Fluctuation statistics in the scrape-off layer of Alcator C-Mod
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-av...
| Main Authors: | , , , , |
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| Format: | Article |
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IOP Publishing
2018
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| Online Access: | http://hdl.handle.net/1721.1/113072 https://orcid.org/0000-0002-7841-9261 |
| _version_ | 1826204193239597056 |
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| author | Kube, R Theodorsen, A Garcia, O E Labombard, Brian Terry, James L |
| author2 | Massachusetts Institute of Technology. Plasma Science and Fusion Center |
| author_facet | Massachusetts Institute of Technology. Plasma Science and Fusion Center Kube, R Theodorsen, A Garcia, O E Labombard, Brian Terry, James L |
| author_sort | Kube, R |
| collection | MIT |
| description | 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 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. |
| first_indexed | 2024-09-23T12:50:22Z |
| format | Article |
| id | mit-1721.1/113072 |
| institution | Massachusetts Institute of Technology |
| last_indexed | 2024-09-23T12:50:22Z |
| publishDate | 2018 |
| publisher | IOP Publishing |
| record_format | dspace |
| spelling | mit-1721.1/1130722022-10-01T11:27:02Z Fluctuation statistics in the scrape-off layer of Alcator C-Mod Kube, R Theodorsen, A Garcia, O E Labombard, Brian Terry, James L Massachusetts Institute of Technology. Plasma Science and Fusion Center Labombard, Brian Terry, James L 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 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. United States. Department of Energy (Agreement DE-FC02-99ER54512) 2018-01-11T21:01:07Z 2018-01-11T21:01:07Z 2016-03 2016-02 2018-01-11T15:57:01Z Article http://purl.org/eprint/type/JournalArticle 0741-3335 1361-6587 http://hdl.handle.net/1721.1/113072 Kube, R et al. “Fluctuation Statistics in the Scrape-Off Layer of Alcator C-Mod.” Plasma Physics and Controlled Fusion 58, 5 (March 2016): 054001 © 2016 IOP Publishing Ltd https://orcid.org/0000-0002-7841-9261 http://dx.doi.org/10.1088/0741-3335/58/5/054001 Plasma Physics and Controlled Fusion Article is made available in accordance with the publisher's policy and may be subject to US copyright law. Please refer to the publisher's site for terms of use. application/pdf IOP Publishing MIT Plasma Science & Fusion Center |
| spellingShingle | Kube, R Theodorsen, A Garcia, O E Labombard, Brian Terry, James L Fluctuation statistics in the scrape-off layer of Alcator C-Mod |
| title | Fluctuation statistics in the scrape-off layer of Alcator C-Mod |
| title_full | Fluctuation statistics in the scrape-off layer of Alcator C-Mod |
| title_fullStr | Fluctuation statistics in the scrape-off layer of Alcator C-Mod |
| title_full_unstemmed | Fluctuation statistics in the scrape-off layer of Alcator C-Mod |
| title_short | Fluctuation statistics in the scrape-off layer of Alcator C-Mod |
| title_sort | fluctuation statistics in the scrape off layer of alcator c mod |
| url | http://hdl.handle.net/1721.1/113072 https://orcid.org/0000-0002-7841-9261 |
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