The construction of a field emission ion source and its adaptation to a mass spectrometer is described. The problems involved and the advantages of such an ion source are discussed. / Thesis / Master of Science (MS)

A laser ion source was investigated for a laser produced spatially bound plasma and the production of cluster ions. The design of a unique geometry for the target was fabricated to have a hollow cylindrical structure where the plasma is ignited inside the narrow volume. The study clarified the difference in the plasma dynamics of the laser ion source operation using the hollow cylinder target as the plasma was allowed to interact with the target surface. The proposed scheme, aiming to produce a low plasma temperature condition, was observed to the increase the probability for agglomeration and generated cluster ions. / 博士(工学) / Doctor of Philosophy in Engineering / 同志社大学 / Doshisha University

During the last decade, the Denton Research Group has made significant advancements in the field of real time direct vapor detection of low volatile explosives under ambient conditions. An ion source plays a crucial role in the sensitive detection of traces of compounds in gas phase by ion mobility spectrometry, but, all the current ionization techniques have significant drawbacks and do not fully satisfy all needs. To overcome the limitations associated with either hazards from a radiogenic ion source or poor reliability from the current non-radiogenic ion sources, the author of this dissertation has undertaken the development of an entirely new ion source based on dielectric barrier discharge technologies. This dissertation describes the development, characterization, and applications of novel dielectric barrier discharge (DBD) ion sources for ion mobility spectrometry. The sources under investigation are non-radiogenic, highly reliable, and provide a high yield of ions. The difficulty of extracting ion current from a traditional dielectric barrier discharge was solved by using an array of tiny discharges formed at the crossing points of two crossed sets of glass coated wires. The relationship of the excitation voltage, frequency, and extraction field for AC excitation on the extracted ion current were studied. The dielectric barrier discharge ion source were also excited in pulse mode by fast-rising and fast-falling high voltage pulses. A high voltage switch using serial MOSFETs was specially designed for driving the dielectric barrier discharge ion source in pulse mode. Application of this dielectric barrier discharge ion source to ion mobility spectrometry was demonstrated with the measurement of limit of detection and direct vapor detection of explosives.

It is the purpose of this paper to present a detailed account of the design and construction of this positive-ion source and associated equipment.

Understanding and improving ionospheric models is important for both military and civilian purposes. This understanding improves prediction of radio propagation used for communication and GPS navigation. Various space-borne instruments, such as retarding potential analyzers (RPAs) and ion traps are routinely flown in low earth orbit (LEO) to provide data for seeding/improve ionospheric models. This thesis describes and characterizes a new ion source that can be used to test and calibrate these space-borne instruments inside a laboratory vacuum chamber. Hot filaments are used to thermionically emit electrons inside the source. These electrons collisionally ionize neutral particles inside the source. Guided by ion-optics simulations, the ion and the electron trajectories inside the source are controlled to provide the required ion beams. A detailed description of the control electronics and the embedded controller for electron emission is discussed within. Using the custom made electronics, the source is able to provide an ion beam with current densities and mean energy comparable to the conditions in LEO. / Master of Science

Prior to launch, space craft instruments need to be tested in a relevant environment to prove operational functionality. Thus, we have developed an ion source to stimulate thermal plasma instruments in a vacuum chamber. This dissertation presents the mechanical design of the source, simulations of the potentials and charged particle trajectories in and around the source, and vacuum chamber measurements of the emitted ion beam. Once the ion source is understood, it is successfully used to test a typical ion instrument. Further aspects of the ion source, efficiency, thermionic filament emission, and collision frequencies are also discussed. / Doctor of Philosophy / Satellites have become a critical resource for business and governments. The study of the high edge of the earth’s atmosphere, where satellites operate, is difficult because the physics of this region is so complex. More measurements of the upper atmosphere are needed to understand exactly how it works and improve the computer models simulating the atmosphere. The instruments used to measure this region need to be validated before they are launched. This dissertation describes the design and testing of an ion source device that can produce charged particles in a very low pressure environment, such as a vacuum chamber, to create conditions similar to those an instrument would encounter in orbit. Computer simulations and physical tests of the source are presented, compared, and found to match. Finally, an actual satellite instrument is successfully validated with the source.

The development of the negative ion source constitutes a crucial step in the construction of the neutral beam injector of ITER. To fulfil the ITER requirements in terms of heating and current drive, the negative ion source should deliver 40 A of D-. The achievement of such a source is challenging from technical and scientific points, and it requires a deeper understanding of the underlying physics. The present knowledge of the ion extraction mechanism from the negative ion source is limited due to the complexity of the problem that involves the comprehension of the behaviour of magnetized plasma sheaths when negative ions and electrons are pulled out from the plasma. Moreover, due to the asymmetry induced by the crossed magnetic configuration used to filter the electrons, any realistic study of this problem must consider the three spatial dimensions. To address this problem in a realistic way, a 3D Particles-in-Cell electrostatic code specifically designed for this system was developed. The code uses Cartesian coordinate system and it can deal with complex boundary geometry as it is the case of the extraction apertures. The complex magnetic field that is applied to deflect electrons is also taken into account. This code, called ONIX, was used to investigate the plasma properties and the transport of negative ions and electrons close to a source extraction aperture. Results on the formation of the plasma meniscus and the screening of the extraction field by the plasma are presented here, as well as negative ions trajectories. Negative ion extraction efficiency from volume and surfaces was investigated showing the capital importance of the surface negative ion production.

<p>The primary goals of this study were to demonstrate and fully characterize a microscale ionization source (i.e. micro-ion source) and to determine the validity of impact ionization theory for microscale devices and pressures up to 100 mTorr. The field emission properties of carbon nanotubes (CNTs) along with Micro-Electro-Mechanical Systems (MEMS) design processes were used to achieve these goals. Microwave Plasma-enhanced CVD was used to grow vertically aligned Multi-Walled Carbon Nanotubes (MWNTs) on the microscale devices. A 4-dimensional parametric study focusing on CNT growth parameters confirmed that Fe catalyst thickness had a strong effect on MWNT diameter. The MWNT growth rate was seen to be a strong function of the methane-to-ammonia gas ratio during MWNT growth. A high methane-to-ammonia gas ratio was selected for MWNT growth on the MEMS devices in order to minimize growth time and ensure that the thermal budget of those devices was met. </p><p>A CNT-enabled microtriode device was fabricated and characterized. A new aspect of this device was the inclusion of a 10 micron-thick silicon dioxide electrical isolation layer. This thick oxide layer enabled anode current saturation and performance improvements such as an increase in dc amplification factor from 27 to 600. The same 3-panel device was also used as an ionization source. Ion currents were measured in the 3-panel micro-ion source for helium, argon, nitrogen and xenon in the 0.1 to 100 mTorr pressure range. A linear increase in ion current was observed for an increase in pressure. However, simulations indicated that the 3-panel design could be modified to improve performance as well as better understand device behavior. Thus, simulations and literature reports on electron impact ionization sources were used to design a new 4-panel micro-ion source. The 4-panel micro-ion source showed an approximate 10-fold performance improvement compared to the 3-panel ion source device. The improvement was attributed to the increased electron current and improved ion collection efficiency of the 4-panel device. Further, the same device was also operated in a 3-panel mode and showed superior performance compared to the original 3-panel device, mainly because of increased ion collection efficiency. </p><p>The effect of voltages applied to the different electrodes in the 4-panel micro-ion source on ion source performance was studied to better understand device behavior. The validity of the ion current equation (which was developed for macroscale ion sources operating at low pressures) in the 4-panel micro-ion source was studied. Experimental ion currents were measured for helium, argon and xenon in the 3 to 100 mTorr pressure range. For comparison, theoretical ion currents were calculated using the ion current equation for the 4-panel micro-ion source utilizing values calculated from SIMION simulations and measured electron currents. The measured ion current values in the 3 to 20 mTorr pressure range followed the calculated ion currents quite closely. A significant deviation was observed in the 20-100 mTorr pressure range. The experimental ion current values were used to develop a corrected empirical model for the 4-panel micro-ion source in this high pressure range (i.e., 3 to 100 mTorr). The role of secondary electrons and electron path lengths at higher pressures is discussed.</p> / Dissertation

The focus of this Ph.D. thesis is the theoretical, computational, and experimental analysis of electrohydrodynamics and ionization in the Array of Micromachined UltraSonic Electrospray (AMUSE) ion source. The AMUSE ion source, for mass spectrometry (MS), is a mechanically-driven, droplet-based ion source that can independently control charge separation and droplet formation, thereby conceptually differing from electrospray ionization (ESI). This aspect allows for low voltage soft ionization of a variety of analytes and flexibility in the choice of solvents, providing a multifunctional interface between liquid chromatography and mass spectrometry for bioanalysis. AMUSE is a versatile device that operates in an array format, enabling a wide range of configurations, including high-throughput and multiplexed modes of operation. This thesis establishes an in-depth understanding of the fundamental physics of analyte charging and electrokinetic charge separation in order to enhance droplet charging and ionization efficiency. A detailed electrohydrodynamic (EHD) computational model of charge transport during the droplet formation cycle in the AMUSE ion source is developed, coupling fluid dynamics, pressure and electric fields, and charge transport in multiphase flow. The developed EHD model presents a powerful tool for optimal design and operation of the AMUSE ion source, providing insight into the microscopic details of physicochemical phenomena, on the microsecond time scale. Analyte charging and electrohydrodynamics in AMUSE are characterized using dynamic charge generation measurements and high-spatial-resolution stroboscopic visualization of ejection phenomena. Specific regimes of charge transport, which control the final droplet charging, have been identified through experimental characterization and simulations. A scale analysis of the ejection phenomena provides a parametric regime map for AMUSE ejection modes in the presence of an external electric field. This analysis identifies the transition between inertia-dominated (mechanical) and electrically-dominated (electrospraying) ejection, where inertial and electric forces are comparable, producing coupled electromechanical atomization. The understanding of analyte charging and charge separation developed through complimentary theoretical and experimental investigations is utilized to improve signal abundance, sensitivity, and stability of the AMUSE-MS response. Finally, these tools and fundamental understanding provide a sound groundwork for the optimization of the AMUSE ion source and future MS investigations.

The development of an accurate and precise geochronology is imperative to understanding archives containing information about Earth’s past. Unable to date all intervals of an archive, researchers use methods of interpolation to approximate age between dates. Sections of the radiocarbon calibration curve can induce larger chronological uncertainty independent of instrumental precision, meaning even a precise date may carry inflated error in its calibration to a calendar age. Methods of interpolation range from step-wise linear regression to, most recently, Bayesian statistical models. These employ prior knowledge of accumulation rate to provide a more informed interpolation between neighboring dates. This study uses a Bayesian statistical accumulation model to inform non-sequential dating of a sediment core using a high-throughput gas-accepting accelerator mass spectrometer. Chronological uncertainty was iteratively improved but approached an asymptote due to a blend of calibration uncertainty, instrument error and sampling frequency. This novel method resulted in a superior chronology when compared to a traditional sediment core chronology with fewer, but more precise, dates from the same location. The high-resolution chronology was constructed for a gravity core from the Pigmy Basin with an overall 95% confidence age range of 360 years, unmatched by the previously established chronology of 460 years. This research reveals that a larger number of low-precision dates requires less interpolation, resulting in a more robust chronology than one based on fewer high-precision measurements necessitating a higher degree of age interpolation.