Advances in measurement and modeling of the high-confinement-mode pedestal on the Alcator C-Mod tokamak

Hughes, J. W.; LaBombard, B.; Mossessian, D.; Hubbard, A.; Terry, J. L.; Biewer, T. M.

doi:10.1063/1.2180748

Cited by 39 publications

(88 citation statements)

References 39 publications

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“…The growing difference in H-mode plasmas between the n B 5+ pedestal location for the LFS and HFS isn't just a simple shift, but rather the pedestal width on the LFS is also increasing with q 95 , similar to electron density pedestal behavior 26 . For all the different regimes, the HFS boron density pedestal remains relatively fixed in width and position, while the LFS boron density pedestal width increases and the pedestal position shifts slightly inward depending on plasma parameters, as shown for the H-mode cases in Figure 8.…”

Section: In-out Asymmetries In the Pedestal Regionmentioning

confidence: 66%

Poloidal asymmetries in edge transport barriers

et al. 2015

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Measurements of impurities in Alcator C-Mod indicate that in the pedestal region, significant poloidal asymmetries can exist in the impurity density, ion temperature, and main ion density. In light of the observation that ion temperature and electrostatic potential are not constant on a flux surface [Theiler et al., Nucl. Fusion 54, 083017 (2014)], a technique based on total pressure conservation to align profiles measured at separate poloidal locations is presented and applied. Gyrokinetic neoclassical simulations with XGCa support the observed large poloidal variations in ion temperature and density, and that the total pressure is approximately constant on a flux surface. With the updated alignment technique, the observed in-out asymmetry in impurity density is reduced from previous publishing [Churchill et al., Nucl. Fusion 53, 122002 (2013)], but remains substantial (nz,H/nz,L∼6). Candidate asymmetry drivers are explored, showing that neither non-uniform impurity sources nor localized fluctuation-driven transport are able to explain satisfactorily the impurity density asymmetry. Since impurity density asymmetries are only present in plasmas with strong electron density gradients, and radial transport timescales become comparable to parallel transport timescales in the pedestal region, it is suggested that global transport effects relating to the strong electron density gradients in the pedestal are the main driver for the pedestal in-out impurity density asymmetry.

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Section: In-out Asymmetries In the Pedestal Regionmentioning

confidence: 66%

Poloidal asymmetries in edge transport barriers

et al. 2015

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“…170 These matches required DIII-D to operate at its lowest feasible H-mode densities, in a regime known to show a dependence of the pedestal width on neutral penetration 171 that is not seen on C-Mod, suggesting that the neutral penetration range can have an effect on sufficiently transparent pedestals. 172 However, C-Mod has the highest neutral opacity of any operating tokamak and should be more prototypical of ITER/ Reactor conditions.…”

Section: Eda H-modementioning

confidence: 99%

20 years of research on the Alcator C-Mod tokamak

et al. 2014

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The object of this review is to summarize the achievements of research on the Alcator C-Mod tokamak [Hutchinson et al., Phys. Plasmas 1, 1511 and Marmar, Fusion Sci. Technol. 51, 261 (2007)] and to place that research in the context of the quest for practical fusion energy. C-Mod is a compact, high-field tokamak, whose unique design and operating parameters have produced a wealth of new and important results since it began operation in 1993, contributing data that extends tests of critical physical models into new parameter ranges and into new regimes. Using only highpower radio frequency (RF) waves for heating and current drive with innovative launching structures, C-Mod operates routinely at reactor level power densities and achieves plasma pressures higher than any other toroidal confinement device. C-Mod spearheaded the development of the vertical-target divertor and has always operated with high-Z metal plasma facing componentsapproaches subsequently adopted for ITER. C-Mod has made ground-breaking discoveries in divertor physics and plasma-material interactions at reactor-like power and particle fluxes and elucidated a) Paper AR1 1, Bull. Am. Phys. Soc. 58, 21 (2013). b) Invited speaker.

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“…Pedestal studies on the Alcator C-Mod tokamak 1 have shown a clear correlation of edge particle transport, and resulting density pedestal structure, with plasma current. 2 Plasma transport appears to clamp edge density gradients, yielding a density pedestal that is strongly resilient to changes in the edge neutral source. 3 Finding mechanisms for breaking the correlation of H-mode density with current, thus allowing improved density control, is a key goal of C-Mod pedestal research.…”

Section: Introductionmentioning

confidence: 99%

Modification of H-mode pedestal structure with lower hybrid waves on Alcator C-Mod

et al. 2010

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The application of lower hybrid waves (LH) in H-mode plasmas on Alcator C-Mod can result in significant reduction of core particle inventory, with no significant degradation of energy confinement. This phenomenon has been observed in steady enhanced D α (EDA) H-mode targets, which are sustained by ion cyclotron RF auxiliary heating, in which pedestal density n ped is usually tied firmly to plasma current I P and shows a strong resilience to changes in the edge neutral source. Upon application of up to 1MW LH power, n ped is reduced by up to 30%, while the temperature profile increases simultaneously such that the pressure pedestal remains constant or is slightly increased.Steady EDA H-mode operation with no edge-localized modes (ELMs) can be maintained while edge collisionality is reduced by factors of reductions of 2-4. Elevation of scrapeoff layer (SOL) density and electric currents accompany the application of LH (at levels as low as 400kW) with a fast time response (~10 -2 s), while full density pedestal relaxation and core density reduction occur on longer time scales (~10 -1 s). A similarly prompt counter-I P change in the edge toroidal velocity is also observed in response to LHRF, followed on longer time scales by a counter-I P change in the central rotation. The range of time scales of the plasma response may indicate that the radial locations of LH interactions (i.e., SOL vs. core), and power deposition mechanisms, are evolving in time.Understanding the responsible physical mechanisms and applying them to a broad range of H-mode discharges could provide a tool for improving density control, affecting edge MHD stability and applying modulation for transport studies.

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Advances in measurement and modeling of the high-confinement-mode pedestal on the Alcator C-Mod tokamak

Cited by 39 publications

References 39 publications

Poloidal asymmetries in edge transport barriers

Poloidal asymmetries in edge transport barriers

20 years of research on the Alcator C-Mod tokamak

Modification of H-mode pedestal structure with lower hybrid waves on Alcator C-Mod

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