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

Permanent lenke
https://hdl.handle.net/10037/10823
DOI
https://doi.org/10.1088/0741-3335/58/5/054001
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article.pdf (1.331Mb)
(PDF)
Dato
2016-03-30
Type
Peer reviewed
Journal article
Tidsskriftsartikkel

Forfatter
Kube, Ralph; Theodorsen, Audun; Garcia, Odd Erik; LaBombard, Brian; Terry, James L.
Sammendrag
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.
Beskrivelse
Link to publishers version: 10.1088/0741-3335/58/5/054001
Sitering
Kube R, Theodorsen A, Garcia OE, LaBombard B, Terry JL. Fluctuation statistics in the scrape-off layer of Alcator C-Mod. Plasma Physics and Controlled Fusion. 2016;58:054001(5):1-13
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