Sawtooth oscillation behavior with varying amounts of applied stellarator rotational transform.

Herfindal, Jeffery L.; Maurer, David A.; Hartwell, Greg J.; Ennis, David A.; Hanson, James D.; Knowlton, Stephen F.; Ma, X.; Pandya, Mihir D.; Roberds, Nicholas; Traverso, Peter J.

doi:10.1063/1.5080242

Sawtooth oscillation behavior with varying amounts of applied stellarator rotational transform.

Journal Article · Tue Mar 05 00:00:00 EST 2019 · Physics of Plasmas

DOI:https://doi.org/10.1063/1.5080242· OSTI ID:1501627

Herfindal, Jeffery L. ^[1]; Maurer, David A. ^[2]; Hartwell, Greg J. ^[2]; Ennis, David A. ^[2]; Hanson, James D. ^[2]; Knowlton, Stephen F. ^[2]; Ma, X. ^[2]; Pandya, Mihir D. ^[2]; Roberds, Nicholas ^[2]; Traverso, Peter J. ^[2]

Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States); Auburn Univ., AL (United States)
Auburn Univ., AL (United States)

In this study, tokamak-like sawtooth oscillations are observed in the Compact Toroidal Hybrid (CTH), a current-carrying stellarator. CTH has the unique ability to change the amount of the applied vacuum rotational transform from external stellarator coils relative to the rotational transform generated by the internal plasma current to investigate the effects of strong three-dimensional magnetic shaping on sawtooth behavior. The observed sawteeth in CTH, for plasmas with monotonically decreasing rotational transform profiles dominated by the plasma current, have characteristics of those observed on tokamaks including (1) a central emissivity rise and then a sudden crash with a well-defined inversion radius, (2) the presence of an m = 1 emissivity fluctuation, and (3) the normalized inversion surface radius scales with the total edge rotational transform. We explore the properties of an ensemble of discharges in CTH in which the fractional rotational transform, defined as the vacuum rotational transform divided by the total rotational transform, is systematically varied from 0.04 to 0.42 to observe changes in sawtooth oscillation dynamics. Over this range of the fractional rotational transform, the measured sawtooth period decreased by a factor of two. At a high fractional rotational transform, the sawtooth amplitude is observed to consist of only low-amplitude oscillations while the measured crash time of the sawtooth oscillation does not appear to have a strong dependence on the amount of the fractional transform applied. Lastly, experimental results indicate that the low-amplitude sawteeth are accompanied by a decrease in the sawtooth period and predominantly correlated with the mean elongation (due to the increasing fractional rotational transform) of the non-axisymmetric plasmas within CTH rather than other global equilibrium parameters.

View Accepted Manuscript (DOE)

Research Organization:: Oak Ridge National Laboratory (ORNL), Oak Ridge, TN (United States); Sandia National Laboratories (SNL-NM), Albuquerque, NM (United States)

Sponsoring Organization:: USDOE National Nuclear Security Administration (NNSA)

Grant/Contract Number:: AC04-94AL85000; AC05-00OR22725; FG02-00ER54610

OSTI ID:: 1501627

Alternate ID(s):: OSTI ID: 1761773

Report Number(s):: SAND--2019-1053J; 672006

Journal Information:: Physics of Plasmas, Journal Name: Physics of Plasmas Journal Issue: 3 Vol. 26; ISSN 1070-664X

Publisher:: American Institute of Physics (AIP)Copyright Statement

Country of Publication:: United States

Language:: English

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