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.
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 ECH-driven counter torque are discussed in some detail. Symmetry breaking by LSN vs. USN asymmetry is related to the origin of intrinsic torque at the separatrix.
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...
In the perspective of operating tungsten monoblocks in WEST, the ongoing major upgrade of the Tore Supra tokamak, a dedicated modelling effort has been carried out to simulate the interaction between the edge plasma and the tungsten wall. A new transport code, SolEdge2D-EIRENE, has been developed with the ability to simulate the plasma up to the first wall. This is especially important for steady state operation, where thermal loads on all the plasma facing components, even remote from the plasma, are of interest. Moreover, main chamber tungsten sources are thought to dominate the contamination of the plasma core. We present here in particular new developments aimed at improving the description of the interface between the plasma and the wall, namely a way to treat sheath physics in a more faithful way using the output of 1D particle in cell simulations. Moreover, different models for prompt redeposition have been implemented and are compared. The latter is shown to play an important role in the balance between divertor and main chamber sources.
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