Ohmic energy confinement saturation and core toroidal rotation reversal in Alcator C-Mod plasmas

Rice, J. E.; Greenwald, M.; Podpaly, Y.; Reinke, M.L.; Diamond, P. H.; Hughes, J.W.; Howard, Nathan; Ma, Y.; Cziegler, I.; Duval, B.P.; Ennever, P.; Ernst, D. R.; Fiore, C. L.; Gao, C.; Irby, J.; Marmar, E.; Porkoláb, M.; Tsujii, N.; Wolfe, S.

doi:10.1063/1.3695213

Cited by 58 publications

(130 citation statements)

References 40 publications

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“…Indeed, spectra analysis has evidenced that TEM are linked with quasi-coherent (QC) modes observed by reflectometry in the core/edge region [6][7][8]. Besides the link established between QC modes and TEM, spectra modifications have also been reported during transitions toward expected TEM-dominated regimes in DIII-D [9][10][11][12][13], and Alcator C-mod [13][14][15][16][17] tokamaks, and in the MST reversed field pinch [18].…”

Section: Introductionmentioning

confidence: 99%

Discriminating the trapped electron modes contribution in density fluctuation spectra

et al. 2015

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Quasi-coherent (QC) modes have been reported for more than 10 years in reflectometry fluctuations spectra in the core region of fusion plasmas. They have characteristics in-between coherent and broadband fluctuations as they oscillate at a marked frequency but have a wide spectrum. This work presents further evidences of the link recently established between QC modes and the trapped electron modes (TEM) instabilities (Arnichand et al 2014 Nucl. Fusion 54 123017). In electron cyclotron resonance heated discharges of Tore Supra, an enhancement of QC modes amplitude is observed in a region where TEM cause impurity transport and turbulence. In JET Ohmic plasmas, QC modes disappear during density ramp-up and current ramp-down. This is reminiscent of Tore Supra and TEXTOR observations during transitions from the linear Ohmic confinement (LOC) to the saturated Ohmic confinement (SOC) regimes. Evidencing TEM activity then becomes experimentally possible via analysis of fluctuation spectra.

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Section: Introductionmentioning

confidence: 99%

Discriminating the trapped electron modes contribution in density fluctuation spectra

et al. 2015

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“…An important feature is the change of sign in the volume averaged toroidal angular momentum, found also in experimental observations [22,23,24], when the toroidal magnetic field, hence the safety factor, is varied (Fig. 31).…”

Section: Influence Of the Safety Factor On The Dynamicsmentioning

confidence: 74%

“…We recall that for all the simulations presented in this manuscript the magnetic Prandtl number is equal to one, P r = 1. In this case the viscous Lundquist number varies because the reference magnetic field used for its calculation is the imposed toroidal component and to modify the safety factor the magnitude of this field is changed, as also done in experiments [22,23]. The parameter q takes four different values.…”

Section: Influence Of the Safety Factor On The Dynamicsmentioning

confidence: 99%

Magnetohydrodynamically generated velocities in confined plasma

et al. 2015

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Abstract. It is shown numerically that both toroidal and poloidal rotational flows are intrinsic to a rigid toroid confining a conducting magnetofluid in which a current is driven by the application of externally-supported electric and magnetic fields. The computation involves no microscopic instabilities or kinetic theory and is purely magnetohydrodynamic (MHD) in nature. The properties and intensity of the rotations are regulated by dimensionless numbers (Lundquist, viscous Lundquist, and Hartmann) that contain the resistivity and viscosity of the magnetofluid. The computation makes use of the recently developed "penalization method" to enforce visco-resistive boundary conditions. The point is not that other more exotic kinetic effects may not also contribute to rotational effects in toroidal laboratory devices, but rather that at the most simple magnetohydrodynamic level (uniform mass density and incompressible magnetofluids), they are not necessary. Rotational flows are an irreducible effect inherent in toroidal, driven MHD by itself, without necessary kinetic complications.

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“…Relative T e fluctuations are reduced after rotation reversals with increases of density in L-mode plasmas. This indicates that the physics related to the reduction of T e fluctuations across ohmic plasmas may be similar to the RF heated plasmas and the change of T e fluctuations might be related to changes in the momentum transport corresponding to rotation reversal, as suggested in [20].…”

Section: T E Fluctuation Measurements Across Rotation Reversalsmentioning

confidence: 92%

“…Thus, it is possible to interpret the reduction of measured temperature fluctuations as being due to a reduction of TEM turbulence due to the increase of collisionality, which would support the hypothesis about the LOC/SOC transition. However, these measurements do not necessarily imply a change of dominant turbulent mode across the LOC/SOC transition, as suggested by past authors [1,10,20]. The change of turbulence across LOC/SOC transition should be studied through gyrokinetic analysis.…”

Section: T E Fluctuation Measurements Across Rotation Reversalsmentioning

confidence: 99%

Correlation ECE diagnostic in Alcator C-Mod

Sung

White

Howard

et al. 2015

EPJ Web of Conferences

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Abstract. Correlation ECE (CECE) is a diagnostic technique that allows measurement of small amplitude electron temperature, T e , fluctuations through standard cross-correlation analysis methods. In Alcator C-Mod, a new CECE diagnostic has been installed [Sung RSI 2012], and interesting phenomena have been observed in various plasma conditions. We find that local T e fluctuations near the edge (ρ ∼ 0.8) decrease across the linearto-saturated ohmic confinement transition, with fluctuations decreasing with increasing plasma density [Sung NF 2013], which occurs simultaneously with rotation reversals [Rice NF 2011]. T e fluctuations are also reduced across core rotation reversals with an increase of plasma density in RF heated L-mode plasmas, which implies that the same physics related to the reduction of T e fluctuations may be applied to both ohmic and RF heated L-mode plasmas. In I-mode plasmas, we observe the reduction of core T e fluctuations, which indicates changes of turbulence occur not only in the pedestal region but also in the core across the L/I transition [White NF 2014]. The present CECE diagnostic system in C-Mod and these experimental results are described in this paper.

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Ohmic energy confinement saturation and core toroidal rotation reversal in Alcator C-Mod plasmas

Cited by 58 publications

References 40 publications

Discriminating the trapped electron modes contribution in density fluctuation spectra

Discriminating the trapped electron modes contribution in density fluctuation spectra

Magnetohydrodynamically generated velocities in confined plasma

Correlation ECE diagnostic in Alcator C-Mod

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