Session JO05: Fundamental Plasmas: Waves and Instabilities

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Non-monotonic double layers, coherent plasma structures which sustain a potential gradient through an asymmetric potential well (or peak), may develop in a plasma subject to current-carrying instabilities. These structures are known to generate electron phase space holes, signifying a major perturbation to electron transport. In this work, 1D1V Vlasov-Poisson simulations are used to study a plasma subject to the ion acoustic instability. Upon saturation of this instability, a secondary growth of small wavenumber ion acoustic waves occurs which ultimately results in non-monotonic double layer formation. Electron reflection off and acceleration through such a double layer generates a two-stream instability, creating large-amplitude (with respect to the already present ion acoustic waves) plasma waves associated with electron holes. The magnitude of the double layer, found to scale inversely with ion mass, dictates the amount of electron reflection and thus the phase speed of the resulting electron plasma waves.   

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The case of a magnetic pulse traveling through a plasma is studied theoretically. Double layer (DL) and solitons were observed in this system before and it's known that certain DLs decay by emitting solitons$^{\mathrm{1}}$. However, the exact trigger mechanism causing this DL-soliton transition is not understood yet. Theoretically, this transition can be treated as a DL-soliton interaction. The soliton and DL KdV equations for the traveling pulse case is derived using the Reductive Perturbation Method (RPM) by considering trapped and free electrons and free ions. RPM is used to reduce nonlinear PDEs using asymptotic expansions. This means the interaction term between the 1$^{\mathrm{st}}$ order soliton and the 2$^{\mathrm{nd}}$ order DL is usually lost and the system of equations is unclosed. In this study, the closure is achieved by assuming an ion density distribution in the DL and back-substituting the DL parameters into the soliton system as a higher order asymptote. The interaction term thus found is directly proportional to the trapped electron density. A higher trapped electron density means an increased DL-soliton interaction and a decreased chance of DL decay. The interaction is also solitary in nature. $^{\mathrm{1}}$Ikezi H, Taylor R J and Baker D R, 1970, Formation and Interaction of Ion-Acoustic Solitons, Phy. Rev. Lett., 25, pp. 11 -- 14.   

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In the solar wind we observe that there is a significant amount of power in high-frequency waves. In the current picture of weakly-collisional plasma turbulence, energy can be transferred to these waves through microinstabilities generated by non-equilibrium features in the velocity distribution functions of the constituent particles. However, it is an open question how much energy in these features is truly available to drive these waves and eventually heat the plasma. In this work we develop an ansatz to quantify the amount of free energy available to microinstabilities that can actually go into heating the plasma relative to other dissipation mechanisms. We apply this metric to a number of electrostatic test cases, and plan to apply the metric to Parker Solar Probe observations of electron distributions and Langmuir waves. Ultimately this metric will be applied to simulations of electromagnetic instabilities as well, and modified to account for the role of multiple types of microinstabilities and sources of free energy.   

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A topological understanding of matter is not only deepening our knowledge of physics, but also leading to novel practical devices and applications. Nontrivial topology in bulk matter has been linked with the existence of topologically protected interfacial states. While these concepts were originally developed in electronic structures and photonic systems, recent advances have demonstrated that continuum fluid systems can support topological waves. We extend these ideas to plasmas and show that a gaseous plasmon polariton (GPP), an electromagnetic surface wave existing at the boundary of magnetized plasma and vacuum, has a topological origin that arises from the nontrivial topology of magnetized plasma. Moreover, we show that the GPP may be found within a gapped spectrum in present-day laboratory devices, suggesting that platforms are currently available for experimental investigation of topological wave physics in plasmas. Experiments to confirm the existence of this wave would open a new frontier in the exploration of the physics of topological waves in plasmas. \\ \\J. B. Parker, J. B. Marston, S. M. Tobias, Z. Zhu, Phys. Rev. Lett. 124, 195001 (2020)   

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Internal dynamo-generated magnetic fields in the stars are inaccessible to direct observations, thus deterring our understanding of their origin and evolution in time. A tool that has proved very useful in analyzing the stellar interiors, called asteroseismology, however, has not yet been able to provide details of internal stellar magnetic fields. Here, we investigate the signatures of subsurface magnetic fields in the dispersion relations of acoustic waves (trapped near the stellar surface). We first begin with an isothermal, stratified atmosphere, permeated by a non-uniform (exponential function of the vertical coordinate) horizontal magnetic field (Ref: \underline {arXiv:1812.06947}). We solve the problem exactly. Next we consider a more realistic polytropic atmosphere. We calculate the dispersion relation numerically and perturbatively. We show that the presence of a horizontal magnetic field breaks the symmetry of rings of constant frequencies over the horizontal wavenumbers. Such asymmetry arising from the magnetic fields eludes the standard helioseismology with its present resolution. Our results hint that internal stellar magnetic fields might be possible to infer based upon stellar surface oscillations.   

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How coherent whistler waves interact with energetic particles is a crucial question in magnetospheric plasma physics, as well as in the context of runaway electrons in fusion devices. A recent study \footnote{P. M. Bellan, Phys. Plasmas, 20, 042117} showed that an exact rearrangement of the relativistic particle equation of motion under a circularly-polarized wave leads to an equation describing the motion of the “frequency mismatch” parameter $\xi$ under a pseudo-potential $\psi$. When the shape of the pseudo-potential is two-valleyed and the particle has enough pseudo-energy to undergo two-valley motion, $\xi$ and the pitch-angle changes greatly. In the present study, the analysis is extended to a distribution of particles. A general condition for two-valley motion is first derived. It is then shown via single-particle simulations that particles which satisfy the two-valley condition indeed undergo large pitch-angle changes. Then, the fraction of two-valley particles are calculated assuming that the particle distribution is Maxwell-J\"{u}ttner, which is the relativistic generalization of the Maxwell-Boltzmann distribution. For magnetospheric parameters, at least 1-5\% of the particles undergo two-valley motion, and this fraction is verified by single-particle simulations   

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The solution by Landau of the one-dimensional linear Vlasov equation, which describes a collisionless plasma, shifts and deforms the Bromwich contour around the poles of the analytically-continued dielectric function. For an unstable equilibrium with growing and decaying normal modes, this procedure results in only the growing modes contributing. However, the van Kampen-Case solution shows that the eigenfunctions of the decaying discrete modes always accompany those of the growing discrete modes, and thus a contradiction seems to arise, since both Landau and van Kampen-Case give valid solutions. In order to reconcile the two seemingly different answers, we present a solution that is equivalent to both and show that the decaying discrete modes do not actually exist; a part of the van Kampen continuum seems to always conspire to exactly cancel the decaying discrete modes. Our solution evaluates the Bromwich integral using properties of Cauchy-type integrals instead of deforming the contour, avoiding the analytic continuation from the Landau method. It also avoids the complicated principal value integrals from the van Kampen-Case method; only a straightforward Laurent series expansion is required.   

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The electric field associated with a planar electrostatic wave has no average momentum, and thus as the waves damp, the momentum of the plasma has to be conserved. We examine various implications of this momentum conservation. In the planar problem, we show how wave-mediated momentum exchange between nonresonant and resonant particles can drive net currents in spite of the constraint [1]. This momentum-exchange current drive can provide a possible magnetogenesis mechanism in astrophysical settings. Adding spatial wave structure, important for application to steady-state laboratory devices, introduces new momentum terms. We discuss the implications of our results for wave-mediated perpendicular momentum transport in magnetized plasmas with hot ion gradients, drawing analogies with collisional momentum transport in rotating ExB plasmas [2-4]. \\ 1) I.E. Ochs and N.J. Fisch, Physics of Plasmas 27(6), 062109 (2020).\\ 2) E.J. Kolmes, I.E. Ochs, M.E. Mlodik, J.M. Rax, R. Gueroult, and N.J. Fisch, Physics of Plasmas 26(8), 082309 (2020).\\ 3) I. E. Ochs and N. J. Fisch, Physical Review Letters 121, 235002 (2018).\\ 4) I. E. Ochs and N. J. Fisch, Physics of Plasmas 25(12), 122306 (2018).   

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Interaction between relativistic electron beams and magnetized plasma is a fundamental and practical problem relevant to many challenging issues in space physics and astrophysics. We present results from a 20~keV beam experiment on the Large Plasma Device (LAPD) at UCLA motivated by the problem of how naturally occurring electron beams may produce type II/III solar radio emissions as well as recent proposals to place compact high-energy electron beam sources on future spacecraft. These spacecraft-borne beams may be used to map magnetic field lines in the Earth's magnetosphere or to generate waves for radiation belt remediation. In the LAPD experiments, electromagnetic emission between the plasma and upper hybrid frequencies is observed by both in-situ probes and by an antenna outside of the plasma. The parallel phase speed of the excited waves is measured to be consistent with generation via a resonance process, while estimates of the radiated power are consistent with incoherent Cherenkov emission. Kinetic modeling suggests that the apparent absence of strong instabilities is due to velocity dispersion imposed by the beam injection conditions. Signatures of nonlinear interactions between fluctuations above the plasma frequency and observed whistler modes are also observed.   

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To conduct the proof-of-principle experiments of high harmonic fast wave (HHFW) coupling and propagation in high beta FRC plasma, a phased-array RF antenna has been designed, built, and installed on the LArge Plasma Device (LAPD). The first experimental campaign of fast wave propagation at low power operation has shown promising results that fast waves can couple and propagate into the plasma core at all antenna phases, even when the antenna is fully retracted close to the wall, and no slow wave excitation has been observed. The magnetic field components of HHFWs in both the near and far fields were measured by B-dot probes, and these measuremental results are used to benchmark the simulation results with the Petra-M full wave code. Meanwhile, a few sets of impedance matching network, decoupler, and antenna phase and impedance measurement diagnostics are currently under construction; this hardware will be deployed on LAPD soon for a new RF campaign, and its experimental results will be presented. *The experiments were performed at UCLA's Basic Plasma Science Facility, which is a collaborative research facility supported by the US DoE, and the NSF.   

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ExB plasma is sustained on a partially magnetized plasma devices such as Hall thruster and magnetron sputtering because electron heating occurs and electron-impact ionization is maintained. A rotating spoke structure was experimentally and numerically captured on ExB device, and mechanism of spoke rotation was discussed. The past experiment for magnetron sputtering device indicated that the direction of spoke propagation is changed depending on the applied voltage, and -ExB drift motion of the rotating spoke was captured. However, a detailed mechanism on -ExB motion of spokes was not revealed and a propagation velocity of -ExB spoke was not characterized. In this study, we conducted a two-dimensional particle-in-cell (PIC) simulation with Monte Carlo collision (MCC) to capture dynamics of -ExB spoke on the magnetron sputtering devices. Our PIC-MCC simulation described that the electron around the double layer is heated by a cross-field motion of gradient B drift. This high temperature electron induces the electron-impact ionization at the double layer, which causes the spoke propagation to the -ExB direction. Our modeling finally concluded that the propagation velocity of rotating spokes can be characterized by the electron diffusion coefficient and ionization frequency.   

Shear Flow‐Interchange Instability in Nightside Magnetotail Observed as "Auroral Beads” at Substorm Onset

Low‐frequency shear flow‐interchange waves transmit sheared zonal flows along magnetic flux tubes toward the ionosphere from the near‐Earth nightside plasma sheet create the "auroral beads” observed in Canada and Alaska observed as the geomagnetic substorm onset. A set of nonlinear pde’s is derived and solved to model the growth and saturation of the auroral beads. The relation with the Kalmoni model (2015, https://doi.org/10.1002/2015JA021470) for shear flow‐ballooning instability is explained. The shear flow‐interchange instability appears to be responsible for substorm onset. The growth starts in the midnight region of the nightside magnetotail producing in the nonlinear stage the auroral beads characteristic of geomagnetic substorm onset.