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

Abstract

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.

The reversed field pinch

“…Viscous heating has also been evaluated, but it is more difficult to make rapid, and therefore powerful [316]. The energetic ion tail is formed, in part, due to runaway ion acceleration [317,318]. A large inductive electric field accompanies the flux change driven by reconnection, which exceeds the Dreicer electric field for ions.…”

Analysis techniques for diagnosing runaway ion distributions in the reversed field pinch

“…An ad hoc perpendicular heating effect has been implemented to heat Maxwellian thermal ions, which is then put through an electric field that is responsible for fast ion runaway. 7 The electric field model is chosen to mimic the induced parallel field from change in equilibrium fields at the reconnection event.…”

Preferential confinement and field-aligned acceleration of energetic ions in a reversed field pinch

“…The confinement variation and runaway process both play an important role in the development of the observed naturally occurring ion distribution [12]. Figure 3 is a measure of the core-localized energetic distribution of discharges without NBI by three NPA views: (a) anti-parallel, (b) perpendicular to and (c) parallel to I p .…”