Dynamics of a reconnection-driven runaway ion tail in a reversed field pinch plasma

Journal Article · Sun May 15 00:00:00 EDT 2016 · Physics of Plasmas

DOI:https://doi.org/10.1063/1.4943525· OSTI ID:22600229

Kim, J.; Bonofiglo, P. J.; Capecchi, W.; Eilerman, S.; Nornberg, M. D.; Sarff, J. S.; Sears, S. H. ^[1]

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1. Department of Physics, University of Wisconsin-Madison, Madison, Wisconsin 53706 (United States)

While reconnection-driven ion heating is common in laboratory and astrophysical plasmas, the underlying mechanisms for converting magnetic to kinetic energy remain not fully understood. Reversed field pinch discharges are often characterized by rapid ion heating during impulsive reconnection, generating an ion distribution with an enhanced bulk temperature, mainly perpendicular to magnetic field. In the Madison Symmetric Torus, a subset of discharges with the strongest reconnection events develop a very anisotropic, high energy tail parallel to magnetic field in addition to bulk perpendicular heating, which produces a fusion neutron flux orders of magnitude higher than that expected from a Maxwellian distribution. Here, we demonstrate that two factors in addition to a perpendicular bulk heating mechanism must be considered to explain this distribution. First, ion runaway can occur in the strong parallel-to-B electric field induced by a rapid equilibrium change triggered by reconnection-based relaxation; this effect is particularly strong on perpendicularly heated ions which experience a reduced frictional drag relative to bulk ions. Second, the confinement of ions varies dramatically as a function of velocity. Whereas thermal ions are governed by stochastic diffusion along tearing-altered field lines (and radial diffusion increases with parallel speed), sufficiently energetic ions are well confined, only weakly affected by a stochastic magnetic field. High energy ions traveling mainly in the direction of toroidal plasma current are nearly classically confined, while counter-propagating ions experience an intermediate confinement, greater than that of thermal ions but significantly less than classical expectations. The details of ion confinement tend to reinforce the asymmetric drive of the parallel electric field, resulting in a very asymmetric, anisotropic distribution.

• OSTI ID: 22600229
• Journal Information: Physics of Plasmas, Vol. 23, Issue 5; Other Information: (c) 2016 AIP Publishing LLC; Country of input: International Atomic Energy Agency (IAEA); ISSN 1070-664X
• Country of Publication: United States
• Language: English

Related Subjects

70 PLASMA PHYSICS AND FUSION TECHNOLOGY
ANISOTROPY
ASTROPHYSICS
ASYMMETRY
BOLTZMANN STATISTICS
CONFINEMENT
DIFFUSION
DISTRIBUTION
ELECTRIC CURRENTS
ELECTRIC FIELDS
HEATING
KINETIC ENERGY
MAGNETIC FIELDS
NEUTRON FLUX
NEUTRONS
PLASMA
REVERSED-FIELD PINCH DEVICES
REVERSE-FIELD PINCH
STOCHASTIC PROCESSES
TAIL IONS
VELOCITY