Characterization of density fluctuations during the search for an I-mode regime on the DIII-D tokamak

Marinoni, A.; Rost, J. C.; Porkolab, M.; Hubbard, A. E.; Osborne, T. H.; White, A. E.; Whyte, D. G.; Rhodes, T. L.; Davis, E. M.; Ernst, D. R.; Burrell, K. H.

doi:10.1088/0029-5515/55/9/093019

Title: Characterization of density fluctuations during the search for an I-mode regime on the DIII-D tokamak

Journal Article · Mon Aug 17 00:00:00 EDT 2015 · Nuclear Fusion

DOI:https://doi.org/10.1088/0029-5515/55/9/093019· OSTI ID:1557846

Marinoni, A. ^[1]; Rost, J. C. ^[1]; Porkolab, M. ^[1]; Hubbard, A. E. ^[1]; Osborne, T. H. ^[2]; White, A. E. ^[1]; Whyte, D. G. ^[1]; Rhodes, T. L. ^[3]; Davis, E. M. ^[1]; Ernst, D. R. ^[1]; Burrell, K. H. ^[2]

Massachusetts Inst. of Technology (MIT), Cambridge, MA (United States)
General Atomics, San Diego, CA (United States)
Univ. of California, Los Angeles, CA (United States)

The I-mode regime, routinely observed on the Alcator C-Mod tokamak, is characterized by an edge energy transport barrier without an accompanying particle barrier and with broadband instabilities, known as weakly coherent modes (WCM), believed to regulate particle transport at the edge. Recent experiments on the DIII-D tokamak exhibit I-mode characteristics in various physical quantities. These DIII-D plasmas evolve over long periods, lasting several energy confinement times, during which the edge electron temperature slowly evolves towards an H-mode-like profile, while maintaining a typical L-mode edge density profile. During these periods, referred to as I-mode phases, the radial electric field at the edge also gradually reaches values typically observed in H-mode. Density fluctuations measured with the phase contrast imaging diagnostic during I-mode phases exhibit three features typically observed in H-mode on DIII-D, although they develop progressively with time and without a sharp transition: the intensity of the fluctuations is reduced; the frequency spectrum is broadened and becomes non-monotonic; two dimensional space-time spectra appear to approach those in H-mode, showing phase velocities of density fluctuations at the edge increasing to about 10 km s^-1. However, in DIII-D there is no clear evidence of the WCM. Preliminary linear gyro-kinetic simulations are performed in the pedestal region with the GS2 code and its recently upgraded model collision operator that conserves particles, energy and momentum. The increased bootstrap current and flow shear generated by the temperature pedestal are shown to decrease growth rates, thus possibly generating a feedback mechanism that progressively stabilizes fluctuations.

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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); USDOE Advanced Research Projects Agency - Energy (ARPA-E)

Grant/Contract Number:: FG02-94ER54235; FC02-0454698; FG02-08ER54984

OSTI ID:: 1557846

Alternate ID(s):: OSTI ID: 1238853

Journal Information:: Nuclear Fusion, Vol. 55, Issue 9; ISSN 0029-5515

Publisher:: IOP ScienceCopyright Statement

Country of Publication:: United States

Language:: English

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Cited by: 33 works

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Gyrokinetic investigation of the ASDEX Upgrade I-mode pedestal Stimmel, K.; Bañón Navarro, A.; Happel, T. Physics of Plasmas, Vol. 26, Issue 12 https://doi.org/10.1063/1.5124986	journal	December 2019
Access and sustainment of naturally ELM-free and small-ELM regimes Viezzer, E. Nuclear Fusion, Vol. 58, Issue 11 https://doi.org/10.1088/1741-4326/aac222	journal	September 2018
Radiative heat exhaust in Alcator C-Mod I-mode plasmas Reinke, M. L.; Brunner, D.; Golfinopoulos, T. Nuclear Fusion, Vol. 59, Issue 4 https://doi.org/10.1088/1741-4326/ab04cf	journal	March 2019
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