Helical core formation and evolution during current ramp-up in the high-field tokamak Alcator C-Mod

A. Wingen; R. S. Wilcox; L. F. Delgado-Aparicio; R. Granetz; S. Houshmandyar; S. Shiraiwa; M. R. Cianciosa; S. K. Seal

doi:10.1063/1.5083055

Helical core formation and evolution during current ramp-up in the high-field tokamak Alcator C-Mod

A. Wingen, R. S. Wilcox, L. F. Delgado-Aparicio, R. Granetz, S. Houshmandyar, S. Shiraiwa, M. R. Cianciosa, S. K. Seal

Research output: Contribution to journal › Article › peer-review

6 Scopus citations

Abstract

Large, spontaneous m/n = 1/1 helical cores are predicted in tokamaks with extended regions of low- or reversed-magnetic shear profiles in a region within the q = 1 surface and an onset condition determined by constant (dp/dρ)/Bt2 along the threshold. These 3D modes occurred frequently in Alcator C-Mod during ramp-up when slow current penetration results in a reversed shear q-profile. The onset and early development of a helical core in C-Mod were simulated using a new 3D time-dependent equilibrium reconstruction, based on the ideal MHD equilibrium code VMEC. The reconstruction used the experimental density, temperature, and soft-X-ray fluctuations. The pressure profile can become hollow due to an inverted, hollow electron temperature profile caused by molybdenum radiation in the plasma core during the current ramp-up phase before the onset of sawteeth, which may also occur in ITER with tungsten. Based on modeling, it is found that a reverse shear q-profile combined with a hollow pressure profile reduces the onset condition threshold, enabling helical core formation from an otherwise axisymmetric equilibrium.

Original language	English
Article number	022501
Journal	Physics of Plasmas
Volume	26
Issue number	2
DOIs	https://doi.org/10.1063/1.5083055
State	Published - Feb 1 2019

Funding

Discussions with Amanda Hubbard, Jerry Hughes, Matt Reinke, and David Pfefferlé are gratefully acknowledged. This material is based upon work supported by the U.S. Department of Energy, Office of Science, Office of Fusion Energy Sciences under Award Nos. DE-AC05-00OR22725, DE-AC02-09CH11466, and DE-FC02-99ER54512. This manuscript was authored by UT-Battelle, LLC, under Contract No. DE-AC05-00OR22725 with the U.S. Department of Energy (DOE). The U.S. Government retains and the publisher, by accepting the article for publication, acknowledges that the U.S. Government retains a nonexclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this manuscript, or allow others to do so, for U.S. Government purposes. DOE will provide public access to these results of federally sponsored research in accordance with the DOE Public Access Plan (http://energy.gov/downloads/doe-public-access-plan).

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U.S. Department of Energy
Office of Science
Fusion Energy Sciences

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abstract = "Large, spontaneous m/n = 1/1 helical cores are predicted in tokamaks with extended regions of low- or reversed-magnetic shear profiles in a region within the q = 1 surface and an onset condition determined by constant (dp/dρ)/Bt2 along the threshold. These 3D modes occurred frequently in Alcator C-Mod during ramp-up when slow current penetration results in a reversed shear q-profile. The onset and early development of a helical core in C-Mod were simulated using a new 3D time-dependent equilibrium reconstruction, based on the ideal MHD equilibrium code VMEC. The reconstruction used the experimental density, temperature, and soft-X-ray fluctuations. The pressure profile can become hollow due to an inverted, hollow electron temperature profile caused by molybdenum radiation in the plasma core during the current ramp-up phase before the onset of sawteeth, which may also occur in ITER with tungsten. Based on modeling, it is found that a reverse shear q-profile combined with a hollow pressure profile reduces the onset condition threshold, enabling helical core formation from an otherwise axisymmetric equilibrium.",

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AU - Houshmandyar, S.

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Helical core formation and evolution during current ramp-up in the high-field tokamak Alcator C-Mod

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