Abstract. A multidiagnostic approach, utilizing Langmuir probes in the midplane, X-point and divertor walls, along with Lithium beam and infrared measurements is employed to evaluate the evolution of the Scrape-off Layer (SOL) of ASDEX Upgrade across the L-mode density transition leading to the formation of a density shoulder. The flattening of the SOL density profiles is linked to a regime change of filaments, which become faster and larger, and to a similar flattening of the q profile. This transition is related to the beginning of outer divertor detachment and leads to the onset of a velocity shear layer in the SOL. Experimental measurements are in good agreement with several filament models which describe the process as a transition from conduction to convection-dominated SOL perpendicular transport caused by an increase of parallel collisionality. These results could be of great relevance since both ITER and DEMO will feature detached divertors and densities largely over the transition values, and might therefore exhibit convective transport levels different to those observed typically in present-day devices.

Velocity fields and density fluctuations of edge turbulence are studied in I-mode [F. Ryter et al., Plasma Phys. Controlled Fusion 40, 725 (1998)] plasmas of the Alcator C-Mod [I. H. Hutchinson et al., Phys. Plasmas 1, 1511(1994] tokamak, which are characterized by a strong thermal transport barrier in the edge while providing little or no barrier to the transport of both bulk and impurity particles. Although previous work showed no clear geodesic-acoustic modes (GAM) on C-Mod, using a newly implemented, gas-puff-imaging based time-delay-estimate velocity inference algorithm, GAM are now shown to be ubiquitous in all I-mode discharges examined to date, with the time histories of the GAM and the I-mode specific [D. Whyte et al., Nucl. Fusion 50, 105005 (2010)] Weakly Coherent Mode (WCM, f ¼ 100-300 kHz, Df =f % 0:5; and k h % 1:3 cm À1 ) closely following each other through the entire duration of the regime. Thus, the I-mode presents an example of a plasma state in which zero frequency zonal flows and GAM continuously coexist. Using two-field (density-velocity and radial-poloidal velocity) bispectral methods, the GAM are shown to be coupled to the WCM and to be responsible for its broad frequency structure. The effective nonlinear growth rate of the GAM is estimated, and its comparison to the collisional damping rate seems to suggest a new view on I-mode threshold physics. V C 2013 AIP Publishing LLC. [http://dx.

Experiments on HL-2A, DIII-D and EAST show that turbulence just inside the last closed flux surface (LCFS) acts to reinforce existing sheared ExB flows in this region. This flow drive gets stronger as heating power is increased in L-mode, and leads to the development of a strong oscillating shear flow which can transition into the H-mode regime when the rate of energy transfer from the turbulence to the shear flow exceeds a threshold. These effects become compressed in time during an L-H transition, but the key role of turbulent flow drive during the transition is still observed. The results compare favorably with a reduced predator-prey type model.

In a wide variety of natural and laboratory magnetized plasmas, filaments appear as a result of interchange instability. These convective structures substantially enhance transport in the direction perpendicular to the magnetic field. According to filament models, their propagation may follow different regimes depending on the parallel closure of charge conservation. This is of paramount importance in magnetic fusion plasmas, as high collisionality in the scrape-off layer may trigger a regime transition leading to strongly enhanced perpendicular particle fluxes. This work reports for the first time on an experimental verification of this process, linking enhanced transport with a regime transition as predicted by models. Based on these results, a novel scaling for global perpendicular particle transport in reactor relevant tokamaks such as ASDEX-Upgrade and JET is found, leading to important implications for next generation fusion devices.

The kinetic energy transfer between shear flows and the ambient turbulence is investigated in the Experimental Advanced Superconducting Tokamak during the L-H transition. As the rate of energy transfer from the turbulence into the shear flow becomes comparable to the energy input rate into the turbulence, the transition into the H-mode occurs. As the observed behavior exhibits several predicted features of zonal flows, the results show the key role that zonal flows play in mediating the transition into H-mode. V C 2012 American Institute of Physics. [http://dx.doi.org/ 10.1063/1.4737612] INTRODUCTION The transition from low (L-mode) to high confinement (H-mode) regime in magnetized confined fusion devices occurs very rapidly at a critical condition, similar, in a sense, to leaning slightly over the side of a canoe causing only a small tilt of the craft, but leaning slightly more may roll you and the craft into the lake. Rapid threshold transitions between distinctly different stable states then require a triggering event, akin to leaning out too far from the canoe. 1 However, the physics that triggers the transition into Hmode is not understood, and thus predictions of the conditions for the transition into the H-mode regime-which are critical for the operation of ITER in the burning plasma regime-are based on empirical scalings with a wide range of uncertainty. Relative to the conditions found in low confinement regimes, H-mode plasmas are characterized by a reduced turbulence level and strong radial electric field (E r ) shear. 2,3 Azimuthally symmetric, bandlike, time-varying, turbulent generated shear flows called zonal flows (ZFs) also appear to be associated with the L-H transition. 4,5 Therefore, the interaction between micro and macroscale turbulent fluctuations has developed into one of the most active research topics in the physics of magnetized plasmas. The main focus has been placed on the generation of zonal flows and the reduction of the ambient turbulence via the nonlinear exchange, or transfer, of energy from the smaller scaled higher frequency turbulent fluctuations into the large scale, low frequency ordered zonal flow. 6-14 SELF-REGULATION OF TURBULENCETheory predicts that the L-H transition can be explained by an intermediate, quasi-periodic transient stage, where turbulence, zonal flow, mean shear flow, and the pressure gradient are coupled. 15,16 In this model, as the input power increases the pressure gradient also increases, resulting in stronger instabilities and fluctuation levels. The turbulence level grows and begins to nonlinearly drives the zonal flow until the zonal flow drive can overcome the flow damping. A finite zonal flow then begins to grow and extract kinetic energy from the turbulence and thereby acts to suppress the turbulence amplitude. Zonal flows can trigger the transition by regulating the turbulence level and associated transport until the mean shear flow is high enough to suppress the remaining turbulence and associated transport, causing the pressure gradient to i...

the ASDEX Upgrade team, 4 and the EUROfusion MST1 team 5

The energetic interaction between drift-wave turbulence and zonal flows is studied experimentally in two-dimensional wave number space. The kinetic energy is found to be transferred nonlocally from the drift waves to the zonal flow. This confirms the theoretical prediction that the parametric-modulational instability is the driving mechanism of zonal flows. The physical mechanism of this nonlocal energetic interaction between and zonal flows and turbulent drift-wave eddies in relation to the suppression of turbulent transport is discussed.Turbulence is responsible for the major part of particle and energy losses in toroidal fusion plasmas. Since the discovery of a transport barrier in 1982 [1] the reduction of turbulent transport by sheared E Â B plasma flows has been intensively investigated. Of special interest is the spontaneous generation of transport barriers triggered by azimuthally symmetric, bandlike shear flows called zonal flows. In magnetized plasmas, zonal flows have the potential to improve confinement mainly due to two mechanisms: (i) the shear decorrelation mechanism [2,3] can reduce turbulent diffusive step width and (ii) the zonal flow is excited by the turbulence and thus is an energy sink for the fluctuations. Since it is impossible for zonal flows to drive radial E Â B flows and, hence, turbulent transport, for the turbulence this energy is lost [2]. Zonal flows as a universal feature are also found in planetary atmospheres and in the interior of the Sun [2]. Hence, the investigation of the generation of zonal flows is also of general interest in physics.The interaction between turbulence and zonal flows has been studied in many experiments ([4] and references therein). Especially, the nonlinear drive of shear flows by Reynolds stress has been demonstrated in the linear device CSDX [5] and the reversed field pinch RFX [6]. Theory predicts that zonal flows are driven nonlocally in k space by the parametric-modulational instability [2]. For an experimental verification of the modulational instability as the zonal-flow driving mechanism a scale resolved analysis is required. To achieve this, usually, a bicoherence analysis is carried out in frequency space. Thus, a nonlocal coupling between turbulence and zonal flows, including the geodesic acoustic mode (GAM), has been demonstrated, e.g., in Refs. [7][8][9][10][11][12]. The GAM is a finite frequency zonal flow. However, a bicoherence analysis yields information on the degree of phase locking of different modes only and, thus, identifies modes that can couple with each other. Driving or damping of zonal flows and the relative importance of the various interactions can only be estimated from an energy transfer analysis. Energy transfer studies of the turbulencezonal-flow interaction and the turbulent cascades have been carried out [13][14][15][16][17]. These studies were done in frequency space, too, using Taylor hypothesis to transform the fluctuations from frequency to k space. Furthermore, the analyses were done in one dimension only. The phys...

Density fluctuations in I-mode discharges in ASDEX Upgrade are studied. The I-mode specific weakly coherent mode (WCM) appears at the transition from L to I-mode. The WCM but also the turbulence in general are strongly modulated by a low frequency mode which can be related to the geodesic acoustic mode (GAM). The GAM induces an energy transfer away from the central WCM frequency, indicating an underlying instability responsible for the WCM. During the I-mode magnetic fluctuations close to the WCM frequency are intensified, which can be assigned to the geodesic Alfvénic oscillation. The geodesic Alfvénic oscillation is present already in L-mode, does not follow changes of frequency of the WCM, therefore it is not responsible for the WCM.