Abstract : This project studies collisional phenomena such as thermalization and diffusion in strongly coupled plasmas. In strongly coupled plasmas, the Coulomb interaction energy per particle exceeds the thermal energy. This occurs in high-energy-density plasmas such as inertial confinement fusion experiments and in nature in the interiors of gas giant planets and the crusts of neutron stars. Classical plasma theory breaks down in strongly coupled systems because of the non-perturbative nature of particle interactions. Improving our understanding of this regime is an important fundamental challenge and it is necessary for modelling plasmas in this regime. We perform our experiments in ultracold neutral plasmas, which are formed by photoionizing laser cooled atoms. These are the coldest neutral plasmas every created, and they allow us to study physics of strongly coupled plasmas in a controlled, well-diagnosed environment.
Abstract : This project studies collisional phenomena such as thermalization and diffusion in strongly coupled plasmas. In strongly coupled plasmas, the Coulomb interaction energy per particle exceeds the thermal energy. This occurs in high-energy-density plasmas such as inertial confinement fusion experiments and in nature in the interiors of gas giant planets and the crusts of neutron stars. Classical plasma theory breaks down in strongly coupled systems because of the non-perturbative nature of particle interactions. Improving our understanding of this regime is an important fundamental challenge and it is necessary for modelling plasmas in this regime. We perform our experiments in ultracold neutral plasmas, which are formed by photoionizing laser cooled atoms. These are the coldest neutral plasmas every created, and they allow us to study physics of strongly coupled plasmas in a controlled, well-diagnosed environment.
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.
Plasmas are normally thought of as high temperature ionized gases or fluids, such as those in the sun's corona or those found in controlled nuclear fusion experiments.Many interesting plasma phenomena can occur, however, in plasmas at low temperature.With the help of laser trapping and cooling, atoms can be photoionized to form neutral plasmas at extremely low temperatures.These plasmas may exist in the so-called strong coupling regime, where the energy of the Coulomb interactions between particles is larger than their thermal energy.In addition to providing a test bed for studying the strongly coupled plasmas such as those found in Jovian planets and white dwarfs, ultracold plasmas play a critical role in understanding the formation of antihydrogen.
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.
This thesis describes the measurements of collective effects in strongly coupled ultra-cold neutral plasmas (UNPs). It shows the implementation of experimental techniques that perturb either the density or velocity distribution of the plasma and it describes the subsequent excitation, observation and analysis of the aforementioned collective phenomena. UNPs are interesting in that they display physics of strongly coupled systems. For most plasma systems, collective effects are well described with classical hydrodynamic or kinetic descriptions. However, for strongly coupled systems, the Coulomb interaction energy between nearest neighbors exceeds the kinetic energy, and these descriptions must be modified as the plasma crosses over from a gas-like to liquid-like behavior. Strongly coupling can be found in exotic plasma systems found astrophysics, dusty plasmas, non-neutral trapped ion plasmas, intense-laser/matter interactions and inertial confinement fusion experiments. Compared to other strongly coupled plasmas, UNPs are ideal for studying collective effects in this regime since they have lower timescales, precisely controllable initial conditions and non-invasive diagnostics. Previous studies of UNPs concentrated on plasma expansion dynamics and some collective effects such as disorder induced heating, but little work had been done in relaxation or collision rates and collective modes in UNPs. This thesis presents a method for measuring collision rates by perturbing the velocity distribution of the plasma, observing plasma relaxation and measuring the relaxation rate. It also presents a new technique for observing collective modes in the plasma by perturbing the initial density of the plasma and how this results in the excitation of ion acoustic waves and a measurement of its dispersion relation. Finally, this thesis presents how this last technique can be used to create a gap in the center of the plasma and how this leads to hole propagation and plasma streaming and presents a characterization of both phenomena. The result of these experiments will be valuable for predicting the behavior of collective effects in other strongly coupled plasmas and for comparison with theories that describe them.
Ultracold neutral plasmas are formed by photoionizing laser-cooled atoms near the ionization threshold. Through the application of atomic physics techniques and diagnostics, these experiments stretch the boundaries of traditional neutral plasma physics. The electron temperature in these plasmas ranges from 1-1000 K and the ion temperature is around 1 K. The density can approach 1011 cm−3. Fundamental interest stems from the possibility of creating strongly-coupled plasmas, but recombination, collective modes, and thermalization in these systems have also been studied. Optical absorption images of a strontium plasma, using the Sr+2S1/2−>2P1/2 transition at 422 nm, depict the density profile of the plasma, and probe kinetics on a 50 ns time-scale. The Doppler-broadened ion absorption spectrum measures the ion velocity distribution, which gives an accurate measure of the ion dynamics in the first microsecond after photoionization.
For interacting many-body systems the two most natural energy scales are the average energy per particle and the average energy per particle and the average nearest-neighbor potential energy. When the ratio of the nearest-neighbor potential energy to the kinetic energy is greater than 1, we say that the system is "strongly coupled," where strong coupling is characterized by the parameter Γ. This dimensionless parameter describes the complete thermodynamic state of a system, making it possible to study the fundamental behavior of strongly coupled systems as manifested in high energy-density plasmas using low energy table-top experiments. In our experiments, we are working with plasmas in this exotic regime. In this paper we report on the progress of an experiment in laser-cooled calcium designed to increase the strong coupling of an ultracold neutral plasma by promoting the plasma ions to the second ionization state. We report preliminary measurements of the change in the Ca+ ion temperature due to the second ionization.