Thesis (Ph. D.)--University of Texas at Austin, 2008. / Vita. Includes bibliographical references.

Thesis (Ph. D.)--University of Washington, 2005. / Vita. Includes bibliographical references (leaves 128-134).

The presented research discusses the design, analysis, and testing of a low current, LaB6 heaterless hollow cathode for space propulsion applications. A heaterless design using LaB6 is chosen to reduce complexity and increase electrical power efficiency and robustness. Argon propellant is used due to its more favorable breakdown voltage characteristics compared to xenon. An original model for the insert region plasma is derived by combining several analyses in literature. This model allows the simultaneous calculation of many plasma and thermal parameters in the cathode using only two completely unobtrusive measurements, and requires several assumptions which are common in hollow cathode research. The design of the cathode and its subsystems are presented in detail. No diagnostics were used in the cathode except direct voltage measurements in the power circuit. A discussion of emitter poisoning and ignition behavior is presented. The cathode is characterized by measuring anode and keeper voltages as a function of anode current and propellant flow rate, with the cathode discharging directly to a flat metal anode. Results are consistent with those obtained by previous investigations of argon hollow cathodes. This data is used with the derived plasma model to calculate the dependence of various parameters on current and flow rate. A discussion of the spot-plume transition behavior is presented. Finally, insights and design improvements are discussed based on the experimental results. / Master of Science / In recent years, the space industry has seen rapidly accelerating growth due to the continuing advancement of technology. A critical area of spacecraft technology is the spacecraft’s propulsion system, which allows the vehicle to achieve and maintain its desired orbit or trajectory through space. One class of propulsion systems known as “electric propulsion” uses electrical power to accelerate the fuel of the spacecraft. These types of propulsion systems are far more efficient than traditional propulsion systems, which use chemical reactions to create thrust. One of the main components of certain types of electric propulsion systems is the hollow cathode, which initiates and sustains the thruster operation. In this research, a hollow cathode with several non-conventional characteristics is developed and tested. First of all, standard hollow cathodes use a heater to bring the cathode up to operational temperature, but this design is heaterless which offers several benefits to the cathode and electrical power system designs. Secondly, the cathode uses a non-conventional choice of material for the “emitter”, which emits electrons when heated and allows the cathode to operate. Lastly, while typical electric propulsion systems use xenon for fuel, this cathode uses argon which has several benefits over xenon including cost. An overview of electric propulsion is presented, as well as a new physics-based model of this type of cathode that allows useful calculations based on simple measurements. The design and test results of the cathode are discussed in detail, with several interesting and insightful behaviors that were noted during testing. Heaterless cathodes have the potential to improve the efficiency, cost, and weight of electric propulsion systems, and this research therefore contributes to an important field for the future of space exploration.

Radical aircraft and propulsion system architecture changes may be required to continue historic performance improvement rates as current civil aircraft and engine technologies mature. Significant fuel-burn savings are predicted to be achieved through the Distributed Propulsion concept, where an array of propulsors is distributed along the span of an aircraft to ingest boundary layer air and increase propulsive efficiency. Studies such as those by NASA predict large performance benefits when integrating Distributed Propulsion with the Blended Wing Body aircraft configuration, as this planform geometry is particularly suited to the ingestion of boundary layer air and the fans can be redesigned to reduce the detrimental distortion effects on performance. Additionally, a conventional aircraft with Distributed Propulsion has not been assessed in public domain literature and may also provide substantial benefits. A conceptual aircraft design code has been developed to enable the modelling of conventional and novel aircraft. A distributed fan tool has been developed to model fan performance, and a mathematical derivation was created and integrated with the fan tool to enable the boundary layer ingestion modelling. A tube & wing Distributed Propulsion aircraft with boundary layer ingestion has been compared with a current technology reference aircraft and an advanced turbofan aircraft of 2035 technology. The advanced tube & wing aircraft achieved a 27.5% fuel-burn reduction relative to the baseline aircraft and the Distributed Propulsion variant showed fuel efficiency gains of 4.1% relative to the advanced turbofan variant due to a reduced specific fuel consumption, produced through a reduction in distributed fan power requirement. The Blended Wing Body with Distributed Propulsion was compared with a turbofan variant reference aircraft and a 5.3% fuel-burn reduction was shown to be achievable through reduced core engine size and weight. The Distributed Propulsion system was shown to be particularly sensitive to inlet duct losses. Further investigation into the parametric sensitivity of the system revealed that duct loss could be mitigated by altering the mass flow and the percentage thrust produced by the distributed fans. Fuel-burn could be further reduced bydecreasing component weight and drag, through decreasing the fan and electrical system size to below that necessary for optimum power or specific fuel consumption.

Ultrafast electron diffraction is a novel pump-probe technique which aims to determine transient structures during photoinduced chemical reactions and other structural transitions. This technique provides structural information at the atomic level of inspection by using an electron pulse as a diffractive probe. The atomic motions of interest happen on the 100 fs = 10^(−13) s time scale. To observe these atomic motions, a probe which matches this time scale is required. In this thesis, I describe the development of an electron diffractometer which is capable of 200 fs temporal resolution while maintaining high signal level per electron pulse. This was made possible by the construction of an ultra-compact photoactivated 60 keV femtosecond electron gun. Traditional electron pulse characterization methods are unsuitable for high number density femtosecond electron pulses such as the pulses produced by this electron gun. I developed two techniques based on the laser ponderomotive force to reliably determine the duration of femtosecond electron pulses into the sub-100 fs range. These techniques produce a direct cross-correlation trace between the electron pulse and a laser pulse. The results of these measurements confirmed the temporal resolution of the newly developed femtosecond electron diffractometer. This cross-correlation technique was also used to calibrate a method for the determination of the temporal overlap of electron and laser pulses. These techniques provide the pulse diagnostics necessary to utilize the temporal resolution provided by femtosecond electron pulses. Owing to their high charge-to-mass ratio, electrons are a sensitive probe for electric fields. I used femtosecond electron pulses in an electron deflectometry experiment to directly observe the transient charge distributions produced during femtosecond laser ablation of a silicon (100) surface. We found an electric field strength of 3.5 × 10^6 V/m produced by the emission of 5.3 × 10^11 electrons/cm^2 just 3 ps after an excitation pulse of 5.6 J/cm^2 . This observation allowed us to rule out Coulomb explosion as the mechanism for ablation under the conditions present in this experiment.

Ultrafast electron diffraction is a novel pump-probe technique which aims to determine transient structures during photoinduced chemical reactions and other structural transitions. This technique provides structural information at the atomic level of inspection by using an electron pulse as a diffractive probe. The atomic motions of interest happen on the 100 fs = 10^(−13) s time scale. To observe these atomic motions, a probe which matches this time scale is required. In this thesis, I describe the development of an electron diffractometer which is capable of 200 fs temporal resolution while maintaining high signal level per electron pulse. This was made possible by the construction of an ultra-compact photoactivated 60 keV femtosecond electron gun. Traditional electron pulse characterization methods are unsuitable for high number density femtosecond electron pulses such as the pulses produced by this electron gun. I developed two techniques based on the laser ponderomotive force to reliably determine the duration of femtosecond electron pulses into the sub-100 fs range. These techniques produce a direct cross-correlation trace between the electron pulse and a laser pulse. The results of these measurements confirmed the temporal resolution of the newly developed femtosecond electron diffractometer. This cross-correlation technique was also used to calibrate a method for the determination of the temporal overlap of electron and laser pulses. These techniques provide the pulse diagnostics necessary to utilize the temporal resolution provided by femtosecond electron pulses. Owing to their high charge-to-mass ratio, electrons are a sensitive probe for electric fields. I used femtosecond electron pulses in an electron deflectometry experiment to directly observe the transient charge distributions produced during femtosecond laser ablation of a silicon (100) surface. We found an electric field strength of 3.5 × 10^6 V/m produced by the emission of 5.3 × 10^11 electrons/cm^2 just 3 ps after an excitation pulse of 5.6 J/cm^2 . This observation allowed us to rule out Coulomb explosion as the mechanism for ablation under the conditions present in this experiment.

The diploma thesis is focused on the study of low-temperature plasma by methods of computational physics, namely particle modelling. The solution focuses on the time- effective execution of simulations, which is concerned, among the other things, with the use of parallelization techniques aimed at maximize shortening of the calculation time. The Langmuir probe was chosen as the most common representative of the probe diagnostics. The main goal of this work was to observe plasma interaction with the embedded solid. Current research on plasma modelling intensively explores dynamic phenomena such as wave propagation or turbulence and instability. For this reason, much of the work is devoted to the dynamics in electropositive and electronegative plasma.

M.Ing. / 50 Hz technology has led the plasma torch converters up to now. This technology was used. The high power levels of plasma torches made it difficult to implement high frequency technology. At this stage it is possible to use high-frequency technology in plasma torch applications. This thesis implements a high frequency converter suitable for plasma applications. The converter used for this application is the Partial Series Resonant Converter. A study launched to get the properties of plasmas showed that the control method used at this stage namely current control is not the ideal control method. Changing the control method of the converter made it possible to see what influence it has on the plasma. A thorough large signal analisis of the Partial Series Resonant Converter was done. From this analisis a transfer function of the converter was developed and the control parameters were calculated. This control parameters made it possible to change the control and to investigate the different control methods. The design of the plasma torch converter was based on the design of a distributed transformer, input and output filter and a non-linear controller. The results of the Partial Series Resonant Converter showed that power control leads to a more stable plasma. This thesis made a positive contribution to the knowledge of plasma torches and the knowledge of plasma torch converters. The thesis forms a summarry of plasmas and plasma-related topics, and futher study fields are defined by this thesis.

When plasmas come in contact with the boundaries that confine them, various complex processes occur between the plasma and the materials in the boundary. These processes, called plasma-material interactions (PMI) lead to physical and chemical modifications in the materials and in the plasma. In the case of a tokamak, a magnetic confinement fusion reactor, the interactions between the plasma and the material in the bounding walls can negatively impact the performance and service life of the reactor. Furthermore, PMI are also found in other areas of significant engineering interest, such as plasma-based spacecraft propulsion engines, where interactions affect the transport properties of the plasma and consequently the performance of the engine. Therefore, gaining a fundamental understanding of the various plasma-material interactions is necessary for the development and improvement of these devices. PMI are dictated by the plasma sheath, a layer of net positive charge that forms at the plasma-boundary interface. The sheath regulates the energy and particle fluxes to the boundary, mediating the interactions. Sheaths, however, are only stable and well-developed when the ions enter the sheath with a speed equal to or greater than the `Bohm speed'. The Bohm speed is a landmark result in sheath theory and various mathematical expressions for it have been derived from fluid and kinetic treatment of plasmas. Although these models are widely used, they are only accurate in cases where the thickness of the sheath is negligible when compared to the scale length of the plasma in consideration. These cases are said to satisfy the `asymptotic limit'. To resolve this, a new Bohm speed model that considers the effects of transport terms such as the electron heat flux, thermal force, and temperature isotropization has been recently proposed [Y. Li et al., Physical Review Letters (2022)]. The model is verified using particle-in-cell (PIC) kinetic simulations and is shown to accurately predict the Bohm speed in cases away from the asymptotic limit. This thesis investigates the new model using the continuum kinetic approach on the Gkeyll software framework. The continuum kinetic approach numerically solves the Vlasov-Maxwell equations using the discontinuous Galerkin method and captures the kinetic phenomena of the plasma without needing to track individual particles. Multiple collisional cases ranging from a Knudsen number of 20 to 5000 are considered in a 1X3V simulation domain using the Lenard-Bernstein collisional operator. The results of the continuum kinetic simulations are benchmarked to the PIC simulation results. It is concluded that across a wide range of collisionalities, the continuum kinetic method captures much of the same physics as the PIC method while offering noise-free results. However, there is a discrepancy between the Bohm speed prediction and the simulation results in the continuum kinetic case. This discrepancy is explored and significant error in the collisional integral derived transport terms between the continuum kinetic method and PIC method is found, suggesting that the difference in collisional operator may be the source of the discrepancy. Nevertheless, the sheath profiles developed in the PIC simulations and the continuum kinetic simulations are in reasonable agreement. / Master of Science / Nuclear fusion is a process in which two light atomic nuclei (like hydrogen) fuse to form a heavier nucleus (like helium) and release tremendous amounts of energy. The resultant energy from these reactions powers the sun and has the potential to provide clean energy for our terrestrial needs. Harnessing fusion energy has been one of the biggest scientific and engineering challenges of our time due to various reasons. One of these reasons is the interaction of plasma, which is the fuel for the fusion reaction, and the materials of the walls that bound the plasma. These interactions are called plasma-material interactions (PMI) and can affect the longevity and performance of fusion reactors. The main governing phenomenon behind these interactions is the plasma sheath, a layer of plasma that is formed when the plasma encounters a boundary. For a sheath to form it is also necessary that ions in the plasma possess a speed greater than the so-called `Bohm speed'. While many expressions have been derived for the Bohm speed, these expressions are only valid when there is a clear sheath entrance that divides the bulk plasma and the sheath. This condition is not satisfied in many cases of interest. Instead, a sheath-transition region is found to exist between the bulk plasma and the sheath. A recently proposed Bohm speed model [Y. Li et al., Physical Review Letters (2022)] resolves this. This model is accurate in cases where the sheath-transition region exists and is derived by considering previously overlooked transport physics. In this work, this new model is studied using a different computational approach known as `continuum kinetics' using an open-source solver called Gkeyll. The results of the continuum kinetic simulations are compared to the results used to verify the model.

Fourier spectral and pseudospectral methods are used in numerical modeling of ionospheric processes, namely macroscopic evolution of naturally and artificially created ionospheric density irregularities. The simulation model consists of two-dimensional electrostatic nonlinear fluid plasma equations that describe the plasma evolution. The spectral and pseudospectral methods are used to solve the spatial dependence of these self-consistent equations. They are chosen over the widely used finite difference and finite element techniques since spectral methods are straightforward to implement on nonlinear equations. They are at least as accurate as finite difference simulations. A potential equation solver is developed to solve the nonlinear potential equation iteratively. Time integration is accomplished using a combination of leapfrog and leapfrog-trapezoidal methods. A FORTRAN program is developed to implement the simulation model. All calculations are performed in the Fourier domain. The simulation model is tested by considering three types of problems. This is accomplished by specifying an initial density (Pedersen conductivity) profile that represents slab model density, density enhancement (due to releases such as barium), or density depletion (due to late times effects of electron attachment material releases) in the presence of a neutral wind. The evolution of the irregularities is monitored and discussed. The simulation results agree with similar results obtained using finite difference methods. A comparison is made between the ionospheric depletion and enhancement problems. Our results show that, given the same parameters and perturbation level, the depletion profiles bifurcate much faster than that of the enhancement. We argue that this is due to the larger growth rate in the E X B interchange instability of the density depletion case. / Master of Science