Bulletin of the American Physical Society
55th Annual Meeting of the APS Division of Plasma Physics
Volume 58, Number 16
Monday–Friday, November 11–15, 2013; Denver, Colorado
Session JI2: Fundamental Plasma Physics |
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Chair: Stephen Vincena, University of California, Los Angeles Room: Plaza E |
Tuesday, November 12, 2013 2:00PM - 2:30PM |
JI2.00001: How the Propagation of Heat-Flux Modulations Triggers $E\times B$ Flow Pattern Formation Invited Speaker: Yusuke Kosuga Recently, a new class of $E\times B$ flow pattern, called an ``$E\times B$ staircase,'' was observed in a simulation study using the full-$f$ flux driven GYSELA code. Here, $E\times B$ staircases are quasi-regular steady patterns of localized shear layers and temperature profile corrugations. The shear layers are interspaced between regions of turbulent avalanching of the size of several correlation length ($\sim 10 \Delta_c$). In this work, a theory to describe the formation of such $E\times B$ staircases from a bath of stochastic avalanches is presented, based on analogy of staircase formation to jam formation in traffic flow. Namely, staircase formation is viewed as a heat flux ``jam'' that causes profile corrugation, which is analogous to a traffic jam that causes corrugations in the local car density in a traffic flow. To model such an effect in plasmas, a finite response time $\tau$ is introduced, during which instantaneous heat flux relaxes to the mean heat flux, determined by symmetry constraints. The response time introduced here is an analogue of drivers' response time in traffic flow dynamics. It is shown that the extended model describes a heat flux ``jam'' and profile corrugation, which appears as an instability, in analogy to the way a clustering instability leads to a traffic jam. Such local amplification of heat and profile corrugations can lead to the formation of $E\times B$ staircases. The scale length that gives the maximum growth rate falls in the mesoscale range and is comparable to the staircase step spacing. [Preview Abstract] |
Tuesday, November 12, 2013 2:30PM - 3:00PM |
JI2.00002: Progress Towards a Practical Multicell Positron Trap Invited Speaker: J.R. Danielson The physics and technology of positron confinement is central to a range of applications at the forefront of antimatter science. Progress in this area has been driven by the development of a suite of novel non-neutral plasma techniques whereby up to $4\times10^9$ positrons have now been trapped and stored.\footnote{D. W. Fitzakerley et al, {\it Bull. Am. Phys. Soc.} {\bf 58}, 176 (2013).} However the next generation of experiments will require orders of magnitude more positrons. This talk describes techniques to increase storage capacity to $\geq 10^{12}$ using a novel multi-cell trap architecture.\footnote{Danielson, Weber, Surko, {\it Phys. Plasmas} {\bf 13}, 123502 (2006).}$^,$\footnote{Danielson, Hurst, Surko, AIP Conf. Proc. {\bf 1521}, 101 (2013).} Plasmas will be stored in separate Penning-Malmberg traps (``cells'') arranged in parallel off the magnetic axis to maximize use of the magnetic field volume while minimizing the required confinement voltages. Experiments with electrons in a test structure will be described to explore the basic physics and technology of the multicell concept and to set the design of a 21-cell trap for $10^{12}$ positrons. Over 50\% of a trapped plasma has been injected into an off-axis cell, and hour-long confinement of $2\times10^8$ particles has been achieved using rotating electric fields. Experiments are under way to identify the limits of the injection process and demonstrate confinement $> 10^{10}$ particles in a single off-axis cell using kilovolt confinement potentials. [Preview Abstract] |
Tuesday, November 12, 2013 3:00PM - 3:30PM |
JI2.00003: Two-Photon Absorption Laser Induced Fluorescence Measurements of Neutral Density in Helicon Plasma Invited Speaker: Matthew Galante Neutral particles play a critical role in nearly all plasmas, from the pedestal region of a tokamak fusion plasma to industrial plasma processing systems. In fusion plasmas, neutrals at the edge serve as both a source of particles and also a sink of momentum and energy. Control of the edge plasma density in tokamaks is critical for the transition to H-mode plasmas and the role of neutrals in modifying the plasma rotation in the edge is an area of active research. However, few methods exist to make localized, direct neutral density measurements. We have developed a new diagnostic based on two-photon absorption laser induced fluorescence (TALIF). We use a high intensity (5 MW/cm$^2$), narrow bandwidth (0.1 cm$^{-1}$) laser to probe the ground state of neutral hydrogen, deuterium and krypton with spatial resolution better than 0.2 cm, a time resolution of 10 ns, and a measurement cadence of 20 Hz. In this talk I will describe proof-of-principle measurements in a helicon plasma source that demonstrate the TALIF diagnostic is capable of measuring neutral densities spanning four orders of magnitude; comparable to the edge neutral gradients predicted in the tokamak pedestal. The measurements are performed in hydrogen and deuterium plasmas and absolute calibration is accomplished through TALIF measurements in neutral krypton. The optical configuration employed is confocal, i.e., both light injection and collection are accomplished through a single optical port in the vacuum vessel. The wavelength resolution of the diagnostic is sufficient to separate hydrogen and deuterium spectra and I will present measurements from mixed hydrogen and deuterium plasmas that demonstrate isotopic abundance measurements are feasible with the TALIF system. Time and spatially resolved measurements also allow us to explore the effects of wall recycling and pulse repetition rates on the neutral density profile in the plasma source. [Preview Abstract] |
Tuesday, November 12, 2013 3:30PM - 4:00PM |
JI2.00004: Destruction of a Magnetic Mirror-Trapped Hot Electron Ring by a shear Alfv\'{e}n Wave Invited Speaker: Yuhou Wang Highly energetic electrons produced naturally or artificially can be trapped in the Earth's radiation belts for months, posing a danger to valuable space satellites. Concepts that can lead to radiation belts mitigation have drawn a great deal of interest. In this work, we demonstrate that a shear Alfv\'{e}n wave (SAW) can effectively de-trap energetic electrons confined by a magnetic mirror field. The experiment is performed in a quiescent afterglow plasma in the Large Plasma Device (LaPD) at UCLA ($n_{e} =0.1-1\times 10^{12}/cm^{3}$, $T_{e} \approx 0.5eV$, $B_{0} =400-1600G$, $L=18m$, and $diameter=0.6m)$. A hot electron ring, along with hard x-rays of energies of $100keV\sim 3MeV$, is generated by 2nd harmonic ECRH and is trapped in a magnetic mirror field ($L=3.5m$, $R_{mirror} =1.1-4)$. A shear Alfv\'{e}n wave ($f\sim 0.5f_{ci} $, $B_{wave} /B_{0} \sim 0.1\% )$ is launched with a rotating magnetic field antenna with arbitrary polarization. Irradiated by the SAW, the electrons are lost periodically with the characteristic frequency of the SAW, and the ring m number changes. The periodical loss of electrons continues even after the termination the wave. The effect is found to be caused only by the right-hand (electron diamagnetic direction) circularly polarized component of the SAW. Hard x-ray tomography, constructed from more than 1000 chord projections at each axial location, shows electrons are lost in both the radial and axial direction. X-ray spectroscopy shows electrons over a broad range of energy de-trapped by the SAW. The de-trapping process is found to be accompanied by electro-magnetic fluctuations in the frequency range of $1\sim 5f_{LH} $, which are also modulated at the frequency of the SAW. To exclude the possible role of whistler waves in this electron de-trapping process, whistler waves at these frequencies are launched with an antenna in absence of the SAW and no significant electron loss found. [Preview Abstract] |
Tuesday, November 12, 2013 4:00PM - 4:30PM |
JI2.00005: A Variational Formulation of Macro-Particle Algorithms for Kinetic Plasma Simulations Invited Speaker: B.A. Shadwick Macro-particle based simulations methods are in widespread use in plasma physics; their computational efficiency and intuitive nature are largely responsible for their longevity. In the main, these algorithms are formulated by approximating the continuous equations of motion. For systems governed by a variational principle (such as collisionless plasmas), approximations of the equations of motion is known to introduce anomalous behavior, especially in system invariants. We present a variational formulation of particle algorithms for plasma simulation based on a reduction of the distribution function onto a finite collection of macro-particles.\footnote{E. G. Evstatiev and B. A. Shadwick, ``Variational Formulation of Particle Algorithms for Kinetic Plasma Simulations,'' J.~Comput. Phys.~\textbf{245}, 376 (2013).} As in the usual Particle-In-Cell (PIC) formulation, these macro-particles have a definite momentum and are spatially extended. The primary advantage of this approach is the preservation of the link between symmetries and conservation laws. For example, nothing in the reduction introduces explicit time dependence to the system and, therefore, the continuous-time equations of motion exactly conserve energy; thus, these models are free of grid-heating. In addition, the variational formulation allows for constructing models of arbitrary spatial and temporal order. In contrast, the overall accuracy of the usual PIC algorithm is at most second due to the nature of the force interpolation between the gridded field quantities and the (continuous) particle position. Again in contrast to the usual PIC algorithm, here the macro-particle shape is arbitrary; the spatial extent is completely decoupled from both the grid-size and the ``smoothness'' of the shape; smoother particle shapes are not necessarily larger. For simplicity, we restrict our discussion to one-dimensional, non-relativistic, un-magnetized, electrostatic plasmas. We comment on the extension to the electromagnetic case. [Preview Abstract] |
Tuesday, November 12, 2013 4:30PM - 5:00PM |
JI2.00006: Confinement and Structural Changes in Vertically Aligned Dust Structures Invited Speaker: Truell Hyde In physics, confinement is known to influence collective system behavior. Examples include coulomb crystal variants such as those formed from ions or dust particles (classical), electrons in quantum dots (quantum) and the structural changes observed in vertically aligned dust particle systems formed within a glass box placed on the lower electrode of a Gaseous Electronics Conference (GEC) rf reference cell. Recent experimental studies have expanded the above to include the biological domain by showing that the stability and dynamics of proteins confined through encapsulation and enzyme molecules placed in inorganic cavities such as those found in biosensors are also directly influenced by their confinement. In this paper, the self-assembly and subsequent collective behavior of structures formed from $n$, charged dust particles interacting with one another and located within a glass box placed on the lower, powered electrode of a GEC rf reference cell is discussed. Self-organized formation of vertically aligned one-dimensional chains, two-dimensional zigzag structures, and three-dimensional helical structures of triangular, quadrangular, pentagonal, hexagonal, and heptagonal symmetries are shown to occur. System evolution is shown to progress from one-dimensional chain structures, through a zigzag transition to a two-dimensional, spindle like structures, and then to various three-dimensional, helical structures exhibiting various symmetries. Stable configurations are shown to be strongly dependent upon system confinement. The critical conditions for structural transitions as well as the basic symmetry exhibited by the one-, two-, and three-dimensional structures that subsequently develop will be shown to be in good agreement with molecular dynamics simulations. [Preview Abstract] |
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