Session JO08: Fundamental Plasma Physics: Reconnection & other topics

Tearing-mediated reconnection in magnetohydrodynamic poorly ionized plasmas. II. Nonlinear evolution

Many plasma environments, such as star-forming molecular clouds, the solar chromosphere, and the diffuse interstellar medium, are poorly ionized and threaded by dynamically important magnetic fields. Recently, we have demonstrated using a combination of analytical arguments and numerical simulations that the onset of magnetic reconnection in these poorly ionized systems is fundamentally different than what occurs in fully ionized plasmas. In this talk, we present a continuation of this work focused on the non-linear evolution of poorly ionized, tearing current sheets and their progression into steady-state reconnection. As in fully-ionized plasmas, after reconnection onsets in a current sheet, the system enters a nonlinear phase characterized by a stochastic plasmoid chain, but the characteristics of this chain differ from those of a stochastic plasmoid chain in fully ionized plasma. The reconnection rate of the system increases with the recombination rate in the plasma. In addition, the plasma in the plasmoids is characterized by an ionization fraction which is much larger than that of the background plasma. Our results could have significant implications for understanding of several important astrophysical processes, including the transport of cosmic rays in the interstellar medium.   

The Role of Alfvén Resonances in Forced Magnetic Reconnection and its impacts on Kelven Helmholtz Instability

Alfvén resonance and magnetic reconnection are fundamental plasma processes which have received much attention in the past because of their role in heating space and laboratory plasmas. We demonstrate here that magnetic reconnection induced by an external driver, or forced magnetic reconnection, is a limiting case of Alfvén resonance parameterized by the frequency of the driver. In addition to demonstrating the transition between these processes, we determine several scalings, such as the dependence of the constant-psi reconnected flux on the driver frequency, showing that a small but finite frequency above the inverse of the reconnection time strongly suppresses reconnection. As a non-linear application of this study on Alfven resonances, we extend prior work by analyzing the saturation of a Kelvin-Helmholtz instability in the presence of Alfvén resonances. The width of the island resulting from a KH roll-up is parameterized as a function of the initial Mach number and the ratio of velocity and magnetic field shear widths. Finally, we study the analytic form of the saturated equilibrium state to understand the nonlinear behavior of this system.   

Laboratory observation of lower hybrid drift waves during the formation of current sheet in anti-parallel magnetic reconnection

Magnetic reconnection is widely recognized as one key process to power most energetic and explosive phenomena throughout the universe by reconfiguring magnetic field topology in plasmas, while releasing the stored magnetic energy. A build-up phase must precede the sudden onset of fast reconnection to accumulate the required magnetic energy to be released. Numerical simulations suggest that lower hybrid drift waves (LHDWs) can facilitate the initiation of fast reconnection. In MRX, an additional reconnection electric field is applied to the initially weak reconnection field to study the dynamic current sheet formation process. LHDWs are excited during the current sheet thinning and the reconnection magnetic field pileup. These LHDWs are spatially and temporally correlated to the electron pressure gradient, suggesting the diamagnetic drift as a free energy source of LHDWs. Initial results suggest that observed LHDWs can provide additional electron heating in the current sheet during the current sheet formation. This may enhance kinetic effects in the electron diffusion region, thereby facilitating fast reconnection.    

Lower Hybrid Drift Waves and Energy Transfer near the Electron Diffusion Regions of Magnetic Reconnection

Kinetic plasma waves are considered to play important roles in the dissipation process during collisionless magnetic reconnection beyond its standard two-dimensional and laminar models. One of the candidate waves is lower hybrid drift waves (LHDW) which are often observed within or nearby the electron diffusion regions (EDR) both in the laboratory and in space [1]. Here we present the results from analyzing measurements during 17 magnetopause and 10 magnetotail reconnection events by Magnetospheric MultiScale (MMS) mission. Our analysis shows that LHDW type depends on the electron beta, as electron beta increases LHDWs become more electromagnetic in nature, consistent with previous measurements in the lab [2,3] and in space [4]. The acceleration/heating is higher in electrostatic LHDWs and it is mostly in parallel direction, consistent with the recent results from the lab [5]. LHDW properties from MMS observations are compared quantitatively with linear dispersion theory [4], and its role during reconnection will be discussed.

[1] H. Ji et al., Space Science Reviews 219, 76 (2023).

[2] T. Carter et al., Phys. Rev. Lett. 88, 015001 (2002).

[3] H. Ji et al., Phys. Rev. Lett. 92, 115001  (2004).

[4] J. Yoo et al., Geophys. Res. Lett. 47, e2020GL087192 (2020).

[5] J. Yoo et al., Phys. Rev. Lett. 132, 145101 (2024).   

Electron-only reconnection and inverse magnetic-energy transfer at sub-ion scales

Electron-only reconnection is a type of magnetic reconnection occurring in sufficiently small regions where ions remain unresponsive to reconnection dynamics. In this work, we derive, and validate numerically, an analytical model for electron-only magnetic reconnection applicable to strongly magnetized (low-beta) plasmas. Our model predicts sub-ion-scale reconnection rates significantly higher than those pertaining to MHD scale reconnection, aligning with recent observations and simulations. We then apply this reconnection model to the problem of inverse magnetic-energy transfer at sub-ion scales. We derive time-dependent scaling laws for the magnetic energy decay that differ from those previously found in the MHD regime. These scaling laws are validated via two- and three-dimensional simulations, demonstrating that sub-ion scale magnetic fields can reach large, system-size scales via successive coalescence. The properties of the associated plasma turbulence, including the energy spectrum and typical magnetic structure dimensions are investigated.   

A transport-like approach to turbulence-mediated magnetic reconnection

A transport-like framework for the study of magnetic reconnection mediated by self-driven turbulence is proposed, based on timescale separation between the reconnection time and the characteristic timescale of the turbulent fluctuations which arise in the reconnection layer. We argue that the mean fields remain on MHD scales even in collisionless cases. These observations provide theoretical justification for an efficient computational approach to this problem, which we discuss.   

Nonthermal particle acceleration in simulations of relativistically hot plasma turbulence

In weakly collisional plasmas, particle interactions with turbulent fluctuations can result in plasma heating and nonthermal particle acceleration. Numerical simulations have shown that the particle energy probability density function (pdf) develops power-law tails at ultrarelativistic energies in both driven relativistically hot plasma turbulence [1] and decaying magnetically dominated plasma turbulence [2]. In [3], we presented a phenomenological model of nonthermal acceleration where particles are energized stochastically in magnetic traps and showed it to be in good agreement with 2.5D simulations of decaying magnetically dominated plasma turbulence. In this presentation, we extend our analysis to 3D simulations of both the driven and decaying cases. The theoretical spectral index of the particle energy pdf is compared to numerical observations and we discuss whether particle acceleration in driven relativistically hot turbulence that is only moderately magnetized should be attributed to particles interacting with turbulent fluctuations in the inertial range or the magnetic field on the outer scale.

[1] – Zhdankin, V., Uzdensky, D. A., Werner, G. R., Begelman, M. C. 2017, PhRvL, 118, 055103.

[2] – Comisso, L., Sironi, L. 2018, PhRvL, 121, 255101.

[3] – Vega, C., Boldyrev, S., Roytershteyn, V., & Medvedev, M. 2022, ApJL, 924, L19.   

Evidence for radiative collapse and magnetic flux pile-up in a radiatively cooled magnetic reconnection experiment

The Magnetic Reconnection on Z (MARZ) platform achieves strong cooling in a magnetic reconnection experiment driven by pulsed power. Two inverse aluminum wire arrays, simultaneously fielded on the Z machine (20 MA peak current, 300 ns rise time, Sandia National Labs), generate oppositely-directed supersonic and super-Alfvénic plasma flows with anti-parallel magnetic fields. Interaction of the magnetized flows generates a reconnection layer (S_L ≈ 120, normalized reconnection rate ≈ 0.3), where line and recombination-bremsstrahlung losses provide a cooling rate much larger than the Alfvénic transit rate (τ_cool^-1/ τ_A^-1  ≈ 50). Time-resolved characterization of the rapidly-falling >1keV X ray emission, and simultaneously rising visible emission from the reconnection layer, demonstrates decreasing layer temperature and rising density, indicative of radiative collapse. X ray diode and spectroscopic measurements provide constraints on the layer temperature. The reconnection layer is further visualized using multi-frame self-emission imaging of optical (540-650 nm) emission. These images show planar discontinuous regions of enhanced emission upstream of the reconnection layer, providing evidence for shock-mediated magnetic flux pile-up, consistent with the super-Alfvénic inflows.

*SNL is managed and operated by NTESS under DOE NNSA contract DE-NA0003525   

Forced reconnection in 3D fields

We present a theory of the 3D magnetic reconnection that occurs when an Alfvén-wave packet is introduced to separating field lines. The separation causes magnetic-flux tubes to narrow and thus the perpendicular (to the local field) wavenumber of the packet to increase, allowing magnetic diffusion to be accessed. Under resistive magnetohydrodynamics (MHD), we show that field lines that separate linearly with distance (linear magnetic shear) reconnect on a timescale proportional to S^3/5, where S is the Lundquist number. This the same as in the 2D tearing-mode theory of Furth, Killeen & Rosenbluth (1963). If the field-line separation is instead exponential with distance, we show that MHD reconnection is suppressed: its timescale becomes proportional to S log S. In the case that the driven Alfvén waves are kinetic (i.e., their perpendicular wavelength is smaller than the ion Larmor radius), we show that resistive reconnection is faster than in the MHD case, with timescale proportional to S^1/3. We discuss the application of our theory to reconnection at a separatrix in a magnetic-confinement-fusion device.   

Two-Fluid Effects on Linear Tearing Mode Stability

In a two-fluid model, equilibrium rotation is tied to “flow surfaces” that are close to, but distinct from, magnetic surfaces. These flow surfaces produce a small but nonzero radial velocity. This is expected to create qualitative modifications in the behavior of modes localized on magnetic surfaces such as tearing modes. We report progress on an exploratory investigation of linear tearing mode behavior when equilibrium poloidal rotation is included in a two-fluid model. Using slab geometry, we were able to show that the layer equation for the fundamental mode must include new terms that depend on the third derivative of the mode sidebands. The direct solution of the coupled system of equations has proven extremely challenging. For this reason, an approximate solution using asymptotic matching is currently underway. Working in Fourier space allows all equations to combine into one. The boundary layer thickness is not as trivially determined as in the single-mode case but can be calculated efficiently by constructing a "Kruskal-Newton diagram", a tool underutilized in the current literature. Preliminary results have indicated that solutions may take the form of exotic special functions which have different asymptotic properties than in the single fluid case. With these tools, we are progressing towards quantifying changes in tearing mode stability in the presence of two-fluid equilibrium flow.   

A 10-moment multi-fluid model for partially magnetized plasmas

Fluid moment models are attractive to plasma modellers because of their significant computational advantage over higher-fidelity kinetic models. However, real plasmas exhibit a number of phenomena that traditional equilibrium fluid models are incapable of capturing, e.g., Landau damping, Bernstein modes and the Weibel instability. Especially in the presence of magnetic field, anisotropic transport can lead to deviation from local equilibrium (Maxwellian velocity distribution function), when collisional timescales are comparable to the dynamical timescales. In this study, we present the results from applying a 10-moment multi-fluid model, which can capture the evolution of a full pressure tensor, in addition to density and bulk velocity. The model is applied to a low temperature plasma test case, the (electrostatic) discharge plasma of a Hall-effect thruster [1], and a high temperature plasma test case, the electromagnetic Weibel instability. The results are compared to previous studies [2,3] and theory [4].

[1] Kuldinow, Derek A., et al. J. Comp. Phys. 508 (2024): 113030.

[2] Sahu, Mansour, and Hara, Phys. Plasmas 27, 113505 (2020)

[3] Yamashita, Lau, and Hara, J. Phys. D Appl. Phys. 56, 384003 (2023)

[4] Basu, B. Phys. Plasmas 9.12 (2002): 5131-5134.   

Mixed resolution of the identity compressed exchange for predicting the electronic structure of warm dense matter and hot dense plasmas from first principles

Mixed deterministic-stochastic density functional theory (mDFT) allows for the prediction of electronic structure and transport properties in warm dense matter (WDM) and hot dense plasma (HDP) with high efficiency. Predicting excited electronic state energies in WDM/HDP requires wavefunction-based methods with one of the simplest yet most effective being hybrid exact exchange functionals. Compressed hybrid functionals have come to prominence in computational materials science but must be reformulated for application to mDFT for WDM/HDP. We formulate a mixed resolution of the identity compressed exchange (mRICE) to compress the exact exchange operator and implement this approach in the SHRED planewave DFT code. We demonstrate the strengths of mRICE by calculating the electronic density of states of warm dense neon and carbon between temperatures 10 and 50 eV. We can achieve significant compression of the exchange operator to nearly 50%, rendering a significant speedup in calculations of electronic structure with hybrid exchange in WDM/HDP while maintaining accuracy. We compare the accuracy of our approach to the fully deterministic adaptively compressed exchange formulated by Lin. We find that mRICE is a powerful compression scheme to perform hybrid exact exchange calculations in WDM and HDP.    

High Voltage Generation for a Cryogenic Neutron Electric Dipole Experiment

An in-situ voltage multiplier was designed to generate a static electric field in superfluid helium-4 at 400 mK for an experiment to search for the neutron electric dipole moment. The voltage required on a central electrode is about 650 kV. Direct supply of such a voltage from a room temperature power supply into the sub-1 Kelvin environment would be very difficult in terms of cryogenic compatibility, so this experiment aims to use concepts from 18th century Cavallo's Multiplier to amplify a moderate supply voltage up to a sufficient level. In this talk, results from testing this apparatus at room-temperature, prior to its installation in a cryostat for low temperature testing, will be presented.   

Collective plasma effects and strong field QED

The fast development of laser facilities in the past few decades has opened the door for testing several strong field QED effects in recent and future facilities. This has increased the theoretical interest in studying strong QED effects in both plasma and vacuum. In this work, we study the effects of quantum relativistic mechanisms such as Schwinger pair-creation together with classical collective plasma effects. We use a fully quantum kinetic model [1-4] to study the interplay between classical plasma physics and quantum mechanics for ultra-strong fields. In addition, we use a classical but relativistic kinetic equation to show how the predictions from classical theory differ from those from fully quantum theory. The conclusions from this work can be used to identify when semi-classical theories start to be inappropriate to describe the dynamics of plasma in ultrastrong electromagnetic fields.