290 research outputs found
Emergence of Kinetic Behavior in Streaming Ultracold Neutral Plasmas
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 is compared with results from a one-dimensional, two-fluid
numerical simulation.Comment: 8 pages, 6 figure
Creating Non-Maxwellian Velocity Distributions in Ultracold Plasmas
We present techniques to perturb, measure and model the ion velocity
distribution in an ultracold neutral plasma produced by photoionization of
strontium atoms. By optical pumping with circularly polarized light we promote
ions with certain velocities to a different spin ground state, and probe the
resulting perturbed velocity distribution through laser-induced fluorescence
spectroscopy. We discuss various approaches to extract the velocity
distribution from our measured spectra, and assess their quality through
comparisons with molecular dynamic simulationsComment: 13 pages, 8 figure
Velocity Relaxation in a Strongly Coupled Plasma
Collisional 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.Comment: 5 pages, 5 figure
Ion Acoustic Waves in Ultracold Neutral Plasmas
We 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
Demonstrating Universal Scaling in Quench Dynamics of a Yukawa One-Component Plasma
The Yukawa one-component plasma (OCP) is a paradigm model 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,
. As a result, systems of similar show the same dynamics,
regardless of the underlying parameters (e.g., density and temperature). We
demonstrate this behavior using ultracold neutral plasmas (UNP) created by
photoionizing a cold ( 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 from to a final value set by the electron
density and temperature. We demonstrate experimentally that the post-quench
dynamics are universal in 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 post-quench kinetic
energy evolution, such as disorder-induced heating and kinetic-energy
oscillations, can be used to determine the plasma density and the electron
temperature.Comment: 10 pages, 12 figures, to be submitted to Physical Review
Ion temperature evolution in an ultracold neutral plasma
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
High Resolution Ionization of Ultracold Neutral Plasmas
Collective effects, such as waves and instabilities, are integral to our
understanding of most plasma phenomena. We have been able to study these in
ultracold neutral plasmas by shaping the initial density distribution through
spatial modulation of the ionizing laser intensity. We describe a relay imaging
system for the photoionization beam that allows us to create higher resolution
features and its application to extend the observation of ion acoustic waves to
shorter wavelengths. We also describe the formation of sculpted density
profiles to create fast expansion of plasma into vacuum and streaming plasmas
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