Session BI3: Invited Basic: Rosenbluth Award, Basic Plasma Physics Experiments

Marshall N. Rosenbluth Outstanding Doctoral Thesis Award Talk: Dynamics of two-dimensional electron vortices subject to applied $E \times B$ strain flows

Presented here is a novel experimental technique to study the two-dimensional (2D) fluid dynamics of magnetized, non-neutral pure electron plasmas subject to applied boundary conditions [1,2]. Details of the experimental procedure are given, and data regarding elliptical distortions of the plasmas are discussed. The $E \times B$ drift dynamics of the plasmas studied here are directly analogous to that of the vorticity in a 2D inviscid, incompressible fluid [3]. The experimental apparatus is designed so that the electric potential on the boundary can be specified in the plane perpendicular to the magnetic field while preserving translational symmetry along the field. This allows one to study vortex dynamics in the presence of external flows. Initially axisymmetric electron vortices are prepared with radial vorticity profiles ranging from smooth (e.g., Gaussian) to quasi-flat. When quasi-flat profiles are subjected to constant external strain flows, the vortices are either destabilized or distorted elliptically in a periodic fashion, depending on the strain-to-vorticity ratio. The data are in quantitative agreement with a theoretical fluid model where the vorticity is treated as piecewise constant in an elliptical region [4]. However, when the profile is sufficiently smooth, the amplitude of the oscillation can decay due to spatial Landau damping [5]. Finally, adiabatic behavior of the system is studied by gradually increasing the strain rate over time using both smooth and piecewise ramp functions. [1] N. C. Hurst, et. al., Phys. Rev. Lett. 117, 235001 (2016) [2] N. C. Hurst, et. al., J. Fluid Mech. 848, 256 (2018) [3] C. F. Driscoll and K. S. Fine, Phys. Fluids B 2, 1359 (1990) [4] S. Kida, J. Phys. Soc. Japan 50, 3517 (1981) [5] D. Schecter, et. al., Phys. Fluids 12, 2397 (2000) This work is done in collaboration with J. R. Danielson, D. H. E. Dubin, and C. M. Surko.   

Pair Plasma Trapping by Higher Order RF Multipole Structures

Charged particle traps such as Penning and Paul traps may be used to confine non-neutral plasma, but space charge repulsion limits stable trapping to low plasma density. This paper demonstrates reducing space charge effects by loading the trap with neutral plasma. 2D and 3D particle-in-cell simulations [1] of RF Multipole Plasma Trap (MPT) structures are conducted [2], showing attainment of neutral plasma trapping up to the critical density set by the trapped species' plasma frequency---several orders of magnitude higher than the space charge-limited non-neutral plasma density. Positive and negative particles of the same charge-to-mass ratio are trapped symmetrically by the multipole field, and studies of pair plasma trapping are presented (100 MHz drive, 10$^{\mathrm{17}}$ m$^{\mathrm{-3}}$ pair ion plasma density; 250 MHz drive, 10$^{\mathrm{14}}$ m$^{\mathrm{-3}}$ electron-positron plasma density). The plasma quasi-neutrality and reduced space charge allows large trapped volumes (greater than 10 cm trap radius) and high total particle content, and higher order multipoles (e.g. n $=$ 8 or 16, compared to n $=$ 2 for the quadrupole Paul trap) become advantageous due to their nearly field-free interior and possibility of design with driving frequency above the plasma frequency. Effective trapping potentials in the MPT on the order of 100 V are achieved while still maintaining the essential adiabatic condition at the trap boundary [3], such that particle trajectories remain stable. This capability makes a compelling platform for experimental pair plasma studies, since the achievable parameters compare favorably with other current approaches [4]. Trapping of ion-electron plasma is also demonstrated, in which the electrons are responsive to the RF and in turn trap the positive ion species electrostatically. PIC simulations and experimental design considerations for loading and diagnostics are presented, as well as studies with magnetic fields at and around ponderomotive gyroresonance. [1] C. Nieter and J. R. Cary, J. Comput. Phys. \textbf{196}, 448 (2004) [2] N. K. Hicks, (submitted to \textit{Phys. Rev. Lett.}) (2019) [3] D. Gerlich, in \textit{Adv. Chem. Phys. State\textunderscore Selected State\textunderscore To\textunderscore State Ion\textunderscore Molecule React. Dyn. Part 1. Exp. Vol. 82}, edited by C.-Y. Ng and M. Baer (John Wiley {\&} Sons, Inc, 1992), pp. 1--176. [4] E. V. Stenson et al., \textit{Phys Rev. Lett.} \textbf{121}, 235005 (2018)   

Milestones on the way to matter/antimatter plasmas: Efficient injection and extended confinement of positrons in a dipole trap

Plasmas that consist of negatively and positively charged particles of equal mass (``pair plasmas'') have been a topic of theoretical and computational studies for more than forty years and are predicted to exhibit many significant differences from electron/ion plasmas. Electron/positron plasmas, which can be magnetized easily, are of particular interest, and there have recently been major advancements in experimental methods to study them in the laboratory. This talk will report on achievements of the APEX (A Positron Electron eXperiment) collaboration that are key steps toward its goal of magnetically confined e+/e- pair plasmas. APEX uses an intense source of cold positrons (the NEPOMUC beam line at the neutron source FRM-II), which will ultimately be accumulated in a series of non-neutral plasma traps, then injected with electrons into the confining magnetic field of a levitated dipole trap. First, a prototype trap based on a supported permanent magnet has been used to demonstrate injection and trapping of the externally produced positrons in a dipole field. Lossless transport of the positrons into the trap has been achieved with ExB drifts, induced by strategically applying voltages to electrodes at the edge of the confinement region\footnote{E. V. Stenson et al., PRL 121, 235005 (2018)}. Switching off these voltages results in injected positrons being trapped for more than a second, corresponding to hundreds of thousands of toroidal transits\footnote{J. Horn-Stanja et al., PRL 121, 235003 (2018)}. Simulations have guided and reproduced injection experiments and tentatively identified elastic scattering off residual background gas (causing diffusion in position and velocity space) as the main limit on confinement, suggesting that longer trapping will be readily achievable.   

Electromagnetic Ion/Ion Beam Instabilities in the Laboratory, the Magnetosphere, and Simulation

Collisionless electromagnetic ion/ion beam instabilities play an important role in the formation of quasi-parallel bow shocks in planetary magnetospheres. One particular instability, the Right-Hand Resonant Instability (RHI), is responsible for generating large-amplitude ($\delta B/B_0 > 1$) ultra-low frequency (ULF) electromagnetic waves in the quasi-parallel foreshock, which interact nonlinearly to create structures such as shocklets and current filaments. Demonstrating the detailed microphysics of these processes through experiment or observation remains an open challenge. \\ Spacecraft observations of these features are well complemented by dimensionless-parameter scaled laboratory experiments. A recent series of experiments at the University of California, Los Angeles has made volumetric measurements of waves produced by the RHI that are directly comparable to those observed in the terrestrial magnetosphere. In the experiment, a high-energy laser ablates a super-Alfv\' enic ($M_A = 5$) laser-produced plasma (LPP) from a plastic (HDPE) target embedded in the highly repeatable magnetized ambient plasma of the Large Plasma Device (LAPD). The LPP, which consists primarily of several carbon charge states, expands quasi-parallel to the background magnetic field and is unstable to the RHI. As the instability grows it generates waves which are measured by an array of 3-axis magnetic flux probes. Moving the probes between shots allows the collection of volumetric data sets comprising thousands of shots. \\ Waves generated in the experiment exhibit spectral properties and circular polarization consistent with linear theory for the RHI. 2D planes transverse to the background magnetic field far from the growth region reveal spiraling current filaments, followed in time by an alternating axial current structure. Mutual comparison with linear theory and 2D hybrid simulations shows that the observed waves closely match spacecraft measurements from the terrestrial quasi-parallel foreshock, although waves in the experiment reach much lower amplitude ($\delta B/B_0 < 0.01$). This result demonstrates that parameter-scaled laboratory experiments can reproduce magnetospheric waves previously only observed \textit{in situ}.   

Model for emissive cathode operation in a large magnetized plasma and controlled generation of plasma flows

This talk reports on a new model that describes the effects produced by thermionic electron injection into a magnetized plasma in which the separation between cathode and anode is much larger than the mean free-path \footnote{M. J. Poulos, \textbf{Phys. Plasmas} 26, 022104 (2019)}. Analytic expressions are found for the spatial pattern of the global current system, the partition of potential across the plasma sheath, and the effective plasma resistance. Formulas for the threshold conditions leading to virtual cathode operation are obtained. Predictions of the model are found to be in excellent quantitative agreement with measurements performed in the Large Plasma Device (LAPD) at the University of California, Los Angeles (UCLA) \footnote{S. Jin, M. J. Poulos, B. Van Compernolle, and G. J. Morales, \textbf{Phys. Plasmas} 26, 022105 (2019)}. It is demonstrated experimentally by selective biasing of the cathode structure that flow-shear generated by thermionic emission, under externally controlled conditions, can suppress the growth of drift-Alfvén instabilities. Experimental measurements and comparisons to model predictions are presented.   

Self-Organization and Turbulent Transport in Partially Magnetized Plasmas with Crossed Electric and Magnetic Fields

Partially magnetized plasmas with crossed electric and magnetic fields are of interest for a number of applications in plasma material processing, electric propulsion, and space physics. In such plasmas, external electric field and weak ion magnetization result in large equilibrium flows of electrons and ions that lead to a number of instabilities and turbulent transport. In this talk, nonlinear simulations demonstrating self-organization and anomalous transport in partially magnetized plasma with crossed electric and magnetic field are presented. The turbulence simulations show complex interaction of small scale modes with large scale zonal flow modes, vortices, and streamers resulting in strongly intermittent anomalous transport that significantly exceeds the classical collisional values. The development of large scale structures and flows is shown to occur as a result of the inverse energy cascade from short wavelength instabilities. The turbulence driven secondary instabilities and large scale structures are shown to dominate the anomalous electron current which is strongly intermittent. Such anomalous transport and structures are consistent with a number of experimental observations in laboratory plasmas, in particular, the plasmas relevant to electric propulsion devices.