A key issue in the development of theory and models for plasma propulsion devices is to describe the instabilities and fluctuations of the devices. It has been widely recognized that many Hall effect thrusters (HETs) exhibit oscillations at frequencies in the range of ∼ 20 kHz. These ionization-related oscillations are generally referred to as Breathing Mode oscillations and have been the subject of considerable research. Here, for the first time, we report direct temporally resolved measurements of the ground state neutral density variation during the period of the oscillation. We used the laser-based Two-Photon Absorption Laser Induced Fluorescence (TALIF) technique to measure neutrals within the plume of a 1.5 kW HET operating on krypton (Kr). Our TALIF scheme employs a frequency-doubled, pulsed dye laser operating at ∼ 212 nm to probe ground state Kr atoms. A novel phase-binning approach is used to recover the time-dependent signal by assigning the timing of each collected TALIF signal (laser shot) relative to the phase of the discharge current. We find that the neutral density fluctuates quite strongly over the period of the oscillation, and that this fluctuation leads the current fluctuation as expected.

A key issue in the development of theory and models for plasma propulsion devices is to describe the instabilities and fluctuations of the devices. It has been widely recognized that many Hall effect thrusters (HETs) exhibit oscillations at frequencies in the range of similar to 20 kHz. These ionization-related oscillations are generally referred to as Breathing Mode oscillations and have been the subject of considerable research. Here, for the first time, we report direct temporally resolved measurements of the ground state neutral density variation during the period of the oscillation. We used the laser-based Two-Photon Absorption Laser Induced Fluorescence (TALIF) technique to measure neutrals within the plume of a 1.5 kW HET operating on krypton (Kr). Our TALIF scheme employs a frequency-doubled, pulsed dye laser operating at similar to 212 nm to probe ground state Kr atoms. A novel phase-binning approach is used to recover the time-dependent signal by assigning the timing of each collected TALIF signal (laser shot) relative to the phase of the discharge current. We find that the neutral density fluctuates quite strongly over the period of the oscillation, and that this fluctuation leads the current fluctuation as expected.

Hollow cathodes (HC) are important components of several electric propulsion devices acting as electron sources and neutralizers. Widely used barium oxide (BaO) HCs are prone to evaporation and sputtering due to plasma-heating and collisions of ions within the channel of the HC. Measuring the number density of the barium atoms that are emitted in the plasma plume of the HC will help inform models of HC physics and lifetime. In this work, cavity ringdown spectroscopy (CRDS) measurements of barium from the thermionic emitter of a heaterless BaO HC are presented. The CRDS scheme employs a pulsed laser at ~553.7 nm (vacuum wavelength) to access a ground state transition of barium and allows a Ba density detection limit of ~4x10^5 (cm-3). We have performed spatial scans of barium density within the plasma plume at a position 0.5 cm downstream from the orifice of the cathode at a current of 5 A and flow-rate of 10 sccm. An Abel inversion was used to find radial Ba density yielding a peak value of ~1.8x10^6 cm-3. We have also performed initial measurements of barium density versus axial distance, anode current, and krypton flow.

Krypton tagging velocimetry (KTV) is an emerging flow diagnostic that investigates the bulk movement of high-speed, low-pressure gas flows. The present contribution expands the typical KTV method to the lower density (vacuum) conditions of interest in electric propulsion research. Our KTV scheme utilizes a two-photon (Write) excitation at 214.7 nm from a pulsed dye laser, followed by (Read) re-excitation of the ensuing metastable at 769.5 nm with an optical parametric oscillator. For the case of cold krypton gas expanding from a hollow cathode into vacuum, we find a bulk velocity of 452 ± 37 m/s and temperature of 20 ± 16 K. We also study the flow in the plume with the hollow cathode plasma operating for which we find bulk velocity of 1200 ± 130 m/s and temperature of 880 ± 370 K. Measurements are performed at background pressures down to ∼10−5 Torr.

A current trend in electric propulsion (EP) is a shift from legacy propellant xenon to alternative propellants including krypton, driving a need for new plasma diagnostics targeted at these emerging propellants including neutral density measurement. The present work advances the Kr Two-Photon Absorption Laser Induced Fluorescence (TALIF) technique for spatially resolved, ground-state neutral density measurements and shows its application to a Barium Oxide (BaO) hollow cathode plasma. This study compares the three main laser wavelengths used for Kr TALIF, 212.6 nm, 214.7 nm, and 216.7 nm. We use a dye laser operating at 212.6 nm to induce TALIF signals in both cold flow krypton and the krypton plasma plume of the BaO cathode, comparing the cold flow results to output from a D2Q16 Lattice Boltzmann method cold flow model. The resultant fluorescence is detected at 758.7 nm. The cathode is movable within the vacuum chamber allowing spatially resolved measurements of krypton neutral densities. Spatial maps are obtained for a cathode flow rate of 7.5 SCCM and both 5 A and 13 A anode currents. These data can contribute to the understanding of facility effects in relation to cathode coupling as well as cathode physics such as collisional damping of instabilities. As the ratio of flow rate of Kr to HC current decreases, “plume” mode appears to be observed, and neutral depletion regions appear beneath the keeper orifice of the HC.

View Video Presentation: https://doi.org/10.2514/6.2023-1863.vid The increased cost of xenon has led to a growing interest in the use of alternate propellants, in particular krypton, for the operation of electric propulsion (EP) devices including Hall Effect thrusters (HETs). An important diagnostic need to investigate basic plasma physics within the devices, as well as phenomena connected to lifetime and facility effects, is to measure spatial profiles of the neutral propellant atoms. In this work, we focus on the development of a krypton two-photon absorption laser induced fluorescence (TALIF) setup suitable for probing low density conditions including within cold-flow and in the plume of a barium oxide (BaO) hollow cathode. The TALIF setup uses a dye-laser operating at 214.7 nm to excite krypton from its ground state to an excited state from which fluorescence is detected at 760 nm. The signal collected from fluorescence of the excited atom can be used to determine the local neutral density. Our results include spatially resolved measurements of TALIF signals in the radial direction of the BaO hollow cathode plasma plume 1 cm axially downstream of the keeper orifice plate. Additionally, we show a method to determine the ionization fraction from TALIF signals, finding an ionization fraction of ~0.06 ± 0.02 in the plume region (with the relatively large uncertainty due to our lack of knowledge of the neutral temperature in the plume).

High-voltage laser-triggered switches (HV-LTSs) are used in pulsed-power applications where low jitter and precise timing are required. The switches allow operation in the megaampere, megavolt regime while maintaining low insertion losses. Currently, there is a lack of detailed plasma measurements in these switches, yet such measurements are needed to elucidate the detailed physics, which include a range of processes such as laser breakdown, streamer formation and growth, current flow, plasma evolution, and cooling. Detailed spatially- and temporally resolved measurements of plasma properties within the switches could contribute to validating and advancing numeric models of these systems. This contribution presents laser Thomson scattering measurements of the electron number density and temperature evolution in a HV-LTS. The switch was operated at 6 kV with current flow for a duration of 145 ns and a peak current density of 0.2 MA/cm2 into a matched load. The Thomson scattering diagnostic system uses a 532 nm probe from an Nd:YAG laser allowing a temporal resolution of ∼10 ns. We find that during the switch current pulse, the plasma electron temperature rose from a starting value of 8.1 ± 1.6 eV (due to cooling of the earlier trigger laser plasma) to a peak value of 26 ± 5 eV with an associated increase in the electron density from 8.6 ± 1.7 × 1017 to 3.1 ± 0.6 × 1018 cm−3.

High-voltage laser-triggered switches (HV-LTS) can deliver kiloampere magnitude currents in nanosecond time frames while having a jitter on the order of only picoseconds to nanoseconds. The most popular model used to simulate the behavior of HV-LTS is the Martin Switch model. However, recent designs of HV-LTS have found inconsistencies in the Martin Switch model that prohibit modeling switches designed to have on-times of hundreds of nanoseconds. The suspected reason for these inconsistencies is the assumptions T. H. Martin used in his derivation, specifically, that the plasma conductivity is constant both spatially and temporally. This study investigates the behavior of plasma conductivity via temporally and spatially-resolved measurements of electron temperature and density by Laser Thomson scattering. We find that the plasma conductivity does have a spatial and temporal dependency during the decay of the plasma channel. Further improvements to the diagnostic setup are necessary to allow earlier time measurements during the switch discharge phase to inform the Martin Model.