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1. Time-resolved turbulent dynamo in a laser plasma

   https://doi.org/10.1073/pnas.2015729118  

   Bott, Archie F.; Tzeferacos, Petros; Chen, Laura; Palmer, Charlotte A.; Rigby, Alexandra; Bell, Anthony R.; Bingham, Robert; Birkel, Andrew; Graziani, Carlo; Froula, Dustin H.; et al (March 2021, Proceedings of the National Academy of Sciences)

   null (Ed.)

   Understanding magnetic-field generation and amplification in turbulent plasma is essential to account for observations of magnetic fields in the universe. A theoretical framework attributing the origin and sustainment of these fields to the so-called fluctuation dynamo was recently validated by experiments on laser facilities in low-magnetic-Prandtl-number plasmas ( P m < 1 ). However, the same framework proposes that the fluctuation dynamo should operate differently when P m ≳ 1 , the regime relevant to many astrophysical environments such as the intracluster medium of galaxy clusters. This paper reports an experiment that creates a laboratory P m ≳ 1 plasma dynamo. We provide a time-resolved characterization of the plasma’s evolution, measuring temperatures, densities, flow velocities, and magnetic fields, which allows us to explore various stages of the fluctuation dynamo’s operation on seed magnetic fields generated by the action of the Biermann-battery mechanism during the initial drive-laser target interaction. The magnetic energy in structures with characteristic scales close to the driving scale of the stochastic motions is found to increase by almost three orders of magnitude and saturate dynamically. It is shown that the initial growth of these fields occurs at a much greater rate than the turnover rate of the driving-scale stochastic motions. Our results point to the possibility that plasma turbulence produced by strong shear can generate fields more efficiently at the driving scale than anticipated by idealized magnetohydrodynamics (MHD) simulations of the nonhelical fluctuation dynamo; this finding could help explain the large-scale fields inferred from observations of astrophysical systems. 

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2. Synchrotron Firehose Instability

   https://doi.org/10.3847/1538-4357/acaf54  

   Zhdankin, Vladimir; Kunz, Matthew W.; Uzdensky, Dmitri A. (February 2023, The Astrophysical Journal)

   Abstract We demonstrate using linear theory and particle-in-cell (PIC) simulations that a synchrotron-cooling collisionless plasma acquires pressure anisotropy and, if the plasma beta is sufficiently high, becomes unstable to the firehose instability, in a process that we dub the synchrotron firehose instability (SFHI). The SFHI channels free energy from the pressure anisotropy of the radiating, relativistic electrons (and/or positrons) into small-amplitude, kinetic-scale, magnetic-field fluctuations, which pitch-angle scatter the particles and bring the plasma to a near-thermal state of marginal instability. The PIC simulations reveal a nonlinear cyclic evolution of firehose bursts interspersed by periods of stable cooling. We compare the SFHI for electron–positron and electron–ion plasmas. As a byproduct of the growing electron-firehose magnetic-field fluctuations, magnetized ions gain a pressure anisotropy opposite to that of the electrons. If these ions are relativistically hot, we find that they also experience cooling due to collisionless thermal coupling with the electrons, which we argue is mediated by a secondary ion-cyclotron instability. We suggest that the SFHI may be activated in a number of astrophysical scenarios, such as within ejecta from black hole accretion flows and relativistic jets, where the redistribution of energetic electrons from low to high pitch angles may cause transient bursts of radiation. 

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3. Eppur Si Muove : self-sustained streaming motions in multiphase MHD

   https://doi.org/10.1093/mnras/staf1968  

   Wang, Chaoran; Oh, S Peng; Jiang姜燕飞, Yan-Fei; Kaul, Ish (November 2025, Monthly Notices of the Royal Astronomical Society)

   ABSTRACT Radiative cooling can drive dynamics in multiphase gas. A dramatic example is hydrodynamic ‘shattering’, the violent pressure-driven fragmentation of a cooling cloud that falls drastically out of pressure balance with its surroundings. We run magnetohydrodynamic (MHD) simulations to understand how shattering is influenced by magnetic fields. In MHD, clouds do not ‘shatter’ chaotically. Instead, after initial fragmentation, both hot and cold phases coherently ‘stream’ in long-lived field-aligned self-sustaining gas flows, at high speed ($$\sim 100 \, {\rm km \, s^{-1}}$$). MHD thermal instability also produces such flows. They are due to the anisotropic nature of MHD pressure support, which only operates perpendicular to B-fields. Thus, even when $$P_{\rm B} + P_{\rm gas} \approx$$const, pressure balance only holds perpendicular to B-fields. Field-aligned gas pressure variations are unopposed, and results in gas velocities $$v \sim (2 \Delta P/\rho)^{1/2}$$ from Bernoulli’s principle. Strikingly, gas in adjacent flux tubes counter-stream in opposite directions. We show this arises from a cooling-induced MHD version of the thin shell instability. Magnetic tension is important both in enabling corrugational instability and modifying its non-linear evolution. Even in high $$\beta$$ hot gas, streaming can arise, since magnetic pressure support grows as gas cools and compresses. Thermal conduction increases the sizes and velocities of streaming cloudlets, but does not qualitatively modify dynamics. These results are relevant to the counter-streaming gas flows observed in solar coronal rain, as well as multiphase gas cooling and condensation in the interstellar medium, circumgalactic medium, and intracluster medium. 

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4. Observations of pressure anisotropy effects within semi-collisional magnetized plasma bubbles

   https://doi.org/10.1038/s41467-020-20387-7  

   Tubman, E. R.; Joglekar, A. S.; Bott, A. F.; Borghesi, M.; Coleman, B.; Cooper, G.; Danson, C. N.; Durey, P.; Foster, J. M.; Graham, P.; et al (December 2021, Nature Communications)

   null (Ed.)

   Abstract Magnetized plasma interactions are ubiquitous in astrophysical and laboratory plasmas. Various physical effects have been shown to be important within colliding plasma flows influenced by opposing magnetic fields, however, experimental verification of the mechanisms within the interaction region has remained elusive. Here we discuss a laser-plasma experiment whereby experimental results verify that Biermann battery generated magnetic fields are advected by Nernst flows and anisotropic pressure effects dominate these flows in a reconnection region. These fields are mapped using time-resolved proton probing in multiple directions. Various experimental, modelling and analytical techniques demonstrate the importance of anisotropic pressure in semi-collisional, high- β plasmas, causing a reduction in the magnitude of the reconnecting fields when compared to resistive processes. Anisotropic pressure dynamics are crucial in collisionless plasmas, but are often neglected in collisional plasmas. We show pressure anisotropy to be essential in maintaining the interaction layer, redistributing magnetic fields even for semi-collisional, high energy density physics (HEDP) regimes. 

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5. Global Electron Thermodynamics in Radiatively Inefficient Accretion Flows

   https://doi.org/10.3847/1538-4357/acef12  

   Satapathy, Kaushik; Psaltis, Dimitrios; Özel, Feryal (September 2023, The Astrophysical Journal)

   Abstract In the collisionless plasmas of radiatively inefficient accretion flows, heating and acceleration of ions and electrons are not well understood. Recent studies in the gyrokinetic limit revealed the importance of incorporating both the compressive and Alfvénic cascades when calculating the partition of dissipated energy between the plasma species. In this paper, we use a covariant analytic model of the accretion flow to explore the impact of compressive and Alfvénic heating, Coulomb collisions, compressional heating, and radiative cooling on the radial temperature profiles of ions and electrons. We show that, independent of the partition of heat between the plasma species, even a small fraction of turbulent energy dissipated to the electrons makes their temperature scale with a virial profile and the ion-to-electron temperature ratio smaller than in the case of pure Coulomb heating. In contrast, the presence of compressive cascades makes this ratio larger because compressive turbulent energy is channeled primarily into the ions. We calculate the ion-to-electron temperature in the inner accretion flow for a broad range of plasma properties, mass accretion rates, and black hole spins and show that it ranges between 5 ≲T_i/T_e≲ 40. We provide a physically motivated expression for this ratio that can be used to calculate observables from simulations of black hole accretion flows for a wide range of conditions. 

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