Plasma dynamics near the magnetic X-point of the two-wire model: Theory and Simulation

Abstract

The two-wire model (TWM) is a magnetic configuration generated by two parallel current-carrying wires, and it contains an X-point at its center and a separatrix. Since the TWM magnetic field is described by a closed-form analytic expression and contains no guide field, it offers a tractable setting for studying how a true magnetic null shapes plasma dynamics. This work investigates two complementary regimes: collisionless charged particle dynamics and collisional low temperature plasma transport. In the collisionless regime, a Lagrangian analysis identifies two particle motion invariants: the total kinetic energy and the base field line value, which is derived from the conserved axial canonical momentum. Collisionless test particle simulations show that the magnetic moment undergoes shifts when the particle traverses the large gradient region near the null. These shifts enable particles to migrate, the phenomenon in which a particle gyrating about one branch of a base field line jumps to the corresponding branch on the other side of the X-point. A threshold energy for migration is derived, and an empirical expression for the migration confinement time is formulated. In the collisional regime, reduced drift-diffusion models for low temperature plasmas are developed in a conformal field-aligned coordinate system, and they predict density plateau formation near the separatrix in the strongly magnetized regime. Self-consistent particle-in-cell simulations are performed to verify the predicted density plateau. The two complementary studies establish a fundamental understanding of plasma dynamics near a true magnetic null for both the collisionless and collisional regimes.