Pedestal profiles and fluctuations in C-Mod enhanced D-alpha H-modes

Hubbard, A.; Boivin, R. L.; Granetz, R.; Greenwald, M.; Hughes, J. W.; Hutchinson, I. H.; Irby, J.; LaBombard, B.; Lin, Y.; Marmar, E.; Mazurenko, A.; Mossessian, D.; Nelson‐Melby, E.; Porkoláb, M.; Snipes, J. A.; Terry, J. L.; Wolfe, S.; Wukitch, S.J.; Carreras, B. A.; Klein, V.; Pedersen, T. S.

doi:10.1063/1.1348329

Cited by 88 publications

(98 citation statements)

References 33 publications

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“…the width and height, and pedestal dynamics are quantified by least squares mtanh fits to ELM synchronised HRTS profiles. Fitting an mtanh function to radial kinetic profiles is a common technique used on many machines such as JET [19,20], AUG [23,26], DIII-D [22,27], Alcator C-Mod [45,46], MAST [24,28,29,47], NSTX [25] and JT-60 [48]. The mtanh fits to the JET HRTS profiles presented in this section are used in Section 5 when evaluating the Peeling Ballooning stability and comparing experimental results to EPED1 predictions for the pedestal pressure.…”

Section: Pedestal Measurementsmentioning

confidence: 99%

Pedestal study across a deuterium fuelling scan for highδELMy H-mode plasmas on JET with the carbon wall

Leyland¹,

Beurskens²,

Frassinetti³

et al. 2013

Nucl. Fusion

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We present the results from a new fuelling scan database consisting of 14 high triangularity ( ~ 0.41), Type I ELMy H-mode JET plasmas. As the fuelling level is increased from low, ( D ~ 0.2x10 22 el/s, n e,ped /n gw =0.7), to high dosing ( D ~ 2.6x10 22 el/s, n e,ped /n gw =1.0) the variation in ELM behaviour is consistent with a transition from 'pure Type I' to 'mixed Type I/II' ELMs [1]. However, the pulses in Scattering (HRTS) system. We continue by presenting, for the first time, the role of pedestal structure, as quantified by a least squares mtanh fit to the HRTS profiles, on the performance across the fuelling scan. A key result is that the pedestal width narrows and peak pressure gradient increases during the ELM cycle for low fuelling plasmas, whereas at high fuelling the pedestal width and peak pressure gradient saturates towards the latter half of the ELM cycle. An ideal MHD stability analysis shows that both low and high fuelling plasmas move from stable to unstable approaching the ideal ballooning limit of the finite peeling ballooning stability boundary. Comparison to EPED predictions show on average good agreement with experimental measurements for both pedestal height and width however when presented as a function of pedestal density, experiment and model show opposing trends. The measured pre-ELM pressure pedestal height increases by ~ 20% whereas EPED predicts a decrease of 25% from low to high fuelling. Similarly the measured pressure pedestal width widens by ~ 55%, in poloidal flux space, whereas EPED predicts a decrease of 20% from low to high fuelling. We give two possible explanations for the disagreement. First, it may be that EPED under-predicts the critical density, which marks the transition from kink-peeling to ballooning limited plasmas. Second, the stronger broadening of the experimental pedestal width than predicted by EPED is an indication that other transport related processes contribute to defining the pedestal width such as enhanced inter-ELM transport as observed at high fuelling, for mixed Type I/II ELMy pulses.M. Leyland

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Section: Pedestal Measurementsmentioning

confidence: 99%

Pedestal study across a deuterium fuelling scan for highδELMy H-mode plasmas on JET with the carbon wall

Leyland¹,

Beurskens²,

Frassinetti³

et al. 2013

Nucl. Fusion

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“…In the "standard" C-Mod operating regime (I p = 0.6 -1.2 MA, B t = 5.3 T), with shape parameters described above, two types of H-mode are generally observed -ELM-free and EDA [3,4]. ELM-free H -mode is characterized by the absence of any kind of edge pedestal relaxation mechanisms and, therefore, by an enhanced impurity confinement.…”

Section: Eda/elm-free Boundarymentioning

confidence: 99%

“…On the other hand, lower pedestal density at constant input power corresponds to higher T e ped . This correlation of pedestal temperature and density can mask the temperature dependence of the EDA/ELM-free threshold in the experiment when plasma target density (and, therefore, H-mode pedestal density) is scanned at constant q, as discussed in [4]. However, analysis of multiple C-Mod discharges in a wide range of pedestal parameters shows, that the type of obtained H mode is correlated with pedestal temperature rather than density, in other words the EDA regime can be attained in lower pedestal density discharges with T e ped below ~ 400 eV (low input power shots).…”

Section: Eda/elm-free Boundarymentioning

confidence: 99%

H-mode pedestal characteristics and MHD stability of the edge plasma in Alcator C-Mod

Mossessian

Snyder

Greenwald

et al. 2002

Plasma Phys. Control. Fusion

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Under most operational conditio ns of Alcator C -Mod the dominant type of H-mode is the steady state enhanced D α mode (EDA), characterized by good energy confinement, continuously degraded impurity confinement and absence of regular ELMs. In this regime a quasicoherent (QC) electromagnetic mode (k θ~5 cm -1 , f~100 kHz) is observed, localized in the region of the density pedestal. Experimental evidence suggests that the mode is responsible for enhancement of particle transport. It is shown experimentally that the QC mode can exist in a well defined region in edge temperature-safety factor space, favoring high edge q values (q95 ≥ 3.5) and requiring moderate pedestal temperature Te ped ≤ Tc (Tc~400 eV). As edge temperature and pressure gradient increase, the quasicoherent mode is replaced by br oadband low frequency fluctuations (f<50 kHz). Small grassy ELMs are observed in these discharges. Analysis of ideal ballooning stability of the C-Mod edge shows that the edge pressure gradient is not limited by infinite n ideal ballooning mode if edge bootstrap current, even reduced by high edge collisionality, is taken into account. Linear stability analysis of coupled ideal peeling/ballooning medium n modes driven by combination of edge pressure and current gradients, shows that the modes become marginally unstable at C-Mod edge in the range of pressure gradients where the grassy ELMs are observed (∇P ≥ 1.2x10 7 Pa/(Wb/rad)).

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“…Quiescent ELM-free H-mode barriers have also been observed in the C-Mod tokamak [29,30,31], where they are referred to as the "enhanced D α " (EDA) H-mode. Density and impurity control in the EDA H-mode in C-Mod occurs as a result of a quasi-coherent (QC) oscillation [31,32] at the plasma edge which drives a large particle flux into the SOL.…”

Section: Introductionmentioning

confidence: 99%

Recent experimental studies of edge and internal transport barriers in the DIII-D tokamak

Gohil

Baylor

Burrell

et al. 2003

Plasma Phys. Control. Fusion

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Results from recent experiments on the DIII-D tokamak have revealed many important details on transport barriers at the plasma edge and in the plasma core. These experiments include: (a) the formation of the H-mode edge barrier directly by pellet injection; (b) the formation of a quiescent H-mode edge barrier (QH-mode) which is free from edge localized modes (ELMs), but which still exhibits good density and radiative power control; (c) the formation of multiple transport barriers, such as the quiescent double barrier (QDB) which combines a internal transport barrier with the quiescent H-mode edge barrier. Results from the pellet-induced H-mode experiments indicate that: (a) the edge temperature (electron or ion) is not a critical parameter for the formation of the H-mode barrier, (b) pellet injection leads to an increased gradient in the radial electric field, E r , at the plasma edge; (c) the experimentally determined edge parameters at barrier transition are well below the predictions of several theories on the formation of the H-mode barrier, (d) pellet injection can lower the threshold power required to form the H-mode barrier. The quiescent H-mode barrier exhibits good density control as the result of continuous magnetohydrodynamic (MHD) activity at the plasma edge called the edge harmonic oscillation (EHO). The EHO enhances the edge particle transport whilst maintaining a good energy transport barrier. The ability to produce multiple barriers in the QDB regime has led to long duration, high performance plasmas with β NH 89 values of 7 for up to 10 times the confinement time. Density profile control in the plasma core of QDB plasmas has been demonstrated using on-axis ECH.

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Pedestal profiles and fluctuations in C-Mod enhanced D-alpha H-modes

Cited by 88 publications

References 33 publications

Pedestal study across a deuterium fuelling scan for highδELMy H-mode plasmas on JET with the carbon wall

Pedestal study across a deuterium fuelling scan for highδELMy H-mode plasmas on JET with the carbon wall

H-mode pedestal characteristics and MHD stability of the edge plasma in Alcator C-Mod

Recent experimental studies of edge and internal transport barriers in the DIII-D tokamak

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