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.Collective wave phenomena are central to the transport and thermodynamic properties of plasmas, and the presence of a rich spectrum of collective modes is a distinctive feature that separates this state of matter from a simple collection of charged particles [1]. In ultracold neutral plasmas (UNPs) [2,3], which are orders of magnitude colder than any other neutral plasma and can be used to explore the physics of strongly coupled systems [4][5][6], little work has been done to study collective modes [7][8][9][10]. Here we employ a new technique for creating controlled density perturbations to excite ion acoustic waves (IAWs) in an UNP and measure their dispersion relation. This flexible technique for sculpting the density distribution will open new areas of plasma dynamics for experimental study, including the effects of strong coupling on dispersion relations [11][12][13][14] and non-linear phenomena [3,10,15,16] in the ultracold regime.UNPs are formed by photoionizing laser-cooled atoms near the ionization threshold. They stretch the boundaries of traditional neutral plasma physics and have extremely clean and controllable initial conditions that make them ideal for studying phenomena seen in more complex systems, such as plasma expansion and equilibration in high-energy-density laser-matter interactions [5] and quark-gluon plasmas [6]. UNPs have shown fascinating dynamics, such as kinetic energy oscillations that directly reflect the strong coupling of ions [5,17]. Strong coupling arises when particle interaction energies exceed the kinetic energy [4]. It is important in many fields of physics spanning classical to quantum behavior [5,6,18,19] and gives rise to phase transitions and the establishment of spatial correlations of particles [4]. These studies complement experiments probing strong coupling in dusty plasmas [18] and non-neutral plasmas of pure ions or electrons [19].Previous experimental studies of collective modes in UNPs were limited to excitations of Langmuir (electron density) oscillations with radio frequency electric fields [7,8] which did not determine a dispersion relation and were relatively insensitive to dynamics of the strongly coupled ions. A high-frequency electron drift instability was observed in an UNP in the presence of crossed electric and magnetic fields [9]. Spherically symmetric ion density modulations were shown to excite IAWs in numerical simulations of UNPs [10]. Here we excite IAWs through direct imprinting of ion density modulations during plasma formation and image them in situ with time resolved laser-induced fluorescence [20].Low frequency electrostatic, o...

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We have used the free expansion of ultracold neutral plasmas as a time-resolved probe of electron temperature. A combination of experimental measurements of the ion expansion velocity and numerical simulations characterize the crossover from an elastic-collision regime at low initial ÿ e , which is dominated by adiabatic cooling of the electrons, to the regime of high ÿ e in which inelastic processes drastically heat the electrons. We identify the time scales and relative contributions of various processes, and we experimentally show the importance of radiative decay and disorder-induced electron heating for the first time in ultracold neutral plasmas. DOI: 10.1103/PhysRevLett.99.075005 PACS numbers: 52.27. Gr, 52.65.ÿy Ultracold neutral plasmas (UNPs) [1,2] occupy an exotic regime of plasma physics in which electron and ion temperatures are orders of magnitude colder than in conventional neutral plasmas. The electron temperature in these systems evolves under the influence of many factors, which can occur on very different time scales, such as disorderinduced heating [3], three-body recombination [4,5], and adiabatic cooling [6,7]. The relative importance of the various effects depends critically upon initial conditions, and this has complicated the experimental study of the electron temperature [8][9][10][11][12] and leads to much theoretical debate [3,6,7,13,14]. We present here detailed experimental measurements and numerical simulations that untangle the time scales and contributions of the various competing effects and characterize the transition from elastic-collision-dominated to inelastic-collision-dominated behavior.UNPs are of fundamental interest because they can be in or near the strongly coupled regime, which is characterized by the existence of spatial correlations between particles and a Coulomb coupling parameter ÿ e 2 =4" 0 ak B T > 1, where T refers to the temperature of the particles and a 4n=3 ÿ1=3 is the Wigner-Seitz radius. Ions in UNPs equilibrate with ÿ i 3 [10,15]. The initial electron temperature is under experimental control and can be set such that a naïve calculation of ÿ e suggests that electrons are also strongly coupled. However, electrons rapidly leave the strongly coupled regime due to various heating mechanisms [3,6,14] that are central to studies presented here.To create a UNP, strontium atoms from a Zeemanslowed beam are trapped and cooled in a magneto-optical trap operating on the 1 S 0 -1 P 1 atomic transition at 461 nm [16]. A 10 ns pulse from a dye laser then excites about 20% of the atoms just above the ionization threshold. The temperature of the resulting ions is initially a few millikelvin, which is similar to the temperature of the lasercooled neutral atoms, but ions heat within 1 s to about 1 K due to disorder-induced heating [15,17]. The initial electron kinetic energy (E e ) equals the difference between the energy of the ionizing photon and the ionization threshold. With a tunable pulsed-dye laser, 2E e =3k B can be set from 1-1000 K. Electrons thermalize l...

Traditional molecular and biochemical methods, such as schizodeme analysis, karyotyping, DNA fingerprinting, and enzyme electrophoretic profiles, have shown a large variability among Trypanosoma cruzi isolates. In contrast to those results, polymerase chain reaction (PCR) amplification of sequences from the 24S␣ ribosomal RNA gene and from the mini-exon gene nontranscribed spacer indicated a dimorphism among T. cruzi isolates, which enabled the definition of two major parasite lineages. In the present study, 86 T. cruzi field stocks (68 isolated from humans with defined presentations of Chagas' disease and 18 from triatomines) derived from four Brazilian geographic areas were typed by the PCR assay based on the DNA sequences of the mini-exon and 24S␣ rRNA genes. These stocks were ordered into the two major T. cruzi lineages. Lineage 1 was associated mainly with human isolates and lineage 2 with the sylvatic cycle of the parasite.

We study the expansion of ultracold neutral plasmas in the regime in which inelastic collisions are negligible. The plasma expands due to the thermal pressure of the electrons, and for an initial spherically symmetric Gaussian density profile, the expansion is self-similar. Measurements of the plasma size and ion kinetic energy using fluorescence imaging and spectroscopy show that the expansion follows an analytic solution of the Vlasov equations for an adiabatically expanding plasma. DOI: 10.1103/PhysRevLett.99.155001 PACS numbers: 52.38. Kd, 52.27.Gr, 52.65.Ff Exactly solvable problems are rare in physics and serve as ideal models that provide intuition for understanding more complex systems. Here, we report the experimental realization of a laser-produced plasma whose dynamics can be described by an analytic solution to the Vlasov equations [1,2], which are central equations in the kinetic theory of plasmas. Expansion into a surrounding vacuum is fundamentally important and typically dominates the dynamics of laser-produced plasmas [3], such as in inertial confinement fusion experiments [4], x-ray lasers [5], or production of energetic (> MeV) ions through irradiation of solids [6,7], foils [8][9][10][11][12][13][14], and clusters [15].We study plasma expansion with ultracold neutral plasmas (UNPs) [16], which are created by photoionizing laser-cooled strontium atoms [17] just above the ionization threshold. The well-controlled initial conditions and relatively slow dynamics of these systems [17,18] provide distinct advantages for studying this problem. In addition, we use fluorescence imaging and spectroscopy for the first time in UNPs. Although the typical energy and density of UNPs is much lower than in traditional hot plasmas produced by intense laser irradiation, the ratio of electron kinetic to potential energy and the resulting expansion dynamics is similar.The investigation of plasma expansion dates back many decades [19,20]. Recently, exact solutions for spatially finite plasmas expanding into vacuum were identified for one-dimensional plasmas [1] and later extended to three dimensions [2,21]. This work was motivated by plasmas produced with short-pulse lasers.The Vlasov equations, along with Poisson's equation, describe the evolution of electron ( e) and ion ( i) distribution functions, f r; v. The Vlasov equations neglect radiative processes and collisional phenomena such as electron-ion thermalization and three-body recombination [22], but they describe many types of plasmas and are part of the foundation of kinetic theory.Among broad classes of general analytic solutions to the Vlasov equations [21], UNPs realize a particular solution that is valid for a quasineutral plasma with spherically symmetric Gaussian distribution functionsQuasineutrality is defined by n e n i , where electron and ion densities are n r R dvf r; v. T are electron and ion temperatures, and the local average velocity varies in space according to ur; t tr. The temperatures must scale as 2 T const [1], which is expected f...

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.More than half a century ago, Landau [1] and Spitzer [2] derived simple expressions for Coulomb collision rates that have become fundamental to modern plasma physics. Precise knowledge of collisional relaxation rates is essential for understanding plasmas of all varieties. It is fundamental to energy exchange in multi-species systems [2] and determines transport properties, such as selfdiffusion rates as well as thermal and electric conductivities [3]. The underlying theory, however, breaks down in strongly coupled systems such as Jovian planet interiors [4] and dense-plasma experiments [5], which display strong correlations between particles. Here, we present the first direct measurement of thermalization rates in an unmagnetized, strongly coupled plasma. Exploiting the very low temperatures in ultracold neutral plasmas [6,7], we realize strong coupling conditions at low enough densities to enable direct time-resolved measurements via optical manipulation and imaging. The observations are supported by numerical simulations that moreover highlight the importance of non-Markovian relaxation effects.In weakly interacting systems, that are either very hot and/or very dilute, relaxation is dominated by binary small-angle scattering events of distant particles. Consequently, a test charge traversing a single-species plasma of temperature T and density ρ, undergoes Brownian motion with a corresponding damping coefficient [1,2] where the factor R(v) derives from the so-called Rosenbluth potential [8] and m denotes the mass of the test particle and the plasma charges. The term ln Λ, known as the Coulomb logarithm, is determined by an upper cutoff for possible impact parameters that ensures convergence of the relaxation rate. In the original Landau-Spitzer derivation it is set equal to the Debye screening length, beyond which interactions are collectively screened by the surrounding plasma charges. Equation (1) is applied to a wide range of plasmas, but it is only valid when spatial correlations in the system are weak. The degree of particle correlations can be characterized by the ratio of their average potential and thermal energy, as expressed by the Coulomb coupling Two counter-propagating, circularly polarized lasers, detuned by ∆pp/2π = −20 MHz from the 5s 2 S 1/2 − 5p 2 P 1/2 transition, optically pump population between the two ground-state magnetic sublevels (m = ±1/2) of ions in an ultracold strontium plasma. The corresponding level-scheme is shown in (b), also indicating excited-state decay with the spontaneous emission rate γ. The optical pumping produces skewed velocity distributions f±(v) for eac...

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Ultracold neutral plasmas provide a new and valuable system in which to study fundamental plasma physics and the effects of strong coupling. In this paper, we provide a brief overview of the field of ultracold neutral plasmas. Then we describe new results from the use of fluorescence from a sheet of laser light as a probe of ion equilibration and expansion dynamics in these systems. This new probe opens many possibilities for studying thermal transport and equilibration in a strongly coupled plasma.