Ion runaway during magnetic reconnection in the reversed-field pinch.

摘要

The anomalous heating and energization of ions during magnetic reconnection events in astrophysical and laboratory plasmas has been an ongoing research topic for decades. Numerous measurements have been made during impulsive bursts of reconnection in the reversed-field pinch to further our understanding of the heating process. Discoveries have been made regarding the conditions necessary for heating, its scaling with various plasma parameters, and the anisotropies and other features associated with the resulting distribution; however, no one mechanism for the conversion of magnetic to kinetic energy has been definitively identified that explains all of the observed phenomena. This work introduces new information to the ongoing study by using a neutral beam injector to study the effects of reconnection on a well-known population of ions with initial energies much higher than the bulk population. The acceleration/energization of the fast ions is measured using a new neutral particle analyzer which collects the charge exchange products of the fast ions and the background neutral gas. Measurements indicate an energy gain of 1-10 keV, depending on plasma conditions and initial ion energy (10-25 keV). The acceleration is well described by runaway in a parallel electric field that is inductively generated due to the change in global magnetic flux during reconnection-driven current relaxation. Equilibrium reconstructions indicate this electric field can range from 50-100 V/m and typically lasts around 200 mus.;The effect of the electric field on fast particles has been modeled using a test particle formulation and using the CQL3D Fokker-Planck solver. Both models predict particle acceleration in agreement with measurements. However, the predicted evolution of the bulk and impurity ion distributions from the same electric field greatly differ from previous measurements. It is well known that ⟨v˜ x b˜⟩ and other turbulent electromotive forces are of great importance to the dynamics of the thermal particles; however, the large gyro-orbit and altered rotational transform of the magnetically decoupled fast ions allow them to largely ignore the magnetic fluctuations and directly accelerate in the presence of the inductive electric field. This work motivates the consideration of multiple mechanisms of heating and energization for particles in different regimes of susceptibility to fluctuation-based terms in the parallel force balance.