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

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

doi:10.1088/0741-3335/58/5/054001

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

Journal Article · Wed Mar 30 00:00:00 EDT 2016 · Plasma Physics and Controlled Fusion

DOI:https://doi.org/10.1088/0741-3335/58/5/054001· OSTI ID:1557615

Kube, R. ^[1]; Theodorsen, A. ^[1]; Garcia, O. E. ^[1]; LaBombard, B. ^[2]; Terry, J. L. ^[2]

UiT The Arctic Univ. of Norway, Tromso (Norway). Dept. of Physics and Technology
Massachusetts Inst. of Technology (MIT), Cambridge, MA (United States). Plasma Science and Fusion Center

We study long time series of the ion saturation current and floating 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 $${{\bar{n}}_{\text{e}}}/{{n}_{\text{G}}}=0.15$$ to 0.42, where $$n_ G$$ 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.

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Research Organization:: Massachusetts Inst. of Technology (MIT), Cambridge, MA (United States)

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

Grant/Contract Number:: FC02-99ER54512

OSTI ID:: 1557615

Journal Information:: Plasma Physics and Controlled Fusion, Vol. 58, Issue 5; ISSN 0741-3335

Publisher:: IOP ScienceCopyright Statement

Country of Publication:: United States

Language:: English

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Cited by: 27 works

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