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 NI01: Fundamental: Waves and InstabilitiesInvited Session Live
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Chair: Fernando Garcia Rubio, LLE Room: Ballroom B |
Wednesday, November 10, 2021 9:30AM - 10:00AM |
NI01.00001: Characterizing Intermittent Turbulent Wave Kinetics and Three-Wave Coupling in Dipole-Confined Plasma Invited Speaker: Mel Abler Plasmas confined by a dipole magnetic field exhibit interchange and entropy mode turbulence causing bursty intermittent transport of particles and energy [1]. On the Collisionless Terrella Experiment (CTX), this turbulence is dominated by low-frequency, long-wavelength modes with amplitudes and phases that vary chaotically in time [2]. We present a new paradigm for characterizing this turbulence by measuring the time-evolution of the fluctuation power spectrum and the instantaneous bispectrum using the continuous wavelet transform [3,4] and computing the statistical properties of turbulent wave kinetics. We observe that both the fluctuation power and the energy transfer by three-wave coupling, or bispectrum, between these fluctuations can be intermittent. When antenna are used to actively launch waves into the turbulence, the intermittency of the driven waves decreases, while the intermittency of other waves increases. Similarly, application of active feedback [5] to amplify the turbulence decreases the intermittency of the wave energy, while suppressing feedback increases this intermittency. Measurements based on this new paradigm show that the transfer of wave energy to larger and smaller scales in a turbulent plasma is not steady but occurs in short and intense bursts, analogous to the better-known short bursts of particle transport in magnetized plasma. |
Wednesday, November 10, 2021 10:00AM - 10:30AM |
NI01.00002: Scaling of Small-scale Dynamo Properties in Fluid Instabilities and Stably Stratified Turbulence Invited Speaker: Valentin Skoutnev Fluid instabilities in astrophysical plasmas are ubiquitous and understanding their efficacy for triggering the dynamo process is essential for understanding the origin of magnetic fields in space and astrophysical plasmas. The first part of this talk proposes simple, theoretical scalings of small-scale dynamo properties for the Rayleigh-Taylor instability and tests the predictions with direct numerical simulations. The scaling relations allow a quantitative prediction of the net magnetic amplification and time dependence of the dynamo growth rate. The second half of the talk focuses on small-scale dynamo in stably-stratified turbulence. Stratified turbulence in stellar radiative zones driven by shear instabilities or breaking internal waves should conceivably drive small-scale dynamo action due to the high conductivity of stellar plasma. This mechanism could provide a source of magnetization in more massive stars with extended radiative zones. We present investigations into three principle properties of the small-scale dynamo in stably stratified turbulence–the onset criterion, the growth rate, and the nature of the magnetic field anisotropy. Using our Sun as a representative star, we find that the stratification is strong enough to make the small-scale dynamo active in the solar tachocline for thermal Prandtl number Pr=1. |
Wednesday, November 10, 2021 10:30AM - 11:00AM |
NI01.00003: Study of heliospheric-relevant magnetic turbulence in a laboratory plasma wind tunnel Invited Speaker: David A Schaffner A novel laboratory environment for the study of heliospheric-relavant magnetic turbulence is presented. The Bryn Mawr Experiment (BMX) at the Bryn Mawr Plasma Laboratory (BMPL) is explores magnetically-dynamic turbulence in a wind-tunnel-like apparatus. The turbulence observed here is more akin to space and astrophysical settings such as the solar wind and the magnetosphere that that seen in other plasma turbulence experiments (such as edge-turbulence in fusion reactor experiments) where there is a strong guide field. Magnetized plasmas structures are generated using a coaxial-electrode source and then launched down a cylindrical flux-conserving chamber through a rail-gun-like mechanism. With no background magnetic field, both magnetic field and flow fluctuate freely within the chamber. Simultaneous multi-position magnetic measurements of these structures exhibit broadband spectra with power-law scaling typically steeper than the traditionally Kolmogorov -5/3 scaling. Experimental analyses have mirrored techniques used in spacecraft analysis of solar wind turbulence and comparisons between these regimes and the laboratory environment are made. |
Wednesday, November 10, 2021 11:00AM - 11:30AM |
NI01.00004: Electron Heating and Acceleration during Magnetic Reconnection in the PHASMA Experiment Invited Speaker: Peiyun Shi Magnetic reconnection, a universal process converting magnetic energy into thermal and kinetic energy in space and laboratory plasmas, is often governed by processes that operate at kinetic scales. Measurements of particle velocity distribution functions (VDFs) at kinetic scales are routinely acquired in space plasmas and numerical simulations, but are still relatively rare in space-relevant laboratory experiments. One incoherent Thomson scattering system was implemented in the PHASMA (PHAse Space MApping) facility to enable non-perturbative and localized measurements of electron VDF (EVDF) during reconnection at the electron kinetic scale. We present EVDFs for electron-only magnetic reconnection between two argon flux ropes, showing electron heating of 0.5-1.0 eV. The electron energy gain corresponds to > 50% of the available reconnecting magnetic energy, which is larger than is normally observed in standard magnetic reconnection and consistent with the electron-only magnetic reconnection observations without the ion coupling in the terrestrial magnetosheath [Phan et al., Nature 557, 202, 2018]. As predicted for a finite guide field, electron heating is stronger along one separatrix, with Hall magnetic field weakening the guide field. Electron beams with velocities around the electron Alfvén speed are observed. We argue the existence of beams rather than bulk flows results from the marginally collisionless regime of PHASMA and possible electron trapping mechanisms. Particle-in-cell simulations designed specifically for PHASMA conditions will be introduced and compared to experimental results. Particle VDF measurements in laboratory reconnection complement observations from satellites and numerical simulations and provide new opportunities for validating theory and simulation via systematically and controllably varying related parameters. |
Wednesday, November 10, 2021 11:30AM - 12:00PM |
NI01.00005: Measurements of Weibel magnetic fields in optical-field ionized plasmas Invited Speaker: Chaojie Zhang Generation and amplification of magnetic fields in plasmas is a long-standing topic that is of great interest to both fundamental and applied physics. Weibel instability is a well-known mechanism responsible for self-generating magnetic fields in anisotropic plasmas and has been extensively investigated in both theory and simulations, yet its experimental verification has proven challenging. Recently, we have demonstrated a new experimental platform that enables the initialization of the plasma electron velocity distribution in a controllable manner and then measures the evolution of Weibel magnetic fields using an external ultrashort relativistic electron probe. Here we will first present experimental results on time-resolved measurements of Weibel magnetic fields in non-relativistic plasmas produced by optical field ionization using an ultrashort IR laser (0.8 μm). It was found that the Weibel magnetic fields self-organize into a quasistatic structure consistent with a helicoid topology within a few picoseconds and such a structure lasts for tens of picoseconds. The magnetic fields show a well-defined wave vector. The growth rate and saturated magnetic field magnitude were measured and agree well with kinetic theory predictions. We then discuss the feasibility of extending the study to the quasi-relativistic regime by using intense CO2 (10 μm) lasers to produce much hotter anisotropic plasmas. The platform we have demonstrated is suitable for exploring a broad range of plasma phenomena such as magnetic reconnection, annihilation, and island formation thereby opening a new avenue for studying astrophysical phenomena in the laboratory. |
Wednesday, November 10, 2021 12:00PM - 12:30PM |
NI01.00006: Stabilization of liquid instabilities with plasma jets at the gas-liquid boundary Invited Speaker: Wonho Choe Impinging gas jets can induce depressions in liquid surfaces. A dimple-like stable cavity on a liquid surface forms owing to the balance of forces among the gas jet impingement, gravity and surface tension. With increasing gas jet speed, the cavity becomes unstable and shows oscillatory motion, bubbling (Rayleigh instability) and splashing (Kelvin–Helmholtz instability). However, despite its scientific and practical importance—particularly in regard to reducing cavity instability growth in certain gas-blown systems—little attention has been given to the hydrodynamic stability of a cavity in such gas–liquid systems so far. Here we demonstrate the stabilization of such instabilities by weakly ionized gas for the case of a gas jet impinging on water, based on shadowgraph experiments and computational two-phase fluid and plasma modelling. We focus on the interfacial dynamics relevant to electrohydrodynamic (EHD) gas flow, so-called electric wind, which is induced by the momentum transfer from accelerated charged particles to neutral gas under an electric field. A weakly ionized plasma consisting of periodic pulsed ionization waves, i.e. plasma bullets, exerts more force via electrohydrodynamic flow on the water surface than a neutral gas jet alone, resulting in cavity expansion without destabilization. Furthermore, both the bidirectional electrohydrodynamic gas flow and electric field parallel to the gas–water interface produced by plasma interacting 'in the cavity' render the surface more stable. This case study demonstrates the dynamics of liquids subjected to a plasma-induced force, offering insights into physical processes and revealing an interdependence between weakly ionized plasma and deformable dielectric matter, including plasma–liquid systems. |
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