Burst statistics in Alcator C-Mod SOL turbulence

Bursty fluctuations in the scrape-off layer (SOL) of Alcator C-Mod have been analyzed using gas puff imaging data. This reveals many of the same fluctuation properties as Langmuir probe measurements, including normal distributed fluctuations in the near SOL region while the far SOL plasma is dominated by large amplitude bursts due to radial motion of blob-like structures. Conditional averaging reveals burst wave forms with a fast rise and slow decay and exponentially distributed waiting times. Based on this, a stochastic model of burst dynamics is constructed. The model predicts that fluctuation amplitudes should follow a Gamma distribution. This is shown to be a good description of the gas puff imaging data, validating this aspect of the model.Comment: 8 pages, 6 figure

Turbulence and transport phenomena in edge and scrape-off-layer plasmas

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 2011.Cataloged from PDF version of thesis.Includes bibliographical references (p. 171-178).This thesis reports recent experimental studies of edge turbulence structure using gas-puff-imaging at high time resolution in a radially broad zone at the low-field-side of Alcator C-Mod[1], and highlights the connections between its characteristics and particle transport. Within the framework of this thesis a new detector system has been designed and built for viewing the steep gradient outboard midplane region at minor radii 0.9 < r/rma, < 1.1, where turbulence is postulated to be driven [2, 3]. The design is optimized for the study of edge and scrape-off-layer turbulence, based on previous studies of these regions. Analysis of the data from this diagnostic in Ohmic L-modes from the electron diamagnetic drift propagating turbulence in the closed field line region of the plasma edge shows a strong scaling of the fluctuation power 5/n with the density normalized to the tokamak density limit ne/ng; in addition, a critical physical scale is found at ... ~ 0.1, where the critical wavenumber is normalized to ps, the ion Larmor radius at the sound speed, also known as the drift scale. The characteristic size scale shows very little variation with operating parameters. The analysis of the physical scale includes spectral analysis, yielding robust spectral indices beta= ~4.5 for larger wavenumbers and a sensitive spectral shape for lower wavenumbers, and the first spectral transfer measurement of turbulence on C-Mod, showing that the critical scale feeds power into the turbulence. A connection is found to the quasi-coherent mode (QCM), a modelike edge fluctuation characteristic of the Enhanced D-Alpha H-mode, which also appears at ... ~ 0.1 and has decreased inverse transfer but greatly enhanced forward transfer compared to the L-mode levels.by István Cziegler.Ph.D

Turbulence Nonlinearities Shed Light on Geometric Asymmetry in Tokamak Confinement Transitions

A comprehensive study of fully frequency-resolved nonlinear kinetic energy transfer has been performed for the first time in a diverted tokamak, providing new insight into the parametric dependences of edge turbulence transitions. Measurements using gas puff imaging in the turbulent L-mode state illuminate the source of the long known but as yet unexplained "favorable-unfavorable" geometric asymmetry of the power threshold for transition to the turbulence-suppressed H mode. Results from the recently discovered I mode point to a competition between zonal flow (ZF) and geodesic-acoustic modes (GAM) for turbulent energy, while showing new evidence that the I-to-H transition is still dominated by ZFs. The availability of nonlinear drive for the GAM against net heat flux through the edge corresponds very well to empirical scalings found experimentally for accessing the I mode

Comparison of velocimetry techniques for turbulent structures in gas-puff imaging data

Recent analysis of Gas Puff Imaging (GPI) data from Alcator C-Mod found blob velocities with a modified tracking time delay estimation (TDE). These results disagree with velocity analysis performed using direct Fourier methods. In this paper, the two analysis methods are compared. The implementations of these methods are explained, and direct comparisons using the same GPI data sets are presented to highlight the discrepancies in measured velocities. In order to understand the discrepancies, we present a code that generates synthetic sequences of images that mimic features of the experimental GPI images, with user-specified input values for structure (blob) size and velocity. This allows quantitative comparison of the TDE and Fourier analysis methods, which reveals their strengths and weaknesses. We found that the methods agree for structures of any size as long as all structures move at the same velocity and disagree when there is significant nonlinear dispersion or when structures appear to move in opposite directions. Direct Fourier methods used to extract poloidal velocities give incorrect results when there is a significant radial velocity component and are subject to the barber pole effect. Tracking TDE techniques give incorrect velocity measurements when there are features moving at significantly different speeds or in different directions within the same field of view. Finally, we discuss the limitations and appropriate use of each of methods and applications to the relationship between blob size and velocity.National Science Foundation (U.S.) (1122374

Blob distortion by radio-frequency induced sheared flow

Blob transport properties in the plasma edge in the presence and absence of radio-frequency (RF) convective cells are compared. For the first time, the interactions between RF convective cells and intermittent plasma blobs in the scrape-off layer (SOL) are observed with gas puff imaging in the ASDEX Upgrade tokamak. It is found that the RF convective cells induce a sheared flow in the far SOL, which is able to stretch, distort and even split the blobs poloidally. The observed phenomena indicate that an externally generated sheared flow in the SOL can be considered as a method to modify blob transport in a favorable way

Scrape-off Layer Transport and Filament Characteristics in High-density Tokamak Regimes

A detailed cross-device investigation on the role of filamentary dynamics in high-density regimes has been performed within the EUROfusion framework, comparing the ASDEX Upgrade (AUG) and TCV tokamaks. Both devices run density ramp experiments at different levels of plasma current, keeping the toroidal field or q 95 constant in order to disentangle the role of the parallel connection length and the current. During the scan at a constant toroidal field, in both devices the scrape-off layer (SOL) profiles tend to develop a clear SOL density shoulder at a lower edge density whenever the current is reduced. Different current behaviour is substantially reconciled in terms of the edge density normalized to the Greenwald fraction. During the scan at constant q 95 AUG exhibits similar behaviour, whereas in TCV no upstream profile modification signature has been observed at lower current levels. The latter behaviour has been ascribed to the lack of target density rollover. The relation between the upstream density profile modification and detachment condition has been investigated. For both devices the relation between blob size and the SOL density e-folding length is found independent of the plasma current, with the observation of a clear increase in blob size and the edge density normalized to a Greenwald fraction. ASDEX Upgrade has also explored filamentary behaviour in the H-mode. The experiments in AUG have focused on the role of neutrals, performing discharges with and without cryogenic pumps, highlighting how high neutral pressure, not only in the divertor but also at the midplane, is needed in order to develop an H-mode SOL profile shoulder in AUG.Euratom research and training programme 2014-2018 633053Euratom research and training programme 2019-2020 633053Swiss National Science Foundation AT-0063United States Department of Energy DE-SC001052

Rotation Reversal Bifurcation and Energy Confinement Saturation in Tokamak Ohmic L-mode Plasmas

Direction reversals of intrinsic toroidal rotation have been observed in diverted Alcator C-Mod Ohmic L-mode plasmas following electron density ramps. For low density discharges, the core rotation is directed cocurrent, and reverses to countercurrent following an increase in the density above a certain threshold. Such reversals occur together with a decrease in density fluctuations with 2 cm(-1)≤k(θ)≤11 cm(-1) and frequencies above 70 kHz. There is a strong correlation between the reversal density and the density at which the Ohmic L-mode energy confinement changes from the linear to the saturated regime