Browsing by Author "Dumitrache, Ciprian, committee member"

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Open Access
Barium sensing in hollow cathode plasma using cavity ring-down spectroscopy (CRDS)
(Colorado State University. Libraries, 2024) Antozzi, Seth, author; Yalin, Azer, advisor; Dumitrache, Ciprian, committee member; Aristoff, David, committee member
Hollow cathodes (HCs) are ion propulsion devices commonly paired with Hall Effect Thrusters (HETs), which are devices of increasing importance in the ion propulsion community. Barium Oxide (BaO) cathodes are known to emit barium when operating under high-temperature conditions. Understanding barium densities in the cathode plasma provides experimental guidance for NASA barium modeling, including understanding of the physical characteristics and lifetime of the cathode. Based on modeling work, expected barium densities are ~1010 cm-3. A sensitive diagnostic is required such as CRDS. In this work, the detection of barium from the thermionic emitter of the Mark II 25 A BaO HC using the laser diagnostic technique of cavity ring-down spectroscopy (CRDS) is presented. CRDS detects ground state neutral barium via absorption of the probe laser beam in the vicinity of 553.548 nm (air wavelength). The cathode CRDS measurements are performed along the axis of the cathode since that is the control volume of interest. We report barium density as a function of heater current (plasma off) with results showing an approximately exponential density increase with current. Further parameters of study include keeper current, anode current (with the cathode operating), and propellant flow values. The measured signal-to-noise allows estimation of the barium density detection limit as ~106 cm-3 in the present configuration. An appendix to this work addresses the need for a diagnostic technique to measure krypton neutrals in HC plumes. In the krypton study, we enhance the krypton Two-Photon Absorption Laser Induced Fluorescence (TALIF) technique and apply it to a BaO HC plasma. We utilize a dye laser at 212.6 nm to excite TALIF fluorescence within the plume, with the fluorescence detected at 758.7 nm. We present spatial maps for krypton neutral densities at a cathode flow rate of 7.5 sccm and anode currents of 5A and 13A. These measurements provide insights into facility effects related to cathode coupling and cathode physics, such as the collisional damping of instabilities. Additionally, we discuss how plasma characteristics, including spot versus plume mode, and plasma luminosity, are influenced.
Open Access
Characterization of plasma conductivity by laser Thomson scattering in a high-voltage laser-triggered switch
(Colorado State University. Libraries, 2023) Gottfried, Jacob A., author; Yalin, Azer P., advisor; Dumitrache, Ciprian, committee member; Rocca, Jorge, committee member
High-voltage laser-triggered switches (HV-LTSs) are used in pulsed-power applications where low jitter and high current are required. The switches allow operation in the mega-ampere, megavolt regime while maintaining low insertion losses. Low inductance HV-LTS designs have shown discrepancies between modeled and experimental behavior, reinvigorating interest in the physics of HV-LTS operation. Detailed spatially- and temporally- resolved measurements of plasma properties within the switches could contribute to validating and advancing numeric models of these systems by checking the assumptions used in their derivation. To date, there is minimal experimental data detailing the evolution of plasma properties during switch operation. This work investigates HV-LTS plasma channel conductivity (the assumption within current models drawing the most critique) during the rising edge of the current pulse through both derivative (V-Dot) electrical probes and electron temperature measurements via laser Thomson scattering. A HV-LTS testbed utilizing an aqueous (variable impedance) resistive load was designed to produce experimental conditions found in larger pulsed power applications. This work describes the design of the load and experimental results under a variety of load conditions and operating voltages of 5 - 6 kV. The results indicate the electron temperature increases during the rising edge of the current pulse, suggesting that the plasma conductivity is temporally evolving. Further, electrical measurements show an increase in plasma conductivity during the rising edge of the current pulse. Evidence from both optical and electrical measurements calls into question the assumption of a temporally constant plasma conductivity as both the optical and electrical diagnostics show a temporally increasing plasma conductivity during the rising edge of the current pulse.
Open Access
Experimental evaluation of a standalone hollow cathode apparatus with a magnetic field
(Colorado State University. Libraries, 2024) Ku, Emily X., author; Williams, John, advisor; Dumitrache, Ciprian, committee member; Thornton, Christopher, committee member
Testing hollow cathode assemblies independently from their use in Hall or gridded ion thrusters offers advantages such as reduced test facility size, lower power requirements, and improved diagnostic access. Standalone tests can reveal important cathode characteristics like ignition time, keeper ignition voltage, tip temperature, and current capability. Replicating the plasma phenomena that occur when a cathode operates within a thruster is challenging but essential, as these phenomena can generate energetic ions that erode cathode and keeper surfaces, limiting thruster lifespan. The primary challenge is to accurately emulate thruster conditions in standalone tests and verify this emulation through comparison with cathode-thruster operations. This thesis presents data on a standalone hollow cathode operated with magnetic fields that emulate those in electric propulsion devices, testing it both without an applied magnetic field and with permanent and solenoidal magnetic fields. Measurements of keeper, anode, and cathode-to-ground voltages were conducted over a range of anode currents and flow rates. At certain conditions, the plasma discharge transitioned to a less stable mode known as plume mode, with higher flow rates shifting this transition to higher anode currents. Introducing a magnetic field decreased the anode current at which this voltage shift occurred. Important findings in this work include: (1) Repeat tests with no magnetic field show that the transition behavior was different from one test to another, indicating that transition behavior may be affected by minute changes in cathode apparatus, or there are significant uncertainties associated with the transition and (2) Significant hysteresis in plume mode transition was observed when increasing and then decreasing anode current. These two findings along with the deleterious effects of the magnetic field have important implications on cathodes operating with Hall thrusters, which often exhibit large, rapid oscillations in discharge current.
Open Access
Thermal management of discretized heaters using CuW microchannel heat sinks and FC3283 for laser diode applications
(Colorado State University. Libraries, 2024) Amyx, Isabella Gascon, author; Bandhauer, Todd M., advisor; Dumitrache, Ciprian, committee member; Venayagamoorthy, Subhas Karan, committee member
Single-phase cooling using microchannel heat sinks (MCHS) has become a popular approach for overcoming the thermal challenges associated with high-powered microelectronic devices. Thermal management is one of the largest barriers to higher power densities in electronics and frequently limits overall device performance. The implementation of forced convective cooling via single-phase liquid cooling in MCHS reduces the thermal resistance resulting in lower device temperatures at high-power conditions, which can decrease the package size and extend the lifespan of devices. The goal of this effort was to investigate practical cooling solutions for laser diode bars. This study examined the effectiveness of a copper tungsten (CuW) microchannel heat sink paired with a dielectric coolant (FC3283) for dissipating both discrete and uniform heat fluxes up to 600 W/cm2 across a 0.25 cm2 surface area through a numerical and experimental study. CuW was chosen as the MCHS material because it is thermal expansion matched to GaAs, which is a common laser diode substrate. FC3283 serves as a dielectric coolant that is compatible with power electronics cooling. Reasonable agreement was found between the numerical model and the experimental results. The resulting thermal resistance ranged from 0.15 cm2 K/W at the highest flow rate to 0.26 cm2 K/W at the lowest flow rate. The resulting thermal performance from this study proved to be insufficient for maintaining optimal temperatures for laser diode applications. Using the validated model, cooling fluid and geometry modifications proved to have a significant impact on the heat transfer coefficients. This study revealed the importance of considering discrete heat sources separately from uniform heat sources and proved that CuW microchannels can be a promising cooling option toward future advancements of laser diode bars and other high-power microelectronics when using a low viscosity and high thermal conductivity, dielectric cooling fluid and an optimized geometry.