Fluorescence measurements of expanding strongly coupled neutral plasmas

Structure formation by electrostatic interactions in strongly coupled medium

The formation of correlated structures is of importance in many diverse contexts such as strongly coupled plasmas, soft matter, and even biological mediums. In all these contexts the dynamics are mainly governed by electrostatic interactions and result in the formation of a variety of structures. In this study, the process of formation of structures is investigated with the help of molecular dynamics (MD) simulations in two and three dimensions. The overall medium has been modeled with an equal number of positive and negatively charged particles interacting via long-range pair Coulomb potential. A repulsive short-range Lennard-Jones (LJ) potential is added to take care of the blowing up of attractive Coulomb interaction between unlike charges. In the strongly coupled regime, a variety of classical bound states form. However, complete crystallization of the system, as typically observed in the context of one-component strongly coupled plasmas, does not occur. The influence of localized perturbation in the system has also been studied. The formation of a crystalline pattern of shielding clouds around this disturbance is observed. The spatial properties of the shielding structure have been analyzed using the radial distribution function and Voronoi diagram. The process of accumulation of oppositely charged particles around the disturbance triggers a lot of dynamic activity in the bulk of the medium. As a result of this, close encounters are possible even between those particles/clusters which were initially and/or at some point of time widely separated. This leads to the formation of a larger number of bigger clusters. There are, however, also instances when bound pairs break up and the electrons from bound pairs contribute to the shielding cloud, whereas ions bounce back into the bulk. A detailed discussion of these features has been provided in the manuscript.

Design and characterization of a resonant microwave cavity as a diagnostic for ultracold plasmas

We present the design and commissioning of a resonant microwave cavity as a novel diagnostic for the study of ultracold plasmas. This diagnostic is based on the measurements of the shift in the resonance frequency of the cavity, induced by an ultracold plasma that is created from a laser-cooled gas inside. This method is simultaneously non-destructive, very fast (nanosecond temporal resolution), highly sensitive, and applicable to all ultracold plasmas. To create an ultracold plasma, we implement a compact magneto-optical trap based on a diffraction grating chip inside a 5 GHz resonant microwave cavity. We are able to laser cool and trap (7.25 ± 0.03) × 10⁷ rubidium atoms inside the cavity, which are turned into an ultracold plasma by two-step pulsed (nanosecond or femtosecond) photo-ionization. We present a detailed characterization of the cavity, and we demonstrate how it can be used as a fast and sensitive probe to monitor the evolution of ultracold plasmas non-destructively. The temporal resolution of the diagnostic is determined by measuring the delayed frequency shift following femtosecond photo-ionization. We find a response time of 18 ± 2 ns, which agrees well with the value determined from the cavity quality factor and resonance frequency.

Ion friction at small values of the Coulomb logarithm

Transport properties of high-energy-density plasmas are influenced by the ion collision rate. Traditionally, this rate involves the Coulomb logarithm, lnΛ. Typical values of lnΛ are ≈10-20 in kinetic theories where transport properties are dominated by weak-scattering events caused by long-range forces. The validity of these theories breaks down for strongly coupled plasmas, when lnΛ is of order one. We present measurements and simulations of collision data in strongly coupled plasmas when lnΛ is small. Experiments are carried out in the first dual-species ultracold neutral plasma (UNP), using Ca^{+} and Yb^{+} ions. We find strong collisional coupling between the different ion species in the bulk of the plasma. We simulate the plasma using a two-species fluid code that includes Coulomb logarithms derived from either a screened Coulomb potential or a the potential of mean force. We find generally good agreement between the experimental measurements and the simulations. With some improvements, the mixed Ca^{+} and Yb^{+} dual-species UNP will be a promising platform for testing theoretical expressions for lnΛ and collision cross-sections from kinetic theories through measurements of energy relaxation, stopping power, two-stream instabilities, and the evolution of sculpted distribution functions in an idealized environment in which the initial temperatures, densities, and charge states are accurately known.

Ultracold neutral plasmas

By photoionizing samples of laser-cooled atoms with laser light tuned just above the ionization limit, plasmas can be created with electron and ion temperatures below 10 K. These ultracold neutral plasmas have extended the temperature bounds of plasma physics by two orders of magnitude. Table-top experiments, using many of the tools from atomic physics, allow for the study of plasma phenomena in this new regime with independent control over the density and temperature of the plasma through the excitation process. Characteristic of these systems is an inhomogeneous density profile, inherited from the density distribution of the laser-cooled neutral atom sample. Most work has dealt with unconfined plasmas in vacuum, which expand outward at velocities of order 100 m/s, governed by electron pressure, and with lifetimes of order 100 μs, limited by stray electric fields. Using detection of charged particles and optical detection techniques, a wide variety of properties and phenomena have been observed, including expansion dynamics, collective excitations in both the electrons and ions, and collisional properties. Through three-body recombination collisions, the plasmas rapidly form Rydberg atoms, and clouds of cold Rydberg atoms have been observed to spontaneously avalanche ionize to form plasmas. Of particular interest is the possibility of the formation of strongly coupled plasmas, where Coulomb forces dominate thermal motion and correlations become important. The strongest impediment to strong coupling is disorder-induced heating, a process in which Coulomb energy from an initially disordered sample is converted into thermal energy. This restricts electrons to a weakly coupled regime and leaves the ions barely within the strongly coupled regime. This review will give an overview of the field of ultracold neutral plasmas, from its inception in 1999 to current work, including efforts to increase strong coupling and effects on plasma properties due to strong coupling.

Using higher ionization states to increase Coulomb coupling in an ultracold neutral plasma

We report measurements and simulations of the time-evolving rms velocity distribution in an ultracold neutral plasma. A strongly coupled ultracold neutral Ca+ plasma is generated by photoionizing laser-cooled atoms close to threshold. A fraction of these ions is then promoted to the second ionization state to form a mixed Ca+-Ca2+ plasma. By varying the time delay between the first and the second ionization events, a minimum in ion heating is achieved. We show that the Coulomb strong-coupling parameter Γ increases by a factor of 1.4 to a maximum value of 3.6. A pure Ca2+ plasma would have Γ=6.8, moving these strongly coupled systems closer to the regime of liquid-like correlations.

Strongly coupled plasmas via Rydberg blockade of cold atoms

We propose and analyze a new scheme to produce ultracold neutral plasmas deep in the strongly coupled regime. The method exploits the interaction blockade between cold atoms excited to high-lying Rydberg states and therefore does not require substantial extensions of current ultracold plasma experiments. Extensive simulations reveal a universal behavior of the resulting Coulomb coupling parameter, providing a direct connection between the physics of strongly correlated Rydberg gases and ultracold plasmas. The approach is shown to reduce currently accessible temperatures by more than an order of magnitude, which opens up a new regime for ultracold plasma research and cold ion-beam applications with readily available experimental techniques.

Spontaneous avalanche ionization of a strongly blockaded Rydberg gas

We report the sudden and spontaneous evolution of an initially correlated gas of repulsively interacting Rydberg atoms to an ultracold plasma. Under continuous laser coupling we create a Rydberg ensemble in the strong blockade regime, which at longer times undergoes an ionization avalanche. By combining optical imaging and ion detection, we access the full information on the dynamical evolution of the system, including the rapid increase in the number of ions and a sudden depletion of the Rydberg and ground state densities. Rydberg-Rydberg interactions are observed to strongly affect the dynamics of plasma formation. Using a coupled rate-equation model to describe our data, we extract the average energy of electrons trapped in the plasma, and an effective cross section for ionizing collisions between Rydberg atoms and atoms in low-lying states. Our results suggest that the initial correlations of the Rydberg ensemble should persist through the avalanche. This would provide the means to overcome disorder-induced heating, and offer a route to enter new strongly coupled regimes.

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.

"Ultracold" neutral plasmas at room temperature

We report a measurement of the electron temperature in a plasma generated by a high-intensity laser focused into a jet of neon. The 15 eV electron temperature is determined using an analytic solution of the plasma equations assuming local thermodynamic equilibrium, initially developed for ultracold neutral plasmas. We show that this analysis method accurately reproduces more sophisticated plasma simulations in our temperature and density range. While our plasma temperatures are far outside the typical "ultracold" regime, the ion temperature is determined by the plasma density through disorder-induced heating just as in ultracold neutral plasma experiments. Based on our results, we outline a pathway for achieving a strongly coupled neutral laser-produced plasma that even more closely resembles ultracold neutral plasma conditions.

Electronic detection of collective modes of an ultracold plasma

Using a new technique to directly detect current induced on a nearby electrode, we measure plasma oscillations in ultracold plasmas, which are influenced by the inhomogeneous and time-varying density and changing neutrality. Electronic detection avoids heating and evaporation dynamics associated with previous measurements and allows us to test the importance of the plasma neutrality. We apply dc and pulsed electric fields to control the electron loss rate and find that the charge imbalance of the plasma has a significant effect on the resonant frequency, in excellent agreement with recent predictions suggesting coupling to an edge mode.

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.

Recombination fluorescence in ultracold neutral plasmas

We present the first measurements and simulations of recombination fluorescence from ultracold neutral calcium plasmas. This method probes three-body recombination at times less than 1 micros, shorter than previously published time scales. For the lowest initial electron temperatures, the recombination rate scales with the density as n0(2.2), significantly slower than the predicted n0(3). Recombination fluorescence opens a new diagnostic window in ultracold plasmas. In most cases it probes deeply bound level populations that depend critically on electron energetics. However, a perturbation in the calcium 4snd Rydberg series allows our fluorescence measurements to probe the population in weakly bound levels that result just after recombination.

Ultracold plasma expansion in a magnetic field

We measure the expansion of an ultracold plasma across the field lines of a uniform magnetic field. We image the ion distribution by extracting the ions with a high-voltage pulse onto a position-sensitive detector. Early in the lifetime of the plasma (<20 micros), the size of the image is dominated by the time-of-flight Coulomb explosion of the dense ion cloud. For later times, we measure the 2D Gaussian width of the ion image, obtaining the transverse expansion velocity as a function of the magnetic field (up to 70 G). We observe that the expansion velocity scales as B(-1/2), explained by a nonlinear ambipolar diffusion model with anisotropic diffusion in two different directions.

Using three-body recombination to extract electron temperatures of ultracold plasmas

Three-body recombination, an important collisional process in plasmas, increases dramatically at low electron temperatures, with an accepted scaling of Te(-9/2). We measure three-body recombination in an ultracold neutral xenon plasma by detecting recombination-created Rydberg atoms using a microwave-ionization technique. With the accepted theory (expected to be applicable for weakly coupled plasmas) and our measured rates, we extract the plasma temperatures, which are in reasonable agreement with previous measurements early in the plasma lifetime. The resulting electron temperatures indicate that the plasma continues to cool to temperatures below 1 K.

Electron-temperature evolution in expanding ultracold neutral plasmas

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 Gamma(e), which is dominated by adiabatic cooling of the electrons, to the regime of high Gamma(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.

Ultracold Neutral Plasmas

Ultracold neutral plasmas occupy an exotic regime of plasma physics in which electrons form a swarming, neutralizing background for ions that sluggishly move in a correlated manner. Strong interactions between the charged particles give rise to surprising dynamics such as oscillations of the average kinetic energy during equilibration and extremely fast recombination. Such phenomena offer stimulating and challenging problems for computational scientists, and the physics can be applied to other environments, such as the interior of gas giant planets and plasmas created by short-pulse laser irradiation of solid, liquid, and cluster targets.

Observation of collective modes of ultracold plasmas

Applying a radio-frequency electric field to an expanding ultracold neutral plasma leads to the observation of as many as six peaks in the emission of electrons from the plasma. These are identified as collective modes of the plasma and are in qualitative agreement with a model of Tonks-Dattner resonances, electron sound waves propagating in a finite-sized, inhomogeneous plasma. Such modes may provide an accurate method to determine the time-dependent electron temperature.