Testing image-velocimetry methods for turbulence diagnostics.

Two image-based velocity-inference techniques, cross-correlation time-delay estimation (CCTDE) and dynamic time warping (DTW), were tested. These techniques are conventionally used in the study of plasma dynamics, but they can be applied to any data where features propagate across the image field-of-view. Differences between the techniques were investigated, which showed that the shortcomings of each technique are complemented well by the strengths of the other. Thus, the techniques should be used in conjunction with each other for optimal velocimetry. For ease of use, an example workflow that applies the results in this paper to experimental measurements is provided for both techniques. The findings were based on a thorough analysis of the uncertainties for both techniques. Specifically, the accuracy and precision associated with inferred velocity fields were systematically tested using synthetic data. Novel findings are presented that strongly improve the performance of both techniques, some of which are as follows: CCTDE was able to operate accurately under most conditions with an inference frequency as short as 1 per 32 frames, as opposed to the typical 1 per ≥256 frames used in the literature; an underlying pattern in CCTDE accuracy depending on the magnitude of the underlying velocity was found; spurious velocities due to the barber pole illusion can now be predicted prior to CCTDE velocimetry through a simple analysis; DTW was more robust against the barber pole illusion than CCTDE; DTW performance with sheared flows was tested; DTW was able to reliably infer accurate flow fields from data with as low as 8 × 8 spatial channels; and however, if the flow direction was unknown prior to DTW analysis, DTW could not reliably infer any velocities.

Fast Low-to-High Confinement Mode Bifurcation Dynamics in a Tokamak Edge Plasma Gyrokinetic Simulation.

Transport barrier formation and its relation to sheared flows in fluids and plasmas are of fundamental interest in various natural and laboratory observations and of critical importance in achieving an economical energy production in a magnetic fusion device. Here we report the first observation of an edge transport barrier formation event in an electrostatic gyrokinetic simulation carried out in a realistic diverted tokamak edge geometry under strong forcing by a high rate of heat deposition. The results show that turbulent Reynolds-stress-driven sheared E×B flows act in concert with neoclassical orbit loss to quench turbulent transport and form a transport barrier just inside the last closed magnetic flux surface.

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.

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.