The development of atom trapping and cooling techniques has been a boon for the field of low temperature plasma physics. By forming plasmas through photoionization (PI) of cold atoms, researchers are able to create cold, dense plasmas that often lie in the strong-coupling regime. This is especially remarkable considering that many naturally-occurring plasmas in the strong-coupling regime are in inaccessible environments (e.g. the insides of stars and gas planets), whereas these laboratory-based plasmas prepared in cold-atom systems are relatively easy to produce and analyze with standard AMO techniques. In this thesis, I present my work from several projects on photoionization, plasma formation and dynamics, and plasma trap development. I first discuss an experiment that I spearheaded on expansion dynamics in cold, non-neutral plasmas. In this experiment, cold plasmas are formed by photoionizing rubidium atoms in a MOT, and the resulting plasma ions are allowed to expand for a variable wait time before being imaged with a single-ion imaging system. This ion imaging system, along with the rest of the experimental setup, was originally used to study Rydberg atoms. I have adapted the setup for plasmas by installing a new laser and changing the experimental timings. I report on plasma features seen in this experimental setup, such as shock shells and time-evolved ion pair correlations. I also present findings from a computational study of wavelength-dependent PI of Rydberg atoms, namely Rydberg-state rubidium and cesium. The main goal of the study is to find PI minima, wavelengths where the atom’s PI cross section decreases dramatically. These minima can be caused by Cooper minima or shape resonances, which each have their own telltale signs. This computational study paves the way for an informed experimental measurement of the locations of some of the identified PI minima, and comparing their experimental locations with the computational results. This is, in part, interesting because the latter depend on whether length or velocity gauge is used. An experimental measurement of PI cross sections could be a good way to explore the significance of gauge dependence in l-dependent potentials. I conclude with a discussion of a project centered on creating a small MOT with diverging cooling beams inside of a metal enclosure. This project was a collaboration with researchers at Eastern Michigan University. The idea to make a MOT with these design features was born out of plans for cryostatic plasma experiments. Now, it has potential applications in plasma experiments, precision measurements, and quantum information. Design-wise, the diverging beams are formed by sending narrow laser beams into 1.5 mm ball lenses, which cause the beams to diverge rapidly. Six ball lenses, one for each cooling beam, are mounted on a metal structure called the ball lens optical box (BLOB), which encases the trapped atom cloud. This enclosure provides immediate benefits for plasma experiments, as it offers Faraday shielding from stray electric fields. The first build successfully produced and maintained a MOT, making it the first known case of a MOT made with diverging beams from ball lenses. No further experiments are planned for this first build, but the concept of a ball lens MOT will certainly be implemented into future experimental efforts. Subsequent designs may include more design features, such as interior electrodes for custom field configurations.
The development of atom trapping and cooling techniques has been a boon for the field of low temperature plasma physics. By forming plasmas through photoionization (PI) of cold atoms, researchers are able to create cold, dense plasmas that often lie in the strong-coupling regime. This is especially remarkable considering that many naturally-occurring plasmas in the strong-coupling regime are in inaccessible environments (e.g. the insides of stars and gas planets), whereas these laboratory-based plasmas prepared in cold-atom systems are relatively easy to produce and analyze with standard AMO techniques. In this thesis, I present my work from several projects on photoionization, plasma formation and dynamics, and plasma trap development. I first discuss an experiment that I spearheaded on expansion dynamics in cold, non-neutral plasmas. In this experiment, cold plasmas are formed by photoionizing rubidium atoms in a MOT, and the resulting plasma ions are allowed to expand for a variable wait time before being imaged with a single-ion imaging system. This ion imaging system, along with the rest of the experimental setup, was originally used to study Rydberg atoms. I have adapted the setup for plasmas by installing a new laser and changing the experimental timings. I report on plasma features seen in this experimental setup, such as shock shells and time-evolved ion pair correlations. I also present findings from a computational study of wavelength-dependent PI of Rydberg atoms, namely Rydberg-state rubidium and cesium. The main goal of the study is to find PI minima, wavelengths where the atom’s PI cross section decreases dramatically. These minima can be caused by Cooper minima or shape resonances, which each have their own telltale signs. This computational study paves the way for an informed experimental measurement of the locations of some of the identified PI minima, and comparing their experimental locations with the computational results. This is, in part, interesting because the latter depend on whether length or velocity gauge is used. An experimental measurement of PI cross sections could be a good way to explore the significance of gauge dependence in l-dependent potentials. I conclude with a discussion of a project centered on creating a small MOT with diverging cooling beams inside of a metal enclosure. This project was a collaboration with researchers at Eastern Michigan University. The idea to make a MOT with these design features was born out of plans for cryostatic plasma experiments. Now, it has potential applications in plasma experiments, precision measurements, and quantum information. Design-wise, the diverging beams are formed by sending narrow laser beams into 1.5 mm ball lenses, which cause the beams to diverge rapidly. Six ball lenses, one for each cooling beam, are mounted on a metal structure called the ball lens optical box (BLOB), which encases the trapped atom cloud. This enclosure provides immediate benefits for plasma experiments, as it offers Faraday shielding from stray electric fields. The first build successfully produced and maintained a MOT, making it the first known case of a MOT made with diverging beams from ball lenses. No further experiments are planned for this first build, but the concept of a ball lens MOT will certainly be implemented into future experimental efforts. Subsequent designs may include more design features, such as interior electrodes for custom field configurations.
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.
In a Magneto-Optical Trap (MOT), realized for the first time in 1987, one can trap and cool neutral atoms to temperatures below a mK. The invention of this device caused a revolution in atomic physics. With an MOT collision and spectroscopy experiments could be performed with unprecedented accuracy. The main breakthrough came about 10 years later when the atom from an MOT were used to create a Bose Einstein Condensate (BEC), a source of coherent matter. All this work has been rewarded with no less than two Nobel prizes: in 1997, Chu, Cohen-Tannoudji and Phillips received the Nobel prize for the practical and theoretical development of laser cooling, and in 2001 Cornell, Ketterle and Wieman received the Nobel prize for the realization of a Bose Einstein Condensate in a dilute gas. In 1999 Killian et al. took trapped atoms in an entirely new direction when they succeeded in creating the very first Ultracold Plasma (UCP) from the atoms in an MOT. The temperature of such a plasma is more than 1000 times lower than the temperatures in conventional cold plasmas. As a result, this type of plasmas received a lot of attention, both theoretically and experimentally. So far studies of UCPs have mainly been of a fundamental kind. In this thesis we study both the fundamental aspects of atoms excited from a magneto-optical as well as the feasibility of using the electrons and ions from a UCP as a source for charged particle accelerators. The work in this thesis therefore pioneers the line between atom physics, plasma physics and accelerator physics. The first part of this thesis deals with an experimental study of the effect of an ionizing laser on cold neon atoms. Here we study the loss of atoms from a Magneto-Optical Trap caused by photionization. From these measurements we deduce a new value for the near threshold photoionization cross section 2:05±0:25x10-18 cm2 at ¿ = 351 nm and 2:05±0:25x10-18 cm2 at ¿ = 364 nm, which is a factor of four more accurate than previous measurements. The measured valuesagree with earlier measurements but differ signi¯cantly from theoretical values. The value of this cross section has a practical relevance for the realization of a continuous ion source. The second part of this thesis has been conducted at the University of Virginia. We study the role of dipole-dipole interactions in the formation of a UCP from a gas of cold Rydberg atoms, and the effect of dipole-dipole interactions on the broadening of microwave transitions. We find that the broadening scales linearly with the density en scales with the fourth power of n, the principal quantum number. These results show the great importance of dipole-dipole interactions in gas of cold Rydberg atoms and are also relevant for the possible realization of a quantum computer from Rydberg atoms. In a third part we studied the potential of using a UCP as a source for electron accelerators. This was done based upon elaborate simulations performed with the General Particle Tracer Code. The main result of these simulations is that, based upon realistic parameters, the brightness, i.e., the figure of merit for charged particle sources defined as the current divided by the emittance squared, of such a source can be up to two orders of magnitude larger than the brightness of conventional sources, which is a major advance. The emittance is a measure for the the focusability of the bunch, and scales as p T, with T the electron temperature. The increase in brightness is mainly attributed to the low electron temperatures of approximately 10 K in an UCP, resulting in an initial emittance of 0.01 mm mrad, compared to 1 mm mrad in a conventional source. With experimentally proven techniques it is possible to extract 10 nC/s from the source, which is a competitive repetition rate. In the next part we describe the design, construction and testing of the first setup with which we want to build an electron accelerator based upon cold atoms. For this we first built a vapor-cell magneto-optical trap, in which we can trap over 109 rubidium atoms with a temperature of 200 ¹K, straight from a background vapor, we also designed an atomic beam that will be implemented in the near future and can provide an increased repetition rate. In the last chapter we show how Rydberg atoms excited from an MOT, evolve into an UCP spontaneously. We extracted a minimum of 1 pC from the plasma. As a main result, we were one of the ¯rst to show how the ions and electrons from a UCP can be imaged on a phosphor screen. From these images we can deduct an upper value for the electron and the ion temperature, we extract for both the electrons and the ions an upper temperature of 40 K, when we translate this temperature to an initial emittance, we obtain a value of 0.01 mm mrad, proving the potential of the electrons extracted from atoms excited out of an MOT as a low emittance source. Our measured current of 1 ¹A is not yet competitive and results from the small electric field, but is expected to increase dramatically in the near future. We will implement a 30 kV 10 ns risetime accelerator with which it should be possible to get a current of about 50 mA. Furthermore we will implement the proper diagnostics to measure the proper charge, current and emittance. In the very last chapter we speculate charged particle sources from cold atoms might advance the state of the art in achievable brightness for both electron and ion beams.
Ultracold neutral plasmas (UNPs) are highly correlated, charged-particle systems studied in standard atomic-molecular optical physics laboratories. Advances in ultracold physics techniques have led to the development of magneto-optical traps that permit the localization of micro-Kelvin temperature atomic samples (10⁸ - 10¹⁰ cm -³ that can be photoionized with a defined excess laser energy. Magneto-optical trapping of molecules proves difficult, and to date, lags in reaching conditions attainable for atoms. Molecules possess additional degrees of freedom where energy can flow during laser trapping and cooling. Alternatively, elastic collisions with inert gases offer another route to low temperatures. Super-sonic expansions of target molecules seeded in a noble gas can reduced the translational temperature to values as low as 0.1 K. Here I report on the formation of a molecular ultracold plasma in a molecular beam of nitric oxide seeded in an inert gas. Our method does not directly form a plasma, but instead, I create a Rydberg gas that spontaneously evolves to a plasma. I have verified the transition to a collective plasma state by applying pulsed-fields to the field-free flight region, and observe a signal that persists well beyond classical field amplitudes required to ionize individual Rydberg states. Electric field screening is a collective effect occurring over a characteristic distance, quantified by the Debye length, λ_D. Molecular ultracold plasmas described here exhibit Debye lengths smaller than the overall size, λ_D<<σ. Plasmas of this nature offer an exotic environment for which to study fundamental phenomena. While plasmas are generally thought of as hot, UNPs exist at low enough temperatures that electrostatic interaction energy between particles is greater than their kinetic energy. At this point, charged particles begin to find themselves in local potential energy minima. Over the entire sample this translates to long-range order, or correlation. It is of great interest for researchers studying UNPs to maximize particle correlation, which requires UNPs to remain very cold, and dense through their lifetime.
We have studied the development of gases of ultra-cold Rydberg atoms in Magneto Optic Traps, and discovered that such gases exhibit a range of density-dependent phenomena. In particular, we have found that at the densities of ∼ 10 7 cm -3 the Rydberg atoms spontaneously evolve into long-lived high angular momentum states. These states slowly decay due to microwave background radiation over tens of milliseconds. We have numerically simulated the effects leading to such behavior, and investigated numerically the l- and n-mixing collisions of Rydberg atoms with electrons in the range of 10 meV. These calculations show that the l- and n-mixing cross-sections follow the n 5 scaling law, expected from simple Stark map considerations, but also depend on the velocities of the colliding electrons, reflecting the adiabaticity of the atom-electron interactions. We have developed a novel technique of measuring the electric fields inside plasmas by using Rydberg excitation spectroscopy of embedded atoms, based on the disappearance of regions of zero oscillator strength in the presence of such fields. We have applied this technique to study the evolution of ultra-cold non-neutral plasmas excited from the MOT, and have found that it expands over 1 mus. We have simulated such an expansion by considering only Coulomb repulsion between the constituent ions, and have found that such a model is consistent with the experimental observations.
The recent development of laser cooling and trapping techniques has made possible the controlled realization of dense and cold atomic samples, thus opening the way for spectroscopic investigations in the low and ultra-low temperature regimes not accessible with conventional techniques. In our lab, an apparatus for laser cooling and Bose-Einstein condensation of rubidium atoms is in operation, producing a sample large enough to perform reliable investigations by absorption or emission techniques. Moving from previous photoionization experiments of laser-cooled atoms in a magneto-optical trap irradiated by cw-laser radiation [1] and stimulated by the interest grown recently on ultra-cold plasma [2], we have started a series of experiments aimed at investigating the interaction of intense laser pulses with the ultra-cold neutral sample, boty aboeand below the condensation temperature. Radiation from an excimer-pumped dye laser was used at a wavelength close to 594 nm, corresponding to the ionization threshold of rubidium for two photon non-resonant absorption from the ground state, with intensities in the tens of MW/cm^2 range and a pulse duration of 16 ns. Preliminary results, attained through the dynamical analysis (atom number, temperature) of the cold atom sample after the arrival of the laser pulse, suggest the occurrence of different processes, initiated by multi-photon ionization, involving collisions between cold electrons and ultra-cold neutrals, recombination at low temperatures and collective processes in the weak and cold plasma. Interpretation of those results demonstrates that this class of experiments, based on a variety of topics typical of atomic, plasma and statistical physics, can open the way for novel and intriguing investigations of laser-matter interaction. Further experimental developments will include duplication of the laser pulse frequency and one photon ionization of ground state rubidium laser-cooled atoms. [1] O. Maragò et al., Phys. Rev. A 57 R4110 (1998); E., Arimondo et al., Appl. Surf. Sci. 154-155 527 (2000); F. Fuso et al., Opt. Commun. 173 223 (2000). [2] T.C. Killian et al., Phys. Rev. Lett. 83 4776 (1999); S. Kulin et al., Phys. Rev. Lett. 85 318 (2000).
This Dissertation details the development of state-of-the-art hybrid atom-ion trapping architecture and technique towards increasing the quantum control of matter and the detection of chemical processes at cold temperatures. Experimental work discussed herein is performed primarily using the second-generation of the MOTion trap, a hybrid atom-ion trap consisting of a co-located magneto-optical trap (MOT) and a linear quadrupole trap (LQT), with which 40Ca and a variety of atomic and molecular ions (Yb+, Ba+, BaCl+, and others) can be co-trapped and cooled. Such a device enables the immersion of laser-cooled, ionic Coulomb crystals in diffuse gases of laser-cooled, highly-polarizable neutral atoms, allowing for studies in sympathetic cooling, cold atom-ion chemistry, and nonequilbrium collisional dynamics.In the first of these studies, I discuss our method of sympathetic cooling in which we use a laser-cooled gas of Ca atoms to sympathetically cool BaCl+ ions confined in the LQT. Because of the large polarizability of Ca, this method is found to be extremely efficient at cooling the notoriously difficult vibrational degree of freedom. This technique opens the door to research with general, ultracold, highly-localized molecular ions in the quantum ground-state which are not able to be produced using the more traditional methods of Doppler-cooling or photoassociation.In the study of cold chemistry in the hybrid trap, I characterize a fundamental reaction, known as charge exchange, at the single-particle level in which an electron is transferred from a neutral atom to an atomic ion during an inelastic collision. This low-temperature process is thought to be foundational to the chemistry leading to the formation of stars, planets and interstellar clouds in our universe. Herein I discuss how improved experimental architecture has enabled the determination of reaction rate constants for individual electronic states of the reactants.Our theoretical study of the collisional processes in hybrid atom-ion traps has led to the discovery of surprising nonequilibrium physics which exists in the hybrid trap environment. Herein I outline the collisional dynamics between neutral atoms and atomic ions in the presence of a time-dependent confining potential and describe how these dynamics manifest themselves in the emergence of nonequilbrium phenomena as indicated by bifurcations in steady-state energies of trapped ions. I show how the these dynamics can potentially limit the efficiency and overall viability of using neutral buffer gases to sympathetically cool the translational motion of trapped ions to ultracold temperatures.In addition to these experimental studies, the development of architecture is discussed with focus given to the implementation of a novel time-of-flight mass spectrometer (TOF-MS) for use with hybrid atom-ion traps as well as standard LQTs. This simple, modular device couples radially with the LQT and enables the straightforward mass analysis of trapped species without complications derived from typical experimental difficulties, engineering constraints, and financial concerns. With this robust device we precisely characterize chemical processes which can take place in these hybrid atom-ion traps. Additionally, I describe how this new architecture is used to perform the action spectroscopy of several previously unstudied molecular ions in order to further establish the field of cold molecular ion research and the widespread use of hybrid atom-ion traps.