Abstract Details
Abstracts
Author: Audun Theodorsen
Requested Type: Poster
Submitted: 2017-02-28 13:24:34
Co-authors: O. E. Garcia
Contact Info:
UIT - The Arctic University of Norway
224 Chestnut St Unit 2
Cambridge, Massachuse 02139
United States
Abstract Text:
Scrape-off layer (SOL) turbulence in tokamaks is characterized by intermittent transport events carrying particles and heat radially outwards. Single point measurements in the far SOL suggest that such events can reach magnitudes far above the mean density and temperature, leading to enhanced erosion and sputtering of material walls. It is therefore of interest to describe how often SOL measurement signals cross a given threshold value, and how long, on average, the signal remains above the threshold.
Single point measurements in the far SOL of numerous tokamak experiments, including the Alcator C-Mod tokamaks[1, 2] have revealed fluctuations that are far from normally distributed, with positive skewness and flatness moments. Large amplitude fluctuations have been shown to have exponentially distributed amplitudes and waiting time between events, as well as a double exponential waveform. A stochastic model, based on exponentially decaying pulses with exponentially distributed amplitudes arriving according to a Poisson process, has been proposed[3] and has been shown to be in agreement with SOL fluctuation statistics[1, 2]. From this stochastic model, predictions for the rate of threshold crossings and average time above threshold, can be derived[3, 4].
In this contribution, we present predictions for the rate of threshold crossings and the average time above threshold for the stochastic model. In addition, simulations of the probability density function of the time above threshold are presented. These simulations are shown to be in agreement with analytic results in the case of both strong and weak intermittency.
References
[1] R.Kube, A.Theodorsen, O.E.Garcia, B.LaBombard and J.L.Terry, PPCF 58, 054001 (2016).
[2] A. Theodorsen and O. E. Garcia, submitt. to Nucl. Fusion arXiv:1702.01104 [physics.plasm-ph]
[3] O.E.Garcia, A.Theodorsen, R.Kube and H.L.PeĢcseli,
POP 23, 052308(2016).
[4] A.Theodorsen and O.E.Garcia, POP 23, 040702(2016).
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