We report optical absorption imaging of ultracold neutral strontium plasmas. The ion absorption spectrum determined from the images is Doppler broadened and thus provides a quantitative measure of the ion kinetic energy. For the particular plasma conditions studied, ions heat rapidly as they equilibrate during the first 250 ns after plasma formation. Equilibration leaves ions on the border between the weakly coupled gaseous and strongly coupled liquid states. On a longer time scale of microseconds, pressure exerted by the trapped electron gas accelerates the ions radially.

We study equilibration of strongly coupled ions in an ultracold neutral plasma produced by photoionizing laser-cooled and trapped atoms. By varying the electron temperature, we show that electron screening modifies the equilibrium ion temperature. Even with few electrons in a Debye sphere, the screening is well described by a model using a Yukawa ion-ion potential. We also observe damped oscillations of the ion kinetic energy that are a unique feature of equilibration of a strongly coupled plasma. DOI: 10.1103 There has been significant theoretical study of the equilibration of strongly coupled plasmas [6 -12], especially in the context of plasmas produced with high-intensity lasers. In addition to generating fundamental interest, this problem challenges computational resources and techniques. Experimental results have been lacking, however, because of the fast time scales involved and limited diagnostics.Ultracold neutral plasmas [13], produced by photoionizing clouds of laser-cooled and trapped atoms, are ideal for experimental studies. The equilibration of the plasma is relatively slow ( 100 ns) due to lower plasma density. Ultracold neutral plasmas also offer a high level of control and diagnostics. By varying laser intensities and wavelengths, it is possible to accurately set the initial density and energy of the system. Optical imaging [14] provides an in situ probe of plasma properties with excellent spatial, temporal, and spectral resolution.In this Letter, we explore ion equilibration during the first microsecond after the plasma is created. The density sets the time and the energy scale for equilibration, but electron screening effects are evident. Even when the number of electrons per Debye sphere is small, the equilibration temperature of the ions agrees with a model [15] that uses a Yukawa ion-ion potential.We also observed oscillations of the ion kinetic energy. For many years, this phenomenon has been the subject of intense study through analytic calculations [7] and simulations [6,[8][9][10][11][12]] of one-component strongly coupled plasmas, but it has not previously been observed experimentally. The oscillations and their damping reflect universal dynamics of a Coulomb system with spatial correlations.Details on laser cooling, plasma formation, and imaging are given in [14,16]. The experiment starts with strontium atoms that are cooled and trapped in a magneto-optical trap (MOT). The neutral atom cloud is characterized by a temperature of about 10 mK, 2 10 8 atoms, and a Gaussian density distribution. We vary the atom density by changing the MOT parameters, or by turning the MOT off and releasing the atoms in a ballistic expansion. Up to 30% of the neutral atoms are then ionized with one photon from the cooling laser and one photon from a pulsed dye laser. The ion density distribution equals the atom distribution at the time of photoionization and is given by n i r n 0i exp ÿr 2 =2 2 , with from 0.6 to 1 mm and n 0i from 2 10 9 to 1:4 10 10 cm ÿ3 . The electron density, n e r , closely follows ...

We report the magnetic trapping of metastable 3 P 2 atomic strontium. Atoms are cooled in a magneto-optical trap ͑MOT͒ operating on the dipole-allowed 1 S 0 -1 P 1 transition at 461 nm. Decay via 1 P 1 → 1 D 2 → 3 P 2 continuously loads a magnetic trap formed by the quadrupole magnetic field of the MOT. Over 10 8 atoms at a density of 8ϫ10 9 cm Ϫ3 and temperature of 1 mK are trapped. The atom temperature is significantly lower than what would be expected from the kinetic and potential energies of atoms as they are transferred from the MOT. This suggests the occurrence of thermalization and evaporative cooling in the magnetic trap. Laser-cooled alkaline-earth-metal atoms offer many possibilities for practical applications and fundamental studies. The two valence electrons in these systems give rise to triplet and singlet levels connected by narrow intercombination lines that are utilized for optical frequency standards ͓1͔.Laser cooling on such a transition in strontium may lead to a fast and efficient route to all-optical quantum degeneracy ͓2,3͔, and there are abundant bosonic and fermionic isotopes for use in this pursuit. The lack of hyperfine structure in the bosonic isotopes and the closed electronic shell in the ground states make alkaline-earth-metal atoms appealing testing grounds for cold-collision theories ͓4 -6͔, and collisions between metastable alkaline-earth-metal atoms is a relatively new and unexplored area for research ͓7͔.In this paper we characterize a technique that should benefit all these experiments-the continuous loading of metastable 3 P 2 atomic strontium ( 88 Sr) from a magneto-optical trap ͑MOT͒ into a purely magnetic trap. This idea was discussed in a recent theoretical study of alkaline-earth-metal atoms and ytterbium ͓8͔. Katori et al. ͓9͔ and Loftus et al. ͓10͔ have also reported observing this phenomenon in their strontium laser-cooling experiments. Continuous loading of a magnetic trap from a MOT was recently described for chromium atoms ͓11͔.This scheme should allow for collection of large numbers of atoms at high density since atoms are shelved in a dark state and are less susceptible to light-assisted collisional loss mechanisms ͓4,6,12͔. It is an ideal starting place for many experiments such as sub-Doppler laser cooling on a transition from the metastable state, as has been done with calcium ͓13͔, production of ultracold Rydberg gases ͓14͔ or plasmas ͓15͔, and evaporative cooling to quantum degeneracy. Optical frequency standards based on laser-cooled alkaline-earthmetal atoms, which are currently limited by high sample temperatures ͓1͔, may benefit from the ability to trap larger numbers of atoms and evaporatively cool them in a magnetic trap.We will first describe the operation of the Sr MOT and how this loads the magnetic trap with 3 P 2 atoms. Then we will characterize the loading and decay rates of atoms in the magnetic trap. Finally, we will present measurements of the 3 P 2 sample temperature.Sr atoms are loaded from a Zeeman-slowed atomic beam ͓16͔ and cooled and tr...

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...

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...