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

Journal Article · Mon Jul 13 00:00:00 EDT 2015 · Physics of Plasmas

DOI:https://doi.org/10.1063/1.4926518· OSTI ID:1897673

Ennever, P. ^[1]; Porkolab, M. ^[1]; Candy, J. ^[2]; Staebler, G.  ^[2]; Reinke, M. L. ^[3]; Rice, J. E. ^[1]; Rost, J. C.  ^[1]; Ernst, D. ^[1]; Fiore, C. ^[1]; Hughes, J. ^[1]; Terry, J.  ^[1]

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

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 SOC 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.

• Research Organization: Massachusetts Inst. of Technology (MIT), Cambridge, MA (United States). Plasma Science and Fusion Center
• Sponsoring Organization: USDOE Office of Science (SC), Fusion Energy Sciences (FES)
• Contributing Organization: Alcator C-Mod Team
• Grant/Contract Number: FG02-94ER54235; FC02-99ER54512; AC02-05CH11231; FC02-99-ER54512; FG02-94-ER54235
• OSTI ID: 1897673
• Alternate ID(s): OSTI ID: 1228666
• Journal Information: Physics of Plasmas, Vol. 22, Issue 7; ISSN 1070-664X
• Publisher: American Institute of Physics (AIP)
• Country of Publication: United States
• Language: English

Related Subjects

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