Department of Physics

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This digital collection includes theses, dissertations, and faculty publications (specifically open educational resource videos) from the Department of Physics.

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Open Access
Ion extraction from a plasma
(Colorado State University. Libraries, 1980) Aston, Graeme, author; Kaufman, Harold R., advisor; Wilbur, Paul J., advisor; Fairbank, William M., Jr., committee member; Sites, James R., committee member
An experimental investigation of the physical processes governing ion extraction from a plasma is presented. The screen hole plasma sheath of a multi-aperture ion accelerator system is defined by equipotential plots for a variety of accelerator system geometries and operating conditions. A sheath thickness of at least fifteen Debye lengths is shown to be typical. The electron density variation within the sheath satisfies a Maxwell-Boltzmann density distribution at an effective electron temperature dependent on the discharge plasma primary-to-Maxwellian electron density ratio. Plasma ion flow up to and through the sheath is predominately one dimensional and the ions enter the sheath with a modified Bohm velocity. Low values of the screen grid thickness to screen hole diameter ratio give good ion focusing and high extracted ion currents because of the effect of screen webbing on ion focusing.
Open Access
Soft x-ray laser interferometry of dense plasmas
(Colorado State University. Libraries, 2007) Filevich, Jorge, author; Rocca, Jorge J. G., advisor
This Dissertation presents the results of the study of plasmas using soft x-ray laser interferometry. The use of soft x-ray wavelengths (14.7 nm and 46.9 nm) permits probing plasmas that are denser and that have steeper density gradients than those that can be probed using optical interferometry. The use of diffraction gratings as beam splitters permitted the construction of a novel interferometer design that is robust, stable and with high throughput. The measurements conducted include the first demonstration of soft x-ray laser interferometry with picosecond resolution. The first set of results presented herein are the observation of an unexpected on-axis density depression in narrow-focus laser-created plasmas. It is caused by plasma-radiation-induced ablation of target material outside of the region irradiated by the plasma-heating laser. This colder material expands at a slower velocity than the hotter central region, resulting in the observed on-axis density depression. The effect is shown to be a general phenomenon, present in many narrow focus plasmas under different irradiation conditions. The second set of results unveiled the significant contribution of bound electrons to the index of refraction of multiply ionized plasmas. Experiments that mapped the density of aluminum plasmas using a λ=14.7 nm laser beam showed interference fringes that bent in the direction opposite to that expected, contradicting the widely accepted assumption that the index of refraction for multiply ionized plasmas at soft x-ray wavelengths only depends on the free electrons. The contribution of bound electrons to the index of refraction is shown to be significant, and to affect a broad range of wavelengths due to numerous bound-bound and bound-free transitions present in the plasma. Moreover, the contribution of bound electrons to the index of refraction was shown to be important in several materials at different probe soft x-ray wavelengths, in particular for tin, silver and carbon plasmas probed at λ=46.9 nm. This fundamental result affects not only the interpretation of soft x-ray interferograms for plasma density measurements, but also the propagation of soft x-ray light in plasmas in general.
Open Access
Excited electronic state decomposition mechanisms and dynamics of nitramine energetic materials and model systems
(Colorado State University. Libraries, 2007) Greenfield, Margo, author; Guo, Yuanqing, advisor; Bernstein, Elliot R., advisor
Energetic materials play an important role in aeronautics, the weapon industry, and the propellant industry due to their broad applications as explosives and fuels. RDX (1,3,5-trinitrohexahydro-s-triazine), HMX (octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine), and CL- 20 (2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane)-j are compounds which contain high energy density (J/cm3) or (J/g). Although RDX and HMX have been studied extensively over the past several decades, a complete understanding of their decomposition mechanisms and dynamics is unknown. This work describes the novel approach taken to assist in the overall understanding of the decomposition of these energetic materials, namely their gas phase single molecule excited state decomposition. Excited electronic states can be generated by shock and compression and therefore play an important role in the initiation/decomposition of RDX, HMX, and CL-20. Energy (ns lasers) and time resolved (fs lasers) UV-photodissociation experiments have been performed to elucidate the mechanisms and dynamics of gas phase energetic material decomposition from excited electronic states. Time of flight mass spectroscopy (TOFMS), laser induced fluorescence (LIF), and pump-probe experiments performed on three energetic materials, as well as five model systems, illustrate the unique behavior of energetic materials. TOFMS UV photodissociation (ns) experiments of gas phase RDX, HMX, and CL-20 generate the NO molecule as the initial decomposition product. Four different vibronic transitions of the initial decomposition product, the NO molecule, are observed: A2Σ(υ'=0)<—X2Π(υ"=0,l,2,3). Simulations of the rovibronic intensities for the A<— Xtransitions demonstrate that NO dissociated from RDX, HMX, and CL-20 is rotationally cold (~ 20 K) and vibrationally hot (~ 1800 K). Conversely, experiments on the five model systems (nitromethane, dimethylnitramine, nitropyrrolidine, nitropiperidine and dinitropiperazine) produce rotationally hot and vibrationally cold NO spectra. LIF experiments are performed to rule out the possible decomposition product OH, generated along with NO, perhaps from the suggested HONO elimination mechanism. The OH radical is not observed in the fluorescence experiments, indicatingthe HONO decomposition intermediate is not an important pathway for the excited electronic state decomposition of cyclic nitramines. The NO molecule is also employed to measure the dynamics of the excited statedecomposition. A 226 nm, 180 fs light pulse is utilized to photodissociate the gas phase systems. Stable ion states of DMNA and nitropyrrolidine are observed while the energetic materials and remaining model systems present the NO molecule as the only observed product. Pump-probe transients of the resonant A<—X (0-0) transition of the NO molecule show a constant signal indicating these materials decompose faster than the time duration of the 226 nm laser light. Comparison of NO from the three energetic materials to NO from NO2 gas generated by a 180 fs light pulse at 226 nm indicates that NO2 is not an intermediate product of the excited electronic state photodissociation of RDX, HMX, or CL-20. Two possible excited state decomposition mechanisms are suggested for the three energetic materials. The first mechanism involves a dissociative excited electronic statein which the nitramine moieties (CNNO2) in the electronically excited energetic material isomerize (CNONO) and further dissociate. In the second possible decomposition mechanism the electronically excited molecules undergo internal conversion to very highly excited (~5 eV of vibrational energy) vibrational states of their ground electronic state. Once in the ground state, isomerization of the nitramine moieties occurs and thematerial further decomposes. Calculational results together with the experimental results indicate the energetic materials decompose according to the second mechanism, relaxation to the ground state, while the model systems follow the excited electronic state decomposition pathway. An additional path in which the -NO2 moiety loses an O atom, becomes linear with the CN attachment, and then NO is released, is also consistent with experimental observations but is, as yet, not supported by calculations. The keys to generating better cyclic nitramine energetic materials would then beto enhance the propensity to form Si - So conical intersections, improve Si - So Franck-Condon factors for internal conversion near the Si zero point level, and to enhance the So density of vibronic states at high So vibrational energy. Additionally, one would like to generate NO with less internal vibrational excitation, so altering the NONO vibrational excitation in the dissociation process could be important. These ideas would suggest that more flexible cyclic nitramines, with increased internal degrees of freedom, might be useful to explore for new energetic systems. Perhaps larger ring structures along the lines of CL-20 might be useful compounds to explore.
Open Access
Oblique pumping, resonance saturation, and spin wave instability processes in thin Permalloy films
(Colorado State University. Libraries, 2008) Olson, Heidi M., author; Patton, Carl E., advisor
The study of nonlinear dynamics in metal films is of increasing importance as advancements are made in magnetic recording. In this dissertation, these interactions are examined by the study of first order spin wave instability (SWI) processes that occur for external static magnetic fields well below ferromagnetic resonance (FMR), and second order SWI processes that occur for static fields over the full FMR field range. This work is concerned specifically with the study of the high power resonance saturation and oblique pumping responses in thin Permalloy films, the microwave threshold amplitudes at which the instabilities occur, and the theoretical analysis of the relevant SWI processes. To greatly increase measurement accuracy and reduce measurement time, the high power FMR system has been modified and new calibration techniques implemented. The modifications to the system allow for fully automated and calibrated microwave threshold amplitude vs. static field measurements, termed butterfly curves. Resonance saturation butterfly curves have been measured for an in-plane field configuration for 35 - 123 nm thin Permalloy films. The butterfly curves show a jump on the low field side associated with a low field shift of the FMR profile and a foldover like asymmetry development. Apart from the jump, the second order Suhl SWI theory, suitably modified for thin films, provides good fits to the butterfly curve data through the use of constant spin wave relaxation rates that are on the same order as expected for intrinsic magnon-electron scattering processes. The FMR in-plane precession cone angles at threshold are small. Oblique pumping butterfly curves have been measured at different in-plane field configurations for 104 and 123 nm thin Permalloy films. The butterfly curves show thickness dependent high field cutoffs that agree with the field points at which the bottom of the spin wave band moves above one half the pump frequency. A combination of parallel and perpendicular first order SWI theory, suitably modified for thin films, shows good fits to the data except at low fields where the thin film approximation is not applicable. The damping trial functions used for the fits correspond to magnon-electron and three-magnon scattering processes.
Open Access
Effects of contact-based non-uniformities in cadmium sulfide/cadmium telluride thin-film solar cells
(Colorado State University. Libraries, 2008) Davies, Alan R., author; Sites, James R., advisor
To strongly contribute to the near-term electricity supply, CdTe-based photovoltaic devices must continue to improve in performance under the constraint of simple and cost efficient fabrication methods. This dissertation focuses on characterization and modeling of devices with non-uniform performance induced by the cell contacts. Devices were obtained from a commercially viable pilot-scale fabrication system at Colorado State University. Current versus voltage (J-V), quantum efficiency (QE) and laser-beam-induced current (LBIC) were the main characterization techniques applied in this work. The p-type CdTe semiconductor has a large work-function and thus tends to form a Schottky barrier when the back-electrode is formed. A common strategy of mitigating the performance-limiting contact barrier is to prepare the CdTe surface with a chemical etch, and include Cu to reduce the effective barrier. Non-uniformity of the etch or Cu inclusion, or insufficient application of Cu can result in a non-uniform contact, with regions of high- and low-energy Schottky barriers participating in the cell performance. Barrier non-uniformities in devices with little or no Cu were identified with the LBIC measurement and a model for their influence was developed and tested using PSpice circuit modeling software. Because of their superstrate configuration, CdTe cells feature front contacts made from transparent-conducting oxides (TCOs). Fluorine-doped tin oxide (F:SnO2) is a common choice because of its availability and acceptable optical and electrical properties. When the n-CdS layer of the CdS/CdTe structure is thinned to encourage greater current generation, non-uniformities of the solar cell junction arise, as CdTe comes into sporadic contact with the TCO layer. Device simulations suggest that the SnO2/CdTe junction is weaker than CdS/CdTe because of a large conduction-band offset induced by the differing electron affinities in the heterojunction. LBIC was used to verify increasing non-uniformity in devices with thin CdS and whole-cell performance followed the trends predicted by simulations. An empirical relationship between CdS thickness and relative influence the weaker junction was developed. The practical limit of CdS thickness was determined to be about 120 nm for CSU devices.
Open Access
Mesospheric momentum flux studies over Fort Collins CO (41N, 105W)
(Colorado State University. Libraries, 2009) Acott, Phillip Edward, author; She, Chiao Yao, advisor; Krueger, David A., advisor
System upgrades to the Colorado State University Sodium Lidar have enabled over 300 hours of night-time gravity wave momentum flux measurements with concurrent 24-hour measurements of the mean and tidal wind and temperature fields of the mesosphere and lower thermosphere (MLT) region of the atmosphere above Fort Collins, CO. Results include the vertical profile of nighttime zonal momentum flux divided by density (MF/ρ), as well as nighttime wind and temperature variances; the results also provide some insight into the accompanying gravity wave-tidal interactions.
Open Access
The cosmic ray energy spectrum from 1-10 EXA electron volts measured by the Pierre Auger Observatory
(Colorado State University. Libraries, 2009) Knapik, Robert, author; Harton, John L., advisor
The observed decrease in flux of cosmic rays as the energy increases can be described by power law with an almost constant spectral index for 12 decades of energy. Observing spectral index changes are used to constrain models for the sources of cosmic rays. The Pierre Auger Observatory was built to study the highest energy cosmic rays and combines two complementary techniques, a fluorescence detector and a surface detector. The surface detector is 100% efficient for energies above 3 EeV allowing for a flux measurement with low systematic uncertainties. This thesis describes the techniques developed to measure the flux of cosmic rays below 3 EeV while maintaining low uncertainties. The resulting energy spectrum confirms the previously measured change in spectral index observed by other experiments. Systematic differences in the measured energy spectra between experiments exist. Possible reasons for these differences and the astrophysical implications are discussed.
Open Access
Development of a very compact high repetition rate soft x-ray laser
(Colorado State University. Libraries, 2010) Furch, Federico Juan Antonio, author; Rocca, Jorge J., advisor; Marconi, Mario, advisor
Over the last 25 years, the field of soft x-ray lasers has evolved from facility size devices delivering a few shots per day, to table-top lasers operating at several shots per second. In these lasers the gain medium is a highly ionized, hot and dense plasma created by a sequence of short, high energy pulses from an optical laser. Current table-top soft x-ray lasers have enabled numerous applications such as nano-scale imaging, nano-fabrication and dense plasma diagnosis among others. However these lasers are still limited in repetition rate, and therefore average power, owing to thermal effects originated in the flash lamp pumped amplifiers of the optical driver laser. Direct diode-pumping of the driver laser opens the possibility of developing more compact, higher repetition rate optical laser systems to pump soft x-ray lasers. Directly pumping small quantum defect materials such as Yb:YAG with a narrow bandwidth source of the optimum wavelength allows to significantly increase the efficiency and then reduce the thermal load in the gain materials. In addition, cryogenic cooling of the laser materials significantly improves their thermal performance. This approach will allow for soft x-ray laser operation at much higher repetition rates. In this work I present the results of the demonstration of an all diode-pumped soft x-ray laser that constitutes the first of a new generation of more compact, higher repetition rate soft x-ray lasers in the spectral region between 10 and 20 nm. To pump these lasers we developed an all diode-pumped chirped pulse amplification laser system based on cryogenically cooled Yb:YAG. This optical laser generates pulses of 1 J of energy in 8.5 ps pulses at 10 Hz, the highest energy per pulse for sub-10 ps pulses from a diode-pumped system at the present time. This soft x-ray laser has the potential to operate at unsurpassed repetition rates in a reduced footprint.
Open Access
Electron-reflector strategy for CdTe thin-film solar cells
(Colorado State University. Libraries, 2010) Hsiao, Kuo-Jui, author; Sites, James R., advisor; Sampath, W. S. (Walajabad S.), committee member; Gelfand, Martin Paul, committee member; Leisure, Robert Glenn, 1938-, committee member
The CdTe thin-film solar cell has a large absorption coefficient and high theoretical efficiency. Moreover, large-area photovoltaic panels can be economically fabricated. These features potentially make the CdTe thin-film solar cell the leading alternative energy source. However, the record CdTe efficiency (16.5%) is much less than its theoretical maximum efficiency (29%), primarily because the open-circuit voltage (0.845 V) is well below what is expected for its band gap (1.5 eV). The incorporation of an electron reflector is a strategy to improve the open-circuit voltage of solar cells, and thus a strong possibility to improve the efficiency of CdTe thin-film solar cells. An electron reflector is a conduction-band energy barrier at the back surface of the solar cell, which can reduce the recombination due to the electron flow to the back surface. Different methods to create an electron reflector are explained in the thesis: (1) expanded band gap, either an expanded-band-gap layer or a bulk-band-gap reduction, and (2) alteration to the band bending through a reversed back barrier or a heavily-doped back surface. Investigation shows that the expanded-band-gap layer is the most efficient and practical mechanism for an electron reflector, and the combination of any two mechanisms does not yield additional improvement. To have the optimal effect from the electron-reflector strategy, reasonable CdTe lifetime (1 ns or above) and full depletion of the CdTe layer are required to ensure high carrier collection. Furthermore, a good-quality reflector interface between the p-type CdTe layer and the electron-reflector layer is essential. Preliminary experimental evidence has shown that CdTe cells with a ZnTe back layer do have a slightly higher open-circuit voltage. An electron reflector should be particularly beneficial for thin (less than 2 μm) CdTe cells which have a fully-depleted CdTe absorber layer. Thin CdTe cells can also benefit from the optical reflection at the back surface. To investigate the possibility of still higher efficiency, both electron and optical reflection were numerically applied to the CdTe record-cell baseline model. However, there is little improvement for CdTe thicknesses greater than 2 μm. To have the optimal effect from combined electron and optical reflection, cells approximately one micron thick are required. Even without the improvement to the current quality of CdTe, cell efficiency above 19% should be achievable with a 0.2-eV electron reflector. Moreover, efficiency above 20% should be possible if one can also achieve large optical back reflection. At the same time, competitive CdTe cell performance at a thickness as thin as 0.4 um should be possible. This thesis gives a comprehensive numerical investigation of the electron-reflector strategy for CdTe thin-film solar cells.
Open Access
Simultaneous trapping of 85Rb & 87Rb in a far off resonant trap
(Colorado State University. Libraries, 2010) Gorges, Anthony R., author; Roberts, Jacob Lyman, advisor; Leisure, Robert Glenn, 1938-, committee member; Eykholt, Richard Eric, 1956-, committee member; Marconi, Mario C., committee member
The experiments described in this thesis were focused on the physics of simultaneous trapping of 85Rb and 87Rb into a Far Off Resonant Trap (FORT), with a view towards the implementation of a non-evaporative cooling scheme. Atoms were first trapped in a Magneto Optical Trap (MOT) and from there loaded into the FORT. We investigated the effects of loading the FORT from a MOT vs. an optical molasses; observing that the molasses significantly improved the trapped atom number. The ultimate number of atoms trapped is determined by a balance between efficient laser cooling into the FORT and light-assisted collisional losses from the FORT. We have studied and measured the loss rates associated with light-assisted collisions for our FORT, measuring both heteronuclear and homonuclear collisions. It was discovered that induced long range dipole-dipole interactions between 85Rb and 87Rb have a significant impact on FORT loading. This interaction interferes with the loading into the trap and thus limits the number of atoms which can be trapped in the FORT under simultaneous load conditions. Despite this limitation, all required experimental parameters for our future measurements have been met. In addition to these FORT studies, we have found a technique which can successfully mitigate the effects of reabsorption in optically thick clouds, which is a limitation to the ultimate temperature an atom cloud will reach during light-based cooling. Planned future measurements for this project include the creation of a variable aspect ratio FORT; along with investigating collision assisted Zeeman cooling.
Open Access
The Faraday filter-based spectrometer: an instrument to study sodium nightglow and associated sodium and oxygen chemistry in the mesopause region
(Colorado State University. Libraries, 2010) Harrell, Sean David, author; She, Chiao-Yao, advisor; Krueger, David A., advisor; Roberts, Jacob Lyman, committee member; Reising, Steven C., committee member
The newly developed Faraday Filter-Based Spectrometer (FFBS) makes possible spectroscopic study of the sodium nightglow in the mesopause region (80-110 km) of the atmosphere. This dissertation details the theory, design, and initial results of this instrument. The ratio of various combinations of NaD2 and NaD1 emission intensities can provide information on the oxygen and sodium chemistry in the mesosphere and lower thermosphere (MLT) region. Early understanding of the production of sodium nightglow utilized the series of chemical reactions known as the Chapman Mechanism. This mechanism involves both sodium and various oxygen species to produce excited states of sodium, which then relax to the ground state and emit light. The emitted light is centered at two wavelengths: D2 (589.158 nm) and D1 (589.756 nm). If the excited states are populated according to the statistical weights of their spin-orbit coupling the RD= D2/D1 intensity ratio should be 2; however there is no a priori reason that the spin-orbit states should be populated statistically in the Chapman mechanism. While early measurement of RD yields a value of 1.98±0.1, more recent measurements show a variation from 1.3 to 1.8; it peaks at the equinoxes and reaches minimum at the solstices. A possible explanation for this variation utilizes a modification to the Chapman Mechanism, which relates the RD value to variations in the atomic oxygen to molecular oxygen, [O]/[O2], concentration ratio through two different chemical pathways for sodium nightglow production. The FFBS is designed to measure RD, the fractional contribution of the two chemical pathways of the modified Chapman Mechanism, and other parameters which are directly proportional to [O]/[O2]. These parameters will help to test the validity of the modified Chapman mechanism. The delineation of the two chemical pathways requires an instrument with a spectral resolution of 0.0002 nm, something that is not possible with traditional spectroscopic instruments. The solution presented here utilizes two ultra narrowband sodium vapor Faraday filters. These utilize the Faraday rotation of light due to the Zeeman splitting of energy levels of sodium atoms in a vapor in an axial magnetic field between crossed polarizers to create an optical filter near both the D1 and D2 resonances with a full width, half maximum bandwidth of approximately 4 GHz (0.004 nm). This leads to a resolution that is good enough to distinguish the two different sodium nightglow spectral linewidths produced by the two pathways of the modified Chapman Mechanism which differ by 1.7 GHz. As a result, the FFBS is able to determine the fractional contribution from each pathway, as well as RD. Therefore the FFBS provides a new method of investigating [O]/[O2] in the mesopause region remotely from the ground. Data from this instrument supports the previous conclusion of a varying RD, with 2009 autumnal equinox measurements averaging around 1.68 and 2010 vernal equinox measurements averaging around 1.52, qualitatively in agreement with previous results by Slanger et al. (2005). In addition to RD, the first known measurements of parameters specific to the modified Chapman mechanism are presented and discussed. The dissertation concludes with a discussion of future work needed to convert the FFBS measurements into [O]/[O2] values, as well as future plans for the FFBS instrument.
Open Access
Vortex phases in type-I superconductors
(Colorado State University. Libraries, 2010) Sweeney, Mark Charles, author; Gelfand, Martin Paul, advisor; Mueller, Jennifer L., committee member; Bradley, Richard M., committee member; Field, Stuart B., committee member
Sufficiently thin films of type-I superconductor in a perpendicular magnetic field exhibit a triangular vortex lattice, while thick films develop an intermediate state. To elucidate what happens between these two regimes, precise numerical calculations have been made within Ginzburg-Landau theory at κ=0.5 and 0.25 for a variety of vortex lattice structures with one flux quantum per unit cell. The phase diagram in the space of mean induction and film thickness includes a narrow wedge in which a square lattice is stable, surrounded by the domain of stability of the triangular lattice at thinner films/lower fields and, on the other side, rectangular lattices with continuously varying aspect ratio. The vortex lattice has an anomalously small shear modulus within and close to the square lattice phase. Solutions of the Ginzburg-Landau equations have also been obtained for bulk systems and thin films for vortex lattices with one vortex but two flux quanta per square or triangular unit cell. These lattices of double fluxoid vortices are thermodynamically unstable in bulk in both type-I and type-II superconductors, as expected. In type-I films the situation is less clear, because the corresponding calculations for more complicated vortex lattice structures are not yet possible.
Open Access
Rydberg spectroscopy of Fr-like thorium and uranium ions
(Colorado State University. Libraries, 2010) Hanni, Mark Earl, author; Lundeen, Stephen R., advisor; Bradley, R. Mark, committee member; Roberts, Jacob Lyman, committee member; Yalin, Azer P., 1972-, committee member
The binding energies of high L Rydberg levels of Th3+ were measured using the resonant excitation Stark ionization spectroscopy (RESIS) technique. When analyzed using the long range polarization model the measured energies determine the dipole and quadrupole polarizability of the Th4+ ion: αd = 7.61(6) a.u. and αQ = 45(4) a.u. The RESIS technique and apparatus constructed for this study are discussed in this work. Modifications to the original design of the detector are presented. The modifications to the detector increased the energy resolution of the beams in the detector. It was determined that a significant source of background present in the observations of the U5+ Rydberg fine structure is due to the presence of auto-ionizing Rydberg states attached to metastable excited ion cores. These auto-ionizing states severely limit the fine structure observations, preventing the successful observation of any U5+ Rydberg fine structure. Also discussed are future directions that could lead to an increase in the signal to noise in the Th3+ fine structure observations and to a successful measurement of the dipole polarizability of U6+.
Open Access
Analysis of impact of non-uniformities on thin-film solar cells and modules with 2-D simulations
(Colorado State University. Libraries, 2010) Koishiyev, Galymzhan Temirkhanovich, author; Sites, James R., advisor; Lear, Kevin L., committee member; Leisure, Robert Glenn, 1938-, committee member; Harton, John L., committee member
Clean and environmentally friendly photovoltaic (PV) technologies are now generally recognized as an alternative solution to many global-scale problems such as energy demand, pollution, and environment safety. The cost ($/kWh) is the primary challenge for all PV technologies. In that respect, thin-film polycrystalline PV technology (CdTe, Cu(In,Ga)Se2, etc), due to its fast production line, large area panels and low material usage, is one of the most promising low-cost technologies. Due to their granular structure, thin-film solar cells are inherently non-uniform. Also, inevitable fluctuations during the multistep deposition process of large area thin-film solar panels and specific manufacturing procedures such as scribing result in non-uniformities. Furthermore, non-uniformities can occur, become more severe, or increase in size during the solar-panel's life cycle due to various environmental conditions (i.e. temperature variation, shading, hail impact, etc). Non-uniformities generally reduce the overall efficiency of solar cells and modules, and their effects therefore need to be well understood. This thesis focuses on the analysis of the effect of non-uniformities on small size solar cells and modules with the help of numerical simulations. Even though the 2-D model developed here can analyze the effect of non-uniformities of any nature, only two specific types of microscopic non-uniformities were addressed here: shunts and weak-diodes. One type of macroscopic non-uniformity, partial shading, was also addressed. The circuit model developed here is a network of diodes, current-sources, and transparent-conductive-oxide (TCO) resistors. An analytic relation between the TCO-resistor, which is the primary model parameter, and TCO sheet resistance ρS, which is the corresponding physical parameter, was derived. Based on the model several useful general results regarding a uniform cell were deduced. In particular, a global parameter δ which determines the performance of a uniform solar cell depending on sheet resistance ρS, cell length L, and other basic parameters, was found. The expression for the lumped series resistance in terms of physical parameters was also derived. Primary power loss mechanisms in the uniform case and their dependence on ρS, L, and light generated current JL were determined. Similarly, power losses in a small-area solar cell with either a shunt or a weak-microdiode were identified and their dependence on ρS, JL, and location of the non-uniformity with respect to the current collecting contact was studied. The impact of multiple identical non-uniformities (shunts or weak-diodes) on the performance of a module was analyzed and estimates of efficiency loss were presented. It was found that the efficiency of the module strongly depends not only on the severity and fractional area of non-uniformities but also on their distribution pattern. A numerical parameter characterizing distribution pattern of non-uniformities was introduced. The most and least favorable distribution patterns of shunts and weak-diodes over the module area were determined. Experimentally, non-uniformities may be detected with the help of spatially resolved measurements such as electroluminescence (EL). The 2-D circuit model was also used to develop the general framework to extract useful information from experimental EL data. In particular, a protocol that can help distinguish a shunt from a weak-diode and estimate the severity of the non-uniformity based on the EL data was developed. Parts of these simulation results were verified with experimental EL data obtained by other authors. The thesis also discusses the effect of partial shading (a macroscopic non-uniformity) on the operation and safety of thin-film solar panels. A detailed analysis of the current-voltage characteristics of partially shaded module was performed. Conditions that result in a shaded cell experiencing high reverse voltage were shown. A mathematical formalism was developed to distinguish two extremes: when reverse-bias shunting or breakdown dominates. It was shown that in the shunt-dominated case in extreme situations the voltage across the shaded cell can be quite large (~ 20V). High voltage across the shaded cell results in both high power dissipation and elevated temperature. Depending on the light generated current, the temperature above ambient of the shaded cell can be as high as ~100-300°C, implying potential safety issues. The analysis covered all basic rectangular shade configurations.
Open Access
Nonlinear spin wave instability processes in manganese substituted zinc y-type hexagonal ferrites
(Colorado State University. Libraries, 2010) Cox, Richard Garner, author; Patton, Carl E., advisor; Eykholt, Richard Eric, 1956-, committee member; Kabos, Pavel, committee member; Leisure, Robert Glenn, 1938-, committee member; Menoni, Carmen S., committee member; Robinson, Raymond S., committee member
The large magnetocrystalline anisotropy observed in hexagonal ferrites makes these materials ideally suited for high frequency millimeter-wave applications. However, the large microwave losses observed at low-power levels and the high-power handling capabilities of hexagonal ferrites need to be addressed prior to their wide acceptance in real devices. In order to address the above issues, measurements and analyses of the microwave field amplitude (hcrit) required to parametrically excite nonlinear spin wave amplitude growth were performed on single crystal easy plane disks of Mn substituted Zn Y-type hexagonal ferrites at 9 GHz and room temperatures. Plots of the hcrit dependence on the static magnetic field, termed "butterfly curves," were obtained and analyzed for the resonance saturation (RA), subsidiary absorption (SA), and parallel pumping (PP) configurations. In order to obtain the butterfly curve data and perform the analyses: (1) a state-of-the-art computer-controlled high-power microwave spectrometer was constructed, (2) the classical spin wave instability theory, originally developed by Suhl and Schloemann, was extended, and (3) instability measurements were performed on multiple Zn Y-type hexagonal ferrites samples for several pumping configurations and static field settings. The measurements and analyses performed here constitute the first time RS, SA, and PP spin wave instability butterfly curve analysis have all been performed in planar hexagonal ferrite samples. This work also corresponds to the first time that resonance saturation measurements and analyses were performed for static magnetic fields both at and in the vicinity of the ferromagnetic resonant field in a hexagonal ferrite. The data obtained as part of this work show that the microwave field amplitude required to parametrically excite nonlinear spin wave amplitude growth in hexagonal ferrites is similar to polycrystalline cubic ferrites, which are currently in use in microwave devices. Follow-up measurements, motivated by this work, revealed that hcrit can be varied by manipulating the sample dimensions. The analyses performed here indicate that two-magnon scattering is likely not the dominant source of the large low-power microwave losses observed in these hexagonal ferrites; rather that these losses may be an intrinsic property of the material. The theoretical work performed here identified a sign problem with the anti-Larmor uniform mode complex damping terms in several past publications and provides an improved methodology of treating the uniform mode anti-Larmor complex frequency damping.
Open Access
Extending single molecule spectroscopic techniques to multi-particle systems of semiconductor nanocrystals
(Colorado State University. Libraries, 2011) Shepherd, Douglas Parker, author; Gelfand, Martin Paul, advisor; Van Orden, Alan K., advisor; Roberts, Jacob Lyman, committee member; Prieto, Amy Lucia, committee member
This dissertation describes the application of single molecule spectroscopic techniques to individual semiconductor nanocrystals (NCs), small clusters of NCs, and NCs used as the light harvesting layer in sensitized solar cells. We first examine how coupling between close-packed NCs may alter their photophysical properties by studying isolated NCs and small clusters of NCs via single molecule time-correlated single-photon counting, from which fluorescence intensity trajectories, autocorrelation functions, decay histograms, and lifetime-intensity distributions have been constructed. These measurements confirm that NC clusters exhibit unique photoluminescence behavior not observed in isolated NCs. In particular, the NC clusters exhibit a short-lifetime component in their photoluminescence decay that is correlated with low photoluminescence intensity of the cluster. A model based on radiative energy transfer to NCs within a cluster that have smaller energy gaps, combined with independent blinking for the NCs in a cluster, accounts for the main experimental features. This energy transfer may lead to energy sinks when an excitation is transferred to a NC that is in the off state. We then examine a model photovoltaic system where a sub-monolayer film of NCs is chemically coupled to a single crystal semiconductor (TiO2 or ZnO) substrate through a variety of capping ligands. Again, utilizing time-correlated single photon counting and internal photon conversion efficiency we have studied both the photoluminescence intensity, photoluminescence decay time, and sensitized photocurrents. We find that for all configurations of capping ligands and substrate the photoluminescence decay rate is quenched compared to the free NCs in solution; whereas, only the short chain capping ligands that promote electron coupling to the substrate produce photocurrents. The longer chain capping groups both inhibit the electron injection and promote NC clustering on the surface where interactions between the individual NCs or the NCs and substrate alter the radiative rate. This result confirms that the possibility of NC clusters leading to a loss of energy due to inter-NC coupling is present in devices and warrants further study.
Open Access
Barium tagging in solid xenon for the EXO experiment
(Colorado State University. Libraries, 2011) Mong, Brian, author; Fairbank, William, Jr., advisor; Lundeen, Stephen, committee member; Berger, Bruce, committee member; Van Orden, Alan, committee member
Neutrinoless double beta decay experiments are searching for rare decay modes never before observed to uncover the absolute mass of the neutrino, as well as to discover if it is a Majorana fermion. Detection of the daughter nucleus can help provide positive identification of this event over most radioactive backgrounds. The goal of the Enriched Xenon Observatory (EXO) is to measure the rate of 0νββ decay in 136Xe, incorporating 136Ba daughter identification by laser induced fluorescence spectroscopy. Here, we investigate a technique in which the 136Ba daughter is grabbed with a cryogenic probe by freezing it in solid xenon ice, and detected directly in the solid xenon. The absorption and fluorescence spectra of barium in solid xenon were observed for the first time in this work. Identification of the 6s2 1S0 → 6s6p 1P1 transition in both absorption (558 nm) and emission spectra (594 nm) were made. Additional blue absorption and emission lines were observed, but their transitions were not identified. Saturation of the 6s2 1S0 → 6s6p 1P1 transition was not observed with increased excitation rates using resonance excitation at 558 nm. From this a limit on the metastable decay rate was deduced to be greater than 104 s-1. Finally a fluorescence spectrum was obtained from a sample with only 20,000 atoms in the laser beam. With potential improvements of 107 in detection efficiency, single barium atom detection seems possible in solid xenon. A fiber probe detector based on a bare single mode fiber was also constructed and tested with fluorescing dye molecules. Successful detection of a few dye molecules in solution at the probe tip was demonstrated.
Open Access
Detection of small numbers of barium ions implanted in solid xenon for the EXO experiment
(Colorado State University. Libraries, 2012) Cook, Shon, author; Fairbank, William, advisor; Lee, Siu Au, committee member; Roberts, Jacob, committee member; Bartels, Randy, committee member
In an effort to discover the yet-unknown absolute masses of neutrinos, the goal of the Enriched Xenon Observatory is to observe neutrinoless double beta decay of 136Xe. Identification of this very rare decay may be difficult even with the best conventional efforts to reduce and reject radioactive background, thus requiring additional background rejection via detection of the daughter 136Ba nucleus. One method of detection is laser-induced fluorescence of the barium atom in solid xenon. Spectra of very small numbers of barium atoms in solid xenon, as few as 3 atoms, are reported for the first time. Demonstration of detection of Ba atoms with large fluorescence efficiencies gives promise for detecting single atoms in the near future. Results from experiments involving implantation of Ba+ ions in solid xenon are discussed. One narrow excitation peak was discovered from ion beam deposition that was not found in neutral deposits. Five new emission lines are found with this same excitation spectrum. Bleaching, annealing, and laser dependence of these lines are studied. The identification of the new Ba species as Ba+ or as a barium molecule is discussed.
Open Access
Constraints on the galactic magnetic field with two-point cumulative autocorrelation function
(Colorado State University. Libraries, 2012) Petrov, Yevgeniy, author; Harton, John L., advisor; Mostafá, Miguel A., committee member; Berger, Bruce, committee member; Burns, Patrick J., committee member
The fact that ultra high energy cosmic rays are charged particles complicates identication of their sources due to deflections by the intervening cosmic magnetic fields. The information about the fields is encoded in the amount of deflection experienced by a charged particle. Unfortunately, the positions of sources are unknown as is the structure of the magnetic field. However, it is possible to deduce the most favorable galactic magnetic field by examining the parameter space of different models of the galactic magnetic field. The method presented in this work is valid under some plausible assumptions, such as extragalactic origin of the UHECR, pure protonic composition above 50 EeV and sufficiently weak randomly oriented galactic and extragalactic components of the magnetic field. I use a two point cumulative autocorrelation function combined with the backtracking method to find regions in the parameter space that are compatible with statistically significant clustering on the extragalactic sky. This approach is independent of any catalog of sources. The ratio between the number of pairs within a certain angular window at the Earth sky and at the extragalactic sky after backtracking serves to indicate focusing or de-focusing properties of a particular field configuration. The results suggest that among several tested fields, the Harari-Mollerach-Roulet model with a bi-symmetric spiral and even vertical symmetry favors clustering of arrival directions at the extragalactic sky with the probability of 2.5% being from an isotropic distribution. Addition of the toroidal halo field improves clustering for the Harari-Mollerach-Roulet field for both bi-symmetric and axisymmetric spirals with even vertical symmetry, and the isotropic probabilities are 2.5% and 5.3% correspondingly. The bi-symmetric and axisymmetric spirals with odd vertical symmetry are disfavored, as well as the models with annular structure.
Open Access
In-situ laser tagging of barium ions in liquid xenon for the EXO experiment
(Colorado State University. Libraries, 2012) Hall, Kendy, author; Fairbank, William, advisor; Toki, Walter, committee member; Marconi, Mario, committee member; Roberts, Jacob, committee member
The goal of the Enriched Xenon Observatory (EXO) collaboration is to measure the half-life of neutrino-less double beta decay using a ton size liquid 136Xe detector with zero back-ground. Zero background detection can only be achieved if the daughter nucleus, 136Ba, can be tagged. The EXO collaboration is investigating several techniques to tag the 136Ba daughter. The goal of this thesis is to investigate the prospects of directly observing a single 136Ba+ ion in the liquid using a laser aimed at the decay site, hence in-situ laser tagging. Because the energy levels of Ba+ ions are expected to be altered from the vacuum configuration, in-situ laser tagging can only be accomplished if the spectroscopy of the Ba+ ions in liquid xenon is understood. An ultra-pure liquid xenon test apparatus with a liquid xenon purity monitor has been built to study the spectroscopy of the Ba+ ions. An unexpected discovery of the nonresonant multiphoton ionization of liquid xenon using pulsed UV lasers was made while characterizing the purity monitor. The discovery was vital to the ability to accurately measure the purity of the liquid xenon. The spectroscopy of Ba+ ions in liquid xenon and the multiphoton ionization studies are the two key topics that are presented in this thesis.