Laser Diagnostics Map Rare Gas Dynamics in Gridded Ion Thruster Experiments

Optical diagnostics utilizing laser-induced fluorescence spectroscopy have been successfully implemented to characterize neutral rare gas distributions in gridded radiofrequency ion sources, establishing critical experimental baselines for assessing accelerator grid erosion. Researchers from the Leibniz Institute of Surface Engineering (IOM) in Leipzig, in collaboration with ArianeGroup GmbH, established single-photon and two-photon absorption laser-induced fluorescence diagnostic set-ups to probe neutral krypton and xenon atoms without physically perturbing the plasma environment. Their investigative series, carried out in a 3-meter-long vacuum facility, focused on mapping neutral atom densities in the plume of an operational radiofrequency thruster as well as resolving neutral velocity distributions directly inside an active discharge chamber.

Understanding neutral particle dynamics is essential for the long-term operational qualification of electric space propulsion systems. While gridded ion engines and Hall-effect thrusters rely on heavy noble gases such as xenon or krypton as efficient propellants, un-ionized neutral atoms escaping or lingering within the acceleration zone give rise to charge-exchange collisions with high-velocity primary ions. These interactions generate slow secondary ions that are subsequently pulled toward the negatively biased accelerator grid by electrostatic attraction. The energetic bombardment of these secondary ions sputters away grid material, progressively widening aperture holes and ultimately dictating the operational lifespan of the engine. Because ground-based experimental lifetime qualification can require tens of thousands of operating hours at enormous financial cost, predictive numerical simulations like the IOM-developed DYNASIM code are routinely deployed, creating a stringent demand for spatially resolved empirical data on neutral gas flows and temperatures to serve as simulation inputs and benchmarks.

To non-intrusively interrogate ground-state neutral populations of rare gases, the researchers turned to two-photon absorption laser-induced fluorescence, designated as TALIF. By utilizing simultaneous absorption of two UV photons, accessible energy states can be reached using commercially available frequency-doubled pulsed dye lasers.

Initial TALIF verification was performed in the expansion plume of an ArianeGroup RIT-10 radiofrequency ion thruster equipped with a two-grid screen-accelerator assembly. In these trials, the thruster was operated using krypton at a flow rate of 8 standard cubic centimeters per minute and a beam current of 210 milliamperes. The investigators first confirmed that the acquired fluorescence intensity scaled linearly with neutral krypton density across the expected plume density regime. Spectral scans targeting the krypton transition at 758.7 nanometers were recorded perpendicular to the beam axis using a gated photomultiplier and boxcar averager. The excited state lifetime was measured to be 26 nanoseconds in the plume, corroborating established literature values and confirming that collisional quenching remained negligible at the dilute local densities of 10 to the 17th power to 10 to the 18th power particles per cubic meter.

Axial scans along the center axis of the RIT-10 thruster revealed the rapid decline of neutral krypton density downstream of the engine exit plane. Due to physical constraints imposed by the thruster housing, which projected approximately 6 centimeters beyond the accelerator grid plane, the closest optical measurement possible in the perpendicular optical configuration was 7 centimeters from the grid. Comparisons between active thruster firing and cold gas flows demonstrated that background facility gas made a measurable contribution to the overall signal at these distances, highlighting the operational necessity for higher pumping speeds or relocated optical delivery heads to capture near-grid phenomena. Furthermore, radial profiles acquired at an axial distance of 8 centimeters under a 12 sccm cold gas flow showed an expected symmetric drop in neutral density away from the central axis, displaying qualitative alignment with analytical isotropic gas expansion models while emphasizing the need for comprehensive geometric modeling that incorporates thruster housing shadows and grid curvature.

Investigating the influence of electrical operating parameters, the researchers tracked TALIF signal intensities at an axial distance of 8 centimeters while varying the beam current at a steady krypton flow of 7 standard cubic centimeters per minute. The empirical data demonstrated that increasing the extraction beam current led to a steady drop in detected neutral fluorescence along the central axis. Extrapolating the measured fluorescence signal toward the theoretical equivalent current corresponding to total propellant mass flow yielded background signal levels in strong agreement with passive facility gas tests.

Moving beyond external plume expansion, the research team demonstrated optical probing directly inside an active plasma environment using the ISQ40RF radiofrequency ion source. Developed at IOM, this source features a discharge chamber constructed from ultraviolet-grade fused silica, offering comprehensive optical access between the exterior turns of the induction radiofrequency coil. In an initial axially aligned TALIF arrangement, neutral xenon fluorescence at 788.7 nanometers was collected through the grid apertures while the excitation laser entered radially through the silica wall. Strong background plasma luminescence accompanied the active discharge, limiting reliable TALIF signal extraction to source operating regimes with mass utilization efficiencies below 25 percent and pointing to future optical filtering and angled detection refinements.

To overcome plasma background interference and interrogate neutral kinetic states inside the discharge chamber, single-photon absorption laser-induced fluorescence (LIF) was deployed. Unlike TALIF, which targets ground-state populations, single-photon LIF exploits transitions from lower metastable or collisionally excited states populated within the non-equilibrium discharge. Probing neutral xenon via an absorption line at 834.7 nanometers with fluorescence detection at 473.4 nanometers, the team utilized a master-oscillator power-amplifier continuous-wave diode laser system. The narrow laser linewidth of approximately 200 kilohertz allowed sub-picometer wavelength scans across a 32-gigahertz mode-hop-free range, yielding resolved absorption lineshapes free from instrumental broadening artifacts.

The measured xenon LIF absorption spectra reflect the convolution of natural lifetime broadening, Doppler broadening from thermal particle motion, and complex hyperfine structure splitting induced by nuclear moments across nine stable and long-lived xenon isotopes. To isolate kinetic information, the investigators implemented a Fourier deconvolution procedure that systematically removed the tabulated hyperfine lineshapes and natural Lorentzian contributions without imposing an assumed profile onto the raw data.

From the Doppler full-width at half-maximum of the deconvolved profiles, the kinetic temperature of the neutral xenon atoms inside the discharge chamber was deduced as a function of supplied radiofrequency power. At the lowest tested rf power setting, the neutral gas exhibited a kinetic temperature of approximately 560 Kelvin, which climbed systematically to 740 Kelvin as the absorbed radiofrequency discharge power was increased. A constant apparent Doppler redshift of approximately minus 100 megahertz was detected across the operating conditions, which the authors attributed to a systematic wavelength measurement offset associated with multimode optical fiber coupling into the Fizeau wavemeter rather than a net physical drift away from the optical probe.

These experimental milestones confirm the viability of integrating non-intrusive laser spectroscopy into comprehensive thruster qualification programs. The vacuum-compatible LIF probe hardware was specifically designed to interface directly with the Advanced Electric Propulsion Diagnostics platform, an integrated suite developed by IOM and partner institutions incorporating pyrometers, tele-microscopes, Faraday cups, and retarding potential analyzers.

Subject of Research: Laser-induced fluorescence diagnostics for neutral rare gases in electric space propulsion systems

Article Title: Single- and two-photon absorption laser-induced fluorescence spectroscopy in rare gases for gridded ion thruster diagnostics

Article References: Eichhorn, C., Pietzonka, L., Scholze, F., Bundesmann, C., Spemann, D., Neumann, H., & Leiter, H. J. (2022). Single- and two-photon absorption laser-induced fluorescence spectroscopy in rare gases for gridded ion thruster diagnostics. EPJ Techniques and Instrumentation, 9(1), Article 2. https://doi.org/10.1140/epjti/s40485-022-00077-y

Image Credits: AI Generated

DOI: 10.1140/epjti/s40485-022-00077-y

Keywords: electric propulsion, gridded ion thrusters, laser diagnostics, laser-induced fluorescence, two-photon absorption, rare gases, krypton, xenon, charge exchange, grid erosion, RIT-10, ISQ40RF, plume diagnostics, discharge chamber, Doppler broadening, velocity distribution