Full particle orbit effects in regular and stochastic magnetic fields
Here we present a numerical study of charged particle motion in a timeindependent magnetic field in cylindrical geometry. The magnetic field model consists of an unperturbed reversedshear (nonmonotonic qprofile) helical part and a perturbation consisting of a superposition of modes. Contrary to most of the previous studies, the particle trajectories are computed by directly solving the full Lorentz force equations of motion in a sixdimensional phase space using a sixthorder, implicit, symplectic GaussLegendre method. The level of stochasticity in the particle orbits is diagnosed using averaged, effective Poincare sections. It is shown that when only one mode is present, the particle orbits can be stochastic even though the magnetic field line orbits are not stochastic (i.e., fully integrable). The lack of integrability of the particle orbits in this case is related to separatrix crossing and the breakdown of the global conservation of the magnetic moment. Some perturbation consisting of two modes creates resonance overlapping, leading to Hamiltonian chaos in magnetic field lines. Then, the particle orbits exhibit a nontrivial dynamics depending on their energy and pitch angle. It is shown that the regions where the particle motion is stochastic decrease as the energy increases. The nonmonotonicity of the qprofile impliesmore »
 Authors:

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 AixMarseille Univ., and CNRS/IN2P3, Marseille (France). Centre de Physique Theorique (CPT); Univ. of Toulon, Marseille (France); Alternative Energies and Atomic Energy Commission (CEA), Provence (France). Inst. for Magnetic Fusion Research (IRFM)
 AixMarseille Univ., and CNRS/IN2P3, Marseille (France). Centre de Physique Theorique (CPT); Univ. of Toulon, Marseille (France)
 Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States)
 Alternative Energies and Atomic Energy Commission (CEA), Provence (France). Inst. for Magnetic Fusion Research (IRFM)
 Publication Date:
 Grant/Contract Number:
 AC0500OR22725
 Type:
 Accepted Manuscript
 Journal Name:
 Physics of Plasmas
 Additional Journal Information:
 Journal Volume: 23; Journal Issue: 7; Journal ID: ISSN 1070664X
 Publisher:
 American Institute of Physics (AIP)
 Research Org:
 Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States)
 Sponsoring Org:
 USDOE Office of Science (SC), Fusion Energy Sciences (FES) (SC24); French National Research Agency (ANR)
 Country of Publication:
 United States
 Language:
 English
 Subject:
 72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS; particle trajectory; magnetic fields; chaos; particle orbits; internal transport barrier
 OSTI Identifier:
 1333068
 Alternate Identifier(s):
 OSTI ID: 1263709
Ogawa, Shun, Cambon, Benjamin P., Leoncini, Xavier, Vittot, Michel, DelCastilloNegrete, Diego B, DifPradalier, Guilhem, and Garbet, Xavier. Full particle orbit effects in regular and stochastic magnetic fields. United States: N. p.,
Web. doi:10.1063/1.4958653.
Ogawa, Shun, Cambon, Benjamin P., Leoncini, Xavier, Vittot, Michel, DelCastilloNegrete, Diego B, DifPradalier, Guilhem, & Garbet, Xavier. Full particle orbit effects in regular and stochastic magnetic fields. United States. doi:10.1063/1.4958653.
Ogawa, Shun, Cambon, Benjamin P., Leoncini, Xavier, Vittot, Michel, DelCastilloNegrete, Diego B, DifPradalier, Guilhem, and Garbet, Xavier. 2016.
"Full particle orbit effects in regular and stochastic magnetic fields". United States.
doi:10.1063/1.4958653. https://www.osti.gov/servlets/purl/1333068.
@article{osti_1333068,
title = {Full particle orbit effects in regular and stochastic magnetic fields},
author = {Ogawa, Shun and Cambon, Benjamin P. and Leoncini, Xavier and Vittot, Michel and DelCastilloNegrete, Diego B and DifPradalier, Guilhem and Garbet, Xavier},
abstractNote = {Here we present a numerical study of charged particle motion in a timeindependent magnetic field in cylindrical geometry. The magnetic field model consists of an unperturbed reversedshear (nonmonotonic qprofile) helical part and a perturbation consisting of a superposition of modes. Contrary to most of the previous studies, the particle trajectories are computed by directly solving the full Lorentz force equations of motion in a sixdimensional phase space using a sixthorder, implicit, symplectic GaussLegendre method. The level of stochasticity in the particle orbits is diagnosed using averaged, effective Poincare sections. It is shown that when only one mode is present, the particle orbits can be stochastic even though the magnetic field line orbits are not stochastic (i.e., fully integrable). The lack of integrability of the particle orbits in this case is related to separatrix crossing and the breakdown of the global conservation of the magnetic moment. Some perturbation consisting of two modes creates resonance overlapping, leading to Hamiltonian chaos in magnetic field lines. Then, the particle orbits exhibit a nontrivial dynamics depending on their energy and pitch angle. It is shown that the regions where the particle motion is stochastic decrease as the energy increases. The nonmonotonicity of the qprofile implies the existence of magnetic ITBs (internal transport barriers) which correspond to shearless flux surfaces located in the vicinity of the qprofile minimum. It is shown that depending on the energy, these magnetic ITBs might or might not confine particles. That is, magnetic ITBs act as an energydependent particle confinement filter. Magnetic field lines in reversedshear configurations exhibit topological bifurcations (from homoclinic to heteroclinic) due to separatrix reconnection. Finally, we show that a similar but more complex scenario appears in the case of particle orbits that depend in a nontrivial way on the energy and pitch angle of the particles.},
doi = {10.1063/1.4958653},
journal = {Physics of Plasmas},
number = 7,
volume = 23,
place = {United States},
year = {2016},
month = {7}
}