Bulletin of the American Physical Society
63rd Annual Meeting of the APS Division of Plasma Physics
Volume 66, Number 13
Monday–Friday, November 8–12, 2021; Pittsburgh, PA
Session BI02: Fundamental: Analytical and ComputationalInvited Live
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Chair: Evdokiya Kostadinova, Auburn University Room: Ballroom C |
Monday, November 8, 2021 9:30AM - 10:00AM |
BI02.00001: Helical Plasma Wave Damping and Magnetic Field Generation Invited Speaker: David R. R Blackman An accurate description of plasma waves is essential to the understanding of many aspects of plasma phenomena. We demonstrate both analytically, and using 3D Particle-In-Cell (PIC) simulations, that it is possible to twist a plasma wave such that, in addition to having longitudinal motion, it can possess quantized orbital angular momentum. This feature implies a helix like density perturbation that rotates about a central axis and can be described using a Laguerre-Gaussian mode. Such perturbations can be driven by Laguerre-Gaussian laser modes. This work is of increasing importance due to developing capabilities for generating such laser modes at high-energy high-intensity laser facilities. |
Monday, November 8, 2021 10:00AM - 10:30AM |
BI02.00002: Atomistic Modeling of Beryllium and Helium Implantation in Tungsten Using Machine Learned Interatomic Potentials Invited Speaker: Mary Alice Cusentino Tungsten and beryllium have been chosen for the divertor and first wall, respectively, of future tokamaks like ITER due to their favorable material properties [1]. However, the divertor will be subject to high fluxes of a variety of plasma species, including eroded beryllium, which can lead to microstructural changes of the tungsten surface. In particular, beryllium implantation in tungsten has been experimentally observed to form stable W-Be intermetallics [2]. These intermetallics have a significantly lower melting temperature than pure tungsten, making the divertor more susceptible to melting. Mixed W-Be layers have also been shown to affect both hydrogen retention [3] and helium fuzz growth [2]. Fundamental understanding of how W-Be intermetallics form and affect the interaction of other plasma species in tungsten is critical. |
Monday, November 8, 2021 10:30AM - 11:00AM |
BI02.00003: Slow manifold reduction as a systematic tool for revealing the geometry of phase space Invited Speaker: Joshua W Burby Classically, the phase space geometry underlying an ideal model was revealed through identification of canonical coordinates, in which Hamilton's equations take their most elementary form, but where simple observables like velocity and scaling laws may become obscure. The pioneering work of Littlejohn, Morrison, and Greene established a new state-of-the-art known as Bracketology that began to uncover the geometry of phase space for plasma models using noncanonical variables. This enabled novel approaches to the study of stability, as embodied by dynamically-accessible free-energy principles, and new paradigms for asymptotic model reduction, as exemplified by variational gyrokinetic theory. In this talk, I will describe a new perspective on the phase space geometry of non-dissipative plasma physics that represents a step beyond Bracketology in terms of power and systematism. After highlighting the ubiquitous connection between reduced plasma models and approximate invariant manifolds, I will explain how each submanifold in an ambient phase space inherits a natural (pre) symplectic geometry of its own, much as submanifolds in Euclidean space inherit metric tensors by restricting the usual dot product. This submanifold geometry may be computed systematically using perturbation theory and provides a general, uniform explanation for a variety of Hamiltonian structures in plasma physics, both new and old. To illustrate the power of this perspective, I will describe how it has lead to (a) discovery of the Hamiltonian structure underlying the 70-year-old kinetic MHD model, (b) a Hamiltonian formulation of the nonlinear WKB method for Eulerian fluid models, and (c) derivation of the first post-Darwin kinetic plasma model, along with its Hamiltonian structure. |
Monday, November 8, 2021 11:00AM - 11:30AM |
BI02.00004: Metaplectic geometrical optics for ray-based modeling of caustics Invited Speaker: Nicolas A Lopez The optimization of radiofrequency-wave systems for fusion applications is often performed using ray-tracing codes, which rely on the geometrical-optics (GO) approximation. However, GO fails at caustics such as cutoffs and focal points, erroneously predicting the wave intensity to be infinite. This is a critical shortcoming of GO-based methods, as often the wave intensity at a caustic is precisely the quantity being optimized, for example, when a wave is focused on a resonance to provide plasma heating. It is commonly believed that accurate modeling of waves in such regions is impossible without full-wave simulations, which are computationally expensive and thereby limit the speed at which such optimizations can be performed. We show that there is a less expensive alternative that we call metaplectic geometrical optics (MGO) [1, 2]. Instead of evolving the electric field E in the usual x (coordinate) or k (spectral) representation, MGO uses a mixed q = Ax + Bk representation. By continuously adjusting the matrix coefficients A and B along the rays, one can ensure that GO remains valid in the q variables, so E(q) can be calculated efficiently and without caustic singularities. The result is then mapped back onto the original x space using integrals (called metaplectic transforms) that can be efficiently computed using Gauss—Freud quadrature along the steepest-descent contours [3]. Our MGO-based calculations successfully reproduce wave structures in paradigmatic (e.g., Airy and cusp) caustics with high fidelity. These results open a path toward speeding up radiofrequency-wave simulations and might also be useful for modeling intensity-dependent laser-plasma interactions. |
Monday, November 8, 2021 11:30AM - 12:00PM |
BI02.00005: Kinetic Theory of Strongly Magnetized Plasmas Invited Speaker: Louis Jose Novel transport properties exhibited by plasmas that are strongly magnetized in the sense that the gyrofrequency exceeds the plasma frequency are not well understood. Here, we develop a generalized kinetic theory that can treat Coulomb collisions in plasmas across all magnetization strength regimes and which asymptotes to the traditional Boltzmann kinetic theory in the weakly magnetized limit. The theory also spans the weak to strong Coulomb coupling regimes by incorporating the mean force kinetic theory concept. To demonstrate the utility of the generalized theory, it is used to compute the friction force on a massive test charge moving through a strongly magnetized one-component plasma. Recent works studying weakly coupled plasmas have shown that strong magnetization leads to a transverse component of the friction force that is perpendicular to both the Lorentz force and velocity of the test charge; in addition to the stopping power component. Recent molecular dynamics simulations have also shown that strong Coulomb coupling in addition to strong magnetization gives rise to a third ``gyrofriction'' component of the friction force in the direction of the Lorentz force. Here, we show that the theory captures these effects and agrees well with the molecular dynamics simulations over a broad range of Coulomb coupling and magnetization strengths. The transverse force is found to strongly influence the average motion of a test charge by changing the gyroradius and the gyrofriction force is found to slightly change the gyrofrequncy of the test charge resulting in a phase shift. |
Monday, November 8, 2021 12:00PM - 12:30PM |
BI02.00006: Coulomb collisions in strongly anisotropic electron-positron plasmas Invited Speaker: Daniel T Kennedy The canonical pair plasma, consisting solely of electrons and positrons, is an exciting new frontier in basic plasma physics. Electron-positron plasmas are an attractive proxy for complex physics due to the ``remarkable stability properties'' they exhibit as a result of mass symmetry between the two species; a symmetric pair plasma is gyrokinetically stable in a constant magnetic field. As such, laboratory pair plasmas ought to enjoy splendid confinement and such a plasma could provide a robust benchmark against theoretical predictions. Positrons are difficult to source terrestrially and this places stringent conditions on the values of plasma density $n$ that can be attained in the laboratory at fixed volume. It is thus advantageous to keep the plasma temperature, $T$ (and thus $\lambda_{D}$), as small as possible. Fortunately, the relatively small $n$ also renders the plasma optically thin to cyclotron radiation. One plan is to exploit relatively high magnetic fields; making use of cyclotron cooling to keep the plasma cold. In this talk, the behaviour of a strongly magnetised collisional electron-positron plasma that is optically thin to cyclotron radiation is considered, and the distribution functions accessible to it on the various timescales in the system are calculated. Particular attention will be paid to the limit in which the collision time exceeds the radiation emission time, making the electron distribution function strongly anisotropic. The constraint of strong magnetisation adds an additional complication in that long-range Coulomb collisions, usually negligible, must now be considered. Nevertheless, we show that the collisional scattering can be accounted for without knowing the explicit form of this collision operator. The rate of radiation emission is calculated and it is found that the loss of energy from the plasma is proportional to the parallel collision frequency multiplied by a factor that only depends logarithmically on plasma parameters. |
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