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Robustness of predator-prey models for confinement regime transitions in fusion plasmas
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Zhu, Hao, Chapman, Sandra C. and Dendy, R. O. (2013) Robustness of predator-prey models for confinement regime transitions in fusion plasmas. Physics of Plasmas, Volume 20 (Number 4). Article number 042302. doi:10.1063/1.4800009
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Official URL: http://dx.doi.org/10.1063/1.4800009
Abstract
Energy transport and confinement in tokamak fusion plasmas is usually determined by the coupled nonlinear interactions of small-scale drift turbulence and larger scale coherent nonlinear structures, such as zonal flows, together with free energy sources such as temperature gradients. Zero-dimensional models, designed to embody plausible physical narratives for these interactions, can help to identify the origin of enhanced energy confinement and of transitions between confinement regimes. A prime zero-dimensional paradigm is predator-prey or Lotka-Volterra. Here, we extend a successful three-variable (temperature gradient; microturbulence level; one class of coherent structure) model in this genre [M. A. Malkov and P. H. Diamond, Phys. Plasmas 16, 012504 (2009)], by adding a fourth variable representing a second class of coherent structure. This requires a fourth coupled nonlinear ordinary differential equation. We investigate the degree of invariance of the phenomenology generated by the model of Malkov and Diamond, given this additional physics. We study and compare the long-time behaviour of the three-equation and four-equation systems, their evolution towards the final state, and their attractive fixed points and limit cycles. We explore the sensitivity of paths to attractors. It is found that, for example, an attractive fixed point of the three-equation system can become a limit cycle of the four-equation system. Addressing these questions which we together refer to as “robustness” for convenience is particularly important for models which, as here, generate sharp transitions in the values of system variables which may replicate some key features of confinement transitions. Our results help to establish the robustness of the zero-dimensional model approach to capturing observed confinement phenomenology in tokamak fusion plasmas.
| Item Type: | Journal Article | ||||
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| Subjects: | Q Science > QA Mathematics Q Science > QC Physics |
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| Divisions: | Faculty of Science > Physics | ||||
| Library of Congress Subject Headings (LCSH): | Plasma (Ionized gases), Nuclear fusion, Tokamaks, Phase transformations (Statistical physics), Lotka-Volterra equations | ||||
| Journal or Publication Title: | Physics of Plasmas | ||||
| Publisher: | American Institute of Physics | ||||
| ISSN: | 1070-664X | ||||
| Official Date: | 2013 | ||||
| Dates: |
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| Volume: | Volume 20 | ||||
| Number: | Number 4 | ||||
| Article Number: | Article number 042302 | ||||
| DOI: | 10.1063/1.4800009 | ||||
| Status: | Peer Reviewed | ||||
| Publication Status: | Published | ||||
| Access rights to Published version: | Restricted or Subscription Access | ||||
| Funder: | Engineering and Physical Sciences Research Council (EPSRC), Research Councils UK (RCUK), European Commission (EC), Euratom, Culham Centre for Fusion Energy (CCFE) | ||||
| Grant number: | EP/I501045 (EPSRC/RCUK) |
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