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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. High-resolution mapping of the radial electric field in RF plasma sheath using a single optically trapped particle

   https://doi.org/10.1088/1361-6595/ae13be  

   Ekanayaka, Pubuduni; Wang, Chuji; Chakraborty_Thakur, Saikat; Thomas, Edward (October 2025, Plasma Sources Science and Technology)

   Abstract Mapping individual components of the electric field,E, with high spatial resolution around the plasma sheath remains challenging, as mostin situplasma probes are generally too intrusive for reliable plasma potential measurements. An optically trapped micron-sized particle in a plasma environment represents the smallest, nonintrusive, or minimally intrusive diagnostic tool because it causes minimal disturbance to the plasma environment in which it is suspended. These microparticles can be used to map the electric fieldin situwith high spatial resolution in a radio frequency (RF) plasma sheath region. In this study, we optically trapped micron-sized single particles and precisely transported a single trapped particle within the cylindrically symmetric capacitively coupled plasma (CCP), over a radial distance of ∼ 0–15 mm, a 2400-fold displacement relative to the particle size, corresponding to a spatial resolution of tens of micrometers. We measured |E| and its spatial distributions in the examined range by analyzing the particle’s trajectory in the plasma after the optical forces were turned off. The radial component of the electric field, |E_r|, was measured at multiple locations parallel to the electrode at 6.7 Pa. The |E_r| was strongest near the circular electrode edges, reaching 0.478 ± 0.005 V mm⁻¹, and decreased to 0.458 ± 0.001 V mm⁻¹toward the center of the electrode. We reconstructed the radial plasma potential, yielding a center-to-edge potential decrease of 4.6 V over one centimeter. This ΔVis consistent with reported CCP trends of small mid-plane radial potential gradients and serves as the confining potential for dust crystal and dust cluster studies. These results highlight the use of optical trapping of a single particle as anin situ, nonintrusive microprobe for quantitative mapping ofE_r(r) andV(r) in RF plasmas. 

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3. Interacting dust grains in complex plasmas: Ion wake formation and the electric potential

   https://doi.org/10.1063/5.0203902  

   Vermillion, K; Banka, R; Mendoza, A; Wyatt, B; Matthews, L; Hyde, T (July 2024, Physics of Plasmas)

   Dust grains have been used as minimally invasive probes to determine plasma parameters including the plasma density, temperature, and electric field in a plasma discharge. However, the dust grains in a plasma generate local potential disturbances due to the collection of charge and the subsequent electrostatic interactions between the dust and charged plasma particles. Dust grains in close proximity to one another exhibit interesting non-reciprocal interactions and self-organize into structures such as one-dimensional filamentary chains, two-dimensional “zigzags,” and three-dimensional helices, among others. The formation of these structures suggests that although the dust grains may be less invasive than traditional plasma probes, the disturbance to the local plasma environment introduced by dust grains is non-trivial. Commonly used analytic forms of the electric potential describing complex plasmas have failed to resolve the near-dust region, and as a result are insufficient to provide insight about the formation of complex dust structures. Here, we use an N-body simulation to compute the electric potential from ion densities near various dust grain configurations. We provide an alternative description to the standard analytic model for the electric potential of dust and ion wakes based on a Gaussian shaped cloud of ions. The electric potential obtained from simulations is used to identify minimum energy configurations for two and three dust grains. It is further demonstrated that the minimum potential region identified for N dust grains and their associated ion wakes does not predict the minimum-energy configuration of N + 1 dust grains. 

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4. Electron scale coherent structure as micro accelerator in the Earth’s magnetosheath

   https://doi.org/10.1038/s41467-024-45040-5  

   Xie, Zi-Kang; Zong, Qiu-Gang; Yue, Chao; Zhou, Xu-Zhi; Liu, Zhi-Yang; He, Jian-Sen; Hao, Yi-Xin; Ng, Chung-Sang; Zhang, Hui; Yao, Shu-Tao; et al (December 2024, Nature Communications)

   Abstract Turbulent energy dissipation is a fundamental process in plasma physics that has not been settled. It is generally believed that the turbulent energy is dissipated at electron scales leading to electron energization in magnetized plasmas. Here, we propose a micro accelerator which could transform electrons from isotropic distribution to trapped, and then to stream (Strahl) distribution. From the MMS observations of an electron-scale coherent structure in the dayside magnetosheath, we identify an electron flux enhancement region in this structure collocated with an increase of magnetic field strength, which is also closely associated with a non-zero parallel electric field. We propose a trapping model considering a field-aligned electric potential together with the mirror force. The results are consistent with the observed electron fluxes from ~50 eV to ~200 eV. It further demonstrates that bidirectional electron jets can be formed by the hourglass-like magnetic configuration of the structure. 

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5. Radially resolved optical emission spectral imaging study of an atmospheric pressure μDBD jet for elucidating the effect of sample surface material on the underlying mechanisms

   https://doi.org/10.1039/D0JA00522C  

   Shi, Songyue; Finch, Kevin; Gamez, Gerardo (May 2021, Journal of Analytical Atomic Spectrometry)

   null (Ed.)

   Dielectric barrier discharges are receiving increasing attention as sampling/ionization sources for ambient mass spectrometry. Nevertheless, the underlying mechanisms are not completely understood, particularly when the plasma plume is exposed to a sample surface. Herein, an atmospheric pressure helium micro-dielectric barrier discharge (μDBD), flowing into open-air and onto a sample surface (floating Cu or LDPE), is studied via optical emission spectroscopy (OES) with radial information extracted through Abel's inversion. Radially resolved optical emission profiles along the axis are shown to shift with respect to the line-of-sight counterparts, for some species. The OES images, as well as vibrational and rotational temperature maps, indicate the energy transfers mainly via Penning ionization of N 2 with He metastables to produce N 2 + , as the plasma plume exits the capillary into open air, while charge transfer with He 2 + is dominant further downstream, as well as toward the periphery of the plume. In addition, the sample surface is shown to play an important role in the energy transfer mechanisms. For the LDPE sample, the spatial distribution sequence of excited species is similar to the free-flowing counterpart but disappears into the surface, which indicates that excited N 2 further downstream/outer periphery is produced from electron recombination with N 2 + . On the other hand, the presence of the floating Cu sample results in an intensity peak at the plasma/surface interface for most species. We propose that the temporal evolution of the half-cycle dynamics have a great effect, where the resulting higher electron temperature and density towards the surface of metallic samples favors electron impact excitation. Furthermore, profilometry of the resulting craters in the floating Cu samples revealed a close correlation between their diameter and the width of the N 2 + emission. 

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