Applying a short electric field pulse to an ultracold plasma induces an electron plasma oscillation. This manifests itself as an oscillation of the electron center of mass around the ion center of mass in the ultracold plasma. In general, the oscillation can damp due to either collisionless or collisional mechanisms, or a combination of the both. To investigate the nature of oscillation damping in ultracold plasmas, we developed a molecular dynamics model of the ultracold plasma electrons. Through this model, we found that depending on the neutrality of the ultracold plasma and the size of an applied DC electric field, there are some parameter ranges where the damping is primarily collisional and some primarily collisionless. We conducted experiments to compare the measured damping rate with theory predictions and found them to be in good agreement. Extension of our measurements to different parameter ranges should enable studies for strong-coupling influence on electron-ion collision rates.
Applying a short electric field pulse to an ultracold plasma induces an electron plasma oscillation. This manifests itself as an oscillation of the electron center of mass around the ion center of mass in the ultracold plasma. In general, the oscillation can damp due to either collisionless or collisional mechanisms, or a combination of the both. To investigate the nature of oscillation damping in ultracold plasmas, we developed a molecular dynamics model of the ultracold plasma electrons. Through this model, we found that depending on the neutrality of the ultracold plasma and the size of an applied DC electric field, there are some parameter ranges where the damping is primarily collisional and some primarily collisionless. We conducted experiments to compare the measured damping rate with theory predictions and found them to be in good agreement. Extension of our measurements to different parameter ranges should enable studies for strong-coupling influence on electron-ion collision rates.
tion of the electrons. This oscillation will damp after the application of the electric eld pulse. We have found that under certain achievable experimental conditions, this damping can be dominated by the electron-ion collision rate. We have measured this damping rate experimentally under these conditions and thus can compare it to theoretical predictions. We will present our measurement technique and results. In addition, we will discuss extensions of this technique to measurements of electron temperature, to investigating strong-coupling physics in the electron component of an ultracold plasma, and to measuring the electron-ion collision rate when the electrons are highly magnetized.
In this dissertation I describe a number of different computational investigations which I have undertaken during my time at Colorado State University. Perhaps the most significant of my accomplishments was the development of a general molecular dynamic model that simulates a wide variety of physical phenomena in ultracold plasmas (UCPs). This model formed the basis of most of the numerical investigations discussed in this thesis. The model utilized the massively parallel architecture of GPUs to achieve significant computing speed increases (up to 2 orders of magnitude) above traditional single core computing. This increased computing power allowed for each particle in an actual UCP experimental system to be explicitly modeled in simulations. By using this model, I was able to undertake a number of theoretical investigations into ultracold plasma systems. Chief among these was our lab's investigation of electron center-of-mass damping, in which the molecular dynamics model was an essential tool in interpreting the results of the experiment. Originally, it was assumed that this damping would solely be a function of electron-ion collisions. However, the model was able to identify an additional collisionless damping mechanism that was determined to be significant in the first iteration of our experiment. To mitigate this collisionless damping, the model was used to find a new parameter range where this mechanism was negligible. In this new parameter range, the model was an integral part in verifying the achievement of a record low measured UCP electron temperature of 1.57 ± 0.28K and a record high electron strong coupling parameter, Γ, of 0.35 ± 0.08. Additionally, the model, along with experimental measurements, was used to verify the breakdown of the standard weak coupling approximation for Coulomb collisions. The general molecular dynamics model was also used in other contexts. These included the modeling of both the formation process of ultracold plasmas and the thermalization of the electron component of an ultracold plasma. Our modeling of UCP formation is still in its infancy, and there is still much outstanding work. However, we have already discovered a previously unreported electron heating mechanism that arises from an external electric field being applied during UCP formation. Thermalization modeling showed that the ion density distribution plays a role in the thermalization of electrons in ultracold plasma, a consideration not typically included in plasma modeling. A Gaussian ion density distribution was shown to lead to a slightly faster electron thermalization rate than an equivalent uniform ion density distribution as a result of collisionless effects. Three distinct phases of UCP electron thermalization during formation were identified. Finally, the dissertation will describe additional computational investigations that preceded the general molecular dynamics model. These include simulations of ultracold plasma ion expansion driven by non-neutrality, as well as an investigation into electron evaporation. To test the effects of non-neutrality on ion expansion, a numerical model was developed that used the King model of the electron to describe the electron distribution for an arbitrary charge imbalance. The model found that increased non-neutrality of the plasma led to the rapid expansion of ions on the plasma exterior, which in turn led to a sharp ion cliff-like spatial structure. Additionally, this rapid expansion led to additional cooling of the electron component of the plasma. The evaporation modeling was used to test the underlying assumptions of previously developed analytical expression for charged particle evaporation. The model used Monte Carlo techniques to simulate the collisions and the evaporation process. The model found that neither of the underlying assumption of the charged particle evaporation expressions held true for typical ultracold plasma parameters and provides a route for computations in spite of the breakdown of these two typical assumptions.
One of the properties of ultracold plasmas that make them interesting objects of study is that they are cold enough that strong coupling effects can be made manifest at their typical densities. In order to study strong coupling effects, sufficiently low temperatures need to be obtained. In turn, this means that the limitations to the lowest achievable temperatures for the electrons and ions in ultracold plasmas are worth investigating as they determine the degree to which strong coupling can be achieved. In addition, understanding these limitations also illuminates the basic physics of ultracold plasmas. A DC electric field applied during ultracold plasma formation can result in significant heating of the electron component. In the work presented here, we use molecular dynamics simulations to study this heating process and determine its impact as a function of ultracold plasma parameters such as electron temperature and density. We find that this heating can have a significant impact on the lowest achievable temperatures for lower-density ultracold plasmas in particular.
Electrons in an expanding ultracold plasma are expected to be in quasiequilibrium since the collision times are short compared to the plasma lifetime, yet electron evaporation is observed as the ion density decreases during expansion. A small electric field that shifts the electron cloud with respect to the ions increases the evaporation rate. Treating the electrons as a zero-temperature fluid, their spatial distribution is calculated as a function of the applied field strength and the ion density. The zero-temperature approximation gives the maximum number of electrons the plasma can hold in the absence of evaporation. By applying this calculation at all times during expansion, the flux of cold electrons from the plasma is predicted and found to be in good agreement with the observed electron signal.
Ultracold plasmas (UCPs) are created by photo-ionization of a cloud of laser-cooled atoms, and have initial electron temperatures in the range 1-100 K and initial ion temperatures in the range 0.001-1 K. As a consequence UCPs can be in the strongly coupled regime, where the typical Coulomb interaction between the particles exceeds the thermal energy of the particles; a clear distinction with conventional plasmas. UCPs are not stable systems and the electron and ion temperature will rise during their evolution. The introduction of a disturbing rf-field to the plasma is expected to speed up the heating of the plasma.In this thesis the intrinsic electron heating mechanisms are studied as well as the heating mechanisms induced by an external rf field. Numerical simulations were performed with the General Particle Tracer code and compared to analytical theories.Two intrinsic heating mechanisms were studied: disorder-induced heating (DIH) and heating by three-body recombination (TBR). DIH arises due the random initial positions of the electrons. An excess of potential energy exists in the electron distribution which is rapidly converted into thermal energy. The time scale of DIH was found to be on the order of the inverse Mie-frequency, confirming analytical theories. TBR was identified in the simulations and the TBR heating rate was found to agree well with analytical models. Two rf-induced heating mechanisms were studied: collisionless energy absorption and collisional absorption. The first of these processes was found to depend strongly on the ratio between the rf field frequency and the oscillation frequency of the electrons in the plasma. Collisional absorption was studied in a regime that collisionless absorption is negligible. Absorption arises because the electrons, which are oscillating at the rf field frequency, are deflected by the Coulomb fields of the ions. The amount of collisional absorption was found to depend strongly on the amplitude of the rf field. It was found that the rf field effectively suppresses the electron-ion collision frequency as a function of increasing field amplitude, confirming analytical theories. For low to moderate amplitudes the amount of energy absorption increases, but less than one might intuitively expect due to the decrease in collision frequency. For very strong field amplitudes the energy absorption even decreases as a function of field amplitude.The understanding achieved, provides new roads to control these type of plasmas experimentally. Ultracold plasmas (UCPs) are created by photo-ionization of a cloud of laser-cooled atoms, and have initial electron temperatures in the range 1-100 K and initial ion temperatures in the range 0.001-1 K. As a consequence UCPs can be in the strongly coupled regime, where the typical Coulomb interaction between the particles exceeds the thermal energy of the particles; a clear distinction with conventional plasmas. UCPs are not stable systems and the electron and ion temperature will rise during their evolution. The introduction of a disturbing rf-field to the plasma is expected to speed up the heating of the plasma.In this thesis the intrinsic electron heating mechanisms are studied as well as the heating mechanisms induced by an external rf field. Numerical simulations were performed with the General Particle Tracer code and compared to analytical theories.Two intrinsic heating mechanisms were studied: disorder-induced heating (DIH) and heating by three-body recombination (TBR). DIH arises due the random initial positions of the electrons. An excess of potential energy exists in the electron distribution which is rapidly converted into thermal energy. The time scale of DIH was found to be on the order of the inverse Mie-frequency, confirming analytical theories. TBR was identified in the simulations and the TBR heating rate was found to agree well with analytical models. Two rf-induced heating mechanisms were studied: collisionless energy absorption and collisional absorption. The first of these processes was found to depend strongly on the ratio between the rf field frequency and the oscillation frequency of the electrons in the plasma. Collisional absorption was studied in a regime that collisionless absorption is negligible. Absorption arises because the electrons, which are oscillating at the rf field frequency, are deflected by the Coulomb fields of the ions. The amount of collisional absorption was found to depend strongly on the amplitude of the rf field. It was found that the rf field effectively suppresses the electron-ion collision frequency as a function of increasing field amplitude, confirming analytical theories. For low to moderate amplitudes the amount of energy absorption increases, but less than one might intuitively expect due to the decrease in collision frequency. For very strong field amplitudes the energy absorption even decreases as a function of field amplitude.The understanding achieved, provides new roads to control these type of plasmas experimentally.
The expansion of ultracold neutral plasmas (UCP) is driven primarily by the thermal pressure of the electron component and is therefore sensitive to the electron temperature. For typical UCP spatial extents, evaporative cooling has a significant influence on the UCP expansion rate at lower densities (less than 108/cm3). We studied the effect of electron evaporation in this density range. Owing to the low density, the effects of three-body recombination were negligible. We modeled the expansion by taking into account the change in electron temperature owing to evaporation as well as adiabatic expansion and found good agreement with our data. We also developed a simple model for initial evaporation over a range of ultracold plasma densities, sizes, and electron temperatures to determine over what parameter range electron evaporation is expected to have a significant effect. We also report on a signal calibration technique, which relates the signal at our detector to the total number of ions and electrons in the ultracold plasma.