Outer midplane scrape-off layer profiles and turbulence in simulations of Alcator C-Mod inner-wall limited discharges

Halpern, Federico D.; LaBombard, Brian; Terry, James L.; Zweben, Stewart J.

doi:10.1063/1.4989705

Outer midplane scrape-off layer profiles and turbulence in simulations of Alcator C-Mod inner-wall limited discharges

Journal Article · Tue Jun 27 00:00:00 EDT 2017 · Physics of Plasmas

DOI:https://doi.org/10.1063/1.4989705· OSTI ID:1378095

Halpern, Federico D. ^[1]; LaBombard, Brian ^[2]; ^[2]; ^[3]

General Atomics, San Diego, CA (United States); Ecole Polytechnique Federale Lausanne (Switzlerland). Swiss Plasma Center; General Atomics
Massachusetts Inst. of Technology (MIT), Cambridge, MA (United States)
Pacific Northwest National Lab. (PNNL), Richland, WA (United States)

A region of steep plasma gradients, the so-called ”narrow-feature”, has been found in the near scrape-off layer (SOL) of inner-wall limited (IWL) discharges. Dedicated IWL discharges were carried out in Alcator C-Mod [E.S. Marmar et al., Nucl. Fusion 55, (2015)] to study this phenomenon, allowing detailed observations of the plasma profiles and fluctuations. Langmuir probe (LP) measurements show a clear two decay length n_e and T_e profile structure at the outer midplane. The Gas-Puff Imaging (GPI) diagnostic shows large turbulent fluctuations across the last closed flux-surface, hence supporting the hypothesis that turbulent phenomena play a role in setting the profile steepness. We have carried out the flux-driven non-linear turbulence simulations of two C-Mod discharges which allows a three-way comparison between LP, GPI, and simulation data. Observations and simulations correlate the steep gradient region characterizing the narrow feature with sheared poloidal flows and a deviation of the plasma potential from its floating value. Furthermore, the E x B shear rate exceeds the linear ballooning growth rate, indicating that the narrow feature could result from the effects of sheared flows, although causality could not be established. The fluctuation level in the narrow feature remains of order unity across the entire SOL, indicating that the transport reduction in the near-SOL cannot result from a simple quench rule.

View Accepted Manuscript (DOE)

Research Organization:: General Atomics, San Diego, CA (United States)

Sponsoring Organization:: USDOE; USDOE Office of Science (SC), Fusion Energy Sciences (FES) (SC-24)

Grant/Contract Number:: AC02-09CH11466; FG02-95ER54309

OSTI ID:: 1378095

Alternate ID(s):: OSTI ID: 1366571

Journal Information:: Physics of Plasmas, Journal Name: Physics of Plasmas Journal Issue: 7 Vol. 24; ISSN 1070-664X

Publisher:: American Institute of Physics (AIP)Copyright Statement

Country of Publication:: United States

Language:: English

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