Plasma Physics
New submissions
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New submissions for Fri, 3 May 24
- [1] arXiv:2405.00854 [pdf, other]
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Title: Suppression of temperature-gradient-driven turbulence by sheared flows in fusion plasmasComments: 21 pages, 13 figures, 1 tableSubjects: Plasma Physics (physics.plasm-ph)
Starting from the assumption that saturation of plasma turbulence driven by temperature-gradient instabilities in fusion plasmas is achieved by a local energy cascade between a long-wavelength outer scale, where energy is injected into the fluctuations, and a small-wavelength dissipation scale, where fluctuation energy is thermalized by particle collisions, we formulate a detailed phenomenological theory for the influence of perpendicular flow shear on magnetized-plasma turbulence. Our theory introduces two distinct regimes, called the weak-shear and strong-shear regimes, each with its own set of scaling laws for the scale and amplitude of the fluctuations and for the level of turbulent heat transport. We discover that the ratio of the typical radial and poloidal wavenumbers of the fluctuations (i.e., their aspect ratio) at the outer scale plays a central role in determining the dependence of the turbulent transport on the imposed flow shear. Our theoretical predictions are found to be in excellent agreement with numerical simulations of two paradigmatic models of fusion-relevant plasma turbulence: (i) an electrostatic fluid model of slab electron-scale turbulence, and (ii) Cyclone-base-case gyrokinetic ion-scale turbulence. Additionally, our theory envisions a potential mechanism for the suppression of electron-scale turbulence by perpendicular ion-scale flows based on the role of the aforementioned aspect ratio of the electron-scale fluctuations.
- [2] arXiv:2405.00886 [pdf, other]
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Title: Implementation of a Mesh refinement algorithm into the quasi-static PIC code QuickPICAuthors: Q. Su, F. Li, W. An, V. Decyk, Y. Zhao, L. Hildebrand, T. N. Dalichaouch, S. Zhou, E. P. Alves, A. S. Almgren, W. B. MoriSubjects: Plasma Physics (physics.plasm-ph); Computational Physics (physics.comp-ph)
Plasma-based acceleration (PBA) has emerged as a promising candidate for the accelerator technology used to build a future linear collider and/or an advanced light source. In PBA, a trailing or witness particle beam is accelerated in the plasma wave wakefield (WF) created by a laser or particle beam driver. The distance over which the drive beam evolves is several orders of magnitude larger than the wake wavelength. This large disparity in length scales is amenable to the quasi-static approach. Three-dimensional (3D), quasi-static (QS), particle-in-cell (PIC) codes, e.g., QuickPIC, have been shown to provide high fidelity simulation capability with 2-4 orders of magnitude speedup over 3D fully explicit PIC codes. We describe a mesh refinement scheme that has been implemented into the 3D QS PIC code, QuickPIC. We use a very fine (high) resolution in a small spatial region that includes the witness beam and progressively coarser resolutions in the rest of the simulation domain. A fast multigrid Poisson solver has been implemented for the field solve on the refined meshes and a Fast Fourier Transform (FFT) based Poisson solver is used for the coarse mesh. The code has been parallelized with both MPI and OpenMP, and the parallel scalability has also been improved by using pipelining. A preliminary adaptive mesh refinement technique is described to optimize the computational time for simulations with an evolving witness beam size. Several test problems are used to verify that the mesh refinement algorithm provides accurate results. The results are also compared to highly resolved simulations with near azimuthal symmetry using a new hybrid QS PIC code QPAD that uses a PIC description in the coordinates ($r$, $ct-z$) and a gridless description in the azimuthal angle, $\phi$.
- [3] arXiv:2405.00907 [pdf, other]
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Title: Angular momentum gain by electrons under action of intense structured lightSubjects: Plasma Physics (physics.plasm-ph); Optics (physics.optics)
The problem of light waves interaction with charged particles becomes more and more complex starting with the case of plane waves, where the analytical solution is well known, to more natural, though more complicated situations which include focused or structured laser beams. Internal structure may introduce a new degree of freedom and qualitatively change the dynamics of interacting particles. For certain conditions, namely for the dilute plasma, description of single-particle dynamics in the focused structured laser beams is the first step and may serve as a good approximation on the way of understanding the global plasma response. Moreover, the general problem of integrability in complex systems starts from consideration of the integrals of motion for a single particle. The primary goal of this work is an understanding of the physics of the orbital angular momentum (OAM) absorption by a single particle in a focused structured light. A theoretical model of the process, including solutions of Maxwell equations with the required accuracy and a high-order perturbative approach to electron motion in external electromagnetic fields, is developed and its predictions are examined with numerical simulations for several exemplary electromagnetic field configurations. In particular, it was found that for the particles distributed initially with the azimuthal symmetry around the beam propagation direction, the transferred OAM has a smallness of the fourth order of the applied field amplitude, and requires an accurate consideration of the temporal laser pulse envelope.
- [4] arXiv:2405.01042 [pdf, other]
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Title: Theoretical study of dissociative recombination and vibrational excitation of the BF$_2^+$ ion by an electron impactComments: 6 pages, 5 figures, 2 tablesJournal-ref: Plasma Sources Sci. Technol. 27 (2018) 115007 (6pp)Subjects: Plasma Physics (physics.plasm-ph); Atomic Physics (physics.atom-ph)
Cross-sections for dissociative recombination and electron-impact vibrational excitation of the BF$^+_2$ molecular ion are computed using a theoretical approach that combines the normal modes approximation for the vibrational states of the target ion and use of the UK R-matrix code to evaluate electron-ion scattering matrices for fixed geometries of the ion. Thermally-averaged rate coefficients are obtained from the cross-sections for temperatures in the 10-3000 K range.
- [5] arXiv:2405.01265 [pdf, ps, other]
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Title: Derivation of Dirac Exchange Interaction Potential from Quantum Plasma Kinetic TheoryAuthors: Fernando HaasSubjects: Plasma Physics (physics.plasm-ph)
The Dirac exchange interaction is derived from recent quantum kinetic theory for collisionless plasmas. For this purpose, the kinetic equation is written in the semiclassical and long wavelength approximations. The validity of the model for real systems is worked out, in terms of temperature and density parameters. Within the region of applicability, the correlation potential energy is shown to be always smaller than the exchange contribution. From the moments of the quantum kinetic equations, macroscopic, hydrodynamic equations are found, for an electron-ion plasma. The Dirac exchange term is explicitly derived, in the case of a completely degenerate electron gas. These results show, within quantum kinetic theory for charged particle systems, a new view of the Dirac exchange interaction frequently used in density functional theory parametrization. Finally, a simpler form of the quantum plasma exchange kinetic theory is also found.
- [6] arXiv:2405.01355 [pdf, other]
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Title: Neural-Parareal: Dynamically Training Neural Operators as Coarse Solvers for Time-Parallelisation of Fusion MHD SimulationsAuthors: S.J.P. Pamela, N. Carey, J. Brandstetter, R. Akers, L. Zanisi, J. Buchanan, V. Gopakumar, M. Hoelzl, G. Huijsmans, K. Pentland, T. James, G. Antonucci, the JOREK TeamSubjects: Plasma Physics (physics.plasm-ph)
The fusion research facility ITER is currently being assembled to demonstrate that fusion can be used for industrial energy production, while several other programmes across the world are also moving forward, such as EU-DEMO, CFETR, SPARC and STEP. The high engineering complexity of a tokamak makes it an extremely challenging device to optimise, and test-based optimisation would be too slow and too costly. Instead, digital design and optimisation must be favored, which requires strongly-coupled suites of High-Performance Computing calculations. In this context, having surrogate models to provide quick estimates with uncertainty quantification is essential to explore and optimise new design options. Furthermore, these surrogates can in turn be used to accelerate simulations in the first place. This is the case of Parareal, a time-parallelisation method that can speed-up large HPC simulations, where the coarse-solver can be replaced by a surrogate. A novel framework, Neural-Parareal, is developed to integrate the training of neural operators dynamically as more data becomes available. For a given input-parameter domain, as more simulations are being run with Parareal, the large amount of data generated by the algorithm is used to train new surrogate models to be used as coarse-solvers for future Parareal simulations, leading to progressively more accurate coarse-solvers, and thus higher speed-up. It is found that such neural network surrogates can be much more effective than traditional coarse-solver in providing a speed-up with Parareal. This study is a demonstration of the convergence of HPC and AI which simply has to become common practice in the world of digital engineering design.
Replacements for Fri, 3 May 24
- [7] arXiv:2303.16286 (replaced) [pdf, other]
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Title: Accelerating Particle-in-Cell Kinetic Plasma Simulations via Reduced-Order Modeling of Space-Charge Dynamics using Dynamic Mode DecompositionSubjects: Plasma Physics (physics.plasm-ph); Computational Physics (physics.comp-ph)
- [8] arXiv:2403.08179 (replaced) [pdf, other]
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Title: Effects of wave damping and finite perpendicular scale on three-dimensional Alfven wave parametric decay in low-beta plasmasComments: 8 pages, 3 figuresSubjects: Plasma Physics (physics.plasm-ph); Solar and Stellar Astrophysics (astro-ph.SR); Space Physics (physics.space-ph)
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