CASPER

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https://hdl.handle.net/2104/5501

CASPER conducts fundamental research in astrophysics, early universe cosmology, space physics, superstring / M theory, complex (dusty) plasma physics, hypervelocity impact (shock) physics and laser hypervelocity impact simulation physics.

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Agglomeration of Dust Particles in the Lab
(AIP Conference Proceedings, 2011) Matthews, Lorin Swint.; Carmona-Reyes, Jorge; Land, Victor; Hyde, Truell Wayne.
Dust aggregates are formed in a laboratory plasma as monodisperse spheres are accelerated in a self-excited dust density wave. The asymmetric charge on the aggregates causes them to rotate as they interact with the sheath electric field or other aggregates The charge and dipole moment can be estimated and compared to numerical models. “Dust molecules”, where two particles are electrostatically bound but not physically touching, are also observed.
Analysis of Magnetic Field Plasma Interactions Using Micro Particles as Probes
(American Physics Society/ Physical Review E, 2015-08-26) Dropmann, Michael; Laufer, Rene; Herdrich, Georg; Matthews, Lorin Swint; Hyde, T. W.
The interaction between a magnetic field and plasma close to a nonconductive surface is of interest for both science and technology. In space, crustal magnetic fields on celestial bodies without atmosphere can interact with the solar wind. In advanced technologies such as those used in fusion or spaceflight, magnetic fields can be used to either control a plasma or protect surfaces exposed to the high heat loads produced by plasma. In this paper, a method will be discussed for investigating magnetic field plasma interactions close to a nonconductive surface inside a Gaseous Electronics Conference reference cell employing dust particles as probes. To accomplish this, a magnet covered by a glass plate was exposed to a low power argon plasma. The magnetic field was strong enough to magnetize the electrons, while not directly impacting the dynamics of the ions or the dust particles used for diagnostics. In order to investigate the interaction of the plasma with the magnetic field and the nonconductive surface, micron-sized dust particles were introduced into the plasma and their trajectories were recorded with a high-speed camera. Based on the resulting particle trajectories, the accelerations of the dust particles were determined and acceleration maps over the field of view were generated which are representative of the forces acting on the particles. The results show that the magnetic field is responsible for the development of strong electric fields in the plasma, in both horizontal and vertical directions, leading to complex motion of the dust particles.
Anomalous Diffusion in One-Dimensional Disordered Systems: A Discrete Fractional Laplacian Method (Part I)
(IOP Publishing, 2020-04-03) Padgett, J.; Kostadinova, E.; Liaw, C.; Busse, K.; Matthews, L.; Hyde, T.
This work extends the applications of Anderson-type Hamiltonians to include transport characterized by anomalous dffusion. Herein, we investigate the transport properties of a one dimensional disordered system that employs the discrete fractional Laplacian, (-Δ)^s, s ∈(0,2), in combination with results from spectral and measure theory. It is a classical mathematical result that the standard Anderson model exhibits localization of energy states for all nonzero disorder in one-dimensional systems. Numerical simulations utilizing our proposed model demonstrate that this localization effect is enhanced for sub-diffusive realizations of the operator, s ∈(1,2), while the super-diffusive realizations of the operator, s ∈(0,1) can exhibit energy states with less localized features. These results suggest that the proposed method can be used to examine anomalous diffusion in physical systems where strong correlations, structural defects, and nonlocal effects are present.
Branes in the M_D x M_d+ x M_d- Compactification of Type II String on S^1/Z_2 and Their Cosmologicval Applications
(2010-07-09T15:21:00Z) Devin, Michael J.; Ali, Tibra; Cleaver, Gerald B.; Wang, Anzhong.; Wu, Qiang, 1977-
In this paper, we study the implementation of brane worlds in type II string theory. Starting with the NS/NS sector of type II string, we first compactify the (D + d+ + d−)-dimensional spacetime, and reduce the corresponding action to a D-dimensional effective action, where the topologies of Md+ and Md− are arbitrary. We further compactify one of the (D − 1) spatial dimensions on an S1/Z2 orbifold, and derive the gravitational and matter field equations both in the bulk and on the branes. Then, we investigate two key issues in such a setup: (i) the radion stability and radion mass; and (ii) the localization of gravity, and the corresponding Kaluza-Klein (KK) modes. We show explicitly that the radion is stable and its mass can be in the order of GeV . In addition, the gravity is localized on the visible brane, and its spectrum of the gravitational KK towers is discrete and can have a mass gap of T eV , too. The high order Yukawa corrections to the 4-dimensional Newtonian potential is exponentially suppressed, and can be negligible. Applying such a setup to cosmology, we obtain explicitly the field equations in the bulk and the generalized Friedmann equations on the branes.
Charged Grains In Saturn's F-Ring: Interaction With Saturn's Magnetic Field
(Advances in Space Research, 2004) Matthews, Lorin Swint.; Hyde, Truell Wayne.
Saturn’s dynamic F-Ring still presents a challenge for understanding and explaining the kinematic processes that lead to the changing structure visible in our observations of this ring. This study examines the effect of Saturn’s magnetic field on the dynamics of micron-sized grains that may become electrically charged due to interaction with plasma in Saturn’s rigidly corotating magnetosphere. The numerical model calculates the dynamics of charged dust grains and includes forces due to Saturn’s gravitational field, the plasma polarization electric field, a third order harmonic expansion of Saturn’s magnetic field, and the F-Ring’s Shepherding moons, Prometheus and Pandora.
Charging and coagulation of dust in protoplanetary environments
(Astrophysical Journal, 2012-01-01) Matthews, Lorin Swint.; Land, Victor; Hyde, Truell Wayne.
Combining a particle–particle, particle–cluster, and cluster–cluster agglomerationmodel with an aggregate charging model, the coagulation and charging of dust particles in plasma environments relevant for protoplanetary disks have been investigated, including the effect of electron depletion in high dust density environments. The results show that charged aggregates tend to grow by adding small particles and clusters to larger particles and clusters, and that cluster–cluster aggregation is significantly more effective than particle–cluster aggregation. Comparisons of the grain structure show that with increasing aggregate charge the compactness factor, φ_σ , decreases and has a narrower distribution, indicating a fluffier structure. Neutral aggregates are more compact, with larger φ_σ , and exhibit a larger variation in fluffiness. Overall, increased aggregate charge leads to larger, fluffier, and more massive aggregates.
Charging and Growth of Fractal Dust Grains
(IEEE Transactions on Plasma Science, 2008-02) Matthews, Lorin Swint.; Hyde, Truell Wayne.
The structure and evolution of aggregate grains formed within a plasma environment are dependent on the charge acquired by the micron-sized dust grains during the coagulation process. The manner in which the charge is arranged on developing irregular structures can affect the fractal dimension of aggregates formed during collisions, which, in turn, influences the coagulation rate and size evolution of the dust within the plasma cloud. This paper presents preliminary models for the charge and size evolution of fractal aggregates immersed in a plasma environment calculated using a modification to the orbital motion-limited (OML) theory. Primary electron and ion currents that are incident on points on the aggregate surface are determined using a line-of-sight (LOS) approximation: only those electron or ion trajectories that are not blocked by another grain within the aggregate contribute to the charging current. Using a self-consistent iterative approach, the equilibrium charge and dipole moment are calculated for the dust aggregate. The charges are then used to develop a heuristic charging scheme, which can be implemented in coagulation models. While most coagulation heories assume that it is difficult for like-charged grains to oagulate, the OML_LOS approximation indicates that the electric potentials of aggregate structures are often reduced enough to allow significant coagulation to occur.
Charging of aggregate grains in astrophysical environments
(Astrophysical Journal, 2013-02-01) Ma, Qianyu.; Matthews, Lorin Swint.; Land, Victor; Hyde, Truell Wayne.
The charging of dust grains in astrophysical environments has been investigated with the assumption that these grains are homogeneous spheres. However, there is evidence which suggests that many grains in astrophysical environments are irregularly shaped aggregates. Recent studies have shown that aggregates acquire higher charge-to-mass ratios due to their complex structures, which in turn may alter their subsequent dynamics and evolution. In this paper, the charging of aggregates is examined including secondary electron emission and photoemission in addition to primary plasma currents. The results show that the equilibrium charge on aggregates can differ markedly from spherical grains with the same mass, but that the charge can be estimated for a given environment based on structural characteristics of the grain. The “small particle effect” due to secondary electron emission is also important for determining the charge of micron-sized aggregates consisting of nano sized particles.
Coagulation in the Vicinity of a Gap-Opening Jupiter-Mass Planet
(Astrophysical Journal, 2016-06) Carballido, Augusto
We analyze the coagulation of dust in and around a gap opened by a Jupiter-mass planet. To this end, we carry out a high-resolution magnetohydrodynamic (MHD) simulation of the gap environment, which is turbulent due to the magnetorotational instability. From the MHD simulation, we obtain values of the gas velocities, densities, and turbulent stresses (a) close to the gap edge, (b) in one of the two gas streams that accrete onto the planet, (c) inside the low-density gap, and (d) outside the gap. The MHD values are then input into a Monte Carlo dust-coagulation algorithm which models grain sticking and compaction. We also introduce a simple implementation for bouncing, for comparison purposes. We consider two dust populations for each region: one whose initial size distribution is monodisperse, with monomer radius equal to 1 μm, and another one whose initial size distribution follows the Mathis–Rumpl–Nordsieck distribution for interstellar dust grains, with an initial range of monomer radii between 0.5 and 10 μm. Without bouncing, our Monte Carlo calculations show steady growth of dust aggregates in all regions, and the mass-weighted (m-w) average porosity of the initially monodisperse population reaches xtremely high final values of 98%. The final m-w porosities in all other cases without bouncing range between 30% and 82%. The efficiency of compaction is due to high turbulent relative speeds between dust particles. When bouncing is introduced, growth is slowed down in the planetary wake and inside the gap. Future studies will need to explore the effect of different planet masses and electric charge on grains.
Comparison of Plasma Magnetic Field Interactions in a Static and Dynamic Plasma Facility
(Transactions of the Japan Society for Aeronautical and Space Sciences, Aerospace Technology Japan, 2016) Dropmann, Michael; Knapp, A.; Eichhorn, C.; Loehle, S.; Laufer, Rene; Herdrich, Georg; Matthews, Lorin Swint.; Hyde, Truell Wayne.; Fasoulas, Stefanos; Roeser, Hans-Peter
Magnetic fields are a principal/widespread/promising tool/instrument in space technology design for the use in advanced propulsion concepts, shielding from radiation or to aid thermal protection during the atmospheric entry of spacecraft. Two experiments have been conducted to investigate the feasibility of using magnetic fields to reduce the heat flux onto a thermal protection system during atmospheric entry. For this purpose a modified heat flux probe with embedded permanent magnets has been exposed to a plasma jet and the structure of the bow shock in front of the probe has been observed using an emission spectroscopy setup. The intensity ratio of ionized argon lines for the experiment with and without magnets has been determined and used to analyze the magnetic field`s impact on the flow. Complementary experiments in a low power capacitively driven plasma have been conducted using micron sized particles as probes to map electric fields in a magnetically perturbed plasma. The results from both experiments are presented and analogies are drawn from both approaches. The experiments have shown that the interactions of the magnetic field with the plasma can create strong electric fields which strongly influence the ions even though the field is too weak to magnetize the ions.
Cosmic Dust Aggregation with Stochastic Charging
(Astrophysical Journal, 2013-10-04) Matthews, Lorin Swint.; Shotorban, Babak; Hyde, Truell Wayne.
The coagulation of cosmic dust grains is a fundamental process which takes place in astrophysical environments, such as presolar nebulae and circumstellar and protoplanetary disks. Cosmic dust grains can become charged through interaction with their plasma environment or other processes, and the resultant electrostatic force between dust grains can strongly affect their coagulation rate. Since ions and electrons are collected on the surface of the dust grain at random time intervals, the electrical charge of a dust grain experiences stochastic fluctuations. In this study, a set of stochastic differential equations is developed to model these fluctuations over the surface of an irregularly shaped aggregate. Then, employing the data produced, the influence of the charge fluctuations on the coagulation process and the physical characteristics of the aggregates formed is examined. It is shown that dust with small charges (due to the small size of the dust grains or a tenuous plasma environment) is affected most strongly.
Crystallization Dynamics of a Single Layer Complex Plasma
(Physical Review Letters, 2010-09) Hartmann, Peter; Douglass, Angela Michelle.; Carmona-Reyes, Jorge; Matthews, Lorin Swint.; Hyde, Truell Wayne.; Kovacs, Aniko Zs.; Donko, Zoltan
We report a series of complex (dusty) plasma experiments, aimed at the study of the detailed time evolution of the recrystallization process following a rapid quench of a two-dimensional dust liquid. The experiments were accompanied by large-scale (million-particle) molecular dynamics simulations, assuming Yukawa-type interparticle interaction. Both experiment and simulation show a t ^/alpha (power-law) dependence of the linear crystallite domain size as measured by the bond-order correlation length, translational correlation length, dislocation (defect) density, and a direct size measurement algorithm. The results show two stages of order formation. On short time scales, individual particle motion dominates; this is a fast process characterized by alpha = 0.93 +/- 0.1. At longer time scales, small crystallites undergo collective rearrangement, merging into bigger ones, resulting in a smaller exponent /alpha = 0.38 +/- 0.06.
Delocalization in infinite disordered two-dimensional lattices of different geometry
(Physical Review B, 2017-12-06) Kostadinova, Eva; Busse, Kyle; Ellis, Naoki; Padgett, Josh; Liaw, Constanze; Matthews, Lorin S.; Hyde, Truell W.
The spectral approach to infinite disordered crystals is applied to anAnderson-type Hamiltonian to demonstrate the existence of extended states for nonzero disorder in 2D lattices of different geometries. The numerical simulations shown prove that extended states exist for disordered honeycomb, triangular, and square crystals. This observation stands in contrast to the predictions of scaling theory, and aligns with experiments in photonic lattices and electron systems. The method used is the only theoretical approach aimed at showing delocalization. A comparison of the results for the three geometries indicates that the triangular and honeycomb lattices experience transition in the transport behavior for similar levels of disorder, which is to be expected from the planar duality of the lattices. This provides justification for the use of artificially prepared triangular lattices as analogues for honeycomb materials, such as graphene. The analysis also shows that the transition in the honeycomb case happens more abruptly compared to the other two geometries, which can be attributed to the number of nearest neighbors.We outline the advantages of the spectral approach as a viable alternative to scaling theory and discuss its applicability to transport problems in both quantum and classical 2D systems.
Detailed model of the growth of fluffy dust aggregates in a protoplanetary disk: Effects of nebular conditions
(AAS Publishing, 2020-07) Xiang, C.; Carballido, A.; Matthews, L.S.; Hyde, T.W.
Coagulation of dust aggregates plays an important role in the formation of planets and is of key importance to the evolution of protoplanetary disks (PPDs). Characteristics of dust, such as the diversity of particle size, porosity, charge, and the manner in which dust couples to turbulent gas, affect the collision outcome and the rate of dust growth. Here we present a numerical model of the evolution of the dust population within a PPD which incorporates all of these effects. The probability that any two particles collide depends on the particle charge, cross-sectional area and their relative velocity. The actual collision outcome is determined by a detailed collision model which takes into account the aggregate morphology, trajectory, orientation, and electrostatic forces acting between charged grains. Our model is applicable to the epoch of time during which hit-and-stick is the primary collision outcome, the duration of which varies greatly depending on the environment. The data obtained in this research reveal the characteristics of dust populations in different environments at the end of the hit-and-stick growth, which establishes the foundation for the onset of the next growth stage where bouncing, mass transfer and fragmentation become important. For a given level of turbulence, neutral and weakly charged particles collide more frequently and grow faster than highly charged particles. In general, the epoch of hit-and-stick growth is much shorter in high turbulence than it is in regions with low turbulence or highly charged grains. In addition, highly charged particles grow to a larger size before reaching the bouncing barrier especially in environments with low turbulence, and exhibit "runaway" growth, in which a few large particles grow quickly by accreting smaller particles while the rest of the population grows very slowly. In general, highly charged aggregates have a more compact structure and are comprised of larger monomers than neutral/weakly charged aggregates. The differences in the particle structure/composition not only affect the threshold velocities for bouncing and fragmentation,
Determination of the levitation limits of dust particles within the sheath in complex plasma experiments
(Physics of Plasmas, 2012-01-23) Douglass, Angela Michelle.; Land, Victor; Qiao, Ke; Matthews, Lorin Swint.; Hyde, Truell Wayne.
Experiments are performed in which dust particles are levitated at varying heights above the powered electrode in a radio frequency plasma discharge by changing the discharge power. The trajectories of particles dropped from the top of the discharge chamber are used to reconstruct the vertical electric force acting on the particles. The resulting data, together with the results from a self-consistent fluid model, are used to determine the lower levitation limit for dust particles in the discharge and the approximate height above the lower electrode where quasineutrality is attained, locating the sheath edge. These results are then compared with current sheath models. It is also shown that particles levitated within a few electron Debye lengths of the sheath edge are located outside the linearly increasing portion of the electric field.
Digital Imaging and Analysis of Dusty Plasmas
(Advances in Space Research, 2004) Boesse, Carolyn M.; Henry, Michael; Hyde, Truell Wayne.; Matthews, Lorin Swint.
Dust particles immersed within a plasma environment, such as those found in planetary rings or cometary environments, will acquire an electric charge. If the ratio of interparticle potential energy to average kinetic energy is high enough the particles will form either a ‘liquid’ structure with short-range ordering or a crystalline structure with long-range ordering. Since their discovery in laboratory environments in 1994, such crystals have been the subject of a variety of experimental, theoretical, and numerical investigations. Laboratory experiments analyzing the behavior of dust grains in a plasma rely on optical diagnostics to provide data about the system in a non-perturbative manner. In the past, capturing, imaging, and analyzing crystalline structure in dusty plasmas has been a non-trivial problem. Utilizing digital imaging and analysis systems, data capture, image formatting, and analysis can be done quickly. Following data capture, image analysis is conducted using modified Particle Image Velocimetry and Particle Tracking Velocimetry algorithms. The data extracted is then used to construct Voronoi diagrams, calculate particle density, inter-particle spacing, pair correlation functions, and thermal energy. From this data other dust plasma parameters can be inferred such as inter-particle forces and grain charges.
Dipole-dipole interactions of charged magnetic grains
(IEEE, 2010-04) Perry, Jonathan D.; Matthews, Lorin Swint.; Hyde, Truell Wayne.
The interaction between dust grains is an important process in fields as diverse as planetesimal formation or the plasma processing of silicon wafers into computer chips. This interaction depends in large part on the material properties of the grains (for example, whether the grains are onducting, nonconducting, ferrous, or nonferrous). This paper considers the effects that electrostatic and magnetic forces, alone or in combination, can have on the coagulation of dust in various environments. A numerical model is used to simulate the coagulation of charged, charged-magnetic, and magnetic dust aggregates formed from a ferrous material, and the results are compared to each other as well as to those from an uncharged nonmagnetic material. The interactions between extended dust aggregates are also examined, specifically looking at how the arrangement of charge over the aggregate surface or the inclusion of a magnetic material produces dipole–dipole interactions. It will be shown that these dipole–dipole interactions can affect the orientation and structural formation of aggregates as they collide and stick. An analysis of the resulting dust populations will also demonstrate the impact that grain composition and/or charge can have on the structure of the aggregate, as characterized by the resulting fractal dimension.
Discrete stochastic charging of aggregate grains
(Physical Review E, 2018-05) Matthews, Lorin Swint.; Shotorban, Babak; Hyde, Truell Wayne.
Dust particles immersed in a plasma environment become charged through the collection of electrons and ions at random times, causing the dust charge to fluctuate about an equilibrium value. Small grains (with radii less than 1 µm) or grains in a tenuous plasma environment are sensitive to single additions of electrons or ions. Here we present a numerical model that allows examination of discrete stochastic charge fluctuations on the surface of aggregate grains and determines the effect of these fluctuations on the dynamics of grain aggregation. We show that the mean and standard deviation of charge on aggregate grains follows the same trends as those predicted for spheres having an equivalent radius, though aggregates exhibit larger variations from the predicted values. In some plasma environments, these charge fluctuations occur on timescales which are relevant for dynamics of aggregate growth. Coupled dynamics and charging models show that charge fluctuations tend to produce aggregates which are much more linear or filamentary than aggregates formed in an environment where the charge is stationary.
Dust as probe for horizontal field distribution in low pressure gas discharges
(Plasma Sources Science and Technology, 2014-06-23) Hartmann, Peter; Kovacs, Aniko Zs.; Carmona-Reyes, Jorge; Matthews, Lorin Swint.; Hyde, Truell Wayne.
Using dust grains as probes in gas discharge plasma is a very promising, but at the same time very challenging method, as the individual external control of dust grains has to be solved. We propose and demonstrate the applicability of the RotoDust experiment, where the well controlled centrifugal force is balanced by the horizontal confinement field in plane electrode argon radio frequency gas discharges. We have reached a resolution of 0.1 V cm−1 for the electric field. This technique is used to verify numerical simulations and to map symmetry properties of the confinement in dusty plasma experiments using a glass box.
Dust as probes: Determining confinement and interaction forces
(IOP Publishing, 2020-10) Hartmann, Peter; Rosenberg, Marlene; Juhasz, Z.; Matthews, Lorin; Sanford, Dustin; Vermillion, Katrina; Reyes, Jorge; Hyde, Truell
The PK-4 system is a micro-gravity dusty plasma experiment currently in operation on-board the International Space Station. The experiment utilizes a long DC discharge in neon or argon gases. We apply our 2D particle-in-cell with Monte Carlo collisions discharge simulation to compute local plasma parameters that serve as input data for future dust dynamics models. The simulation includes electrons, Ne+ ions, and Ne^m metastable atoms in neon gas and their collisions at solid surfaces including secondary electron emission and glass wall charging. On the time scale of the on-board optical imaging, the positive column appears stable and homogeneous. On the other hand, our simulations show that on microsecond time scales the positive column is highly inhomogeneous: ionization waves with phase velocities in the range between 500 m s−1 and 1200 m s−1 dominate the structure. In these waves, the electric field and charged particle densities can reach amplitudes up to 10 times of their average value. Our experiments on ground-based PK-4 replica systems fully support the numerical findings. In the experiment, the direction of the DC current can be alternated, which has been found to favor dust particle chain formation. We discuss possible mechanisms for how the highly oscillatory plasma environment contributes to the dust particle chain formation.

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