Speaker
Jay Anderson
(University of Wisconsin-Madison)
Description
Reconnection-driven heating of ions is a powerful process in many astrophysical and laboratory plasmas, including CT merging and reversed-field pinch (RFP) discharges. The RFP is 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 RFP, 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 sufficiently energetic ions whose guiding centers drift substantially from magnetic field lines. Second, the confinement of ions varies dramatically as a function of velocity. While thermal ions are governed by stochastic diffusion along tearing-altered field lines (and radial diffusion increases with parallel speed), the energetic ions are well confined, only weakly affected by a stochastic magnetic field. High energy ions traveling mainly in the direction of 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.
Experiments with neutral beam injection elegantly confirm the ion runaway process and fast ion confinement characteristics in MST. Neutral particle analyzers measure an unrestricted parallel acceleration of the fast test particle distribution during the reconnection event. The energy gain is larger for higher initial ion energy (reduced drag), and deceleration is observed with reversed electric field (counter-current injection) according to runaway dynamics and confirmed with Fokker-Planck modeling.
Work supported by USDOE. Portions of the work were accomplished with the use of infrastructure of Complex DOL (BINP, Russia).
Primary author
Jay Anderson
(University of Wisconsin-Madison)
Co-authors
Dr
Alexander Ivanov
(Budker Institute of Nuclear Physics)
Ami Dubois
(University of Wisconsin-Madison)
John Sarff
(University of Wisconsin-Madison)
Jungha Kim
(University of Wisconsin-Madison)
Dr
Sergey Polosatkin
(Budker Institute of Nuclear Physics)
Dr
Vladimir Davydenko
(Budker Institute of Nuclear Physics)
abdulgader almagri
(University of Wisconsin-Madison)
