Physics
Permanent URI for this communityhttp://hdl.handle.net/1903/2269
Browse
31 results
Search Results
Item Dispersion of ion gyrocenters in models of anisotropic plasma turbulence(2009) Gustafson, Kyle Bergin; Dorland, William D; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Turbulent dispersion of ion gyrocenters in a magnetized plasma is studied in the context of a stochastic Hamiltonian transport model and nonlinear, self-consistent gyrokinetic simulations. The Hamiltonian model consists of a superposition of drift waves derived from the linearized Hasegawa-Mima equation and a zonal shear flow perpendicular to the density gradient. Finite Larmor radius (FLR) effects are included. Because there is no particle transport in the direction of the density gradient, the focus is on transport parallel to the shear flow. The prescribed flow produces strongly asymmetric non-Gaussian probability distribution functions (PDFs) of particle displacements, as was previously known. For kρ=0, where k is the characteristic wavelength of the flow and ρ is the thermal Larmor radius, a transition is observed in the scaling of the second moment of particle displacements. The transition separates nearly ballistic superdiffusive dispersion from weaker superdiffusion at later times. FLR effects eliminate this transition. Important features of the PDFs of displacements are reproduced accurately with a fractional diffusion model. The gyroaveraged ExB drift dispersion of a sample of tracer ions is also examined in a two-dimensional, nonlinear, self-consistent gyrokinetic particle-in-cell (PIC) simulation. Turbulence in the simulation is driven by a density gradient and magnetic curvature, resulting in the unstable ρ scale kinetic entropy mode. The dependence of dispersion in both the axial and radial directions is characterized by displacement and velocity increment distributions. The strength of the density gradient is varied, using the local approximation, in three separate trials. A filtering procedure is used to separate trajectories according to whether they were caught in an eddy during a set observation time. Axial displacements are compared to results from the Hasegawa-Mima model. Superdiffusion and ballistic transport are found, depending on filtering and strength of the gradient. The radial dispersion of particles, as measured by the variance of tracer displacements, is diffusive. The dependence of the running diffusion coefficient on ρ for each value of the density gradient is considered.Item H (sub) alpha & Neutral Density Scaling in the Maryland Centrifugal eXperiment(2009) Clary, Ryan; Ellis, Richard; Hassam, Adil; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The Maryland Centrifugal eXperiment (MCX) is a hydrogen plasma confinement experiment with a rotating mirror magnetic configuration. This experiment was designed to test the concepts of centrifugal confinement and velocity shear stabilization which may allow scaleability to a fusion reactor. These two concepts, however, rely on supersonic plasma fluid velocities, which, apart from possible plasma instabilities, could be greatly reduced by fluid drag with neutral hydrogen, leading to decreased confinement. Resonant charge exchange between a hydrogen ion and a hydrogen atom is believed to be the dominant drag mechanism on the rotating plasma. Neutral hydrogen emission lines (particularly the Balmer-alpha line, H (sub) alpha) are therefore of primary interest in diagnosing how neutral hydrogen affects plasma confinement. For this purpose, a multi-chord H (sub) alpha emission detector (multi-chord HED) was designed and constructed by the author in order to measure emissivity profiles. These profiles, together with an atomic collisional-radiative model, provide estimates of neutral hydrogen density and local charge-exchange times. Varied experimental parameters were applied to MCX discharges and the resulting variations in neutral density are compared to theoretical scaling laws. The charge-exchange times are compared to the measured momentum confinement time. We find that the inner and outer-most flux surfaces are not distinctly identified by the emissivity profile and the emissivity is dominant at the vacuum chamber wall. We also find that, while the overall emissivity profile does not match theoretical prediction, neutral density scaling is approximately described by the models. In addition, charge-exchange times are found to be much smaller than the momentum confinement time as well as to scale differently than the momentum confinement time. This dissertation includes a detailed description of the multi-chord HED system and its calibration, both spectrally and absolutely. We also present models based on neutral and plasma interaction which provide the scaling laws used to compare to experimental results.Item Beam halo creation and propagation in the University of Maryland Electron Ring(2009) Papadopoulos, Christos F.; O'Shea, Patrick G; Kishek, Rami A; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)In this thesis we discuss the phenomenon of halo creation in charged particle beams. For this, we combine analytical, numerical and experimental work, which focuses on the University of Maryland Electron Ring, but is applicable to a wide range of accelerators in the same intensity regime. We find that the details of the beam distribution do not affect the structure of the halo, but are nonetheless important as they determine the number of particles in the halo and whether the latter can be regenerated. Furthermore, we show that the halo in configuration and velocity space comprises of the same particles, a prediction that has great importance for halo removal and diagnostics. In particular, we show that even in the case of ideal halo removal in phase space, the complicated internal dynamics of the beam core lead to halo regeneration. Following on previous work, we also construct a theoretical particle-core model for a skew quadrupole focusing channel, and compare the results to PIC simulations as well as measurements on UMER. The agreement between these three approaches is satisfactory, within the constraints of each case.Item Transport in Poygonal Billiard Systems(2009) Reames, Matthew Lee; Dorfman, J. R.; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The aim of this work is to explore the connections between chaos and diffusion by examining the properties of particle motion in non-chaotic systems. To this end, particle transport and diffusion are studied for point particles moving in systems with fixed polygonal scatterers of four types: (i) a periodic lattice containing many-sided polygonal scatterers; (ii) a periodic lattice containing few-sided polygonal scatterers; (iii) a periodic lattice containing randomly oriented polygonal scatterers; and (iv) a periodic lattice containing polygonal scatterers with irrational angles. The motion of a point particle in each of these system is non-chaotic, with Lyapunov exponents strictly equal to zero.Item Non-linear Development of Streaming Instabilities in Magnetic Reconnection with a Strong Guide Field(2009) Che, Haihong; Drake, James F.; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Magnetic reconnection is recognized as a dominant mechanism for converting magnetic energy into the convective and thermal energy of particles, and the driver of explosive events in nature and laboratory. Magnetic reconnection is often modeled using resistive magnetohydrodynamics, in which collisions play the key role in facilitating the release of energy in the explosive events. However, in space plasma the collisional resistivity is far below the required resistivity to explain the observed energy release rate. Turbulence is common in plasmas and the anomalous resistivity induced by the turbulence has been proposed as a mechanism for breaking the frozen-in condition in magnetic reconnection. Turbulence-driven resistivity has remained a poorly understood, but widely invoked mechanism for nearly 50 years. The goal of this project is to understand what role anomalous resistivity plays in fast magnetic reconnection. Turbulence has been observed in the intense current layers that develop during magnetic reconnection in the Earth's magnetosphere. Electron streaming is believed to be the source of this turbulence. Using kinetic theory and 3D particle-in-cell simulations, we study the nonlinear development of streaming instabilities in 3D magnetic reconnection with a strong guide field. Early in time an intense current sheet develops around the x-line and drives the Buneman instability. Electron holes, which are bipolar spatial localized electric field structures, form and then self-destruct creating a region of strong turbulence around the x-line. At late time turbulence with a characteristic frequency in the lower hybrid range also develops, leading to a very complex mix of interactions. The difficulty we face in this project is how to address a long-standing problem in nonlinear kinetic theory: how to treat large amplitude perturbations and the associated strong wave-particle interactions. In my thesis, I address this long-standing problem using particle-in-cell simulations and linear kinetic theory.Some important physics have been revealed. 1: The lower hybrid instability (LHI) dominates the dynamics in low $beta$ plasma in combination with either the electron-electron two-stream instability (ETS) or the Buneman instability (BI), depending on the parallel phase speed of the LHI. 2: An instability with a high phase speed is required to tap the energy of the high velocity electrons. The BI with its low phase speed, can not do this. The ETS and the LHI both have high phase speed. 3: The condition for the formation of stable electron holes requires $|v_p -v_g|< sqrt{2e|phi|/m_e}$, where $|phi|$ is the amplitude of the electric potential, and $v_p$ and $v_g$ are the phase and group velocity of the relevant waves. Like ETS and BI, LHI all can form electron holes. 4: The overlapping resonance in phase space is the dominant mechanism for transporting the momentum and energy from high velocity electrons to low velocity electrons, which then couple to the ions.Item Trinity: A Unified Treatment of Turbulence, Transport, and Heating in Magnetized Plasmas(2009) Barnes, Michael; Dorland, William; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)To faithfully simulate ITER and other modern fusion devices, one must resolve electron and ion fluctuation scales in a five-dimensional phase space and time. Simultaneously, one must account for the interaction of this turbulence with the slow evolution of the large-scale plasma profiles. Because of the enormous range of scales involved and the high dimensionality of the problem, resolved first-principles global simulations are very challenging using conventional (brute force) techniques. In this thesis, the problem of resolving turbulence is addressed by developing velocity space resolution diagnostics and an adaptive collisionality that allow for the confident simulation of velocity space dynamics using the approximate minimal necessary dissipation. With regard to the wide range of scales, a new approach has been developed in which turbulence calculations from multiple gyrokinetic flux tube simulations are coupled together using transport equations to obtain self-consistent, steady-state background profiles and corresponding turbulent fluxes and heating. This approach is embodied in a new code, Trinity, which is capable of evolving equilibrium profiles for multiple species, including electromagnetic effects and realistic magnetic geometry, at a fraction of the cost of conventional global simulations. Furthermore, an advanced model physical collision operator for gyrokinetics has been derived and implemented, allowing for the study of collisional turbulent heating, which has not been extensively studied. To demonstrate the utility of the coupled flux tube approach, preliminary results from Trinity simulations of the core of an ITER plasma are presented.Item Ultracold Plasma Dynamics in a Magnetic Field(2009) Zhang, Xianli; Rolston, Steven L.; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Plasmas, often called the fourth state of matter and the most common one in the universe, have parameters varying by many orders of magnitude, from temperature of a few hundred kelvin in the Earth's ionosphere to 1016 K in the magnetosphere of a pulsar. Ultracold plasmas, produced by photoionizing a sample of laser-cooled and trapped atoms near the ionization limit, have extended traditional neutral plasma parameters by many orders of magnitude, to electron temperatures below 1 K and ion temperatures in the tens of &mu K to a few Kelvin, and densities of 105 cm-3 to 1010 cm-3. These plasmas thus provide a testing ground to study basic plasma theory in a clean and simple system with or without a magnetic field. Previous studies of ultracold plasmas have primarily concentrated on temperature measurements, collective modes and expansion dynamics in the absence of magnetic fields. This thesis presents the first study of ultracold plasma dynamics in a magnetic field. The presence of a magnetic field during the expansion can initiate various phenomena, such as plasma confinement and plasma instabilities. While the electron temperatures are very low in ultracold plasmas, we need only tens of Gauss of magnetic field to observe significant effects on the expansion dynamics. To probe the ultraocold plasma dynamics in a magnetic field, we developed a new diagnostic - projection imaging, which images the ion distribution by extracting the ions with a high voltage pulse onto a position-sensitive detector. Early in the lifetime of the plasma (< 20 &mu s), the size of the image is dominated by the time-of-flight Coulomb explosion of the dense ion cloud. For later times, we measure the 2-D Gaussian width of the ion image, obtaining the transverse expansion velocity as a function of magnetic field (up to 70 G),and observe that the transverse expansion velocity scales as B &minus1/2, explained by a nonlinear ambipolar diffusion model that involes anisotropic diffusion in two different directions. We also present the first observation of a plasma instability in an expanding ultracold plasma. We observe periodic emission of electrons from an ultracold plasma in weak, crossed magnetic and electric fields, and a strong perturbed electron density distribution in electron time-of-flight projection images. We identify this instability as a high-frequency electron drift instability due to the coupling between the electron drift wave and electron cyclotron harmonic, which has large wavenumbers corresponding to wavelengths close to the electron gyroradius.Item Rotating, hydromagnetic laboratory experiment modelling planetary cores(2009) Kelley, Douglas H.; Lathrop, Daniel P.; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This dissertation describes a series of laboratory experiments motivated by planetary cores and the dynamo effect, the mechanism by which the flow of an electrically conductive fluid can give rise to a spontaneous magnetic field. Our experimental apparatus, meant to be a laboratory model of Earth's core, contains liquid sodium between an inner, solid sphere and an outer, spherical shell. The fluid is driven by the differential rotation of these two boundaries, each of which is connected to a motor. Applying an axial, DC magnetic field, we use a collection of Hall probes to measure the magnetic induction that results from interactions between the applied field and the flowing, conductive fluid. We have observed and identified inertial modes, which are bulk oscillations of the fluid restored by the Coriolis force. Over-reflection at a shear layer is one mechanism capable of exciting such modes, and we have developed predictions of both onset boundaries and mode selection from over-reflection theory which are consistent with our observations. Also, motivated by previous experimental devices that used ferromagnetic boundaries to achieve dynamo action, we have studied the effects of a soft iron (ferromagnetic) inner sphere on our apparatus, again finding inertial waves. We also find that all behaviors are more broadband and generally more nonlinear in the presence of a ferromagnetic boundary. Our results with a soft iron inner sphere have implications for other hydromagnetic experiments with ferromagnetic boundaries, and are appropriate for comparison to numerical simulations as well. From our observations we conclude that inertial modes almost certainly occur in planetary cores and will occur in future rotating experiments. In fact, the predominance of inertial modes in our experiments and in other recent work leads to a new paradigm for rotating turbulence, starkly different from turbulence theories based on assumptions of isotropy and homogeneity, starting instead with inertial modes, which are the linear eigenmodes of any rapidly rotating fluid.Item Novel applications of high intensity femtosecond lasers to particle acceleration and terahertz generation(2008-11-20) York, Andrew Gregory; Milchberg, Howard M; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)We have investigated new applications for high intensity femtosecond lasers theoretically and experimentally, including a novel method to accelerate electrons to relativistic energy and a new type of coherent lasing medium for amplification of few-cycle, high energy pulses of terahertz radiation. We report the development of corrugated `slow wave' plasma guiding structures with application to quasi-phase-matched direct laser acceleration of charged particles. These structures support guided propagation at intensities up to 2x10^17 W/cm^2, limited by our current laser energy and side leakage. Hydrogen, nitrogen, and argon plasma waveguides up to 1.5 cm in length with corrugation period as short as 35 microns are generated, with corrugation depth approaching 100%. These structures remove the limitations of diffraction, phase matching, and material damage thresholds and promise to allow high-field acceleration of electrons over many centimeters using relatively small femtosecond lasers. We present simulations that show a laser pulse power of 1.9 TW should allow an acceleration gradient larger than 80 MV/cm. A modest power of only 30 GW would still allow acceleration gradients in excess of 10 MV/ cm. Broadband chirped-pulse amplification (CPA) in Ti:sapphire revolutionized nonlinear optics in the 90's, bringing intense optical pulses out of large government facilities and into the hands of graduate students in small university labs. Intense terahertz pulses (>> 10 microjoules, <5 cycles), however, are still only produced at large accelerator facilities like Brookhaven National Labs. CPA is theoretically possible for terahertz frequencies, but no broadband lasing medium like Ti:sapphire has been demonstrated for terahertz. Dipolar molecular gases such as hydrogen cyanide (HCN), `aligned' by intense optical pulses, are a novel and promising medium for amplification of broadband few-cycle terahertz pulses. We present calculations that show rotationally excited molecules can amplify a few-cycle seed pulse of terahertz radiation: a sequence of short, intense optical pulses aligns a dipolar gas, driving the molecules into a broad superposition of excited rotational states. A broadband seed terahertz pulse following the optical pulses can then be amplified on many pure rotational transitions simultaneously. We also discuss plans and progress towards experimental realization of a few-cycle terahertz amplifier.Item Advanced Lagrangian Simulation Algorithms for Magnetized Plasmas Turbulence(2008-08-05) Broemstrup, Ingmar; Dorland, William; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Nonlinear processes in hot, magnetized plasma are notoriously difficult to understand without the use of numerical simulations. In recent decades, first principles, kinetic simulations have been widely and successfully used to study plasma turbulence and reconnection in weakly collisional systems. In this thesis, extensions of well-known, Lagrangian, particle-in-cell (PIC) simulation algorithms for problems such as these are derived and implemented. The algorithms are tested for multiple species (electrons and ions, with the physical mass ratio) in non-trivial magnetic geometry (cylindrical/toroidal). The advances presented here address two major shortcomings of conventional gyrokinetic PIC algorithms, with demonstrated excellent performance on large, parallel supercomputers. Although the gyrokinetic formalism rigorously describes the evolution of fluctuations which are small compared to a typical Larmor radius, most existing algorithms use low-order approximations of the gyroaveraging operator, and cannot accurately describe small scale fluctuations. The gyroaveraging algorithm presented here accurately and uniquely treats a wide range of fluctuation scales, above and below the thermal gyroradius. The second shortcoming of traditional algorithms relates to the slow loss of accuracy that is associated with the build-up of noise. In this thesis, a PIC pitch-angle scattering collision operator is developed. This collision operator is physically motivated and controls the growth of noise without introducing non-physical dissipation. Basic tests of the new algorithms are presented in linear and nonlinear regimes, using one to thousands of processors simultaneously.