Title: The effects of dilution on turbulence and transport in C-Mod ohmic plasmas and comparisons with gyrokinetic simulations

Abstract

Main ion dilution has been predicted by gyrokinetic simulations to have a significant effect on ion thermal transport in C-Mod ohmic plasmas. This effect was verified experimentally with a specific set of experiments on C-Mod in which ohmic deuterium plasmas across the linear ohmic confinement (LOC) through the saturated ohmic confinement (SOC) regimes were diluted by seeding with nitrogen gas (Z=7) injection. The seeding was observed to increase the normalized ion temperature gradients (ITGs) by up to 30% without a corresponding increase in the gyrobohm normalized ion energy flux, indicating a change in either the stiffness or the critical ion temperature gradient associated with ITG turbulence. The seeding also reversed the direction of the intrinsic toroidal rotation in plasmas slightly above the normal intrinsic rotation reversal critical density. GYRO simulations of the seeded and unseeded plasmas show that the seeding affected both the critical gradient and the stiffness. For plasmas in the LOC regime, the dilution primarily increased the critical gradient, while for plasmas in the SOC regime the dilution primarily decreased the stiffness. At r/a = 0.8, where the experimental fluxes were above marginal stability, local GYRO predicted and experimental energy fluxes agreed, except for Q_i in the SOCmore » regime where GYRO under-predicted the experimental energy flux. At r/a = 0.6, where the experimental fluxes were close to marginally stable, local GYRO predicted ITG modes to be strongly unstable and are responsible for both Q_i and Q_e (with Q_i > Q_e), as opposed to the experiment where Q_i < Q_e. In contrast, global GYRO in this region predicted the ITG modes to be closer to marginal stability, and accurately predict the experimental Q_i when the T_i profile is modified within experimental uncertainties. Further, the fact that Q_e is always less than Q_i in the r/a = 0.6 simulations with k_θρ_s ≤1 indicates that high-k electron temperature gradient driven (ETG) modes must be included in future simulations and may be responsible for the electron energy transport in this case.« less

Authors:

Ennever, P. ^[1]; Porkolab, M. ^[1]; Candy, J. ^[2]; Staebler, G.  ^[2];

• Search DOE PAGES for author "Staebler, G."
• Search DOE PAGES for ORCID "0000-0002-1944-1733"

Reinke, M. L. ^[3]; Rice, J. E. ^[1]; Rost, J. C.  ^[1];

• Search DOE PAGES for author "Rost, J. C."
• Search DOE PAGES for ORCID "0000-0003-4501-8995"

Ernst, D. ^[1]; Fiore, C. ^[1]; Hughes, J. ^[1]; Terry, J.  ^[1]

• Search DOE PAGES for author "Terry, J."
• Search DOE PAGES for ORCID "0000-0003-4255-5509"

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+ Show Author Affiliations

1. Massachusetts Inst. of Technology (MIT), Cambridge, MA (United States)
2. General Atomics, San Diego, CA (United States)
3. Univ. of York (United Kingdom)

Publication Date:

Mon Jul 13 00:00:00 EDT 2015

Research Org.:

Massachusetts Inst. of Technology (MIT), Cambridge, MA (United States). Plasma Science and Fusion Center

Sponsoring Org.:

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

Contributing Org.:

Alcator C-Mod Team

OSTI Identifier:

1897673

Alternate Identifier(s):

OSTI ID: 1228666

Grant/Contract Number:  

FG02-94ER54235; FC02-99ER54512; AC02-05CH11231; FC02-99-ER54512; FG02-94-ER54235

Resource Type:

Accepted Manuscript

Journal Name:

Physics of Plasmas

Additional Journal Information:

Journal Volume: 22; Journal Issue: 7; Journal ID: ISSN 1070-664X

Publisher:

American Institute of Physics (AIP)

Country of Publication:

United States

Language:

English

Subject:

70 PLASMA PHYSICS AND FUSION TECHNOLOGY; plasma confinement; leptons; energy production, transmission and distribution; electronic transport; isotopes; energy flux; chemical elements; control theory; gyrokinetic simulations