Session BO6: Reconnection

Laser-driven magnetic reconnection in the multi-plasmoid regime

Magnetic reconnection is a promising candidate mechanism for accelerating the nonthermal particles associated with explosive astrophysical phenomena. Laboratory experiments are starting to probe multi-plasmoid regimes of relevance for particle acceleration. We have performed two- and three-dimensional particle-in-cell (PIC) simulations to explore particle acceleration for parameters relevant to laser-driven reconnection experiments. We have extended our previous work [1,2] to explore particle acceleration in larger system sizes. Our results show the transition to plasmoid-dominated acceleration associated with the merging and contraction of plasmoids that further extend the maximum energy of the power-law tail of the particle distribution. Furthermore, we have modeled Coulomb collisions and will discuss the influence of collisionality on the plasmoid formation, dynamics, and particle acceleration. [1] Totorica, S. R., T. Abel, and F. Fiuza, Physical Review Letters, 116, 095003 (2016). [2] Totorica, S. R., T. Abel, and F. Fiuza, Physics of Plasmas, 24, 041408 (2017).   

3-D magnetic reconnection in colliding laser-produced plasmas

Recent experiments have demonstrated magnetic reconnection between colliding plasma plumes, where the reconnecting magnetic fields were self-generated in the expanding laser-produced plasmas by the Biermann battery effect. Using fully kinetic 3-D particle in cell simulations, we conduct the first end-to-end simulations of these experiments, including self-consistent magnetic field generation via the Biermann effect through driven magnetic field reconnection. The simulations show rich, temporally and spatially dependent magnetic field reconnection. First, we find fast, vertically-localized “Biermann-mediated reconnection,” an inherently 3-D reconnection mechanism where the sign of the Biermann term reverses in the reconnection layer, destroying incoming flux and reconnecting flux downstream. Reconnection then transitions to fast, collisionless reconnection sustained by the non-gyrotropic pressure tensor. To separate out the role 3-D mechanisms, 2-D simulations are initialized based on reconnection-plane cuts of the 3-D simulations. These simulations demonstrate: (1) suppression of Biermann-mediated reconnection in 2-D; (2) similar efficacy of pressure tensor mechanisms in 2-D and 3-D; and (3) plasmoids develop in the reconnection layer in 2-D, where-as they are suppressed in 3-D.   

The role of current sheet formation in driven plasmoid reconnection in laser-produced plasma bubbles

We conduct a multiparametric study of driven magnetic reconnection relevant to recent experiments on colliding magnetized laser produced plasmas using the PIC code PSC. Varying the background plasma density, plasma resistivity, and plasma bubble geometry, the results demonstrate a variety of reconnection behavior and show the coupling between magnetic reconnection and global fluid evolution of the system. We consider both collision of two radially expanding bubbles where reconnection is driven through an X-point, and collision of two parallel fields where reconnection must be initiated by the tearing instability. Under various conditions, we observe transitions between fast, collisionless reconnection to a Sweet-Parker-like slow reconnection to complete stalling of the reconnection. By varying plasma resistivity, we observe the transition between fast and slow reconnection at Lundquist number $S \approx 10^3$. The transition from plasmoid reconnection to a single X-point reconnection also happens around $S \approx 10^3$. We find that the criterion $\delta / d_i <1$ is necessary for fast reconnection onset. Finally, at sufficiently high background density, magnetic reconnection can be suppressed, leading to bouncing motion of the magnetized plasma bubbles.   

Electron acceleration in pulsed-power driven magnetic-reconnection experiments

We present recent results from pulsed-power driven magnetic reconnection experiments, fielded on the MAGPIE generator (1.2~MA, 250~ns). The setup used in these experiments produces plasma inflows which are intrinsically magnetised; persist for many hydrodynamic time-scales; and are supersonic. Previous work has focussed on characterising the dynamics of bulk plasma flows \footnote{J. D. Hare et al. (2017) \textbf{Phys. Rev. Lett. 118, 085001}} \footnote{L. G. Suttle et al. (2016) \textbf{Phys. Rev. Lett. 116, 225001}}, using a suite of diagnostics including laser interferometry, (imaging) Faraday rotation, and Thompson scattering. Measurements show the formation of a well defined, long lasting reconnection layer and demonstrate a power balance between the power into and out of the reconnection region. \\ \\ The work presented here focuses on diagnosing non-thermal electron acceleration by the reconnecting electric field. To achieve this, metal foils were placed in the path of accelerated electrons. Atomic transitions in the foil were collisionally exited by the electron beam, producing a characteristic X-Ray spectrum. This X-Ray emission was diagnosed using spherically bent crystal X-Ray spectrometry, filtered X-Ray pinhole imaging, and X-Ray sensitive PIN diodes.   

Fast magnetic reconnection supported by sporadic small-scale Petschek-type shocks

Magnetic reconnection is thought to play a core role in explosive energy conversion. According to the Sweet-Parker theory, it is difficult to conduct magnetic reconnection efficiently in highly conductive plasma. Petschek proposed another reconnection theory. However, numerical simulations suggest that Petschek reconnection is not stable in a system with spatially uniform resistivity. Some mechanism is needed to sustain the localized diffusion region. We perform resistive 2D MHD simulation in a large system with a high spatial resolution, and find that small-scale slow mode MHD shocks predicted by Petschek spontaneously form even under a uniform resistivity. In this process, growth of plasmoids in the current sheet play a role of localizing the diffusion region, and slow mode shocks form next to plasmoids. These plasmoids enhance magnetic reconnection intermittently and repeatedly. As a result, the reconnection rate increases up to 0.02. Furthermore, our simulation suggests that the obtained reconnection rate doesn’t depend on the Lundquist number. This is due to a similarity in the evolution of plasmoid in different scale. A part of this study is published in Physics of Plasmas (Shibayama et al. (2015), Physics of Plasmas, 22, 10, 100706).   

Tearing Instability of a Current Sheet Forming by Sheared Incompressible Flow

Sweet-Parker current sheets are unstable to the tearing mode, suggesting they will not form in physical systems. Understanding magnetic reconnection thus requires study of the stability of a current sheet as it forms. Such formation can occur as a result of sheared, sub-Alfv\'{e}nic incompressible flows into and along the sheet. This work presents an analysis of how tearing perturbations behave in a current sheet forming under the influence of such flows, beginning with a phase when the growth rate of the tearing mode is small and the perturbations are governed by ideal MHD. Later, after the tearing growth rate becomes significant relative to the time scale of the driving flows, a multiple scale formulation can be used to describe linear tearing as occurring at the stationary rates with time dependence added. Once the tearing mode enters the nonlinear regime, the flows and changing magnetic configuration add a new term and time dependence to the stationary rates. This analysis allows greater understanding of reconnection in evolving systems, and increases confidence in the application of tools developed in time-independent current sheets to changing current sheets.   

A Nonlinear evolution of tearing mode with resistivity and hyper-resistivity

A quasilinear model has been developed for nonlinear tearing mode with resistivity and hyper-resistivity in which only the quasilinear current effect has been taken into account. The nonlinear evolution equation has been derived analytically by using the perturbation method. It is shown that the nonlinear evolution of flux perturbation depends on both resistivity term and hyper-resistivity term. It is found that the hyper-resistivity plays a destabilizing effect.   

Scaling Study of Reconnection Heating in Torus Plasma Merging Experiments

We have been investigating toroidal plasma merging and reconnection for high-power heating of spherical tokamak (ST) and field-reversed configuration (FRC), using TS-3 (ST, FRC: R$=$0.2m, 1985-), TS-4 (ST, FRC: R$=$0.5m, 2000-), UTST (ST: R$=$0.45m, 2008-) and MAST (ST: R$=$0.9m, 2000-) devices. The series of merging experiments made clear the promising scaling and characteristics of reconnection heating: (i) its ion heating energy that scales with square of the reconnecting magnetic field B$_{\mathrm{rec}}$, (ii) its energy loss lower than 10{\%}, (iii) its ion heating energy (in the downstream) 10 time larger than its electron heating energy (at around X-point) and (iv) low dependence of ion heating on the guide (toroidal) field B$_{\mathrm{g}}$. The B$_{\mathrm{rec}}^{\mathrm{2}}$-scaling was obtained when the current sheet was compressed to the order of ion gyrodadius. When the sheet was insufficiently compressed, the measured ion temperature was lower than the scaling prediction. Based on this scaling, we realized significant ion heating up to 1.2keV in MAST [1,2] after 2D elucidation of ion heating up to 250eV in TS-3 [3,4]. This promising scaling leads us to new high B$_{\mathrm{rec}}$ reconnection heating experiments for future direct access to burning plasma: TS-U (2017-) in Univ. Tokyo and ST-40 in Tokamak Energy Inc. (2017-). This presentation reviews major progresses in those toroidal plasma merging experiments for physics and fusion applications of magnetic reconnection. [1] Y. Ono, et al., Phys. Plasma 22, 055708 (2015). [2] Y. Ono, et al., Phys. Rev. Lett. 107, 185001, (2011).   

Development of a sub-cm high resolution ion Doppler tomography diagnostics for fine structure measurement of guide field reconnection in TS-U

A new type of high-throughput/high-resolution 96CH ion Doppler tomography diagnostics has been developed using "multi-slit" spectroscopy technique for detailed investigation of fine structure formation during high guide field magnetic reconnection. In the last three years, high field merging experiment in MAST pioneered new frontiers of reconnection heating [1]: formation of highly peaked structure around {\it X}-point in high guide field condition ($B_t>0.3$T), outflow dissipation under the influence of better plasma confinement to form high temperature ring structure which aligns with closed flux surface of toroidal plasma, and interaction between ion and electron temperature profile during transport/confinement phase to form triple peak structure ($\tau^E_{ei}\sim4$ms). To investigate more detailed mechanism with in-situ magnetic measurement, the university of Tokyo starts the upgrade of plasma parameters and spatial resolution of optical diagnostics as in MAST. Now, a new type of high-throughput/high-resolution 96CH ion Doppler tomography diagnostics system construction has been completed and it successfully resolved fine structure of ion heating downstream, aligned with closed flux surface formed by reconnected field. [1] H. Tanabe et al., Phys. Rev. Lett. 115, 215004 (2015)   

Nonthermal particle acceleration in 3D relativistic pair reconnection

Magnetic reconnection in relativistic pair plasma may power nonthermal high-energy flares in astrophysical sources (e.g., the Crab Nebula). Recently, 2D particle-in-cell (PIC) simulations have demonstrated nonthermal particle acceleration (NTPA) that could explain the observed nonthermal (i.e., power-law) photon spectra. However, 3D effects, such as the relativistic drift kink instability (RDKI), have the potential to disrupt NTPA. We present a systematic PIC investigation of 3D relativistic reconnection in collisionless pair plasma, showing that key observationally-relevant aspects of reconnection, such as energy dissipation rate and NTPA, are only weakly affected by increasing "3D-ness"--e.g., by increasing the simulation length in the third dimension or decreasing the guide magnetic field. NTPA remains a robust product of 3D reconnection, despite clear manifestation of the RDKI in the absence of strong guide field. While a strong guide field suppresses RDKI as expected, it also suppresses NTPA (in 2D and 3D), yielding power-law particle spectra with steeper slopes and lower cutoff energies; we conjecture that the effect of the guide field may be captured by including its enthalpy in the magnetization $\sigma$, which has previously been shown to affect the NTPA power-law.   

Non-thermal particle acceleration in 3D magnetic reconnection

Non-thermal particle acceleration is one major unresolved problem in space physics and astrophysics. Recent studies have shown that magnetic reconnection is one primary mechanism for particle energization in space and astrophysical plasmas. Using fully kinetic simulations, these studies have shown the formation of power-law particle energy distributions during reconnection. Mostly of those simulations are two-dimensional (2D), causing energetic particles being artificially confined in magnetic islands with closed field lines. By carrying out similar 2D kinetic simulations, we show that the distribution of accelerated particles integrated over the whole simulation box appears highly non-thermal, it is actually the superposition of a series of distributions in different sectors of 2D magnetic islands. To resolve the issue of artificially particle confinement, we carry out 3D kinetic simulations and show that the mixing of particles are enhanced by the development of turbulence and mixing of magnetic field lines. We investigate the local energy distribution as a result of including the 3D physics.   

Relativistic reconnection in near critical Schwinger field

Magnetic reconnection in relativistic pair plasma with QED radiation and pair-creation effects in the presence of strong magnetic fields is investigated using 2D particle-in-cell simulations. The simulations are performed with the QED module [1] of the OSIRIS framework that includes photon emission by electrons and positrons and single photon decay into pairs (non-linear Breit-Wheeler). We investigate~the effectiveness of reconnection as a pair- and gamma-ray production~mechanism across a broad range of~reconnecting magnetic fields, including those approaching~the critical~quantum (Schwinger) field, and we also explore how the radiative cooling and pair-production processes affect reconnection.~We find that in~the extreme field regime, the magnetic energy is mostly converted into radiation rather than into particle kinetic energy. This study is a first concrete step towards better understanding of magnetic reconnection as a possible mechanism powering gamma-ray flares in magnetar magnetospheres [2] [1] T. Grismayer et al., Physics of Plasmas 23, 056706 (2016) [2] D.A. Uzdensky, Space Science Reviews 160, 45-71 (2011)   

Apex Dips of Experimental Flux Ropes: Helix or Cusp?

We present a new theory for the presence of apex dips in certain experimental flux ropes. Previously such dips were thought to be projections of a helical loop axis generated by the kink instability. However, new evidence from experiments and simulations suggest that the feature is a 2D cusp rather than a 3D helix. The proposed mechanism for cusp formation is a density pileup region generated by nonlinear interaction of neutral gas cones emitted from fast-gas nozzles. The results indicate that small density perturbations can result in large distortions of an erupting flux rope, even in the absence of significant pressure or gravity forces. The density pileup at the apex also suppresses the m=1 kink mode by acting as a stationary node. Consequently, more accurate density profiles should be considered when attempting to precisely model the stability and eruption of solar flux ropes such as CME's.   

Relativistic magnetic reconnection driven by a moderately intense laser interacting with a micro-plasma-slab.

Magnetic reconnection (MR) in the relativistic regime is generally thought to be responsible for powering rapid bursts of non-thermal radiation in astrophysical events. It is therefore of significant importance to study how the field energy is transferred to the plasma to power the observed emission. However, due to the difficulty in making direct measurements in astrophysical systems or achieving relativistic MR in laboratory environments, the particle acceleration is usually studied using fully kinetic PIC simulations. Here we present a numerical study of a readily available (TW-mJ-class) laser interacting with a micro-scale plasma slab. The simulations show when the electron beams excited on both sides of the slab approach the end of the plasma structure, ultrafast relativistic MR occurs. As the field topology changes, the explosive release of magnetic energy results in emission of relativistic electron jets with cut-off energy \textasciitilde 12 MeV. The proposed novel scenario can be straightforwardly implemented in experiments, and might significantly improve the understanding of fundamental questions such as field dissipation and particle acceleration in relativistic MR.