Full particle orbit effects in regular and stochastic magnetic fields

Journal Article · Mon Jul 18 00:00:00 EDT 2016 · Physics of Plasmas

DOI:https://doi.org/10.1063/1.4958653· OSTI ID:1333068

Ogawa, Shun ^[1]; Cambon, Benjamin P. ^[2]; Leoncini, Xavier ^[2]; Vittot, Michel ^[2]; Del-Castillo-Negrete, Diego B ^[3]; Dif-Pradalier, Guilhem ^[4]; Garbet, Xavier ^[4]

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1. Aix-Marseille 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)
2. Aix-Marseille Univ., and CNRS/IN2P3, Marseille (France). Centre de Physique Theorique (CPT); Univ. of Toulon, Marseille (France)
3. Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States)
4. Alternative Energies and Atomic Energy Commission (CEA), Provence (France). Inst. for Magnetic Fusion Research (IRFM)

Here we present a numerical study of charged particle motion in a time-independent magnetic field in cylindrical geometry. The magnetic field model consists of an unperturbed reversed-shear (non-monotonic q-profile) 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 six-dimensional phase space using a sixth-order, implicit, symplectic Gauss-Legendre 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 non-monotonicity of the q-profile implies the existence of magnetic ITBs (internal transport barriers) which correspond to shearless flux surfaces located in the vicinity of the q-profile 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 energy-dependent particle confinement filter. Magnetic field lines in reversed-shear 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 non-trivial way on the energy and pitch angle of the particles.

• Research Organization: Oak Ridge National Laboratory (ORNL), Oak Ridge, TN (United States)
• Sponsoring Organization: USDOE Office of Science (SC), Fusion Energy Sciences (FES); French National Research Agency (ANR)
• Grant/Contract Number: AC05-00OR22725
• OSTI ID: 1333068
• Alternate ID(s): OSTI ID: 1263709
• Journal Information: Physics of Plasmas, Vol. 23, Issue 7; ISSN 1070-664X
• Publisher: American Institute of Physics (AIP)
• Country of Publication: United States
• Language: English

Related Subjects

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS
particle trajectory
magnetic fields
chaos
particle orbits
internal transport barrier