Optical Cavity in Relativistic Regime for Laser Propulsion

TL;DR

This paper models a photon-recycling optical cavity for relativistic laser propulsion, demonstrating how optimized multilayer mirrors can significantly enhance thrust while managing thermal stability for interstellar flight.

Contribution

It introduces a delay differential model for the cavity dynamics, incorporating Doppler effects and time delays, and derives spectral conditions for mirror design to optimize thrust and thermal stability.

Findings

Thrust gain is significant with optimized multilayer reflectors.

Thermal management constraints limit cavity performance.

Spectral mirror properties are critical for system optimization.

Abstract

Laser propulsion has been proposed for relativistic interstellar flights, but it faces the significant challenge of requiring extremely powerful laser radiation due to the inherently low momentum transfer between the beam and the sail. The photon-recycling technique enhances thrust by transferring momentum through multiple reflections within a cavity setup, formed by the lightsail and a ground-based mirror in a laser system array. In this work, a delay differential model is developed to describe the evolution of the beam and thrust, incorporating both the Doppler effect and the round-trip time delay experienced by each beam component. With optimized multilayer reflectors, the thrust performance gain is shown to be significant for interstellar flight, though limited by diffraction and the necessity of removing harmful redshifted radiation that could overheat the lightsail. By balancing thrust performance with thermal stability, we derive a simple condition for determining the spectral requirements of the mirrors. Given a selected laser wavelength, this condition fully specifies the necessary properties of the cavity mirrors, enabling the same system to effectively support a range of launch protocols.