Browsing by Author "McQuillen, P."
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Item Creating and studying ion acoustic waves in ultracold neutral plasmas(2012) Killian, T.C.; McQuillen, P.; O'Neil, T.M.; Castro, J.; Rice Quantum InstituteItem Creating Non-Maxwellian Velocity Distributions in Ultracold Plasmas(arXiv, 2011) Castro, J.; Bannasch, G.; McQuillen, P.; Pohl, T.; Killian, T.C.; Rice Quantum InstituteItem Demonstrating universal scaling for dynamics of Yukawa one-component plasmas after an interaction quench(American Physical Society, 2016) Langin, T.K.; Strickler, T.; Maksimovic, N.; McQuillen, P.; Pohl, T.; Vrinceanu, D.; Killian, T.C.The Yukawa one-component plasma (OCP) model is a paradigm for describing plasmas that contain one component of interest and one or more other components that can be treated as a neutralizing, screening background. In appropriately scaled units, interactions are characterized entirely by a screening parameter, κ. As a result, systems of similar κ show the same dynamics, regardless of the underlying parameters (e.g., density and temperature). We demonstrate this behavior using ultracold neutral plasmas (UNPs) created by photoionizing a cold (T≤10 mK) gas. The ions in UNP systems are well described by the Yukawa model, with the electrons providing the screening. Creation of the plasma through photoionization can be thought of as a rapid quench of the interaction potential from κ=∞ to a final κ value set by the electron density and temperature. We demonstrate experimentally that the postquench dynamics are universal in κ over a factor of 30 in density and an order of magnitude in temperature. Results are compared with molecular-dynamics simulations. We also demonstrate that features of the postquench kinetic energy evolution, such as disorder-induced heating and kinetic-energy oscillations, can be used to determine the plasma density and the electron temperature.Item Emergence of kinetic behavior in streaming ultracold neutral plasmas(AIP Publishing LLC, 2015) McQuillen, P.; Castro, J.; Bradshaw, S.J.; Killian, T.C.We create streaming ultracold neutral plasmas by tailoring the photoionizing laser beam that creates the plasma. By varying the electron temperature, we control the relative velocity of the streaming populations, and, in conjunction with variation of the plasma density, this controls the ion collisionality of the colliding streams. Laser-induced fluorescence is used to map the spatially resolved density and velocity distribution function for the ions. We identify the lack of local thermal equilibrium and distinct populations of interpenetrating, counter-streaming ions as signatures of kinetic behavior. Experimental data are compared with results from a one-dimensional, two-fluid numerical simulation.Item Experimental Measurement of Self-Diffusion in a Strongly Coupled Plasma(American Physical Society, 2016) Strickler, T.S.; Langin, T.K.; McQuillen, P.; Daligault, J.; Killian, T.C.We present a study of the collisional relaxation of ion velocities in a strongly coupled, ultracold neutral plasma on short time scales compared to the inverse collision rate. The measured average velocity of a tagged population of ions is shown to be equivalent to the ion-velocity autocorrelation function. We thus gain access to fundamental aspects of the single-particle dynamics in strongly coupled plasmas and to the ion self-diffusion constant under conditions where experimental measurements have been lacking. Nonexponential decay towards equilibrium of the average velocity heralds non-Markovian dynamics that are not predicted by traditional descriptions of weakly coupled plasmas. This demonstrates the utility of ultracold neutral plasmas for studying the effects of strong coupling on collisional processes, which is of interest for dense laboratory and astrophysical plasmas.Item Imaging the evolution of an ultracold strontium Rydberg gas(American Physical Society, 2013) McQuillen, P.; Zhang, X.; Strickler, T.; Dunning, F.B.; Killian, T.C.; Rice Quantum InstituteClouds of ultracold strontium 5s48s1S0 or 5s47d1D2 Rydberg atoms are created by two-photon excitation of laser-cooled 5s21S0 atoms. The spontaneous evolution of the cloud of low orbital angular momentum (low-ℓ) Rydberg states towards an ultracold neutral plasma is observed by imaging resonant light scattered from core ions, a technique that provides both spatial and temporal resolution. Evolution is observed to be faster for the S states, which display isotropic attractive interactions, than for the D states, which exhibit anisotropic, principally repulsive interactions. Immersion of the atoms in a dilute ultracold neutral plasma speeds up the evolution and allows the number of Rydberg atoms initially created to be determined.Item Ion Acoustic Waves in Ultracold Neutral Plasmas(American Physical Society, 2010) Castro, J.; McQuillen, P.; Killian, T.C.; Rice Quantum InstituteWe photoionize laser-cooled atoms with a laser beam possessing spatially periodic intensity modulations to create ultracold neutral plasmas with controlled density perturbations. Laser-induced fluorescence imaging reveals that the density perturbations oscillate in space and time, and the dispersion relation of the oscillations matches that of ion acoustic waves, which are long-wavelength, electrostatic, density waves.Item Ion holes in the hydrodynamic regime in ultracold neutral plasmas(American Institute of Physics, 2013) McQuillen, P.; Castro, J.; Strickler, T.; Bradshaw, S.J.; Killian, T.C.We describe the creation of localized density perturbations, or ion holes, in an ultracold neutral plasma in the hydrodynamic regime, and show that the holes propagate at the local ion acoustic wave speed. We also observe the process of hole splitting, which results from the formation of a density depletion initially at rest in the plasma. One-dimensional, two-fluid hydrodynamic simulations describe the results well. Measurements of the ion velocity distribution also show the effects of the ion hole and confirm the hydrodynamic conditions in the plasma.Item Ion temperature evolution in an ultracold neutral plasma(AIP Publishing, 2015) McQuillen, P.; Strickler, T.; Langin, T.; Killian, T.C.We study the long-time evolution of the ion temperature in an expanding ultracold neutral plasma using spatially resolved, laser-induced-fluorescence spectroscopy. Adiabatic cooling reduces the ion temperature by an order of magnitude during the plasma expansion, to temperatures as low as 0.2 K. Cooling is limited by heat exchange between ions and the much hotter electrons. We also present evidence for an additional heating mechanism and discuss possible sources. Data are described by a model of the plasma evolution, including the effects of ion-electron heat exchange. We show that for appropriate initial conditions, the degree of Coulomb coupling of ions in the plasma increases during expansion.Item Velocity Relaxation in a Strongly Coupled Plasma(American Physical Society, 2012) Bannasch, G.; Castro, J.; McQuillen, P.; Pohl, T.; Killian, T.C.; Rice Quantum InstituteCollisional relaxation of Coulomb systems is studied in the strongly coupled regime. We use an optical pump-probe approach to manipulate and monitor the dynamics of ions in an ultracold neutral plasma, which allows direct measurement of relaxation rates in a regime where common Landau-Spitzer theory breaks down. Numerical simulations confirm the experimental results and display non-Markovian dynamics at early times.