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1. Preliminary study of plasma modes and electron-ion collisions in partially magnetized strongly coupled plasmas

   https://doi.org/10.1103/PhysRevE.109.015201  

   Pak, Chanhyun; Billings, Virginia; Schlitters, Matthew; Bergeson, Scott D; Murillo, Michael S (January 2024, Physical Review E)

   Magnetic fields influence ion transport in plasmas. Straightforward comparisons of experimental measurements with plasma theories are complicated when the plasma is inhomogeneous, far from equilibrium, or characterized by strong gradients. To better understand ion transport in a partially magnetized system, we study the hydrodynamic velocity and temperature evolution in an ultracold neutral plasma at intermediate values of the magnetic field. We observe a transverse, radial breathing mode that does not couple to the longitudinal velocity. The inhomogeneous density distribution gives rise to a shear velocity gradient that appears to be only weakly damped. This mode is excited by ion oscillations originating in the wings of the distribution where the plasma becomes non-neutral. The ion temperature shows evidence of an enhanced electron-ion collision rate in the presence of the magnetic field. Ultracold neutral plasmas provide a rich system for studying mode excitation and decay. 

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2. Multi-dimensional incoherent Thomson scattering system in PHAse Space MApping (PHASMA) facility

   https://doi.org/10.1063/5.0133665  

   Shi, Peiyun; Scime, Earl E. (February 2023, Review of Scientific Instruments)

   A multi-dimensional incoherent Thomson scattering diagnostic system capable of measuring electron temperature anisotropies at the level of the electron velocity distribution function (EVDF) is implemented on the PHAse Space MApping facility to investigate electron energization mechanisms during magnetic reconnection. This system incorporates two injection paths (perpendicular and parallel to the axial magnetic field) and two collection paths, providing four independent EVDF measurements along four velocity space directions. For strongly magnetized electrons, a 3D EVDF comprised of two characteristic electron temperatures perpendicular and parallel to the local magnetic field line is reconstructed from the four measured EVDFs. Validation of isotropic electrons in a single magnetic flux rope and a steady-state helicon plasma is presented. 

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3. Model for temperature anisotropy relaxation in non-neutral plasmas

   https://doi.org/10.1063/5.0281155  

   Jose, Louis; LeVan, Jarett; Baalrud, Scott_D (July 2025, Physics of Plasmas)

   Non-neutral plasma experiments are excellent benchmarks for validating transport models, including in strongly coupled conditions. Experiments with Penning–Malmberg traps operate under the Brillouin limit, which means that the plasma is also strongly magnetized in the sense that the gyrofrequency exceeds the plasma frequency. This is an unusual regime that is not described by traditional plasma kinetic theory, particularly when strong coupling and strong magnetization are both present. Here, we apply a recently developed generalized Boltzmann kinetic theory to compute the temperature anisotropy relaxation rate in this regime. Strong magnetization is found to severely suppress energy exchange during collisions, leading to a drastically reduced anisotropy relaxation rate. The results exhibit good agreement with previous work by Glinsky et al. when the plasma is weakly coupled and extend the calculation to the strongly coupled regime as well. Results are compared with published experimental measurements, demonstrating good agreement. Furthermore, the model is tested using molecular dynamics simulations over a broader range of parameters than the experiments reached. These simulations utilize a new Green–Kubo relation, enabling an equilibrium simulation method that is more accurate than previous non-equilibrium methods that have been applied to this problem. Finally, a discussion of detailed balance in strongly magnetized plasmas is provided. Specifically, it is shown that despite the absence of time-reversal symmetry, which is usually used to mathematically prove detailed balance, the results satisfy detailed balance to a high degree of numerical precision. 

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4. Model for binary collisions in strongly magnetized plasmas: Repulsive interactions

   https://doi.org/10.1063/5.0290568  

   Baalrud, S_D; Jose, L.; Lafleur, T. (October 2025, Physics of Plasmas)

   A reduced model is developed to describe the outcome of collisions between two like-charged particles in the presence of a strong magnetic field. Two cases are considered: large mass ratio (e.g., positron–proton or electron–antiproton) and unity mass ratio (e.g., electron–electron or ion–ion). The model applies to the asymptotic regime of strong magnetization, where the gyroradius of the low-mass particle is small compared to the interaction spatial scale (of the order of the Debye length in a weakly coupled plasma). The ion is assumed to be weakly magnetized in the two-component case. The positron (or electron) magnetic moment is assumed to be conserved during the collision, satisfying the first adiabatic invariant. The model then solves for other aspects of the charged particle motion perturbatively in orders of the inverse magnetic field strength. For the positron–ion case, this includes the velocity vector of the ion, the change in velocity of the positron parallel to the magnetic field, and the spatial shift of the positron gyrocenter. For the identical particle case, this includes the relative speed of the two particles in the parallel direction and the shift of the relative gyrocenters of the particles. An important aspect of the model is the identification of a generalized conserved momentum. The results enable the determination of the outcome of collisions with far lower computational resources than required for full orbit calculations, and can be utilized to rapidly evaluate transport rates for kinetic theories. The regimes considered are expected to be particularly relevant to experiments that trap antimatter. 

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5. On the rheology and magnetization of dilute magnetic emulsions under small amplitude oscillatory shear

   https://doi.org/10.1017/jfm.2022.1019  

   Abdo, Rodrigo F.; Abicalil, Victor G.; Cunha, Lucas H.P.; Oliveira, Taygoara F. (January 2023, Journal of Fluid Mechanics)

   A dilute magnetic emulsion under the combined action of a uniform external magnetic field and a small amplitude oscillatory shear is studied using numerical simulations. We consider a three-dimensional domain with a single ferrofluid droplet suspended in a non-magnetizable Newtonian fluid. We present results of droplet shape and orientation, viscoelastic functions and bulk emulsion magnetization as functions of the shear oscillation frequency, magnetic field intensity and orientation. We also investigate how the magnetic field induces mechanical anisotropy by producing internal torques in oscillatory conditions. We found that, when the magnetic field is parallel to the shear plane, the droplet shape is mostly independent of the shear oscillation frequency. Regarding the viscometric functions, we show how the external magnetic field modifies the storage and loss moduli, especially for a field aligned to the main velocity gradient. The bulk emulsion magnetization is studied in the same fashion as the viscoelastic functions of the oscillatory shear. We show that the in-phase component of the magnetization with respect to the shear rate reaches a saturation magnetization, at the high frequencies limit, dependent on the magnetic field intensity and orientation. On the other hand, we found a non-zero out-of-phase response, which indicates a finite emulsion magnetization relaxation time. Our results indicate that the magnetization relaxation is closely related to the mechanical relaxation for dilute magnetic emulsions under oscillatory shear. 

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