Session NI2: Waves and Instabilities

Summary of initial results from the Magnetized Dusty Plasma Experiment (MDPX) device

Dusty (or complex) plasmas are four-component plasma systems consisting of electrons, ions, neutral atoms and charged, solid particulates. These particulates, i.e., the ``dust,'' become charged through interactions with the surrounding plasma particles and are therefore fully coupled to the background. The study of dusty plasmas began with astrophysical studies and has developed into a distinct area of plasma science with contributions to industrial, space, and fundamental plasma science. However, the vast majority of the laboratory studies are performed without the presence of a magnetic field. This is because, compared to the masses of the electrons and ions, the dust particles are significantly more massive and therefore the charge-to-mass ratio of the dust is very small. As a result, large (B $>$ 1 T) magnetic fields are required to achieve conditions in which the dynamics of electrons, ions, and dust particles are dominated by the magnetic field. This presentation will provide a brief description of the design of the large bore (50 cm diameter x 158 cm long), multi-configuration, 4-Tesla class, superconducting magnet and integrated plasma chamber optimized for the study of dusty plasmas at high magnetic field - the MDPX device [1]. The presentation will then focus on initial results of measurements made using MDPX - including observations of a new type of imposed ordered structures formed by the dust particles in a magnetized plasma [2], E x B driven flows of the particles, and observations of instabilities. This work is a collaboration of the author with Uwe Konopka (Auburn), Robert L. Merlino (Univ. of Iowa), Marlene Rosenberg (UCSD), and the MDPX team at Auburn University.\\[4pt] [1] E. Thomas, et. al, J. Plasma Phys., 81, 345810206 (2015).\\[0pt] [2] E. Thomas Jr, Phys. Plasmas, 22, 030701 (2015).   

Demonstration of Electrostatic to Electromagnetic Conversion through Induced Nonlinear Scattering by Thermal Plasma

The nonlinear conversion of electrostatic (ES) to electromagnetic (EM) waves in the whistler branch through induced scattering by thermal electrons is an important contribution to the evolution of plasmas in weak turbulence when the wave amplitude is large enough for linear/quasi-linear approaches to break down. It has been theoretically shown that in isothermal low beta turbulent plasmas the rate of induced scattering by particles is much larger than three-wave coalescence and decay processes. It is particularly important to near-Earth space plasma evolution during disturbed times when wave amplitudes cross the threshold for nonlinear scattering. The change in k vector and group velocity of the waves resulting from the conversion from ES to EM enhances~the efficiency~of pitch-angle scattering, which plays a dramatic role in regulating the trapped energetic electron fluxes in\textbf{~}the Earth's radiation belts. This nonlinear process is being studied in the NRL Space Physics Simulation Chamber, demonstrating the induced nonlinear scattering of quasi-electrostatic pump waves by thermal electrons. The experimental results support theoretical predictions of the nonlinear interaction.   

Excitation of chirping whistler waves in a laboratory plasma

\noindent Whistler mode chorus emissions with a characteristic frequency chirp largely control the dynamic variability of the Earth's outer radiation belt. They are responsible for the acceleration of outer radiation belt electrons to relativistic energies and also for the scattering loss of these electrons into the atmosphere. Here, we report on the first laboratory experiment where whistler waves exhibiting fast frequency chirping have been artificially produced using a gyrating beam of energetic electrons injected into a cold plasma. It is shown that there is an optimal beam density for frequency chirps, which indicates the existence of optimum wave amplitude for the generation of chirps. Also, frequency chirps only occur for a very narrow range of ratio of $f_{pe}/f_{ce}$, similar to that observed in space. Strong magnetic field gradient, which prohibits the formation of phase space electron hole, disrupts frequency chirps as expected.\footnote{B. Van Compernolle et al., Phys. Rev. Lett. 114, 245002 (2015).} Broadband whistler waves similar to magnetospheric hiss are also observed at relatively high plasma density. Their mode structures are identified by the phase-correlation technique. It is demonstrated that broadband whistlers are excited through Landau resonance, cyclotron resonance and anomalous cyclotron resonance. Wave growth rate and wave normal angle given by linear theory are consistent with experimental results in general. Preliminary particle-in-cell simulation captures the linear theory prediction of broadband whistlers and also gives important information on the evolution of electron distribution function.   

Bounce-harmonic Landau Damping of Plasma Waves

We present measurement of plasma wave damping, spanning the temperature regimes of direct Landau damping, bounce-harmonic Landau damping, inter-species drag damping, and viscous damping. Direct Landau damping is dominant at high temperatures, but becomes negligible as $\bar{v} < v_{ph} / 5$ . The measurements are conducted in trapped pure ion plasmas contained in Penning-Malmberg trap, with wave-coherent LIF diagnostics of particle velocities. Our focus is on bounce harmonics damping, controlled by an applied ``squeeze'' potential, which generates harmonics in the wave potential and in the particle dynamics. A particle moving in $z$ experiences a non-sinusoidal mode potential caused by the squeeze, producing high spatial harmonics with lower phase velocity. These harmonics are Landau damped even when the mode phase velocity $ v_{ph}$ is large compared to the thermal velocity $\bar{v} $, since the $n^{th}$ harmonic is resonant with a particle bouncing at velocity $v_b = v_{ph}/n$. Here we increase the bounce harmonics through applied squeeze potential; but some harmonics are always present in finite length systems. For our centered squeeze geometry, theory shows that only odd harmonics are generated, and predicts the Landau damping rate from $ v_{ph}/n$. Experimentally, the squeeze potential increases the wave damping and reduces its frequency. The frequency shift occurs because the squeeze potential reduces the number of particle where the mode velocity is the largest, therefore reducing the mode frequency. We observe an increase in the damping proportional to $V^2_s$, and a frequency reduction proportional to $V_s$ , in quantitative agreement with theory\footnote{A.Ashourvan, D.H.E. Dubin, Phys. Plas. \textbf{21}, 052109, 2014.}. Wave-coherent laser induced fluorescence allows direct observation of bounce resonances on the particle distribution, here predominantly at $ v_{ph}/3$ . A clear increase of the bounce harmonics is visible on the particle distribution when the squeeze potential is applied.   

Vortex dynamics of electron plasmas in externally imposed $E \times B$ flows

Electron plasmas confined in cylindrical Penning-Malmberg traps may be used to study two-dimensional vorticity dynamics through an isomorphism between the Drift-Poisson equations for $E \times B$ flow and the Euler equations describing an ideal fluid.\footnote{T. B. Mitchell \& C. F. Driscoll, {\it Phys. Fluids} {\bf 8}, 7 (1996).} In the work to be described here, the boundary conditions are varied by biasing segments of a confining electrode. This results in the imposition of ExB flows that advect the electron density (which is the analog of fluid vorticity). In this manner, the response of a stable, coherent 2D vortex to irrotational shear and strain flows can be studied with the precision and control not possible in traditional fluids such as water.\footnote{R. R. Trieling, et al., {\it J. Fluid Mech.} {\bf 360}, 273-294 (1998).} Behavior to be described includes partial or complete vortex destruction through the stripping of peripheral vorticity, vortex fission, and breaking of adiabatic invariance for a vortex in time-dependent external flow fields. It is shown that vortex stripping is the dominant response to external strain, and the conditions necessary for partial stripping, fission, and total decoherence are elucidated. One advantage of electron plasma experiments over those with traditional fluids is the ability to control the relevant timescales, and this enables novel studies of non-adiabatic effects and the fission process. Beyond basic vortex dynamics and 2D turbulence,\footnote{P. Tabeling, {\it Phys. Reports} {\bf 362}, 1-62 (2002).} these studies are potentially relevant to geophysical fluid flows\footnote{D. G. Dritschel \& B. Legras, {\it Phys. Today} 44-51 March (1993).} and shear suppression of turbulence in plasmas.\footnote{P. Terry, {\it Rev. Mod. Phys.} {\bf 72}, 1 (2000).}   

Resonant excitation of waves by a spiraling ion beam on the large plasma device

The resonant interaction between energetic-ions and plasma waves is a fundamental topic of importance in the space, controlled magnetic-fusion, and laboratory plasma physics. We report new results on the spontaneous generation of traveling shear Alfv\'{e}n waves and high-harmonic beam-modes in the lower-hybrid range of frequencies by an intense ion beam. In particular, the role of Landau and Doppler-shifted ion-cyclotron resonances (DICR) in extracting the free-energy from the ion-beam and destabilizing Alfv\'{e}n waves was explored on the Large Plasma Device (LAPD). In these experiments, single and dual-species magnetized plasmas ($n \approx 10^{10}$--$10^{12}$ cm$^{-3}$, T$_e \approx$ 5.0--10.0 eV, B = 0.6--1.8 kG, He$^+$ and H$^+$ ions, 19.0 m long, 0.6 m diameter) were produced and a spiraling hydrogen ion beam (5--15 keV, 2--10 A, beam-speed/Alfv\'{e}n-speed = 0.2--1.5, J $\approx$ 50--150 mA/cm$^2$, pitch-angle $\approx 53^\circ$) was injected into the plasma. The interaction of the beam with the plasma was diagnosed using a retarding-field energy analyzer, three-axis magnetic-loop, and Langmuir probes. The resonance conditions for the growth of shear Alfv\'{e}n waves were examined by varying the parameters of the ion-beam and ambient plasma. The experimental results demonstrate that the DICR process is particularly effective in exciting left-handed polarized shear Alfv\'{e}n waves that propagate in the direction opposite to the ion beam. The high-harmonic beam modes were detected in the vicinity of the spiraling ion beam and contained more than 80 harmonics of Doppler-shifted gyro-frequency of the beam.\\[4pt] [1] Tripathi et. al., Rev. Sci. Instrum. 82, 093501 (2011)\\[0pt] [2] Tripathi et. al., Phys. Rev. E 91, 013109 (2015)