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Comments below this answer suggest that a laser ablation-based propulsion system, where the laser is separate from the object to be accelerated, could accelerate an object to about $\frac{1}{1000}$ of the speed of light, and that the electric field would accelerate ablated atoms to nearly the speed of light and therefore generate substantial momentum transfer.

While it's a hypothetical situation, I'm just curious about this acceleration. I normally think of laser ablation as a thermal process; absorbed light dramatically increases the temperature locally in a very thin layer of strongly absorbing material which then expands due to a large increase in temperature. So it would be somewhat analogous to a rocket in that it's thermal expansion that creates the velocity and therefore momentum, though it's a lot hotter than what you can get from combustion of the reaction mass alone.

But is there really a mechanism where the electric field of the laser beam itself can generate enough acceleration to bring the plasma particles to a velocity remotely close to that of light? The light is "AC" at several times 1014 Hz, how could it provide such a net "DC" acceleration?

Or could it be some kind of self-bias of the plasma due to the short pulse length? Highly mobile electrons escape first, leaving a lot of self-repulsive positive charge that accelerates itself?

Are there sources about the physics behind this kind of proposed propulsion where I can read further?

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  • $\begingroup$ Does the laser actually need to accelerate the gas electromagnetically, or just heat it to an extremely high temperature (possibly with secondary heating of the plasma exhaust separate from the initial vaporization)? $\endgroup$
    – ikrase
    Mar 25, 2020 at 9:38
  • $\begingroup$ @ikrase My question asks about a specific explanation as described in the first sentence. For your question about laser-thermal acceleration I think you can choose a temperature, calculate the characteristic velocity associated with that temperature, plug it in to the rocket equation, and see what you get for a mass ratio when the final velocity is 0.001 c. $\endgroup$
    – uhoh
    Mar 25, 2020 at 9:46

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This should be useful to you. Take especial note of the method of bombarding afoil with a femotsecond pulse beam to emit relativistic ions. Granted, not sure if that would work with beam propulsion from a remote source (you could bounce the pulse off a mirror to strike the foil from the correct direction, I suppose.

https://www.psi.ch/sites/default/files/import/lmx-interfaces/BooksEN/Claude_JPP_2010-1.pdf

To give you my two cents, this seems totally possible with beamed power from a ground or orbital station. The question is, what size is the vehicle, and at what point, given vehicle mass considerations, would you want to use a laser sail rather than a laser ablation engine.

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