Non-linear Development of Streaming Instabilities in Magnetic Reconnection with a Strong Guide Field

Magnetic reconnection is recognized as a dominant mechanism for converting magnetic energy into the convective and thermal energy of particles, and the driver of explosive events in nature and laboratory. Magnetic reconnection is often modeled using resistive magnetohydrodynamics, in which collisions play the key role in facilitating the release of energy in the explosive events. However, in space plasma the collisional resistivity is far below the required resistivity to explain the observed energy release rate. Turbulence is common in plasmas and the anomalous resistivity induced by the turbulence has been proposed as a mechanism for breaking the frozen-in condition in magnetic reconnection. Turbulence-driven resistivity has remained a poorly understood, but widely invoked mechanism for nearly 50 years. The goal of this project is to understand what role anomalous resistivity plays in fast magnetic reconnection. Turbulence has been observed in the intense current layers that develop during magnetic reconnection in the Earth's magnetosphere. Electron streaming is believed to be the source of this turbulence. Using kinetic theory and 3D particle-in-cell simulations, we study the nonlinear development of streaming instabilities in 3D magnetic reconnection with a strong guide field. Early in time an intense current sheet develops around the x-line and drives the Buneman instability. Electron holes, which are bipolar spatial localized electric field structures, form and then self-destruct creating a region of strong turbulence around the x-line. At late time turbulence with a characteristic frequency in the lower hybrid range also develops, leading to a very complex mix of interactions. The difficulty we face in this project is how to address a long-standing problem in nonlinear kinetic theory: how to treat large amplitude perturbations and the associated strong wave-particle interactions. In my thesis, I address this long-standing problem using particle-in-cell simulations and linear kinetic theory.Some important physics have been revealed. 1: The lower hybrid instability (LHI) dominates the dynamics in low $beta$ plasma in combination with either the electron-electron two-stream instability (ETS) or the Buneman instability (BI), depending on the parallel phase speed of the LHI. 2: An instability with a high phase speed is required to tap the energy of the high velocity electrons. The BI with its low phase speed, can not do this. The ETS and the LHI both have high phase speed. 3: The condition for the formation of stable electron holes requires $|v_p -v_g|< sqrt{2e|phi|/m_e}$, where $|phi|$ is the amplitude of the electric potential, and $v_p$ and $v_g$ are the phase and group velocity of the relevant waves. Like ETS and BI, LHI all can form electron holes. 4: The overlapping resonance in phase space is the dominant mechanism for transporting the momentum and energy from high velocity electrons to low velocity electrons, which then couple to the ions

Jet deflection by very weak guide fields during magnetic reconnection

Simulations of antiparallel reconnection have shown collimated electron jets outflowing from the x-point, and associated highly elongated "outer electron diffusion regions." New PIC simulations with ion/electron mass ratios as large as 1836 show that jets are deflected towards the magnetic separatrix by out-of-plane guide fields, Bg, as small as 0.05 times the asymptotic reconnecting field, B0. The outer electron diffusion region is distorted and broken up, but the diffusion rate is unchanged. These results are interpreted in terms of electron dynamics and are compared to recent measurements of reconnection jets in the magnetosheath

Grain engineering of Sb2S3 thin films to enable efficient planar solar cells with high open-circuit voltage

Sb2S3 is a promising environmentally friendly semiconductor for high performance solar cells. But, like many other polycrystalline materials, Sb2S3 is limited by nonradiative recombination and carrier scattering by grain boundaries (GBs). This work shows how the GB density in Sb2S3 films can be significantly reduced from 1068 ± 40 to 327 ± 23 nm µm−2 by incorporating an appropriate amount of Ce3+ into the precursor solution for Sb2S3 deposition. Through extensive characterization of structural, morphological, and optoelectronic properties, complemented with computations, it is revealed that a critical factor is the formation of an ultrathin Ce2S3 layer at the CdS/Sb2S3 interface, which can reduce the interfacial energy and increase the adhesion work between Sb2S3 and the substrate to encourage heterogeneous nucleation of Sb2S3, as well as promote lateral grain growth. Through reductions in nonradiative recombination at GBs and/or the CdS/Sb2S3 heterointerface, as well as improved charge-carrier transport properties at the heterojunction, this work achieves high performance Sb2S3 solar cells with a power conversion efficiency reaching 7.66%. An impressive open-circuit voltage (VOC) of 796 mV is achieved, which is the highest reported thus far for Sb2S3 solar cells. This work provides a strategy to simultaneously regulate the nucleation and growth of Sb2S3 absorber films for enhanced device performance

Suprathermal Electron Transport and Electron Beam Formation in the Solar Corona

Electron beams that are commonly observed in the corona were discovered to be associated with solar flares. These “coronal” electron beams are found ≥300 Mm above the acceleration region and have velocities ranging from 0.1 c up to 0.6 c . However, the mechanism for producing these beams remains unclear. In this paper, we use kinetic transport theory to investigate how isotropic suprathermal energetic electrons escaping from the acceleration region of flares are transported upwardly along the magnetic field lines of flares to develop coronal electron beams. We find that magnetic focusing can suppress the diffusion of Coulomb collisions and background turbulence and sharply collimate the suprathermal electron distribution into beams with the observed velocity within the observed distance. A higher bulk velocity is produced if energetic electrons have harder energy spectra or travel along a more rapidly expanding coronal magnetic field. By modeling the observed velocity and location distributions of coronal electron beams, we predict that the temperature of acceleration regions ranges from 5 × 10 ^6 to 2 × 10 ^7 K. Our model also indicates that the acceleration region may have a boundary where the temperature abruptly decreases so that the electron beam velocity can become more than triple (even up to 10 times) the background thermal velocity and produce the coronal type III radio bursts

How electron two-stream instability drives cyclic Langmuir collapse and continuous coherent emission.

Continuous plasma coherent emission is maintained by repetitive Langmuir collapse driven by the nonlinear evolution of a strong electron two-stream instability. The Langmuir waves are modulated by solitary waves in the linear stage and electrostatic whistler waves in the nonlinear stage. Modulational instability leads to Langmuir collapse and electron heating that fills in cavitons. The high pressure is released via excitation of a short-wavelength ion acoustic mode that is damped by electrons and reexcites small-scale Langmuir waves; this process closes a feedback loop that maintains the continuous coherent emission

How electron two-stream instability drives cyclic Langmuir collapse and continuous coherent emission.

Continuous plasma coherent emission is maintained by repetitive Langmuir collapse driven by the nonlinear evolution of a strong electron two-stream instability. The Langmuir waves are modulated by solitary waves in the linear stage and electrostatic whistler waves in the nonlinear stage. Modulational instability leads to Langmuir collapse and electron heating that fills in cavitons. The high pressure is released via excitation of a short-wavelength ion acoustic mode that is damped by electrons and reexcites small-scale Langmuir waves; this process closes a feedback loop that maintains the continuous coherent emission