Phenomenology and Capabilities of Mutually Guided Laser and Neutral Particle Beams for Deep Space Propulsion

Author: Chris Limbach, Ph.D.

Background: By circumventing the rocket equation, particle and laser beams provide attractive mechanisms for accelerating spacecraft to relativistic velocities for interstellar flight. In both approaches the beam divergence and momentum coupling play a central role in architecture optimization and propulsion capabilities. The possibility of combining both approaches, entailing an overlap of co-propagating laser and particle beams, provides an opportunity to tailor an interaction between the optical field and atoms which allows self-guiding through the space environment. 

Objective: The working principle of self-guiding and potential for long-range guided propagation will be explored. In particular, this work addresses the stability of the overlapped beams and progress toward laboratory studies. 

Methods: Self-channeling in vacuum arises from the interplay of refractive modulation of the laser wavefront and optical dipole forces influencing the motion of the atoms. Analyses of the paraxial wave equation and collisionless Boltzmann equation are applied toward obtaining self-consistent spatial soliton solutions. The application of a Landau stability analysis to the coupled equations is then used to understand the dynamical evolution. These analytical approaches are subsequently connected with laboratory methods with the goal of reproducing key phenomenology in ground-based experiments. Relevant experimental methods include TDLAS for characterization of the atomic beam and ultra-high vacuum technologies for simulating the space environment. 

Results: Axisymmetric soliton solutions are obtained for a range of atomic beam mass flow-rates and laser power, with two non-dimensional parameters governing the soliton spatial profiles. These parameters correspond to 1) the strength of refraction versus diffraction, and 2) the strength of the optical dipole force relative to thermal motion of the atoms. Together, they also play a key role in the dynamics, determining the strength of the feedback loop that amplifies or dampens perturbations in the beam profile. It will be shown that a newly discovered phenomenon analogous to Landau damping results in the decay of high wavenumber (small-scale) perturbations that can serve to stabilize long-range propagation. Using modern experimental methods for generating cold, supersonic atomic beams, we show that this phenomenon should be observable in laboratory experiments.