We present observations of wave steepening and signatures of shock formation during expansion of ultracold neutral plasmas formed with an initial density distribution that is centrally peaked and decays exponentially with distance. The plasma acceleration and velocity decrease at large distance from the plasma center, leading to central ions overtaking ions in the outer regions and the development of a steepening front that is narrow compared to the size of the plasma. The density and velocity change dramatically across the front, and significant heating of the ions is observed in the region of steepest gradients. For a reasonable estimate of electron temperature, the relative velocity of ions on either side of the front modestly exceeds the local sound speed (Mach number M≳1). This indicates that by sculpting steep density gradients, it is possible to create the conditions for shock formation, or very close to it, opening a new avenue of research for ultracold neutral plasmas.
We present observations of wave steepening and signatures of shock formation during expansion of ultracold neutral plasmas formed with an initial density distribution that is centrally peaked and decays exponentially with distance. The plasma acceleration and velocity decrease at large distance from the plasma center, leading to central ions overtaking ions in the outer regions and the development of a steepening front that is narrow compared to the size of the plasma. The density and velocity change dramatically across the front, and significant heating of the ions is observed in the region of steepest gradients. For a reasonable estimate of electron temperature, the relative velocity of ions on either side of the front modestly exceeds the local sound speed (Mach number M≳1). This indicates that by sculpting steep density gradients, it is possible to create the conditions for shock formation, or very close to it, opening a new avenue of research for ultracold neutral plasmas.
Ultracold neutral plasmas (UNPs), which are formed by photoionizing a cloud of cold atoms, have been a powerful platform for studying a wide variety of plasma phenomena. Two areas of interest include expansion into vacuum and the development of shockwaves. Previous UNP experiments have used plasmas with an initial Gaussian density distribution. UNPs with a radially exponentially decaying density distribution, or exponential UNPs, can be formed by photoionizing atoms out of a purely magnetic trap. This dissertation studies the expansion in free space of exponential UNPs as well as the development and characterization of shockwaves in exponential UNPs. The free space expansion of an exponential UNP is compared to the well-known self-similar hydrodynamic expansion of a Gaussian plasma. Similar scaling laws describe the size evolution well for both cases. The evolution of the plasma size and velocity show a characteristic ion acoustic time scale, indicating that the plasma is generally well-described by a hydrodynamic description. While the exponential UNP is predominantly in the hydrodynamic regime, heating at the central peak for low-density, high-electron temperature suggests significant local non-neutrality at early times. This thesis presents the first observation of a shockwave in a UNP. The evolution of the density and velocity for exponential UNPs show signs of wave steepening. A significant density and velocity jump over a narrow region develops. These occur at the same location in the plasma. Additionally, a large spike in the ion temperature occurs at this front. The relative ion velocity across the front modestly surpasses the local sound speed. The initial conditions were varied to characterize the development of shockwaves. Varying the density and electron temperature did not impact shock formation for the ranges used. Changing the shape caused the shocks to disappear as the density gradient decreased. This suggests that the electron thermal pressure gradient plays a significant role in the shock formation. The establishment of shock formation in UNPs opens a new avenue of research for UNPs.
We present the results of a direct simulation of the expansion of a two-component ultracold plasma for various numbers of particles, densities, and electron temperatures. A description of the expansion process common to all plasma parameters is given. After the escape of fast electrons from the plasma cloud, the excess positive charge is localized at the outer boundary, in a narrow layer. This layer has a characteristic front shape with a sharp drop in the charge concentration. The charged layer retains the remaining electrons during the entire expansion process. As the plasma expands, the speed of movement of the charged layer becomes constant and significantly exceeds the sonic speed of ions. In addition, the dependence of the radial velocity of ions on the radius acquires a self-similar character long before the final stage of expansion. Based on the calculation results, equations and self-similar solutions are obtained. General dependencies on plasma parameters are determined, which are compared with experimental data.
This work describes the evolution of an ultracold neutral plasma as it expands freely in vacuum. It presents a comprehensive study of the electron temperature evolution under different initial conditions. Ultracold neutral plasmas are created by photoionizing laser-cooled neutral atoms in ultrahigh vacuum. The ions are typically at a temperature of ∼ 1K while the electron temperature can be set from 1--1000 K. After photoionization, some of the highly energetic electrons escape from the cloud, leaving a net positive charge in the cloud. This creates a Coulomb well which traps the rest of the electrons, and a plasma is formed. Since the electrons have a lot of kinetic energy, they tend to leave the cloud, however, the Coulomb force from the ion pulls the electrons back into the cloud. This exerts a recoil force on the ions, and the whole plasma starts expanding radially outwards. Since the expansion is caused by the thermal pressure of the electrons, a study of the plasma expansion unravels the complicated electron temperature evolution, under different initial conditions. Many collisional processes become significant as a plasma expands. These physical processes tend to heat or cool the ions and electrons, leading to very different kinds of evolution depending on the initial conditions of the plasma. This work demonstrates three different regions of parameter space where the degree of significance of these physical processes is different during the ultracold neutral plasma evolution. The experimental results are verified by theoretical simulations, performed by Thomas Pohl, which untangle the complicated electron temperature evolution.
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.
We present a study of the expansion of an ultracold neutral plasma (UCNP) with an initial density distribution that decays exponentially in space, created by photoionizing atoms shortly after their release from a quadrupole (or biconic cusp) magnetic trap. A characteristic ion acoustic timescale is evident in the evolution of the plasma size and velocity, indicating that the dynamics are reasonably well described by a model of hydrodynamic expansion of a quasi-neutral plasma. However, for low plasma density and high initial electron temperature, excess ion kinetic energy in the vicinity of the central density peak suggests significant local non-neutrality at early times. Observations are compared to the well-understood self-similar expansion of a UCNP with an initial Gaussian density distribution, and a similar scaling law describes the evolution of plasma size for both cases.
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.