbulence radial spreading into the linearly stable zone reduces the turbulent intensity in the linearly unstable zone and introduces an additional dependence on the ρ* ≡ ρi/a to the turbulent intensity and the transport scaling. These are in broad, semi-quantitative agreement with a number of global gyrokinetic simulation results with zonal flows and without zonal flows. Front propagation stops when the radial flux of fluctuation energy from the linearly unstable region is balanced by local dissipation in the linearly stable region.

Geodesic–acoustic-mode in JFT-2M tokamak plasmas

The characteristics of geodesic–acoustic-mode (GAM) are investigated through direct and simultaneous measurement of electrostatic and density fluctuations with a heavy ion beam probe.The amplitude of the GAM changes in relation to the radial position; it is small near the separatrix, reaches a local maximum at 3 cm inside the separatrix and then decreases again to 5 cm inside the separatrix. The frequency is constant in the range, though the predicted GAM frequency varies according to the temperature gradient. The correlation length is about 6 cm and comparable to the structure of the amplitude of the GAM. The results indicate the GAM has a radial structure which reflects the local condition at about 3 m inside the separatrix.The phase relation between the GAM oscillation indicates that the GAM is a radial propagating wave.The interaction between the GAM and the ambient density fluctuation is shown by the high coherence between the GAM oscillation and the temporal behaviour of the ambient density fluctuation. Moreover, the phase relation between the electric field fluctuation of the GAM ( ) and the amplitude of the density fluctuation indicates that the modulation of the ambient density fluctuation delays the . The causality between the GAM and the modulation of the density fluctuation is revealed.

ITER H mode confinement database update

This paper describes an update of the H mode confinement database that has been assembled for the ITER project. Data were collected from six machines of different sizes and shapes: ASDEX, DIII-D, JET, JFT-2M, PBX-M and PDX. The updated database contains better estimates of fast ion energy content and thermal energy confinement times, discharges with RF heating, data using boronization, beryllium and pellets, more systematic parameter scans, and other features. The list of variables in the database has been expanded, and the selection criteria for the standard dataset have been modified. We also present simple scalings of the total and thermal energy confinement time to the new dataset

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.

Anomalous viscosity due to drift wave turbulence

The transport matrix, due to low‐frequency microturbulence, is obtained for a cylindrical plasma including anomalous ion and electron viscosities. Quasilinear theory is used in the electrostatic limit. Particle, momentum, and energy fluxes are represented in terms of three thermodynamical forces, which are generated by the density gradient, the gradient of the parallel flow, the temperature gradient, radial electric field, and the convection of the wave. The characteristics of the matrix are examined. The 3×3 transport matrices for electrons and ions are symmetric but contain off‐diagonal elements. In the drift wave approximation, the ion flux decouples from the thermodynamical force on the electrons, and vice versa. The ion anomalous viscosity, the heat conductivity, and the off‐diagonal elements are found to be comparable. The anomalous electron viscosity due to the drift wave is calculated to be small. The transport equations of ions show that pure heat conductivity, which is the value when the net par...

Bispectral analysis applied to coherent floating potential fluctuations obtained in the edge plasmas on JFT-2M

This paper presents results of bispectral analysis applied to floating potential fluctuations in the edge region of ohmically heated plasmas in the JAERI Fusion Torus-2 Modified (JFT-2M) tokamak. Inside the outermost surface of plasmas, coherent mode fluctuations (CMs) in floating potential were observed around the frequency of the geodesic acoustic mode. The squared bicoherence shows significant nonlinear couplings between the CMs and background fluctuations. The biphase at the frequency of the CMs is localized around π, while that at frequencies of background fluctuations distributes in a wide range. The total bicoherence at the frequency of the CMs is proportional to the squared amplitude of the CMs. These observations are consistent with the theoretical prediction on the drift wave-zonal flow systems. Interpretation of the absolute value of the total bicoherence is also discussed.

Energy channeling from energetic particles to bulk ions via beam-driven geodesic acoustic modes—GAM channeling

A novel mechanism for energy transfer from energetic particles to bulk ions via geodesic acoustic modes (GAMs) is presented. The mechanism involves the excitation of GAMs by energetic particles. The GAMs are damped on ions by the Landau damping, by which wave energy is given to bulk ions. This process of energy exchange is formulated within the framework of quasilinear theory. The rate of energy transfer from energetic particles to GAMs can be comparable to that of energy exchange to particles via collisions, if the E × B velocity of GAMs reaches the level of diamagnetic velocity. Under this circumstance, the partition of energy absorptions by bulk ions and electrons is substantially modified due to the selective ion heating by GAMs.

Confinement improvement in H-mode-like plasmas in helical systems

The reduction of the anomalous transport due to the inhomogeneous radial electric field is theoretically studied for toroidal helical plasmas. The self-sustained interchange-mode turbulence is analysed for a system with magnetic shear and magnetic hill. The ballooning mode turbulence is studied for a system with magnetic well-like conventional stellarators. The influence of the radial electric field inhomogeneity on the transport coefficients and fluctuations is quantitatively shown. Unified theories of the transport coefficients in the L-mode- and H-mode-like plasmas are presented.

Observation of coherent bicoherence and biphase in potential fluctuations around geodesic acoustic mode frequency on JFT-2M

Two types of small-poloidal wavelength low frequency floating potential fluctuations (~1 kHz, and 10–15 kHz) are observed in the edge region of ohmically heated plasmas in the JFT-2M tokamak. The higher frequency fluctuations are inferred as geodesic acoustic modes (GAMs). Significant bicoherences between both modes and the broad-band fluctuations are observed simultaneously; however, frequency ranges are different. A significant bicoherence range is from 70 to 125 kHz at the lower frequency modes (LFM) and is from 20 to 125 kHz at the GAM. Phase angles at the significant bicoherences are coherent around π. These results indicate that both modes are nonlinearly coupled to the broad-band fluctuations. Two spectral peaks are observed around the GAM frequency and the difference in peak frequencies is around the LFM frequency, suggesting nonlinear couplings between the LFM and the GAM. However, the modulation of the GAM by the LFM is not conclusive. In this paper, the results of the letter (Nagashima et al 2005 Phys. Rev. Lett. 95 095002) are presented in detail.