The Design and Characterization of an Atomic Rubidium Source and Apparatus for Coupled Laser and Particle Beam Propulsion Experiments
Abstract
The need for a high specific impulse and innovative space propulsion technology is growing as NASA sets goals for exploring celestial bodies in our solar system and beyond. Directed energy space propulsion has been a candidate for these missions as it removes the need for thrusters and reaction mass systems from the science spacecraft. However, these proposed directed energy concepts, either in the form of electromagnetic (EM) radiation pressure (e.g. laser) or mass momentum beams (e.g. ion beams) suffer from inherent divergences that ultimately lower the total impulse transferred. A new concept for directed energy space propulsion utilizes a neutral atom beam spatially overlapped with a co-propagating laser which is detuned from the resonant frequency of the atom. Through refractive guiding and the optical dipole force, this combined beam shows potential for self-guiding over millions of kilometers.
The next stage for this low Technology Readiness Level (TRL) technology is ground experiment validation of theory and simulations. The present work describes efforts directed toward experimentally studying the combined beam propulsion system in laboratory-scale facilities. This includes the development of an atomic rubidium jet source, encapsulating vacuum facilities, laser overlapping and separation apparatus, and diagnostic techniques for studying the laser and atom beam. Tunable diode laser absorption spectroscopy (TDLAS) is applied toward characterization of the propagation parameters of the atomic jet including density, temperature, and axial velocity. The laser absorption profiles extracted from the TDLAS measurements indicate the presence from a cold group of atoms (< 10 K) and a hot group of atoms (50 - 200 K) which is predicted in literature for an atomic source that is transitioning from the continuum to free-molecular flow regime. The TDLAS data indicate that for a rubidium getter source at high current, the mass flow of the beam is approximately 2 × 10−3 µg/s. Although the mass flow rate from the rubidium source was lower than predicted, calculations of the waveguide V-Parameter indicate a potential light guiding capability for laser detunings less than 30 GHz from resonance. Findings from this initial experimental study to characterize the rubidium jet were used to influence the design of a combined beam study which attempted to produce a sufficient dipole potential to reduce the divergence of an atomic jet.
The present work also describes novel advancements to the measurement capabilities through the development of new experimental apparatuses. The experiment added two additional TDLAS measurement stations and utilized a semi-automated synchronized spatial scanning gantry. Additionally, a method of separating the combined beam into the constituent atomic/laser beams was devised in order to study the final overlapped laser profile. Results from the combined beam experiment indicated that the atom temperature was at or above the 300 K ambient temperature, suggesting a significant degree of rubidium was present as background species in the chamber rather than in a low divergence jet. With the contamination of the jet diagnostics from the background rubidium, the behavior of the atomic jet could not be analyzed. However, the laser overlapping and separation systems did provide valuable data in terms of the interaction of the guiding laser with a rubidium vapor. The overlapped laser profile experienced detectable attenuation as it was tuned about the atomic resonance of the rubidium, which could be used in future experiments for atom jet bulk velocity measurements. Additionally, when the overlapped laser was tuned close to the atomic resonance, the TDLAS diagnostic stations confirmed a depletion of the ground state number density via the measured transmission.
Subject
space propulsionpropulsion
alkali source
vacuum
laser propulsion
particle beam propulsion
laser diagnostic
absorption spectroscopy
rubidium
Citation
Morgan, Hayden Patrick (2021). The Design and Characterization of an Atomic Rubidium Source and Apparatus for Coupled Laser and Particle Beam Propulsion Experiments. Master's thesis, Texas A&M University. Available electronically from https : / /hdl .handle .net /1969 .1 /196098.
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