Showing papers by "Kimitaka Itoh published in 2015"

Towards an emerging understanding of non-locality phenomena and non-local transport

Abstract: In this paper, recent progress on experimental analysis and theoretical models for non-local transport (non-Fickian fluxes in real space) is reviewed. The non-locality in the heat and momentum transport observed in the plasma, the departures from linear flux-gradient proportionality, and externally triggered non-local transport phenomena are described in both L-mode and improved-mode plasmas. Ongoing evaluation of ‘fast front’ and ‘intrinsically non-local’ models, and their success in comparisons with experimental data, are discussed

Towards an emerging understanding of non-locality phenomena and non-local transport

Abstract: In this paper, recent progress on experimental analysis and theoretical models for non-local transport (non-Fickian fluxes in real space) is reviewed. The non-locality in the heat and momentum transport observed in the plasma, the departures from linear flux-gradient proportionality, and externally triggered non-local transport phenomena are described in both L-mode and improved-mode plasmas. Ongoing evaluation of ‘fast front’ and ‘intrinsically non-local’ models, and their success in comparisons with experimental data, are discussed

Density Peaking by Parallel Flow Shear Driven Instability

Abstract: Yusuke KOSUGA1,2), Sanae-I. ITOH2,3) and Kimitaka ITOH3,4) 1)Institute for Advanced Study, Kyushu University, Fukuoka 812-8581, Japan 2)Research Institute for Applied Mechanics, Kyushu University, Fukuoka 816-8580, Japan 3)Research Center for Plasma Turbulence, Kyushu University, Fukuoka 816-8580, Japan 4)National Institute for Fusion Science, Toki 509-5202, Japan (Received 25 November 2014 / Accepted 16 February 2015)

Identification of the energetic-particle driven GAM in the LHD

Abstract: n = 0 modes with frequency chirping have been observed by a heavy ion beam probe and Mirnov coils in the large helical device plasmas, where n is the toroidal mode number. The spatial structures of the electrostatic potential fluctuation and the density fluctuation correspond to those of the geodesic acoustic mode (GAM). The modes are observed only during the tangential neutral beam injection with the energy of 175 keV. The energy spectra of fast ions measured by a neutral particle analyzer implies that the modes are excited by the fast ions through the inverse Landau damping. The absolute values and the temperature dependence of the frequency of the mode can be interpreted by the dispersion relation taking into account the measured energy spectra of the fast ions. Therefore, the observed n = 0 modes are identified as the energetic-particle driven GAM.

Flow damping due to stochastization of the magnetic field

Abstract: Understanding the transport of ions, electrons and heat in magnetized plasmas is important to the development of fusion power as well as our understanding of the behaviour of astrophysical objects. Ida et al. find that stochastization of magnetic field lines in a plasma damps plasma flow more strongly than expected.

Self-regulated oscillation of transport and topology of magnetic islands in toroidal plasmas

Abstract: The coupling between the transport and magnetic topology is an important issue because the structure of magnetic islands, embedded in a toroidal equilibrium field, depends on the nature of the transport at the edge of the islands. Measurements of modulated heat pulse propagation in the DIII-D tokamak have revealed the existence of self-regulated oscillations in the radial energy transport into magnetic islands that are indicative of bifurcations in the island structure and transport near the q = 2 surface. Large amplitude heat pulses are seen in one state followed by small amplitude pulses later in the discharge resulting in a repeating cycle of island states. These two states are interpreted as a bifurcation of magnetic island with high and low heat pulse accessibility. This report describes the discovery of a bifurcation in the coupled dynamics between the transport and topology of magnetic islands in tokamak plasmas.

3D effects of edge magnetic field configuration on divertor/scrape-off layer transport and optimization possibilities for a future reactor

Abstract: This paper assesses the three-dimensional (3D) effects of the edge magnetic field structure on divertor/scrape-off layer transport, based on an inter-machine comparison of experimental data and on the recent progress of 3D edge transport simulation. The 3D effects are elucidated as a consequence of competition between transports parallel () and perpendicular () to the magnetic field, in open field lines cut by divertor plates, or in magnetic islands. The competition has strong impacts on divertor functions, such as determination of the divertor density regime, impurity screening and detachment control. The effects of magnetic perturbation on the edge electric field and turbulent transport are also discussed. Parameterization to measure the 3D effects on the edge transport is attempted for the individual divertor functions. Based on the suggested key parameters, an operation domain of the 3D divertor configuration is discussed for future devices.

On the spatial structure of solitary radial electric field at the plasma edge in toroidal confinement devices

Abstract: The solitary radial electric field in the edge of toroidal plasma is studied based on the electric field bifurcation model. Results are applied to tokamak and helical plasmas, and the dependence of the electric field structure on the plasma parameters and geometrical factors is analyzed. The order of magnitude estimate for tokamak plasma is not far from experimental observations. It is shown that, in helical plasmas, the height of electric field structure is reduced substantially owing to the ripple particle transport, while the width is influenced less. The implications of the results for the limit of achievable gradient in the H-mode pedestal are also discussed.

Overview of transport and MHD stability study: Focusing on the impact of magnetic field topology in the Large Helical Device

Abstract: The progress in the understanding of the physics and the concurrent parameter extension in the large helical device since the last IAEA-FEC, in 2012 (Kaneko O et al 2013 Nucl. Fusion 53 095024), is reviewed. Plasma with high ion and electron temperatures (Ti(0) ~ Te(0) ~ 6 keV) with simultaneous ion and electron internal transport barriers is obtained by controlling recycling and heating deposition. A sign flip of the nondiffusive term of impurity/momentum transport (residual stress and convection flow) is observed, which is associated with the formation of a transport barrier. The impact of the topology of three-dimensional magnetic fields (stochastic magnetic fields and magnetic islands) on heat momentum, particle/impurity transport and magnetohydrodynamic stability is also discussed. In the steady state operation, a 48 min discharge with a line-averaged electron density of 1 × 1019 m−3 and with high electron and ion temperatures (Ti(0) ~ Te(0) ~ 2 keV), resulting in 3.36 GJ of input energy, is achieved.

Edge plasma dynamics during L-H transition in the JFT-2M tokamak*

Abstract: This article presents a radial electric field measurement by a heavy ion beam probe in the JFT-2M tokamak, during the L-H transition. An abrupt increase (time scale of O(100 µs)) of the strong edge radial electric field (localized in the radius with FWHM ~7 mm) results in the increase of density gradient and turbulence reduction. Rapid inward propagation of the turbulence suppression front is observed at the transition. After the transition, the electric field structure in the tiny edge localized modes (ELMs) is analyzed. Transport self-regulation events observed in the vicinity of the L-H transition, i.e. the limit cycle oscillation (LCO) in the L-mode, the tiny ELM in the H-mode, as well as the L-H transition itself, are summarized in a single Lissajous diagram in the electric field-density gradient space, which provides a comprehensive explanation of the transition dynamics.

Formation mechanism of steep wave front in magnetized plasmas

Abstract: Bifurcation from a streamer to a solitary drift wave is obtained in three dimensional simulation of resistive drift waves in cylindrical plasmas. The solitary drift wave is observed in the regime where the collisional transport is important as well as fluctuation induced transport. The solitary drift wave forms a steep wave front in the azimuthal direction. The phase of higher harmonic modes are locked to that of the fundamental mode, so that the steep wave front is sustained for a long time compared to the typical time scale of the drift wave oscillation. The phase entrainment between the fundamental and second harmonic modes is studied, and the azimuthal structure of the stationary solution is found to be characterized by a parameter which is determined by the deviation of the fluctuations from the Boltzmann relation. There are two solutions of the azimuthal structures, which have steep wave front facing forward and backward in the wave propagation direction, respectively. The selection criterion of these solutions is derived theoretically from the stability of the phase entrainment. The simulation result and experimental observations are found to be consistent with the theoretical prediction.

Azimuthal inhomogeneity of turbulence structure and its impact on intermittent particle transport in linear magnetized plasmas

Abstract: Fluctuation component in the turbulence regime is found to be azimuthally localized at a phase of the global coherent modes in a linear magnetized plasma PANTA. Spatial distribution of squared bicoherence is given in the azimuthal cross section as an indicator of nonlinear energy transfer function from the global coherent mode to the turbulence. Squared bicoherence is strong at a phase where the turbulence amplitude is large. As a result of the turbulence localization, time evolution of radial particle flux becomes bursty. Statistical features such as skewness and kurtosis are strongly modified by the localized turbulence component, although contribution to mean particle flux profile is small.

Simulation study of hysteresis in the gradient-flux relation in toroidal plasma turbulence

Abstract: Global nonlinear simulations with heat modulation are carried out to understand the turbulent transport mechanism in toroidal plasmas. Rapid propagation of the heat modulation and a hysteresis in the gradient-flux relation are found in the turbulent simulation of drift-interchange modes. A global mode is excited nonlinearly, and the nonlinear couplings with Reynolds stress take a finite temporal duration for self-consistent redistribution of the energy. The mode also has a seesaw effect: increase of the amplitude of the global mode at one position affects the turbulence at the other radial position not by inducing the radial flux by itself, but by absorbing the energy from microscopic modes. Successive excitations of microscopic modes cause the accelerated propagation of the flux change like turbulence spreading after the onset of modulation. Owing to these non-diffusive processes, the hysteresis appears in the gradient-flux relation, which is compared with experiments.

Topology bifurcation of a magnetic flux surface in toroidal plasmas

Abstract: The bifurcation of magnetic topology is identified by a significant change in the heat pulse propagation properties in magnetized plasmas. Clear evidence of stochastization of the magnetic surfaces near a rational surface is observed in the core plasma with weak magnetic shear in the Large Helical Device by slowly decreasing the magnetic shear and measured by applying heat pulses driven by modulated electron cyclotron heating (MECH). Three topologies of the magnetic flux surfaces (a nested magnetic island and partial and full stochastization) are identified by the patterns of heat pulse propagation observed in the flat temperature region in the plasma. Slow heat pulse propagation exhibiting a non-monotonically increasing delay time is evidence of a magnetic island, while the fast heat pulse propagation observed in the plasma with medium magnetic shear is evidence of the stochastization of the magnetic surfaces. The region with the fast heat pulse propagation varies with a slight change of magnetic shear. There are two types of stochastization of the magnetic surfaces. In one, the stochastization region is localized near the rational surface (partial stochastization) and in the other, the region is extended to the magnetic axis (full stochastization). The appearance of a stochastic magnetic field is not caused by MHD instability. The significant increase of the ratio of electron thermal diffusivity to ion thermal diffusivity is consistent with that expected by stochastization of the magnetic field.

Study of Nonlinear Behavior of Low-Frequency MHD Mode Caused by Transition of Radial Electric Field in LHD

Abstract: The response of low-frequency magnetic fluctuations in a transition of radial electric field (Er) is investigated in the edge region of LHD plasmas. Consequently, the amplitude of a magnetic fluctuation with f 2 5 kHz is suddenly enhanced at the transition timing of Er, and then disappeared after the transition of Er. Furthermore, it is found that the amplitude of Hα signals is enhanced, corresponding to the abrupt diminishing of the magnetic fluctuation with f 2 5 kHz.

Evaluation of Non-Linear Mode Coupling During End-Plate Biasing Experiment in PANTA

Abstract: To understand turbulent transport in magnetized plasmas, an end-plate biasing experiment was performed in a linear magnetized plasma device, PANTA. Here we report the change of bi-coherence among fluctuations during biasing. During biasing, coupling with drift waves is decrease, and that with the mediator is increase.

Contribution of geodesic acoustic modes in the limit cycle oscillation near H-mode transition in JFT-2M plasmas

Abstract: The magnitude of geodesic acoustic modes (GAMs) is analyzed, which are driven by the oscillating Reynolds stress force in poloidal direction in the limit cycle oscillation (LCO) of tokamaks near the condition of H-mode transition. Although the frequency of LCO is much lower than the natural frequency, the forced oscillation of GAMs at the LCO frequency is excited. The amplitude of excited GAMs can be similar to that of low frequency zonal flows. The result is compared with the observation of the JFT-2M tokamak.

The effect of beta-induced Alfvén eigenmode on L-I-H transitions in the HL-2A tokamak

Abstract: The characteristics of beta-induced Alfven eigenmode (BAE) at L–I–H transitions have been investigated in the HL-2A experiments for the first time. It is found that a BAE of fBAE ~ 13.6 kHz coexists with the limit cycle oscillations of fLCO ~ 2.6 kHz and both disappear after the I–H transition in the discharges with L–I–H transitions. It is also found that the ambient turbulence is modulated by the radial electric field at the BAE frequency and that the level of density fluctuation gradually drops in L–I–H transitions, but it remains invariant in L–I–L ones. These results suggest a possible correlation between the BAE and the L–I–H transitions. The sudden disappearance of the BAE in L–I and I–H transitions has the similar effect of sawtooth crashes on triggering L–I–H transitions in the HL-2 A tokamak.

Erratum: Flow damping due to stochastization of the magnetic field

Abstract: Nature Communications 6: Article number: 5816 (2015); Published: 8 January 2015; Updated: 5 March 2015 The affiliation details for A. Isayama, K. Shinohara, P.S. Bawankar, X.L. Huang, M. Ohno, S. Pandya, R. Sano and H. Zhang as part of the LHD Experiment Group were incorrect in this Article. The correct affiliation details for these authors are listed below:

An Application of Hadamard Transform for Plasma Laser-Induced Fluorescence Spectroscopy

Abstract: The importance of measurements of inhomogeneities in plasma is widely recognized to understand the selforganization mechanisms in magnetized plasmas and to improve a quality of products processed by industrial plasmas. There is variety of plasma parameters should be measured. Some diagnostic tools for plasma parameter, e.g. spectroscopy of weak light, have a low signal-to-noise ratio (SNR). A conditional averaging (or lock-in amplifier) method is very powerful to improve SNR if the noise is random. However, a long averaging time is necessary to increase the SNR. To improve the SNR tenfold, a hundredfold averaging time is required. It severely impairs the experimental efficiency to observe the spatial structures by scanning of single detector. In optical measurement, an efficient conditional averaging scheme based on the Hadamard transform method [1], which uses ordered masks either block or transmit light, is frequently used. Here we propose an application of the Hadamard transform method to plasma diagnostics. As a specific diagnostic, we considered the laser-induced fluorescence spectroscopy (LIF) [2, 3], which can derive local absolute ion flow velocity and ion temperature with velocity distribution measurements. Our new method is named as Hadamard laserinduced fluorescence (HLIF). At the spatial structure measurement, conventional LIF methods require realignment of incident laser beam and fluorescence collection optics mechanically, in addition to the laser frequency scan. Thus, the spatial and reproducibility resolution is at most a few mm [4]. Using PMT array detector is a method to increase in the reproducibility resolution. However, the number of spatial measure points is limited by the number of

Simulation Study of Toroidal Flow Generation of Minority Ions by Local ICRF Heating

Abstract: The toroidal flow generation of minority ions by the local ion cyclotron range of frequencies (ICRF) heating is investigated in a tokamak plasma by applying the GNET code, which can solve the drift kinetic equation in the 5-D phase space. An asymmetry of velocity distribution function in the parallel direction is found and two types of toroidal averaged flow of minority ions are observed. One is the sheared flow near the RF power absorption region depending on the sign of k||, and the other is the toroidal flow, which is larger than the previous one, independent of the sign of k||. It is found that the k||-sign-independent toroidal flow is generated by the net toroidal motion of energetic tail ions and that the k||-sign-dependent flow is related to the mechanism proposed by Ohkawa [Phys. Plasmas 12, 094506 (2005)].

Ion scale nonlinear interaction triggered by disparate scale electron temperature gradient mode

Abstract: We have observed that the disparate scale nonlinear interactions between the high-frequency (∼0.4 MHz) electron temperature gradient (ETG) mode and the ion-scale low-frequency fluctuations (∼kHz) were enhanced when the amplitude of the ETG mode exceeded a certain threshold. The dynamics of nonlinear coupling between the ETG mode and the drift wave (DW) mode has already been reported [C. Moon, T. Kaneko, and R. Hatakeyama, Phys. Rev. Lett. (2013)]. Here, we have newly observed that another low-frequency fluctuation with f ≃ 3.6 kHz, i.e., the flute mode, was enhanced, corresponding to the saturation of the DW mode growth. Specifically, the bicoherence between the flute mode and the DW mode reaches a significant level when the ∇Te/Te strength exceeded 0.54 cm−1. Thus, it is shown that the ETG mode energy was transferred to the DW mode, and then the energy was ultimately transferred to the flute mode, which was triggered by the disparate scale nonlinear interactions between the ETG and ion-scale low-frequen...

Theory of Pseudo-Classical Confinement and Transition to L-Mode

Abstract: A theory of the self‐sustained turbulence is developed for resistive plasma in toroidal devices. Pseudoclassical confinement is obtained in the low‐temperature limit. As temperature increases, the current diffusivity prevails upon resistivity, and the turbulence nature changes so as to recover the L‐mode transport. Comparison with experimental observation on this transition is made. The Hartmann number is also given.

Observation of Parallel Force Balance for Drift Wave Fluctuation by a Fast Voltage Sweeping Method in a Linear Plasma

Abstract: Electron parallel force balance for drift-wave fluctuation is evaluated using fluctuation data obtained by a single probe fast voltage sweeping method in a linear plasma. Waveforms of the floating potential obtained by the method were validated by conventional high-impedance floating potential measurement. It is found that consistency in amplitude and phase relationships in the force balance is high when the electron temperature fluctuation is taken into account.