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Title: Use of reconstructed 3D equilibria to determine onset conditions of helical cores in tokamaks for extrapolation to ITER

Abstract

The threshold for spontaneous symmetry breaking, which results in growth of m/n = 1/1 helical cores, in tokamaks is determined using VMEC. This model is based on a DIII-D hybrid discharge with helical core, and it predicts ITER (15MA scenario) to operate far in the helical core formation regime. Helical cores can only exist in tokamak discharges with monotonic but low, or reversed q-shear and qmin 1 in the core. The helical core is a saturated internal kink mode; its onset threshold is proportional to (dp/d)/B 2 t around q = 1. Below the threshold, applied 3-D fields can drive a helical core to finite size, as in DIIID. Above it, a random 3-D kick causes a bifurcation from axisymmetry and excites a spontaneous helical core. Helical cores occur frequently in C-Mod due to impurities; modeling shows the axisymmetric equilibrium becomes unstable at the end of the current ramp up due to the slow current penetration and a resultant reversed shear q-profile, which lowers the threshold. Lastly, a comparison of the helical core onset threshold for discharges from DIII-D, C-Mod and ITER confirms that while DIII-D is marginally stable, C-Mod and especially ITER are highly susceptible to helical core formation.

Authors:
ORCiD logo; ; ; ORCiD logo; ORCiD logo; ; ;
Publication Date:
Research Org.:
General Atomics, San Diego, CA (United States); Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States); Princeton Plasma Physics Lab. (PPPL), Princeton, NJ (United States)
Sponsoring Org.:
USDOE Office of Science (SC), Fusion Energy Sciences (FES) (SC-24)
OSTI Identifier:
1422609
Alternate Identifier(s):
OSTI ID: 1375431
Grant/Contract Number:
FC02-04ER54698; AC05-00OR22725; AC02-09CH11466
Resource Type:
Journal Article: Accepted Manuscript
Journal Name:
Nuclear Fusion
Additional Journal Information:
Journal Volume: 58; Journal Issue: 3; Journal ID: ISSN 0029-5515
Publisher:
IOP Science
Country of Publication:
United States
Language:
English
Subject:
70 PLASMA PHYSICS AND FUSION TECHNOLOGY

Citation Formats

Wingen, A., Wilcox, R. S., Seal, S. K., Unterberg, E. A., Cianciosa, M. R., Delgado-Aparicio, L. F., Hirshman, S. P., and Lao, L. L.. Use of reconstructed 3D equilibria to determine onset conditions of helical cores in tokamaks for extrapolation to ITER. United States: N. p., 2018. Web. doi:10.1088/1741-4326/aaa33d.
Wingen, A., Wilcox, R. S., Seal, S. K., Unterberg, E. A., Cianciosa, M. R., Delgado-Aparicio, L. F., Hirshman, S. P., & Lao, L. L.. Use of reconstructed 3D equilibria to determine onset conditions of helical cores in tokamaks for extrapolation to ITER. United States. doi:10.1088/1741-4326/aaa33d.
Wingen, A., Wilcox, R. S., Seal, S. K., Unterberg, E. A., Cianciosa, M. R., Delgado-Aparicio, L. F., Hirshman, S. P., and Lao, L. L.. Mon . "Use of reconstructed 3D equilibria to determine onset conditions of helical cores in tokamaks for extrapolation to ITER". United States. doi:10.1088/1741-4326/aaa33d.
@article{osti_1422609,
title = {Use of reconstructed 3D equilibria to determine onset conditions of helical cores in tokamaks for extrapolation to ITER},
author = {Wingen, A. and Wilcox, R. S. and Seal, S. K. and Unterberg, E. A. and Cianciosa, M. R. and Delgado-Aparicio, L. F. and Hirshman, S. P. and Lao, L. L.},
abstractNote = {The threshold for spontaneous symmetry breaking, which results in growth of m/n = 1/1 helical cores, in tokamaks is determined using VMEC. This model is based on a DIII-D hybrid discharge with helical core, and it predicts ITER (15MA scenario) to operate far in the helical core formation regime. Helical cores can only exist in tokamak discharges with monotonic but low, or reversed q-shear and qmin 1 in the core. The helical core is a saturated internal kink mode; its onset threshold is proportional to (dp/d)/B2t around q = 1. Below the threshold, applied 3-D fields can drive a helical core to finite size, as in DIIID. Above it, a random 3-D kick causes a bifurcation from axisymmetry and excites a spontaneous helical core. Helical cores occur frequently in C-Mod due to impurities; modeling shows the axisymmetric equilibrium becomes unstable at the end of the current ramp up due to the slow current penetration and a resultant reversed shear q-profile, which lowers the threshold. Lastly, a comparison of the helical core onset threshold for discharges from DIII-D, C-Mod and ITER confirms that while DIII-D is marginally stable, C-Mod and especially ITER are highly susceptible to helical core formation.},
doi = {10.1088/1741-4326/aaa33d},
journal = {Nuclear Fusion},
number = 3,
volume = 58,
place = {United States},
year = {Mon Jan 15 00:00:00 EST 2018},
month = {Mon Jan 15 00:00:00 EST 2018}
}

Journal Article:
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