Session TM9: Mini-Conference on Bridging the Divide Between Space and Laboratory Plasma Physics I

Wave-particle correlation measurements on the Parker Solar Probe mission.

The Parker Solar Probe (PSP) mission will launch in July 2018 and eventually orbit with perihelion at 9.8 solar radii, deep into the solar corona. The FIELDS and SWEAP instruments on PSP will measure in situ electric and magnetic fields and thermal plasmas, respectively. A hardware interface between the FIELDS and SWEAP instuments will allow FIELDS to measure directly individual particle counts from the SWEAP electrostatic analyzer instruments. The FIELDS Time Domain Sampler (TDS) has a channel dedicated to particle counts and sampled simultaneously with electric and/or magnetic field measurements. In this talk, I'll describe the instrument design and show some laboratory test data, and discuss the science that we hope address with the PSP wave- particle correlation measurements.   

The Basic Plasma Science Facility: a platform for studying plasma processes relevant to space and astrophysical settings

The Basic Plasma Science Facility at UCLA is a national user facility for studies of fundamental processes in magnetized plasmas. The centerpiece is the Large Plasma Device, a 20 m, magnetized linear plasma device. Two hot cathode plasma sources are available. A Barium Oxide coated cathode produces plasmas with $n\sim 10^{12}$cm$^{-3}$, $T_e \sim 5$ eV, $T_i \lesssim 1$eV with magnetic field from 400G-2kG. This low-$\beta$ plasma has been used to study fundamental processes, including: dispersion and damping of kinetic and inertial Alfv\'{e}n waves, flux ropes and magnetic reconnection, three-wave interactions and parametric instabilities of Alfv\'{e}n waves, turbulence and transport, and interactions of energetic ions and electrons with plasma waves. A new Lanthanum Hexaboride (LaB$_6$) cathode is now available which produces significantly higher densities and temperatures: $n \lesssim 5\times 10^{13}$cm$^{-3}$, $T_e \sim 12$eV, $T_i \sim 6$eV. This higher pressure plasma source enabled the observation of laser-driven collisionless magnetized shocks and, with lowered magnetic field, provides magnetized plasmas with $\beta$ approaching or possibly exceeding unity. This opens up opportunities for investigating processes relevant to the solar wind and astrophysical plasmas.   

Plasmoid-Mediated Reconnection and Turbulence in Laboratory and Space Plasmas

Among recent new developments, the so-called plasmoid instability of thin current sheets has challenged classical nonlinear reconnection models. Within the framework of the resistive MHD model, this instability alters qualitatively the predictions of the classical Sweet-Parker model, leading to a new nonlinear regime of fast reconnection in which the reconnection rate itself becomes independent of the Lundquist number. This regime has also been seen in Hall MHD as well as fully kinetic simulations. Plasmoids, which can grow by coalescence to large sizes, provide a powerful mechanism for coupling between large (global) and small (kinetic) scales as well as an efficient accelerator of particles to high energies. A new phase diagram of fast reconnection has been proposed, informing the design of experiments (such as the FLARE experiment at Princeton, and TREX at Madison). In 3D, the instability produces self-generated and strongly anisotropic turbulence in which the reconnection rate for the mean magnetic field remains approximately at the 2D value, but the energy spectrum deviates strongly from standard MHD turbulence phenomenology. Applications of the theory to observations in laboratory (including fusion) and space (both magnetospheric and solar) plasmas will be discussed.   

Probes, Moons, and Kinetic Plasma Wakes

Nonmagnetic objects as varied as probes in tokamaks or moons in space give rise to flowing plasma wakes in which strong distortions of the ion and electron velocity distributions cause electrostatic instabilities. Non-linear phenomena such as electron holes are then produced. Historic probe theory largely ignores the resulting unstable character of the wake, but since we can now simulate computationally the non-linear wake phenomena, a timely challenge is to reassess the influence of these instabilities both on probe measurements and on the wakes themselves. Because the electron instability wavelengths are very short (typically a few Debye-lengths), controlled laboratory experiments face serious challenges in diagnosing them. That is one reason why they have long been neglected as an influence in probe interpretation. Space-craft plasma observations, by contrast, easily obtain sub-Debye-length resolution, but have difficulty with larger-scale reconstruction of the plasma spatial variation. In addition to surveying our developing understanding of wakes in magnetized plasmas, ongoing analysis of Artemis data concerning electron holes observed in the solar-wind lunar wake will be featured.   

The Mechanism for Energy Buildup in the Solar Corona

Magnetic reconnection and helicity conservation are two of the most important basic processes determining the structure and dynamics of laboratory and space plasmas. The most energetic dynamics in the solar system are the giant CMEs/flares that produce the most dangerous space weather at Earth, yet may also have been essential for the origin of life. The origin of these explosions is that the lowest-lying magnetic flux in the Sun's corona undergoes the continual buildup of stress and free energy that can be released only through explosive ejection. We perform MHD simulations of a coronal volume driven by quasi-random boundary flows designed to model the processes by which the solar interior drives the corona. Our simulations are uniquely accurate in preserving magnetic helicity. We show that even though small-scale stress is injected randomly throughout the corona, the net result of magnetic reconnection is a coherent stressing of the lowest-lying field lines. This highly counter-intuitive result -- magnetic stress builds up locally rather than spreading out to a minimum energy state -- is the fundamental mechanism responsible for the Sun's magnetic explosions. It is likely to be a mechanism that is ubiquitous throughout laboratory and space plasmas.   

Using field-particle correlations to study auroral electron acceleration in the LAPD

Resonant nonlinear Alfv\'en wave-particle interactions are believed to contribute to the acceleration of auroral electrons. Experiments in the Large Plasma Device (LAPD) at UCLA have been performed with the goal of providing the first direct measurement of this nonlinear process. Recent progress includes a measurement of linear fluctuations of the electron distribution function associated with the production of inertial Alfv\'en waves in the LAPD. These linear measurements have been analyzed using the field-particle correlation technique to study the nonlinear transfer of energy between the Alfv\'en wave electric fields and the electron distribution function. Results of this analysis indicate collisions alter the resonant signature of the field-particle correlation, and implications for resonant Alfv\'enic electron acceleration in the LAPD are considered.   

Magnetic flux rope model for solar coronal loops

Coronal loops on the surface of the sun appear to be magnetic flux ropes, but details are obscured by the difficulty of solar observations. Toroidal magnetic fusion plasmas provide a deep theoretical resource for analyzing their configuration and stability. Curved plasma loops with finite pressure are unstable to expansion in major radius, while the solar gravity and magnetic field can provide stabilizing forces. A MHD model for simple loops\footnote{L. Sugiyama, M. Asgari-Targhi, Phys. Plasmas 24, 022904 (2017).} parametrizes loop steady states in terms of the MHD gravity parameter $\hat{G}=ga/v_A^2$ relative to the plasma beta $\beta$ and inverse aspect ratio $\epsilon=a/R_o$ ($g$ is the acceleration due to gravity and $v_A$ the shear Alfv\'{e}n velocity). At the maximum $\hat{G}/\beta\sim\epsilon^2$, the plasma density varies strongly between the top and the bottom ends of the loop. Comparison to observed thin loops in solar active regions show that the predicted steady states fit the range of observed heights and that height increases with $\hat{G}$ up to the critical limit. The model also describes features of the thicker loops that give can rise to solar flares and coronal mass ejections and provides insight into a number of open questions in solar physics.   

Comparison of fluid, astrophysical, and laboratory turbulence using a permutation entropy and statistical complexity technique

Understanding turbulent processes as having a more random/stochastic mechanism or a more chaotic/deterministic mechanism is important for characterizing and comparing different turbulent systems. A statistical time-series analysis technique that quantifies the permutation entropy and statistical complexity of a signal can be used to distinguish noisy fluctuations from those arising from underlying chaotic behavior. This technique is applied to three turbulent datasets: velocity fluctuations from a fluid wind tunnel, magnetic and velocity fluctuations from satellite observation of the solar wind, and magnetic and velocity fluctuations from a turbulent laboratory plasma. The work aims to develop a more global understanding of turbulent behavior that spans both fluid and plasma regimes. Results show that fluid and astrophysical turbulence exhibit more stochastic like behavior while laboratory plasmas retain higher complexity features. The behavior of complexity and permutation entropy as a function of scale is also examined and such scans are useful for extracting important spatial and temporal scales in the system.   

Laboratory development and testing of spacecraft diagnostics

The Naval Research Laboratory's Space Chamber experiment is a large-scale laboratory device dedicated to the creation of large-volume plasmas with parameters scaled to realistic space plasmas. Such devices make valuable contributions to the investigation of space plasma phenomena under controlled, reproducible conditions, allowing for the validation of theoretical models being applied to space data. However, in addition to investigations such as plasma wave and instability studies, such devices can also make valuable contributions to the development and testing of space plasma diagnostics. One example is the plasma impedance probe developed at NRL. Originally developed as a laboratory diagnostic, the sensor has now been flown on a sounding rocket, is included on a CubeSat experiment, and will be included on the DoD Space Test Program's STP-H6 experiment on the International Space Station. In this talk, we will describe how the laboratory simulation of space plasmas made this development path possible.