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We present that a statistical method known as the Complexity-Entropy analysis is useful to characterize a state of plasma turbulence and flux in the resonant magnetic perturbation (RMP) edge localized mode (ELM) control experiment. The RMP ELM suppression phase with the stochastic pedestal top temperature fluctuation can be distinguished from the natural ELM free phase with the chaotic fluctuation. It is discussed that the stochastic temperature fluctuation localized near the pedestal top can be originated from the narrow layer of the field penetration near the pedestal top. The forced magnetic island can emit the resonant drift wave of comparable sizes (relatively low-k) in the RMP ELM suppression phase, and it can results in the generation of stochastic higher wavenumber fluctuations coupled to tangled fields around the island. The analysis of the ion saturation current measurement around the main outer striking point on the divertor shows that it also becomes more stochastic as the stronger plasma response to the RMP field is expected.

We calculate a set of ITER poloidal field (PF) coil currents at the initial magnetization phase using the quadratic programming algorithm. The optimization is performed by exploiting the total magnetic energy of the ITER PF system as a cost function to be minimized. We also develop a simple analytic model which can make a rapid evaluation of temperature margin of superconducting PF coils for a given set of coil currents. This calculation is necessary for a quick examination of the compatibility of the PF scenario with engineering constraints. For an easy adaptation to the future integrated ITER simulator, all modules are developed using the ITER Integrated Modeling Analysis Suite (IMAS) data structure and the Kepler framework.

Using a statistical correlation analysis, we compute evolution of the magnetic Kubo number during an edge pedestal collapse in nonlinear reduced magnetohydrodynamic simulations. The Kubo number is found not to exceed the unity in spite of performing the simulation with a highly unstable initial pressure profile to the ideal ballooning mode. During the edge pedestal collapse, the Kubo number is within the values of $0.2$ and $0.6$ suggesting that the quasilinear diffusion model is sufficient to explain the energy loss mechanism during the pedestal collapse. Temporal evolution of poloidal correlation lengths of pressure fluctuations resembles with that of the Chirikov parameter and the Kubo number; while radial correlation lengths of the pressure fluctuations are strongly correlated with the radial width of the magnetic stochastic layer., Comment: 10 pages, 13 figures, Submitted to Physics of Plasmas

We evaluate electromagnetic (EM) loads on the main systems of the ITER machine using a single finite element model. The 20° sector of the full ITER machine includes the main in-vessel components as well as the vacuum vessel. Narrow slits of the in-vessel components are effectively modeled by using the element splitting method without significant increase of computation memory and time as well as without sacrificing the accuracy. Furthermore, the halo current is taken into account at the same time together with the plasma current. To apply both currents concurrently, dedicated conversion codes are utilized to transfer the plasma simulation results by DINA to the electromagnetic analysis by ANSYS-EMAG used here. The electromagnetic loads on the ITER machine are calculated for various disruption scenarios. Investigation on the analysis results is made to find the worst plasma disruption case and the design-driving load component for each system as well as to compare load contribution from eddy and halo currents. The effect of the narrow slits on load reduction is also examined.

A simple and efficient modeling technique is presented for a proper analysis of complicated eddy current flows in conducting structures with fine air gaps. It is based on the idea of replacing a slit with the decoupled boundary of finite elements. The viability and efficacy of the technique is demonstrated in a simple problem. Application of the method to electromagnetic load analyses during plasma disruptions in ITER has been successfully carried out without sacrificing computational resources and speed. This shows the proposed method is applicable to a practical system with complicated geometrical structures.

Electromagnetic (EM) loads due to eddy current and halo current during plasma disruptions are evaluated for the ITER diagnostic upper port plug. To reduce strong EM loads acting on the port plug fixed to the vacuum vessel like a cantilever beam, three design options have been considered: removal of the diagnostic first wall, slitting of the diagnostic shield module and recess of the port plug. The main focus of the present study is to examine the efficacy of these options in terms of EM loads on the upper port plug. It is found that making slits is more effective than removing the first wall. It is also shown that the upper port plug needs to be recessed to reduce the EM load induced by halo current.

An analysis is carried out on the three-dimensional modeling and computation of the magnetic field in ITER. The commercial finite element code ANSYS-EM is employed for this study. In particular, an emphasis is put on the analysis of the characteristics of non-axisymmetric magnetic fields produced by ferromagnetic materials, including ferromagnetic inserts (FIs) and helium cooled solid breeder test blanket modules (TBMs). It is found that the ITER design requirement for toroidal field ripple is violated by the presence of TBMs, even in the presence of FIs. Calculations of TBM-produced error fields also show that TBM produces a significant error field at q = 2 surface exceeding the ITER design requirement. Discussions are made of the potential implication of the TBM-produced non-axisymmetric fields on plasma performance and the design of a TBM emulation system.

According to recent updates of ITER shield blanket design, electromagnetic loads during the plasma disruption are being evaluated to verify the mechanical confidence and reliability. As a course of such evaluations, a benchmark activity for the electromagnetic analysis, coordinated by ITER Organization, is underway between ITER parties to compare the calculation results for disruption loads on the blankets. In this paper, we present calculation results for the electromagnetic loads on the simplified but practical model of ITER shield blankets with respect to six representative disruption scenarios of which ITER distributes simulation results based on the DINA code as a reference of the design and analysis. Commercial finite element method software, ANSYS/Emag™, was employed to evaluate the eddy current on the blanket modules with the 40° sector model for major conducting structure of the tokamak including double-walled vacuum vessel, triangular support, and vertical targets of divertors. An interface between ANSYS/Emag™ and plasma simulator was implemented with a conversion tool assigning the plasma current density on the ANSYS elements corresponding to the current filaments in DINA outputs. Discussions are made of the possible improvement of the blanket model taking more realistic blanket configuration into account at the cost of the moderate increase in computational time. A final remark is given of the possibility of incorporating halo currents into ANSYS disruption simulations, which are major sources of electromagnetic loads on in-vessel components including blankets.

In cold pulse experiments in J-TEXT, not only are rapid electron temperature increases in the core observed, but also steep rises in the inner density are found. Moreover, some evidence of acceleration of the core toroidal rotation is also observed during the non-local transport process of electron temperature. These new findings of cold pulse experiments in J-TEXT suggest that turbulence spreading is a possible mechanism for the non-local transport dynamics.

In-vessel control coils (IVCCs) are to be used for the fast plasma position control, field error correction (FEC), and resistive wall mode (RWM) stabilization for the Korea Superconducting Tokamak Advanced Research (KSTAR) device. The IVCC system comprises 16 segments to be unified into a single set to achieve following remarkable engineering advantages; (1) enhancement of the coil system reliability with no welding or brazing works inside the vacuum vessel, (2) simplification in fabrication and installation owing to coils being fabricated outside the vacuum vessel and installed after device assembly, and (3) easy repair and maintenance of the coil system. Each segment is designed in 8 turns coil of 32 mm × 15 mm rectangular oxygen free high conductive copper with a 7 mm diameter internal coolant hole. The conductors are enclosed in 2 mm thick Inconel 625 rectangular welded vacuum jacket with epoxy/glass insulation. Structural analyses were implemented to evaluate structural safety against electromagnetic loads acting on the IVCC for the various operation scenarios using finite element analysis. This paper describes the design features and structural analysis results of the KSTAR in-vessel control coils.

We present a general approach and procedures for developing a Grid-based remote monitoring system when the local control system employs Experimental Physics and Industrial Control System (EPICS). A description is given of the prototype KSTAR remote monitoring system that has been implemented on the basis of the general concept. It is demonstrated that the Grid computing technology combined with Globus toolkit software tools, can be successfully utilized for the realization of a remote monitoring system for tokamak experiments. Discussions are made on requirements and a plan to build a complete remote participation system for future tokamak experiments.

A study is carried out on the real-time plasma shape identification in the KSTAR device. An improved form of the finite current element (FCE) method is utilized in this study. Results are shown that the plasma boundary can be reproduced in 7 mm accuracy for any plasma configuration in ideal cases without invoking measurement errors. A design guideline for magnetic diagnostics (MD) is established when the measurement signals are subject to Gaussian noise. It is found that the measurement errors in poloidal field (PF) coil currents have substantial influence on the determination of the plasma shape.

We study the impact of impurities on turbulence driven intrinsic rotation (via residual stress) in the context of the quasi-linear theory. A two-fluid formulation for main and impurity ions is employed to study ion temperature gradient modes in sheared slab geometry modified by the presence of impurities. An effective form of the parallel Reynolds stress is derived in the center of mass frame of a coupled main ion-impurity system. Analyses show that the contents and the radial profile of impurities have a strong influence on the residual stress. In particular, an impurity profile aligned with that of main ions is shown to cause a considerable reduction of the residual stress, which may lead to the reduction of turbulence driven intrinsic rotation.

© 2015 AIP Publishing LLC. Using three-dimensional nonlinear simulations of tokamak turbulence, we show that an edge transport barrier (ETB) forms naturally once input power exceeds a threshold value. Profiles, turbulence-driven flows, and neoclassical coefficients are evolved self-consistently. A slow power ramp-up simulation shows that ETB transition is triggered by the turbulence-driven flows via an intermediate phase which involves coherent oscillation of turbulence intensity and E × B flow shear. A novel observation of the evolution is that the turbulence collapses and the ETB transition begins when RT> 1 at t = tR(RT: normalized Reynolds power), while the conventional transition criterion (ω E × B > γ l i n where ω E × B denotes mean flow shear) is satisfied only after t = tC( >tR), when the mean flow shear grows due to positive feedback.

We present design concepts and features of the plasma control system (PCS) in Korea Superconducting Tokamak Advanced Research (KSTAR). A design structure of the PCS is proposed as an effort to achieve research objectives of the KSTAR project. The PCS architecture is characterized by the real-time data communication using reflective-memory network, the hierarchical organization of dedicated controllers, and the integrated generation of actuator signals. Discussions are made of the functions and present design choices of the KSTAR PCS.

We calculate eddy current induced vertical forces on poloidal field (PF) coils and plasma facing conductors during a plasma disruption in Korea Superconducting Tokamak Advanced Research (KSTAR). A system of axisymmetric circuit equations, taking into account a saddle-connected three-dimensional configuration of KSTAR passive plates, combined with a prescribed plasma disruption scenario, is solved using a symmetric and anti-symmetric decomposition technique. The maximum vertical force on each independent conductor, which is designed to have its own vertical support, is calculated. It is shown that the effect of initial equilibrium magnetic fields upon maximum vertical forces is significant, and it should be included in the design analysis of vertical supports for tokamak conducting structures.

Simulation studies on magnetics control for the proposed Korea Superconducting Tokamak Advanced Research (KSTAR) plasmas are presented. Here, the magnetics control includes the fast time scale (∼10 ms) plasma position control, the slow time scale (∼1 s) plasma shape and current control, and the control of resistive wall modes (RWM). The plasma position control is realized by using two pairs of inner control coils inside KSTAR vacuum vessel. The plasma shape and current control is achieved by using superconducting poloidal field (PF) coils in KSTAR. The voltage limitations in PF coil power supplies are taken into account in the simulation. A set of 12 in-vessel control coils, which are mounted on the inner wall of KSTAR vacuum vessel, are used for the control of RWMs. The required feedback currents in in-vessel control coils for the stabilization of RWMs are estimated.

International audience; Dedicated experiments have been performed in KSTAR Ohmic plasmas to investigate the detailed physics of the rotation reversal phenomena. Here we adapt the more general definition of rotation reversal, a large change of the intrinsic toroidal rotation gradient produced by minor changes in the control parameters (Camenen et al 2017 Plasma Phys. Control. Fusion 59 034001), which is commonly observed in KSTAR regardless of the operating conditions. The two main phenomenological features of the rotation reversal are the normalized toroidal rotation gradient (u') change in the gradient region and the existence of an anchor point. For the KSTAR Ohmic plasma database including the experiment results up to the 2016 experimental campaign, both features were investigated. First, the observations show that the locations of the gradient and the anchor point region are dependent on q(95). Second, a strong dependence of u' on nu(eff) is clearly observed in the gradient region, whereas the dependence on R/L-Ti, R/L-Te, and R/L-ne is unclear considering the usual variation of the normalized gradient length in KSTAR. The experimental observations were compared against several theoretical models. The rotation reversal might not occur due to the transition of the dominant turbulence from the trapped electron mode to the ion temperature gradient mode or the neoclassical equilibrium effect in KSTAR. Instead, it seems that the profile shearing effects associated with a finite ballooning tilting well reproduce the experimental observations of both the gradient region and the anchor point; the difference seems to be related to the magnetic shear and the q value. Further analysis implies that the increase of u' in the gradient region with the increase of the collisionality would occur when the reduction of the momentum diffusivity is comparatively larger than the reduction of the residual stress. It is supported by the perturbative analysis of the experiments and the nonlinear gyrokinetic simulations. The absence of the sign change of u' even when a much lower collisionality is produced by additional electron cyclotron heating brings further experimental support to this interpretation.

On the basis of three-dimensional nonlinear magnetohydrodynamic simulations, we propose a new dynamical process leading to the stochastization of magnetic fields during an edge pedestal collapse. Primary tearing modes are shown to grow by extracting kinetic energy of unstable ballooning modes, eventually leading to the island overlap. Secondary tearing modes, which are generated through a coherent nonlinear interaction between adjacent ballooning modes, play a key role in this process, mediating the energy transfer between primary ballooning and tearing modes. Explicit calculations of the parallel energy loss through the stochastic field lines show that it can be a likely dominant energy loss mechanism during an edge pedestal collapse., 12 pages, 5 figures

Simulation studies of plasma shape identification and shape control for the proposed Korea superconducting tokamak advanced research (KSTAR) are described. It is shown that the total number of magnetic measurements can be effectively reduced by considering the patterns of magnetic flux and fields, generated by plasma, along a prescribed measurement contour. The effect of eddy currents on shape identification is investigated in dynamic simulations. The isoflux control scheme and a standard PID control law are adopted for the development of a model shape control system. It is shown that appropriate weighting factors of poloidal field coils, incorporating the efficacy of each coil to a shape control point, can significantly reduce the total feedback power required for a shape control action. Finally, a shape control simulation using calculated flux errors, which correspond to a more realistic experimental situation, is presented.

We present the results of fast time scale plasma control simulations for the proposed Korea Superconducting Tokamak Advanced Research (KSTAR) tokamak. Here, the fast plasma control includes stabilization of the vertical instability and rapid radial position control. As a simulation tool, we use the tokamak simulation code (TSC). We evaluate the power supply requirements for both the vertical and radial position control with a similar approach to that which was employed in the analysis of the tokamak physics experiments (TPX). In particular, we investigate the efficacy of control coil locations to minimize the power supply requirements. From this study, it is shown that two separate sets of control coils, one responsible for vertical control and the other responsible for radial control, are required to provide robust control of both vertical and radial position for all the anticipated plasmas in KSTAR experiments.

We present a semi-analytic model for low (L) to high (H) mode transition power threshold (Pth). Two main assumptions are made in our study. First, high poloidal mode number drift resistive ballooning modes (high-m DRBM) are assumed to be the dominant turbulence driver in a narrow edge region near to last closed flux surface. Second, the pre-transition edge profile and turbulent diffusivity at the narrow edge region pertain to turbulent equipartition. An edge power balance relation is derived by calculating the dissipated power flux through both turbulent conduction and convection, and radiation in the edge region. Pth is obtained by imposing the turbulence quench rule due to sheared E × B rotation. Evaluation of Pth shows a good agreement with experimental results in existing machines. Increase of P th at low density (i.e., the existence of roll-over density in P th vs. density) is shown to originate from the longer scale length of the density profile than that of the temperature profile. © 2014 AIP Publishing LLC.

We present an analytic study of momentum transport of tokamak plasmas in the vicinity of minimum safety factor (q) position in reversed magnetic shear configuration. Slab ion temperature gradient modes with an equilibrium flow profile are considered in this study. Quasi-linear calculations of momentum flux clearly show the novel effects of q-curvature on the generation of intrinsic rotation and mean poloidal flow without invoking reflectional symmetry breaking of parallel wavenumber (k∥). This q-curvature effect originates from the inherent asymmetry in k∥ populations with respect to a rational surface due to the quadratic proportionality of k∥ when q-curvature is taken into account. Discussions are made of possible implications of q-curvature induced plasma flows on internal transport barrier formation in reversed shear tokamaks.

We present a model for turbulent electron thermal transport at the edge pedestal in high (H)-mode plasmas based on electron temperature gradient (ETG) turbulence. A quasi-linear analysis of electrostatic toroidal ETG modes shows that both turbulent electron thermal diffusivity and hyper-resistivity exhibits the Ohkawa scaling in which the radial correlation length of turbulence becomes the order of electron skin depth. Combination of the Ohkawa scales and the plasma current dependence results in a novel confinement scaling inside the pedestal region. It is also shown that ETG turbulence induces a thermoelectric pinch, which may accelerate the density pedestal formation. © 2013 AIP Publishing LLC.

An overview of the physics of intrinsic torque is presented, with special emphasis on the phenomenology of intrinsic toroidal rotation in tokamaks, its theoretical understanding, and the variety of momentum transport bifurcation dynamics. Ohmic reversals and electron cyclotron heating-driven counter torque are discussed in some detail. Symmetry breaking by lower single null versus upper single null asymmetry is related to the origin of intrinsic torque at the separatrix. (Some figures may appear in colour only in the online journal)

Two types of experiments were carried out to conduct an intrinsic rotation study in KSTAR. The first was a density ramp-up experiment without neutral beam injection, and the second was an experiment with beam blip technique. In these experiments, some characteristics of the intrinsic rotation were observed in the KSTAR Ohmic L-mode plasmas including: (i) a non-monotonic dependence of the core intrinsic rotation, called U-curve behaviour, with respect to the electron density and the collisionality related to the gradient of the toroidal rotation profile; and (ii) the behaviour of the anchor point in the intrinsic rotation profile for which the region exhibits a roughly flat shape and stays at nearly the same value even if the gradient of the toroidal rotation changes significantly in the core region. The location of the anchor point seems to be related to the q profile, and the toroidal rotation at the anchor point changes with the plasma operation parameters. These observations in the KSTAR Ohmic L-mode plasmas seem to be related to the rotation reversal phenomenon. A transport analysis was performed for the beam blip experiments in order to evaluate the intrinsic torque so that the U-curve behaviour can be further understood. The first results of the transport analysis in the KSTAR Ohmic L-mode plasmas show a correlation of the momentum fluxes and the intrinsic torques with the electron density and the collisionality. The rough magnitude and profiles of the intrinsic torque was experimentally obtained, and their possible mechanism is briefly discussed.

We present an analytic study of the impact of external torque on the formation of ion internal transport barriers (ITBs). A simple analytic relation representing the effect of low external torque on transport bifurcations is derived based on a two field transport model of pressure and toroidal momentum density. It is found that the application of an external torque can either facilitate or hamper bifurcation in heat flux driven plasmas depending on its sign relative to the direction of intrinsic torque. The ratio between radially integrated momentum (i.e., external torque) density to power input is shown to be a key macroscopic control parameter governing the characteristics of bifurcation. © 2012 American Institute of Physics.

Experiments at DIII-D investigated the effects of magnetic error fields similar to those expected from proposed ITER test blanket modules (TBMs) containing ferromagnetic material. Studied were effects on: plasma rotation and locking, confinement, L–H transition, the H-mode pedestal, edge localized modes (ELMs) and ELM suppression by resonant magnetic perturbations, energetic particle losses, and more. The experiments used a purpose-built three-coil mock-up of two magnetized ITER TBMs in one ITER equatorial port. The largest effect was a reduction in plasma toroidal rotation velocity v across the entire radial profile by as much as Δv/v ∼ 60% via non-resonant braking. Changes to global Δn/n, Δβ/β and ΔH98/H98 were ∼3 times smaller. These effects are stronger at higher β. Other effects were smaller. The TBM field increased sensitivity to locking by an applied known n = 1 test field in both L- and H-mode plasmas. Locked mode tolerance was completely restored in L-mode by re-adjusting the DIII-D n = 1 error field compensation system. Numerical modelling by IPEC reproduces the rotation braking and locking semi-quantitatively, and identifies plasma amplification of a few n = 1 Fourier harmonics as the main cause of braking. IPEC predicts that TBM braking in H-mode may be reduced by n = 1 control. Although extrapolation from DIII-D to ITER is still an open issue, these experiments suggest that a TBM-like error field will produce only a few potentially troublesome problems, and that they might be made acceptably small.

PHYSICS OF PLASMAS 21, 012302 (2014) Momentum transport in the vicinity of q min in reverse shear tokamaks due to ion temperature gradient turbulence Rameswar Singh, 1,2,a) R Singh, 1,3 Hogun Jhang, 3 and P. H. Diamond 3,4,5 Institute for Plasma Research, Bhat, Gandhinagar 382 428, India Laboratoire de Physique des Plasmas, Ecole Polytechnique route de Saclay, 91128 Palaiseau Cedex, France WCI Center for Fusion Theory, National Fusion Research Institute, Daejeon 305-333, South Korea Center for Momentum Transport and Flow Organization, University of California, San Diego, California 92093, USA Center for Astrophysics and Space Sciences, University of California San Diego, 9500 Gilman Dr., La Jolla, California 92093-0424, USA (Received 23 October 2013; accepted 20 December 2013; published online 13 January 2014; publisher error corrected 3 March 2014) We present an analytic study of momentum transport of tokamak plasmas in the vicinity of minimum safety factor (q) position in reversed magnetic shear configuration. Slab ion temperature gradient modes with an equilibrium flow profile are considered in this study. Quasi-linear calculations of momentum flux clearly show the novel effects of q-curvature on the generation of intrinsic rotation and mean poloidal flow without invoking reflectional symmetry breaking of parallel wavenumber (k k ). This q-curvature effect originates from the inherent asymmetry in k k populations with respect to a rational surface due to the quadratic proportionality of k k when q-curvature is taken into account. Discussions are made of possible implications of q-curvature C 2014 induced plasma flows on internal transport barrier formation in reversed shear tokamaks. V AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4861625] I. INTRODUCTION Understanding of turbulent momentum transport in toka- mak plasmas has been a key research topic over the past dec- ades. This is because of the profound influence of plasma flows (both toroidal and poloidal) on confinement properties of tokamak plasmas. Radial transport of toroidal flow deter- mines the flow profile which has an influence on confinement enhancement by providing the E ! B shear driven by the ra- dial flow gradient. 1,2 Plasma rotation can also stabilize dan- gerous, magnetohydrodynamics (MHD) instabilities such as resistive wall modes (RWM), which is essential to realize high performance advanced tokamak operation. 3,4 Either external sources, such as neutral beam injection (NBI), or background turbulence (via turbulent Reynolds stress 5,6 ) can drive plasma flows in tokamaks. The latter, known as intrinsic rotation, 7 has been a focus in confinement physics research for many years because of its potential impact on reactor scale experiments where neutral beams cannot penetrate into the core region. One of the mechanisms of intrinsic rotation generation is by conversion of radial inhomogeneity into hk k i (¼spectrally averaged wavenumber in parallel direction) asymmetry driving residual stress. 8 Since background turbulence also gives rise to turbulent mo- mentum diffusivity (viscosity), momentum convection/ pinch, as well as playing as a source of intrinsic torque, the final flow profile will be determined by a “balance” between external and intrinsic torques and turbulent momentum con- duction and pinch. Momentum pinch itself cannot drive intrinsic rotation; however, it is important for flow profile a) rameswar@ipr.res.in 1070-664X/2014/21(1)/012302/12/$30.00 shaping. Symmetry breaking due to magnetic field curvature and gradient results in momentum pinch in tokamaks. 9–14 Conversion of radial inhomogeneity into hk k i asymme- try requires some symmetry breaking mechanisms. 7,8,15–19 Role of mean E ! B shearing in symmetry breaking, hence in off-diagonal momentum flux, was identified by many authors. 16,18 Recently, G€ urcan et al. showed how the mean E ! B shear can lead to the reflectional symmetry breaking of hk k i in ion temperature gradient (ITG) turbulence, result- ing in finite parallel residual stress. 18 Reflectional symmetry breaking of k jj here means generation of eigenmode averaged wave number hk jj i 6 ¼ 0 by symmetry breaking of eigenmode about a rational surface, i.e., by j/j 2 ðxÞ 6 ¼ j/jð%xÞ. Turbulence intensity gradient can also induce hk k i symmetry breaking, 19 which may be an important driver of the intrinsic torque near the top of a transport barrier where a strong fluc- tuation intensity gradient is present. The process of hk k i sym- metry breaking by E ! B shear and/or intensity gradient implies a coupling between intrinsic rotation and transport barrier dynamics, such as internal transport barrier (ITB) for- mation, which involves steep temperature and/or density gra- dients (i.e., strong radial inhomogeneity) and the strong E ! B shear and/or the fluctuation intensity gradient (i.e., hk k i symmetry breaking). Gyrokinetic simulations have shown the generation of intrinsic rotation from the hk k i sym- metry breaking by mean E ! B shear 20–23 and turbulence in- tensity gradient 20 and up-down asymmetry of equilibrium magnetic topology. 24,25 Gyrokinetic simulations have also shown residual stress from profile shearing effects. 26,27 Apart from the symmetry breaking driven by mean E ! B shear, turbulence can also generate mean poloidal and parallel flows via various other mechanisms. Weiland et al. 28 showed toroidal residual stress generation form by C 2014 AIP Publishing LLC V This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 132.239.66.163 On: Tue, 20 May 2014 19:20:05

We show that strong edge magnetic shear, which is generic to divertor plasmas, makes the E × B shearing of turbulence eddys in toroidal geometry more effective. From calculations of the E × B shearing rates for KSTAR edge parameters, we conclude that the enhanced magnetic shear at the diverted KSTAR plasma edge facilitates E × B shear suppression of turbulence and ensuing H-mode transition. (Some figures may appear in colour only in the online journal)

We present a statistical analysis of heat transport in stationary enhanced confinement regimes obtained from flux-driven gyrofluid simulations. The probability density functions of heat flux in improved confinement regimes, characterized by the Nusselt number, show significant deviation from Gaussian, with a markedly fat tail, implying the existence of heat avalanches. Two types of avalanching transport are found to be relevant to stationary states, depending on the degree of turbulence suppression. In the weakly suppressed regime, heat avalanches occur in the form of quasi-periodic (QP) heat pulses. Collisional relaxation of zonal flow is likely to be the origin of these QP heat pulses. This phenomenon is similar to transient limit cycle oscillations observed prior to edge pedestal formation in recent experiments. On the other hand, a spectral analysis of heat flux in the strongly suppressed regime shows the emergence of a 1/f (f is the frequency) band, suggesting the presence of self-organized criticality (SOC)-like episodic heat avalanches. This episodic 1/f heat avalanches have a long temporal correlation and constitute the dominant transport process in this regime. © 2012 American Institute of Physics.

We study the interplay between intrinsic rotation and internal transport barrier (ITB) dynamics through the dynamic change of the parallel Reynolds stress. Global flux-driven gyrofluid simulations are used for this study. In particular, we investigate the role of parallel velocity gradient instability (PVGI) in the ITB formation and the back transition. It is found that the excitation of PVGI is followed by a change in the Reynolds stress which drives a momentum redistribution. This significantly influences E × B shear evolution and subsequent ITB dynamics. Nonlocal interactions among fluctuations are also observed during the PVGI excitation, resulting in turbulence suppression at the ITB. © 2012 American Institute of Physics.