Fluctuation statistics in the scrape-off layer of Alcator C-Mod

Kube, R.; Theodorsen, A.; Garcia, O. E.; LaBombard, B.; Terry, J. L.

doi:10.7910/DVN/LHWGJE

Title: Fluctuation statistics in the scrape-off layer of Alcator C-Mod

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Abstract

We study long time series of the ion saturation current and oating potential, sampled by Langmuir probes dwelled in the outboard mid-plane scrape-off layer and embedded in the lower divertor baffle of Alcator C-Mod. A series of ohmically heated L-mode plasma discharges is investigated with line-averaged plasma density ranging from ne/nG = 0.15 to 0.42, where nG is the Greenwald density. All ion saturation current time series that are sampled in the far scrape-off layer are characterized by large-amplitude burst events. Coefficients of skewness and excess kurtosis of the time series obey a quadratic relationship and their histograms coincide partially upon proper normalization. Histograms of the ion saturation current time series are found to agree well with a prediction of a stochastic model for the particle density fluctuations in scrape-off layer plasmas. The distribution of the waiting times between successive large-amplitude burst events and of the burst amplitudes are approximately described by exponential distributions. The average waiting time and burst amplitude are found to vary weakly with the line-averaged plasma density. Conditional averaging reveals that the radial blob velocity, estimated from floating potential measurements, increases with the normalized burst amplitude in the outboard mid- plane scrape-off layer. For low densitymore »« less

Authors:

Kube, R.; Theodorsen, A.; Garcia, O. E.; LaBombard, B.; Terry, J. L.

OSTI

Publication Date:: 2018-10-16

DOE Contract Number:: FC02-99ER54512

Research Org.:: Massachusetts Inst. of Technology (MIT), Cambridge, MA (United States). Plasma Science and Fusion Center

Sponsoring Org.:: USDOE Office of Science (SC), Fusion Energy Sciences (FES)

Subject:: 70 PLASMA PHYSICS AND FUSION TECHNOLOGY

OSTI Identifier:: 1879096

DOI:: https://doi.org/10.7910/DVN/LHWGJE

Citation Formats


                    Kube, R., Theodorsen, A., Garcia, O. E., LaBombard, B., and Terry, J. L. Fluctuation statistics in the scrape-off layer of Alcator C-Mod.  United States: N. p., 2018. 
        Web.  doi:10.7910/DVN/LHWGJE.

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                    Kube, R., Theodorsen, A., Garcia, O. E., LaBombard, B., & Terry, J. L. Fluctuation statistics in the scrape-off layer of Alcator C-Mod.  United States.  doi:https://doi.org/10.7910/DVN/LHWGJE

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                    Kube, R., Theodorsen, A., Garcia, O. E., LaBombard, B., and Terry, J. L. 2018.  
"Fluctuation statistics in the scrape-off layer of Alcator C-Mod".  United States.  doi:https://doi.org/10.7910/DVN/LHWGJE.  https://www.osti.gov/servlets/purl/1879096. Pub date:Tue Oct 16 04:00:00 UTC 2018

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@article{osti_1879096,

  title        = {Fluctuation statistics in the scrape-off layer of Alcator C-Mod},

  author       = {Kube, R. and Theodorsen, A. and Garcia, O. E. and LaBombard, B. and Terry, J. L.},

  abstractNote = {We study long time series of the ion saturation current and oating potential, sampled by Langmuir probes dwelled in the outboard mid-plane scrape-off layer and embedded in the lower divertor baffle of Alcator C-Mod. A series of ohmically heated L-mode plasma discharges is investigated with line-averaged plasma density ranging from ne/nG = 0.15 to 0.42, where nG is the Greenwald density. All ion saturation current time series that are sampled in the far scrape-off layer are characterized by large-amplitude burst events. Coefficients of skewness and excess kurtosis of the time series obey a quadratic relationship and their histograms coincide partially upon proper normalization. Histograms of the ion saturation current time series are found to agree well with a prediction of a stochastic model for the particle density fluctuations in scrape-off layer plasmas. The distribution of the waiting times between successive large-amplitude burst events and of the burst amplitudes are approximately described by exponential distributions. The average waiting time and burst amplitude are found to vary weakly with the line-averaged plasma density. Conditional averaging reveals that the radial blob velocity, estimated from floating potential measurements, increases with the normalized burst amplitude in the outboard mid- plane scrape-off layer. For low density discharges, the conditionally averaged waveform of the floating potential associated with large amplitude bursts at the divertor probes has a dipolar shape. In detached divertor conditions the average waveform is random, indicating electrical disconnection of blobs from the sheaths at the divertor targets.},

  doi          = {10.7910/DVN/LHWGJE},

  journal      = {},

  number       = ,

  volume       = ,

  place        = {United States},

  year         = {2018},

  month        = {10}

}

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DOI: https://doi.org/10.7910/DVN/LHWGJE

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