Plasma Physics
- [1] arXiv:2405.08173 [pdf, ps, html, other]
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Title: Constructing nested coordinates inside strongly shaped toroids using an action principleComments: 18 pages and 10 figures submitted to the Journal of Plasma PhysicsSubjects: Plasma Physics (physics.plasm-ph)
A new approach for constructing polar-like boundary-conforming coordinates inside a toroid with strongly shaped cross-sections is presented. A coordinate mapping is obtained through a variational approach, which involves identifying extremal points of a proposed action in the mapping space from [0, 2{\pi}] x [0, 2{\pi}] x [0, 1] to a toroidal domain in R3. This approach employs an action built on the squared Jacobian and radial length. Extensive testing is conducted on general toroidal boundaries using a global Fourier-Zernike basis via action minimization. The results demonstrate successful coordinate construction capable of accurately describing strongly shaped toroidal domains. The coordinate construction is successfully applied to the computation of 3D MHD equilibria in the GVEC code where the use of traditional coordinate construction by interpolation from the boundary failed.
- [2] arXiv:2405.08453 [pdf, ps, other]
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Title: Outside and inside a magnetic island: different perspectives to describe the same observablesComments: 13 pages, 4 figures, submitted to J. Plasma PhysSubjects: Plasma Physics (physics.plasm-ph)
We compare three different approaches to describe a magnetic island in a generic toroidal plasma: (i) perturbative, from the perspective of the equilibrium magnetic field and the related action in a variational principle formulation, (ii) again perturbative, based on the integrability of a system with a single resonant mode and the application of a canonical transformation onto a new island equilibrium system, and (iii) non-perturbative, making use of a full geometric description of the island considered as a stand-alone plasma domain. For the three approaches, we characterize some observables and discuss the respective limits.
- [3] arXiv:2405.08472 [pdf, ps, other]
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Title: Impurity Parallel Velocity Gradient instabilityComments: 17 pages, 12 figuresSubjects: Plasma Physics (physics.plasm-ph)
In magnetized plasmas, a radial gradient of parallel velocity, where parallel refers to the direction of magnetic field, can destabilise an electrostatic mode called as Parallel Velocity Gradient (PVG). The theory of PVG has been mainly developed assuming a single species of ions. Here, the role of impurities is investigated based on a linear, local analysis, in a homogeneous, constant magnetic field. To further simplify the analysis, the plasma is assumed to contain only two ion species - main ions and one impurity species - while our methodology can be straightforwardly extended to more species. In the cold-ion limit, retaining polarization drift for both main ions and impurity ions, and assuming Boltzmanian electrons, the system is described by 4 fluid equations closed by quasineutrality. The linearized equations can be reduced to 2 coupled equations: one for the electric potential, and one for the effective parallel velocity fluctuations, which is a linear combination of main ion and impurity parallel velocity fluctuations. This reduced system can be understood as a generalisation of the Hasegawa-Mima model. With finite radial gradient of impurity parallel flow, the linear dispersion relation then describes a new instability: the impurity PVG (i-PVG). Instability condition is described in terms of either the main ion flow shear, or equivalently, an effective flow shear, which combines main ion and impurity flow shears. Impurities can have a stabilising or destabilising role, depending on the parameters, and in particular the direction of main flow shear against impurity flow shear. Assuming a reasonable value of perpendicular wavenumber, the maximum growthrate is estimated, depending on impurity mass, charge, and concentration.
- [4] arXiv:2405.08761 [pdf, ps, other]
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Title: Effect of injection conditions on the non-linear behavior of the ECDI and related turbulent transportComments: 18 pages, 17 figuresSubjects: Plasma Physics (physics.plasm-ph)
The electron-cyclotron drift instability (ECDI) has been proposed as one of the main actors behind the anomalous transport of electrons in Hall thruster devices. In this work, we revisit the theory and perform two-dimensional PIC simulations under several conditions to analyze the non-linear behavior and the induced transport under several boundary conditions. Simulation results with fully-periodic boundaries and conditions faithful to the linear theory show the growth of ECDI modes, ion-wave trapping vortexes and agree with the existing literature in early times. In the long term, however, we observe very mild oscillations and null anomalous current. The evolution towards this new equilibrium is coherent to what can be expected from energy conservation. The quenching of the oscillations seem to be highly related with the distortion of ion-trapping vortexes in phase space after a long-term interaction of ion particles with the electrostatic wave. This result suggests that sustained oscillations and turbulent current could benefit from the renewal of ions by, e.g., removing and injecting particles through axial boundaries instead of applying periodicity. This second type of simulations shows that injection conditions highly impact the late simulation behavior of ECDI oscillations, where we identify several regimes depending on the value of the ion residence time compared to the characteristic saturation time in the fully periodic case. The intermediate regime, where these two times are close, is the only one providing sustained oscillations and electron transport and seems to be the relevant one in Hall devices.
New submissions for Wednesday, 15 May 2024 (showing 4 of 4 entries )
- [5] arXiv:2405.08073 (cross-list from nucl-th) [pdf, ps, html, other]
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Title: Relativistic Guiding-Center Motion: Action Principle, Kinetic Theory, and HydrodynamicsComments: 5 pages + 7 pages of supplementary materialSubjects: Nuclear Theory (nucl-th); High Energy Astrophysical Phenomena (astro-ph.HE); High Energy Physics - Theory (hep-th); Plasma Physics (physics.plasm-ph)
We treat the guiding-center dynamics in a non-uniform external Maxwell field using a manifestly Lorentz covariant action principle which easily reproduces the known Vandervoort equations of motion. We derive the corresponding kinetic theory and ideal hydrodynamic theory. In contrast to conventional five-equation hydrodynamics, the guiding-center hydrodynamics needs only three equations due to a constraint on the motion across magnetic field. We argue that this hydrodynamics applies more generally than the kinetic theory, e.g., for strongly-coupled quark-gluon plasma.
- [6] arXiv:2405.08155 (cross-list from astro-ph.HE) [pdf, ps, html, other]
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Title: On the width of a collisionless shock and the index of the cosmic rays it acceleratesComments: 14 pages, 6 figures, to appear in ApJSubjects: High Energy Astrophysical Phenomena (astro-ph.HE); Solar and Stellar Astrophysics (astro-ph.SR); Plasma Physics (physics.plasm-ph)
Despite being studied for many years, the structure of collisionless shocks is still not fully determined. Such shocks are known to be accelerators of cosmic rays, which, in turn, modify the shock structure. The shock width $\lambda$ is known to be connected to the cosmic rays (CRs) spectral index, $a$. Here, we use an instability analysis to derive the shock width in the presence of CRs. We obtain an analytical expression connecting the shock width to the CRs index and to the fraction of upstream particles that are accelerated. We find that when this fraction becomes larger than $\sim$~30\%, a new instability becomes dominant. The shock undergoes a transition where its width increases by a factor $\sim 8- 10$, and the CRs acceleration effectively ends. Our analysis is valid for strong, non-relativistic and unmagnetized shocks. We discuss the implication of these results to the expected range of CRs spectra and flux observed, and on the structure of non-relativistic collisionless shocks.
- [7] arXiv:2405.08196 (cross-list from astro-ph.SR) [pdf, ps, html, other]
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Title: Fully Kinetic Simulations of Proton-Beam-Driven Instabilities from Parker Solar Probe ObservationsLuca Pezzini, Andrei N. Zhukov, Fabio Bacchini, Giuseppe Arrò, Rodrigo A. López, Alfredo Micera, Maria Elena Innocenti, Giovanni LapentaSubjects: Solar and Stellar Astrophysics (astro-ph.SR); Plasma Physics (physics.plasm-ph); Space Physics (physics.space-ph)
The expanding solar wind plasma ubiquitously exhibits anisotropic non-thermal particle velocity distributions. Typically, proton Velocity Distribution Functions (VDFs) show the presence of a core and a field-aligned beam. Novel observations made by Parker Solar Probe (PSP) in the innermost heliosphere have revealed new complex features in the proton VDFs, namely anisotropic beams that sometimes experience perpendicular diffusion. This phenomenon gives rise to VDFs that resemble a "hammerhead". In this study, we use a 2.5D fully kinetic simulation to investigate the stability of proton VDFs with anisotropic beams observed by PSP. Our setup consists of a core and an anisotropic beam populations that drift with respect to each other. This configuration triggers a proton-beam instability from which nearly parallel fast magnetosonic modes develop. Our results demonstrate that before this instability reaches saturation, the waves resonantly interact with the beam protons, causing significant perpendicular heating at the expense of the parallel temperature. Furthermore, the proton perpendicular heating induces a hammerhead-like shape in the resulting VDF. Our results suggest that this mechanism probably contribute to producing the observed hammerhead distributions.
- [8] arXiv:2405.08480 (cross-list from cond-mat.str-el) [pdf, ps, other]
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Title: Dynamic exchange-correlation effects in the strongly coupled electron liquidSubjects: Strongly Correlated Electrons (cond-mat.str-el); Chemical Physics (physics.chem-ph); Plasma Physics (physics.plasm-ph)
We present the first quasi-exact \textit{ab initio} path integral Monte Carlo (PIMC) results for the dynamic local field correction $\widetilde{G}(\mathbf{q},z_l;r_s,\Theta)$ in the imaginary Matsubara frequency domain, focusing on the strongly coupled finite temperature uniform electron gas. These allow us to investigate the impact of dynamic exchange--correlation effects onto the static structure factor. Our results provide a straightforward explanation for previously reported spurious effects in the so-called \textit{static approximation} [Dornheim \textit{et al.}, \textit{Phys.~Rev.~Lett.}~\textbf{125}, 235001 (2020)], where the frequency-dependence of the local field correction is neglected. Our findings hint at the intriguing possibility of constructing an analytical four-parameter representation of $\widetilde{G}(\mathbf{q},z_l;r_s,\Theta)$ valid for a substantial part of the phase diagram, which would constitute key input for thermal density functional theory simulations.
- [9] arXiv:2405.08665 (cross-list from cond-mat.stat-mech) [pdf, ps, html, other]
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Title: Note: Shoving model and the glass transition in one-component plasmaComments: Brief Report, 3 pagesJournal-ref: The Journal of Chemical Physics 160, 166101 (2024)Subjects: Statistical Mechanics (cond-mat.stat-mech); Soft Condensed Matter (cond-mat.soft); Chemical Physics (physics.chem-ph); Plasma Physics (physics.plasm-ph)
A modified shoving model is applied to estimate the location of the glass transition in a one-component plasma. The estimated value of the coupling parameter $\Gamma\simeq 570$ at the glass transition is compared with other predictions available in the literature.
Cross submissions for Wednesday, 15 May 2024 (showing 5 of 5 entries )
- [10] arXiv:2310.17646 (replaced) [pdf, ps, other]
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Title: Learning the dynamics of a one-dimensional plasma model with graph neural networksComments: title changed, accepted for publication in Machine Learning: Science and Technology, code available at this https URLSubjects: Plasma Physics (physics.plasm-ph); Machine Learning (cs.LG); Computational Physics (physics.comp-ph)
We explore the possibility of fully replacing a plasma physics kinetic simulator with a graph neural network-based simulator. We focus on this class of surrogate models given the similarity between their message-passing update mechanism and the traditional physics solver update, and the possibility of enforcing known physical priors into the graph construction and update. We show that our model learns the kinetic plasma dynamics of the one-dimensional plasma model, a predecessor of contemporary kinetic plasma simulation codes, and recovers a wide range of well-known kinetic plasma processes, including plasma thermalization, electrostatic fluctuations about thermal equilibrium, and the drag on a fast sheet and Landau damping. We compare the performance against the original plasma model in terms of run-time, conservation laws, and temporal evolution of key physical quantities. The limitations of the model are presented and possible directions for higher-dimensional surrogate models for kinetic plasmas are discussed.