Investigator: Hayden Morgan
Beamed propulsion has been a primary candidate for deep space propulsion, due to its numerous advantages, including reducing the mass of the scientific spacecraft. Proposed beamed propulsion methods typically involve a terrestrial or space-based transmitter that sends energy, usually in the form of photons, to the scientific spacecraft which harnesses the momentum transfer. Beamed photonic energy propulsion suffers largely due to divergence, which is the beam spreading out over vast distances. To combat this, several concepts propose very large transmitters or receivers. The concept being researched at the LDPDL, proposed by Dr. Christopher Limbach and Dr. Kentaro Hara , is a combined particle beam and laser beam propulsion method. Combing these two beams practically eliminates the problem of beam divergence and has the added benefit of using mass momentum transfer, compared to only photons. This propulsion method can be utilized to make deep space missions, including exploring our nearest star, Proxima Centauri, more possible in a human lifetime.
This optically coupled particle and laser beam are governed by two fundamental coupling phenomena. The light guiding occurs due to the higher index of refraction inside the particle beam compared to the vacuum of space outside the beam. This same principle can be seen in step index fiber optic cables. The particle guiding requires the particles to be at very low (sub-Kelvin) temperatures to effectively couple the beams. Once the atoms are cold, the high intensity laser creates and potential “well” through a dipole force that the particles will stay trapped in unless they gain enough energy to escape the well . This particle trapping effect has already been investigated by several researchers [5,6] and works through known physics.Currently, Dr. Hara, Dr. Limbach and several of their students have created several simulations of the optically coupled beam over thousands of kilometers . Additionally, the group has focused on the vacuum facility design necessary to provide a reasonable substitute space-like vacuum environment as well as the design, build, and testing of a rubidium particle jet source. The primary goal of this particle source is to develop a high mass-flux jet of low-divergence atoms. This will result in a particle beam that is easier to laser cool and has a high index of refraction in which to guide the overlapped laser beam. This particle source, shown in Figure 2, consists of several key elements:
- A heated rubidium reservoir that creates a source of gaseous rubidium
- A converging-diverging supersonic nozzle with a Ø0.3mm throat
- A vacuum-compatible ceramic insulator
- A cooled intermediate chamber used to condensate gaseous rubidium
- A beam skimmer used pick off the central cone of interest of the gas jet
The purpose of the cooled intermediate chamber and skimmer are to isolate the central region of the plume from the rest of the gas exhaust, which results in a jet of atoms with very low divergence.
The initial vacuum facility, shown in Figure 3, consists of a custom main vacuum chamber using a turbo-molecular pump to reach pressures in the range of 10-7 torr. In addition to the high vacuum compatibility, the chamber has several thermocouple feedthroughs to monitor the temperature in the heated rubidium reservoir and cooling bellows which are the heat removal source for the cooling intermediate chamber of the jet source. Several cartridge heaters wired through electrical feedthroughs are used in conjunction with a silicon-controlled rectifier (SCR) to heat the rubidium reservoir up to 727°C which causes the rubidium to evaporate. The intermediate chamber cooling system is designed such that the inner walls stay cold enough to cryopump the unskimmed rubidium so it will not bounce off the wall and collide with particles in the cone of interest. Another cooled element to the system is the beam dump which is designed to optimize the amount of reflections against a chilled surface where the rubidium will condense at the end of the run.
To characterize the rubidium beam, a diagnostics viewport is installed immediately before the beam dump. This viewport will allow an absorption spectroscopy measurement to be conducted that will gather information on the number density of the beam as well as its bulk velocity and temperature. These measurements can then be used to provide more accurate simulations and predict the behavior of future experiments in which the propagation distance is much greater, such as the planned system shown in Figure 4. This initial experiment also serves as a benchmark for future laser cooled particle beam experiments in which the density and temperature profile of the beam can be compared to quantify improvements.
Check out this NASA video for more information on this project.
References: Limbach, Christopher, and Hara, Kentaro. PROCSIMA: Diffractionless Beamed Propulsion for Breakthrough Interstellar Missions. 2019, NIAC Phase I Report  Limbach, Christopher, and Hara, Kentaro. “Performance Analysis of a Combined Laser and Neutral Particle Beam Propulsion Concept Based on Self-Guiding”. AIAA Propulsion and Energy Forum. Indianapolis, IN. 2019.  Kuldinow, Derek, et al. “Numerical Simulation of Laser and Particle Coupled Beam Propagation“. AIAA Propulsion and Energy Forum. Indianapolis, IN. 2019.  Slowe, Christopher, et al. “High Flux Source of Cold Rubidium Atoms.” Review of Scientific Instruments, vol. 76, 3 Oct. 2005.  Bjorkholm, J. E, et al. “Observations of Focusing of Neutral Atoms by the Dipole Forces of Resonance Radiation Pressure.” The American Physical Society, vol. 41, ser. 20, 13 Nov. 1978, pp. 1361–1364. 20.  Grimm, Rudolf, et al. “Optical Dipole Traps for Neutral Atoms.” Advances in Atomic, Molecular and Optical Physics, vol. 42, 2000, arxiv.org/abs/physics/9902072v1
 Luria, K., et al. “Generation and Propagation of Intense Supersonic Beams.” The Journal of Physical Chemistry, vol. 115, 2011, doi:dx.doi.org/10.1021/jp201342u.