Session JM11: Mini-Conference on Reconnection: Laboratory

Live

Magnetic reconnection is explored on the Terrestrial Reconnection Experiment where the absolute rate of reconnection is set by an external drive. A shock interface between the supersonically driven plasma inflow and a region of magnetic flux pileup permits the normalized reconnection rate to self regulate to a fixed value. In agreement with numerical and theoretical results, the width of the electron diffusion regions is characterized by the kinetic length scale of the electrons. While the reconnection layers are modulated by a current-driven instability, their characteristics remain consistent with a 3D simulation for which off-diagonal stress in the electron pressure tensor is responsible for fast reconnection.   

Live

During magnetic reconnection, the electron fluid decouples from the magnetic field within narrow current layers, and theoretical models for this process can be distinguished in terms of their predicted current layer widths. In agreement with numerical and theoretical results, we present observations from the Terrestrial Reconnection EXperiment (TREX) that confirm the presence of electron scale current layer widths. Although the layers are modulated by a current-driven instability, the narrow experimental current layers are consistent with a 3D simulation for which off-diagonal terms in the electron pressure tensor are responsible for fast reconnection. Further analysis of TREX's new toroidal Bdot probe array will be included.   

Live

In the Magnetic Reconnection Experiment (MRX), there have been observations of various waves such as the quasi-electrostatic lower hybrid drift wave (ES-LHDW), electromagnetic lower hybrid drift wave (EM-LHDW), and whistler wave. Here we provide an overview of these measurements in comparison to space observations. During antiparallel asymmetric reconnection, the whistler wave is observed near the low-density side separatrix together with lower hybrid drift instabilities. This whistler wave is driven by temperature anisotropy of energetic electrons, while the instabilities are caused by multiple, fast-growing ES-LHDW modes, which are driven by density gradients. ES-LHDW becomes stable in the electron diffusion region (EDR) without a guide field due to the stabilization effects of the high plasma beta. EM-LHDW, on the other hand, exists in the EDR but the role of EM-LHDW in fast reconnection is found to be limited. Recently, ES-LHDW has been observed near the electron diffusion region with a significant guide field that lowers the local plasma beta. To explain the excitation of LHDW, a local linear model has been developed. This model explains both space and laboratory observations and provides a theoretical framework for wave-associated anomalous resistivity and electron heating.\\ \\In collaboration with: Hantao Ji, Princeton University; Jonathan Jara-Almonte, Princeton Plasma Physics Laboratory; Sayak Bose, Princeton Plasma Physics Laboratory; Aaron Goodman, Princeton Plasma Physics Laboratory; Andrew Alt, Princeton Plasma Physics Laboratory; Masaaki Yamada, Princeton Plasma Physics Laboratory; Will Fox, Princeton Plasma Physics Laboratory   

Live

Guide field reconnection is studied by varying normalized guide field, $b_{gn}=b_{g}/b_{up}$ from 0 to 1.3, where $b_{g}$ is the guide field at X-point and $b_{up}$ is the upstream reconnecting magnetic field. The current sheet profile resembles a Harris sheet at zero guide field. At a finite guide field, the thickness of the current sheet increases. Reconnection inflow advects the guide field towards the X-point while the boundary conditions of MRX impedes the outflow causing the guide field to pile up in the reconnection region. Theoretically predicted magnetic field profiles for current sheets that are intermediate between Harris and force-free Harris sheets are found to be in good agreement with the experimental data. This suggests that the non-uniform guide field pressure due to the pile up may contribute to the force balance in both inflow and outflow directions, thus affecting the reconnection rate. We also analyze the role of non-uniform guide field in producing a density gradient in the reconnection region which in turn can affect the parallel electric field by Ohm’s law.   

Live

Magnetic reconnection has been widely recognized as a fundamental plasma process underlying many explosive energetic phenomena observed throughout the Universe, within (heliophysics) or beyond (astrophysics) our solar system. These plasmas are characterized by their large Lundquist numbers, $S$, and normalized system sizes, $\lambda$. In two dimensions, different phases of magnetic reconnection can be organized in phase diagrams in terms of these parameters. When at least one of $S$ or $\lambda$ is sufficiently large, magnetic reconnection enters a multiple X-line phase exhibiting its dynamic nature of interacting plasmoids. The statistical properties of these phases are of crucial importance in determining the energetic consequences of magnetic reconnection, including acceleration of electrons to non-thermal values, responsible for observational signatures of magnetic reconnection. This talk provides a concise summary of recent progress in quantifying statistical properties of these regimes from both observations and numerical simulations, including distribution functions of plasmoids or magnetic structures, and magnetic dissipation with regards to magnetic structures. Implications for laboratory experiments will be discussed.   

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Some of the most outstanding challenges in reconnection physics [1] involve the coupling between MHD and kinetic scales, the development of 3D magnetic turbulence via the plasmoid instability, and the influence of kinetic scale plasma instabilities on the dissipation physics. New progress on these difficult issues will require close collaboration across the three main approaches used to study reconnection: theory and simulation, in-situ and remote observations, and laboratory experiments. Here, we present new results from efforts to model the FLARE and TREX reconnection experiments using particle-in-cell simulations. These simulations feature the realistic experimental 3D toroidal geometries, boundary conditions, current drive, and collisions. For the TREX experiment, we will present results from a validated study of collisionless electron-scale reconnection layers that are modulated by a 3D toroidal drift instability. For the new FLARE experiment, we study the role of 3D effects on the transition from collisional-to-kinetic reconnection via the oblique plasmoid instability, and present scoping simulations for the upcoming FLARE experimental campaigns. [1] H. Ji et al. (2020). https://arxiv.org/abs/2004.00079\\ \\In collaboration with: William Daughton, Los Alamos Natl Lab; Ari Le, Los Alamos Natl Lab; Samuel Greess, University of Wisconsin; Jan Egedal, University of Wisconsin Madison; Jonathan Jara-Almonte, Princeton Plasma Physics Laboratory; Hantao Ji, Princeton Plasma Physics Laboratory   

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We report on experiments conducted on the Omega and Omega EP laser facilities of the Laboratory for Laser Energetics (LLE), Rochester, NY, USA. Two neighboring plasma bubbles are produced by intense laser beams and the self-generated Biermann fields reconnect in the process of the bubbles' expansion and collision. In a typical shot, two oppositely directed magnetic fields, each of about $15$\,T, reconnect in a fraction of a nanosecond within a $50\,\mu m$-wide current sheet. The reconnection-induced electric field of about $10^7$\,V/m - $10^8$\,V/m can potentially accelerate the plasma electrons to a high energy. We present measurements of the electron energy spectra obtained in a series of shots that are carefully designed to reduce the effect of other sources of hot electrons, like LPI (Laser-Plasma Interactions). To isolate the reconnection as an acceleration source, the spectra obtained from two-bubble reconnecting shots are compared to the spectra from single-bubble, non-reconnecting shots. The results are compared with numerical PIC simulations and theoretical models.   

Live

Here we present recent progress of our exploration of high field reconnection heating in the ST40 and TS-6 merging spherical tokamak formation experiments. Ion heating in keV regime as in the MAST spherical tokamak has successfully been reproduced in ST40 and supporting heating/transport physics has now been investigated in TS-6. From the formation of first plasma in 2018 in both projects, ST40 demonstrated successful application scenario (connection of merging startup plasma to steady state) in 2019 and TS-6 enables full-2D ion temperature profile measurement with gyro-scale spatial resolution using 96CH/320CH ion Doppler tomography. Based on outflow heating mechanism ($\Delta T_i \propto B_{rec}^2$), magnetic reconnection forms high temperature region in the downstream of outflow jet and forms fine structure. Blob-like structure in the diffusion region is ejected toward downstream and forms clear hot spots after merging, while the hot spots on the closed flux surface formed by reconnected field lines are transported on the field line direction with the weight of $\kappa^i_{\parallel}/ \kappa^i_{\perp}=2(\omega_{ci}\tau_{ii})^2>>1$ under the influence of high guide field and finally forms poloidally ring-like structure.