Strategies for Mitigation of Dust and Charged Ion Impact on Laser-Driven Lightsails

Author: Andrew Higgins, PhD, Aeronautics and Astronautics, Professor, McGill University

Abstract Background: The impact of interplanetary and interstellar dust grains on lightsails is a significant concern for laser-driven interstellar flight. Over a light year of travel, the mean spacing between impact sites on forward-facing surfaces is estimated to be on the order of 100 microns. The fundamental particles of the interstellar medium (ISM: protons, alpha particles, etc.) are also of concern. A particular source of apprehension are impacts on the sail in near-earth space (estimated mean spacing of impact sites ~ 1 mm) which could degrade the low absorptivity/high reflectivity requirements of the sail. In the worst case, laser energy deposition could couple to the sail material, resulting in near-instantaneous destruction of the sail, similar to the well- known phenomena of “fiber fuze” and “laser-supported detonation”.

Abstract Objectives: This study will critically examine a number of strategies to minimize or eliminate the threat to the sail presented by the dust grain and ion impact problems: (1) Use of the drive laser as a means to displace or vaporize interplanetary dust in advance of the sail during the acceleration phase. (2) Design of a “fault-tolerant” sail that can withstand local catastrophic failure of the sail due to laser-coupling, but does not propagate to adjacent regions of the sail. (3) For the interstellar cruise phase (when the sail flies edge-on to the ISM), the use of graded materials (bilayers) on the leading edge to act as channels to direct ions outward from the main sail.

Abstract Methods: Given the embryonic nature of the techniques considered here, the modelling is done via first-order analytic models, including accepted models for laser ablation, laser-supported detonation propagation, and charge particle penetration.

Abstract Results: Dust removal or vaporization in the volume the light sail will traverse during the acceleration phase does not appear feasible due to the large volume that would need to be cleared. Displacement of dust via the laser light transmitted through the sail, as would be the case with thin dielectric sails, may be feasible. A fault-tolerant sail that prevents laser-supported destruction from propagating across the sail appears possible but may necessitate large gaps in the sail, resulting in wasting much of the laser illumination. Charged particle re-direction via graded materials is an established technology that has been demonstrated experimentally in the particle accelerator community.

TRL Assessment: The present level of the technologies considered in this study are Level 1. Completion of the analyses presented will contribute to elevating the TRL to 2-3.

Abstract Development: The talk will conclude with a proposed roadmap of a progressive hierarchy of models and laboratory validation to evolve the more promising of the proposed approaches to TRL 4.

Abstract Near-Term Technical Milestones: Bench-top demonstration of CW-laser-driven ablation of dust should be feasible with 1 W lasers. Demonstration of the ability to prevent propagation of laser-supported sail destruction would necessitate use of 1kW-class lasers. Charged particle re-direct

Abstract Conclusions: The first-order analysis proposed here suggests that the concepts have sufficient potential to warrant additional analysis. It is hoped that this preliminary analysis will stimulate further thinking about nonconventional solutions to the dust grain impact problem.