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1. Kinetic phenomena of helical plasma waves with orbital angular momentum

   https://doi.org/10.1063/5.0095017  

   Blackman, D. R. ; Nuter, R. ; Korneev, Ph. ; Arefiev, A. ; Tikhonchuk, V. T. ( July 2022 , Physics of Plasmas)

   An accurate description of plasma waves is fundamental for the understanding of many plasma phenomena. It is possible to twist plasma waves such that, in addition to having longitudinal motion, they can possess a quantized orbital angular momentum. One such type of plasma wave is the Laguerre–Gaussian mode. Three-dimensional numerical particle-in-cell simulations demonstrate the existence of stable long-lived plasma waves with orbital angular momentum. These waves can be shown to create large amplitude static magnetic fields with unique twisted longitudinal structures. In this paper, we review the recent progress in studies of helical plasma waves and present a new analytical description of a standing Laguerre–Gaussian plasma wave mode along with 3D particle-in-cell simulation results. The Landau damping of twisted plasma waves shows important differences compared to standard longitudinal plasma wave Landau damping. These effects include an increased damping rate, which is affected by both the focal width and the orbital number of the plasma wave. This increase in the damping rate is of the same order as the thermal correction. Moreover, the direction of momentum picked up by resonant particles from the twisted plasma wave can be significantly altered. By contrast, the radial electric field has a subtle effect on the trajectories of resonant electrons. 

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2. Accurate simulation of direct laser acceleration in a laser wakefield accelerator

   https://doi.org/10.1063/5.0152383  

   Miller, Kyle G. ; Palastro, John P. ; Shaw, Jessica L. ; Li, Fei ; Tsung, Frank S. ; Decyk, Viktor K. ; Joshi, C. ; Mori, Warren B. ( July 2023 , Physics of Plasmas)

   In a laser wakefield accelerator (LWFA), an intense laser pulse excites a plasma wave that traps and accelerates electrons to relativistic energies. When the pulse overlaps the accelerated electrons, it can enhance the energy gain through direct laser acceleration (DLA) by resonantly driving the betatron oscillations of the electrons in the plasma wave. The traditional particle-in-cell (PIC) algorithm, although often the tool of choice to study DLA, contains inherent errors due to numerical dispersion and the time staggering of the electric and magnetic fields. Furthermore, conventional PIC implementations cannot reliably disentangle the fields of the plasma wave and laser pulse, which obscures interpretation of the dominant acceleration mechanism. Here, a customized field solver that reduces errors from both numerical dispersion and time staggering is used in conjunction with a field decomposition into azimuthal modes to perform PIC simulations of DLA in an LWFA. Comparisons with traditional PIC methods, model equations, and experimental data show improved accuracy with the customized solver and convergence with an order-of-magnitude fewer cells. The azimuthal-mode decomposition reveals that the most energetic electrons receive comparable energy from DLA and LWFA. 

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3. Fluid models capturing Farley–Buneman instabilities

   https://doi.org/10.5194/angeo-41-281-2023  

   Rojas, Enrique L. ; Burns, Keaton J. ; Hysell, David L. ( January 2023 , Annales Geophysicae)

   Abstract. It is generally accepted that modeling Farley–Buneman instabilities requires resolving ion Landau damping to reproduce experimentally observed features. Particle-in-cell (PIC) simulations have been able to reproduce most of these but at a computational cost that severely affects their scalability. This limitation hinders the study of non-local phenomena that require three dimensions or coupling with larger-scale processes. We argue that a form of the five-moment fluid system can recreate several qualitative aspects of Farley–Buneman dynamics such as density and phase speed saturation, wave turning, and heating. Unexpectedly, these features are still reproduced even without using artificial viscosity to capture Landau damping. Comparing the proposed fluid models and a PIC implementation shows good qualitative agreement.

    

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4. Cascading parametric decay coupling between whistler and ion acoustic waves: Darwin particle-in-cell simulations

   https://doi.org/10.3389/fspas.2022.1007240  

   Karbashewski, Scott ; Sydora, Richard D. ; Agapitov, Oleksiy V. ( November 2022 , Frontiers in Astronomy and Space Sciences)

   We present the results of numerical studies of the whistler wave parametric decay instability in the system with the suppressed Landau damping of ion acoustic waves (IAWs) based on the self-consistent Darwin particle-in-cell (PIC) model. It has been demonstrated that a monochromatic whistler wave launched along the background magnetic field couples to a counter-propagating whistler mode and co-propagating ion acoustic mode. The coupling of the electromagnetic mode to the electrostatic mode is guided by a ponderomotive force that forms spatio-temporal beat patterns in the longitudinal electric field generated by the counter-propagating whistler and the pump whistler wave. The threshold amplitude for the instability is determined to be δB w / B 0 = 0.028 and agrees with a prediction for the ion decay instability: δB w / B 0 = 0.042 based on the linear kinetic damping rates, and δB w / B 0 = 0.030 based on the simulation derived damping rates. Increasing the amplitude of the pump whistler wave, the secondary and tertiary decay thresholds are reached, and cascading parametric decay from the daughter whistler modes is observed. At the largest amplitude ( δB w / B 0 ∼ 0.1) the primary IAW evolves into a short-lived and highly nonlinear structure. The observed dependence of the IAW growth rate on the pump wave amplitude agrees with the expected trend; however, quantitatively, the growth rate of the IAW is larger than expected from theoretical predictions. We discuss the relevant space regimes where the instability could be observed and extensions to the parametric coupling of whistler waves with the electron acoustic wave (EAW). 

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5. Numerical Simulation of Plasma Interfaces Using the Starfish Plasma Simulation Code

   https://doi.org/10.2514/6.2020-3245  

   Senig, James ; Wang, Xiaowen ( June 2020 , AiAA Aviation Forum 2020)

   null (Ed.)

   A particular challenge for low temperature plasma (LTP) research is the diversity of parameter space and conditions. For plasma systems where the densities are not low, the kinetic theory is time-consuming and becomes unrealistic. In this regime, the particle-in-Cell (PIC) method is appropriate where the evolution of a particle system at every time step consists of an Eulerian stage and a Lagrangian stage. The PIC method can deal with complex geometries and large distortions in the field. The PIC solver, Starfish, is a two-dimensional plasma and gas simulation code operating on structured 2D/axisymmetric Cartesian or body fitted stretched meshes. The purpose of this study is to use the Starfish Plasma Simulation Code for numerical simulations of plasma interfaces. Specifically, two applications are considered: 1) the modeling of a large area (30 cm x 30 cm) microwave plasma chemical vapor deposition system, and 2) the understanding of LTP treatment on surface modification of polycaprolactone pellets and thermal properties of extruded filaments. With the exact geometries and experimental results being provided, numerical simulations of these two applications are ongoing. 

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