Research

Plasma Etching of Superconducting Radiofrequency Cavities.
Superconductive materials and the generation of high frequency fields are in the foundation of the next generation of particle accelerators. High frequency fields are generated in the resonant cavities, made of superconductive materials in the bulk sheet or thin film format. Hence the name - superconductive radiofrequency (SRF) cavities. One of the attributes of the next generation of particle accelerators is the compactness, which invites novel cavity designs - more compact but more complex and asymmetric geometry, as opposed to current elliptical cavities with cylindrical symmetry. Thus, the processing of new cavities to provide surfaces with high residual resistance ratio is faced with new challenges that will certainly have to inspire new approaches. Dry etching or cleaning using a variety of plasma discharges are one of the answers to these challenges.
Plasma-based Nb surface treatment provides an excellent opportunity to eliminate surface imperfections, increase cavity quality factor, and push accelerating field distributions to higher levels. At the same time, the developed technology will be more environmentally friendly than the HF-based "wet etching" technology, and will reduce significantly the cavity preparation costs. In the proposed study, plasma etching of bulk Nb is performed on the surface of standard single-cell cavities with the goal of eliminating non-superconducting pollutants in the penetration depth region and the mechanically damaged surface layer.

Cleaning and Modification of Solid Surfaces using Gas Discharges
Plasma cleaning is a viable method of accelerator surface maintenance. While the main mechanism of surface cleaning by baking is thermal desorption, its disadvantage is in the enhanced thermal of diffusion into the bulk, which leads to deterioration of material performance. Plasma cleaning is probably the best way to overcome this disadvantage. The effect of electrical discharge has been recognized relatively early, but the exact match between discharge plasma and contaminated metal layer kinetics at their interface has not yet been understood in detail. It would primarily depend on the character, thickness, and molecular content of the surface contamination layers. With respect to the molecular content we will define at least three molecular groups: (a) hydrocarbons and other organic pollutants; (b) water and other hydrogen based contaminants; (c ) metal oxides. Plasma kinetics is adjusted by composition and regime of operation to address optimally one of these contaminants.

Oxygen Production from Martian Atmosphere
Research activity on in situ resource utilization during Mars exploration is presently focused on finding an effective method for oxygen production. There are many approaches to the extraction of oxygen from the Martian atmosphere, which is composed mainly of carbon dioxide. I am developing an effective method based on carbon dioxide dissociation in a radio frequency glow discharge. This concept does not involve modifying the Martian atmosphere, does not require dust filtration, and no transport of reactive material from Earth is needed. From the plasma kinetic point of view the problem is similar to maintaining an active gas mixture on carbon dioxide laser discharges. However, the requirements are opposite. In addition, oxygen extraction through a selective membrane has to be integrated into the process. Several different techniques for oxygen extraction are currently being developed in my laboratory, with the objective of proving the effectiveness of this method for in situ oxygen production on Mars.

Absolute Cross Sections for Electron Atom (Molecule) Collisions
Low energy electron scattering by atomic species serves as a crucial test of collision theory. My interest was focused on electrons with energies from the threshold of inelastic channels to approximately threefold value. This energy regime is especially important because of the complexity of the theoretical description. In order to understand the scattering process completely, full knowledge of the scattering amplitudes is required. To this end, most of the information can be obtained from measurements with polarized beams, except for the absolute differential cross section of the collision process. Thus, it is important to determine this quantity independently from theory. At the same time, absolute differential cross sections are used to determine momentum transfer and viscosity cross sections, which are needed for plasma modeling, but inaccessible to direct measurements. I developed a method for absolute measurements of electron scattering by gaseous species and studied elastic, inelastic, and ionization processes involving various atoms and molecules. In the case of condensable species, this is an experimentally challenging task, primarily because of the difficulty in determining the absolute number density of target atoms. I overcame this problem by developing a recoil atom technique for differential cross section measurements. In this mode of observation, after complex data analysis, absolute differential cross sections have been routinely determined in my laboratory. As a result, very extensive theoretical work has been done using our experimental results as benchmark data and the understanding of atomic collision processes has been greatly enhanced.

Electron collisions with laser excited atomic target
In this area of interest, the primary goal is to acquire experimental information concerning collisions of electrons with atoms, which have been prepared in a polarized excited state. Our experiments offer fundamental insight into collision dynamics in the low impact energy range, where the thresholds for inelastic channels are located. The low energy range is particularly interesting because it embodies all the possible complexities that can be present in a scattering process (elastic, excitation, or ionization), such as distortion, multiple scattering, correlations, post collision interactions, and exchange effects. Besides these fundamental reasons, the experimental work on electron scattering by excited atoms is important in providing data relevant to more applied fields where excited state atomic collisions play a significant role. Fundamental understanding of electron scattering by coherently prepared excited atoms provides the basic information for polarization plasma spectroscopy.

Electron - Impact Secondary Electron Emission from Solid and Liquid Targets and the Mechanisms of Multipacting in Electron Accelerators
Study of the secondary emission process, still believed to be one of the critical contributors to multipactor discharge phenomena in SRF cavities of linear accelerators, storage rings, and free electron lasers, has a more than a hundred years of history. When a (primary) electron from a beam collides with a surface of a solid, it is reflected or absorbed in the bulk thereby emitting additional electrons. All electrons emitted from the surface are called the secondary electrons. In spite a long history of research and maturity of understanding its phenomenology, there are problems that have remained completely unresolved and some of them are the phenomena related to multipacting. Secondary electron yield depends on the energy and the incident angle of the primary electron beam. It also depends on the material and the chemical, physical and geometric conditions on the surface. On their own, the secondary electrons have an energy distribution which is in the range between 0 and E_Pmax (maximum energy of primary electrons) and the angle distribution that has weak dependence on the incident angle.

Bright Atomic Beam and Atom Optics
The bright, monoenergetic atomic beam (BMAB) has been developed in my laboratory since its establishment at Old Dominion University. Design and construction of the beam involves many challenges. My approach to generating a BMAB has three phases of preparation. These phases include longitudinal velocity compression, transverse spatial and velocity compression (focusing and collimation) and atomic beam deflection. Longitudinal velocity compression is achieved by a Doppler cooling technique, using a Zeeman slower with a decreasing parabolic magnetic field to compensate for the Doppler shift. An optical atom funnel, a two dimensional version of a magneto optical trap, placed at the exit of the slower, is used to focus and collimate the atomic beam. The slowed atomic beam is deflected out of the counterpropagating laser cooling beam, by shining a traveling wave radiation force with a convergent circular wave front slightly detuned below the resonant frequency perpendicular to it. There are several advantages in using the tunable BMAB. A decrease in velocity and, correspondingly, an increase in the deBroglie wavelength can be accomplished with practically no decrease in the atomic intensity since the primary beam remains the same (no change in source temperature). Furthermore, the BMAB in the present scheme is outside of the laser-cooling field. Thus, no restrictions apply to surface interactions or near field interference effects, for instance. The prepared and fine tuned BMAB can be used for experimental studies of atomic collisions, and for experiments in the field of atom optics. The beam has potential applications to a wide range of technical problems related to atomic interactions with surfaces, for example. The BMAB can also be used for development of atomic optical elements. The first optical element to be developed is a magnetic field with lens properties, to focus or deflect the atomic beam. The field will be generated with a variable current hexapole magnet, serving as an atom beam zoom lens.

Wave properties of atomic beam
Some fundamental aspects of QM theory of atoms, such as their wave properties are of natural interest in a laboratory whose principal tools are atomic beams. I have been developing the basic knowledge necessary for developing experiments on wave-particle duality of atoms. My interest in the experiments on non-classical behavior of atoms in an atomic interferometer arises from the fact that this is one of the simplest tools of atom optics where the still lacking understanding of the relation between wave and particle properties can be developed. Transverse momentum distribution of atoms in an interferometer is a phenomenon where this relationship can be studied and tested. Experimental evidence of the transverse momentum distribution would support particle and wave interpretation of quantum interference.

Proton transfer in ion molecule collisions
My recent field of interest is the experimental study of proton transfer between molecular ions and neutral molecules. Interactions of ions and neutrals at energies higher than a few electron volts are usually investigated by beam experiments. Below a few electron volts of ion kinetic energy, i.e. thermal energies, the energy region of interest, it becomes extremely difficult to produce monoenergetic ion beams. Since the data on collision, processes at these low energies are very important for many astrophysical and technical applications, a wide variety of swarm experiments have been developed. One of these experiments is being developed in my laboratory, with the purpose of studying proton transfer between polyatomic molecular ions and neutral molecules. The simplest example of this kind of reaction is the proton transfer between H₃O⁺ and a neutral molecule having proton affinity higher than water. The experiment combines a conventional Selected Ion Flow Drift Tube (SIFDT) with a polyatomic molecular ion source. The proton transfer from primary ions to the investigated molecules is affected in a reaction chamber. Collisional probabilities of the proton transfer reaction will be studied using SIFDT and appropriate ion detection techniques. The advantage of this method is in the fact that the principal constituents of ambient air have lower proton affinity than the molecules involved in the reaction. Proton transfer reactions involving volatile trace constituents of ambient air, such as polyaromatic hydrocarbons, have important application in air pollution monitoring. In that line, our studies will lead to new, more sensitive, real time techniques for ambient measurements of chemically significant trace gases. As a spin off of this research activity, studies of human breath related to lung cancer and environmental tobacco smoke are in preparation. The study of oxidation processes of biogenic and volatile organic compounds and of some selected isotopic effects in plasma, reactions are already proposed.

Effects of Weakly Ionized Gas on Propulsion and Aerodynamics
Weakly ionized molecular gases have drawn increasing attention in recent years, partly because of the complexity of phenomena observed in these media and partly because of their potential for application in aerodynamics. Present understanding of propagation, interaction and dispersion of high amplitude sound waves, shock waves and solitary waves through nonequilibrium, weakly ionized, molecular gases is in a rather primitive stage. Interpretation of experimental results is lacking some crucial details. It is often impossible to predict correctly the outcome of the proposed experiments, because the detailed knowledge of the underlying physical mechanisms is still missing. Discussion of the thermal or plasma kinetic nature of the observed effects is still going on and remains unresolved. However, empirical data are being collected, dispersion of shock waves in weakly ionized gases has been verified, and the effects such as drag reduction have been estimated. The field currently presents an attractive challenge and I am trying to use my expertise in atomic and molecular collision physics to contribute to the understanding of the physical mechanisms of the observed aerodynamic properties of molecular gases by interpreting the existing experimental results and by designing future experiments.

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