skip to main content
OSTI.GOV title logo U.S. Department of Energy
Office of Scientific and Technical Information

Title: Thermal transport dynamics in the quasi-single helicity state

Authors:
ORCiD logo [1];  [2]
  1. Department of Engineering Physics, University of Wisconsin-Madison, Madison, Wisconsin 53706, USA
  2. Department of Physics, University of Wisconsin-Madison, Madison, Wisconsin 53706, USA
Publication Date:
Sponsoring Org.:
USDOE
OSTI Identifier:
1363705
Grant/Contract Number:
FG02-85ER53212
Resource Type:
Journal Article: Publisher's Accepted Manuscript
Journal Name:
Physics of Plasmas
Additional Journal Information:
Journal Volume: 24; Journal Issue: 6; Related Information: CHORUS Timestamp: 2018-02-14 21:09:03; Journal ID: ISSN 1070-664X
Publisher:
American Institute of Physics
Country of Publication:
United States
Language:
English

Citation Formats

McKinney, I. J., and Terry, P. W. Thermal transport dynamics in the quasi-single helicity state. United States: N. p., 2017. Web. doi:10.1063/1.4985317.
McKinney, I. J., & Terry, P. W. Thermal transport dynamics in the quasi-single helicity state. United States. doi:10.1063/1.4985317.
McKinney, I. J., and Terry, P. W. Thu . "Thermal transport dynamics in the quasi-single helicity state". United States. doi:10.1063/1.4985317.
@article{osti_1363705,
title = {Thermal transport dynamics in the quasi-single helicity state},
author = {McKinney, I. J. and Terry, P. W.},
abstractNote = {},
doi = {10.1063/1.4985317},
journal = {Physics of Plasmas},
number = 6,
volume = 24,
place = {United States},
year = {Thu Jun 01 00:00:00 EDT 2017},
month = {Thu Jun 01 00:00:00 EDT 2017}
}

Journal Article:
Free Publicly Available Full Text
Publisher's Version of Record at 10.1063/1.4985317

Save / Share:
  • The nonlinear dynamics of an inviscid magnetohydrodynamic system in a low-{beta} plasma is investigated through a simplified model obtained by a Galerkin projection. We show that a single Fourier mode dominates over the whole energy spectrum, a self-organization mechanism of turbulence being responsible for this nonlinear effect. Our results are in agreement with observations of an analogous phenomenon, say, the emergence of a quasi-single-helicity state both in numerical simulations and real experiments in reversed field pinch laboratory plasmas.
  • The heat transport inside the magnetic island generated in a quasi-single-helicity regime of a reversed-field pinch device is studied by using a numerical code that simulates the electron temperature and the soft x-ray emissivity. The heat diffusivity {chi}{sub e} inside the island is determined by matching the simulated signals with the experimental ones. Inside the island, {chi}{sub e} turns out to be from one to two orders of magnitude lower than the diffusivity in the surrounding plasma, where the magnetic field is stochastic. Furthermore, the heat transport properties inside the island are studied in correlation with the plasma current andmore » with the amplitude of the magnetic fluctuations.« less
  • The operating conditions for obtaining a quasi-single helicity (QSH) state with a good reproducibility are found in a reversed-field pinch (RFP) experiment on the large RFP machine, TPE-RX [Y. Yagi et al., Fusion Eng. Des. 45, 421 (1999)]. In these conditions, the reversal of toroidal magnetic field (B{sub ta}) is maintained at a very shallow value ({approx}-0.2 mT) after the setting up phase and the following fast current rising phase. After a certain period at this shallow reversal ({approx}15-25 ms), the m/n=1/6 mode (m and n being the poloidal and toroidal Fourier mode numbers, respectively) rapidly grows and saturates beforemore » the termination of discharge. The growth of this mode dominates the other modes and the QSH state with m/n=1/6 is finally achieved. This QSH state can be sustained for a long period (up to {approx}45 ms) almost until the end of discharge by applying a delayed reversal of B{sub ta} with appropriate trigger timing and magnitude. The initial setup of the QSH states shows a reproducibility of almost 100%, but its sustainment for a long period shows a slightly reduced reproducibility ({approx}85%). The initial rapid growth of the single dominant mode is compared with the numerical results of linear stability and nonlinear three-dimensional (3D) calculations by assuming the experimental magnetic field profile estimated with a standard model. Linear calculations show that the m/n=1/6 mode has the maximum growth rate to the ideal magnetohydrodynamic instability and can explain the dominant growth of this mode. The 3D calculations also show a qualitative agreement with the experiment, where under some conditions the m/n=1/6 mode becomes dominant after an initial relaxation and continues to the end of the simulation. These results indicate that the present QSH state is the combined result of the linear growth and nonlinear saturation of a particular mode.« less
  • By applying a small positive pulse ({delta}B{sub ta}) in toroidal magnetic field, the quasi-single helicity (QSH) state can be obtained with a controllable and reproducible manner in a reversed-field pinch (RFP) experiment on the large RFP machine, TPE-RX [Y. Yagi et al., Fusion Eng. Des. 45, 421 (1999)]. The QSH state in RFP is one of the states where the improved confinement can be observed, and is important for development toward the pure single helicity (SH) state. In the SH state, the dynamo-action for sustaining the RFP configuration will be driven by a single helical mode and its harmonics, andmore » the anomalous plasma loss can be avoided which is caused by the multi-helicity dynamo action in ordinary RFPs. In the operating condition presented here, the reversal of toroidal magnetic field (B{sub ta}) is maintained at a shallow value ({approx}-1 mT) for a certain period ({approx}20 ms) after the setting up of the RFP configuration and then the positive {delta}B{sub ta} ((less-or-similar sign)5 mT magnitude and {approx}2 ms width) is applied to the B{sub ta}, which is usually negative during the sustaining phase of RFP. Just after applying the pulse, the m/n=1/6 mode (m and n being the poloidal and toroidal Fourier mode numbers, respectively) grows dominantly and the configuration goes into QSH state. This QSH state can be sustained for a long period (up to {approx}45 ms) almost until the end of discharge by applying a delayed reversal of B{sub ta} with appropriate timing and magnitude. The setting up of the QSH states shows a reproducibility of almost 100% with the same timing corresponding to the applied positive pulse. This observation can confirm the interpretation in the former report [Y. Hirano et al., Phys. Plasmas 12, 112501 (2005)], in which it is claimed that the QSH state is obtained when a small positive pulse in toroidal magnetic field spontaneously appears.« less
  • The quasi-single-helicity (QSH) state of a reversed-field pinch (RFP) plasma is a regime in which the RFP configuration can be sustained by a dynamo produced mainly by a single tearing mode and in which a helical structure with well-defined magnetic flux surfaces arises. In this Letter, we show that spontaneous transitions to the QSH regime enhance the particle confinement. This improvement is originated by the simultaneous and cooperative action of the increase of the magnetic island and the reduction of the magnetic stochasticity.