Can Dusty Plasma radiators protect against lasers?

Question

From Project Rho: http://www.projectrho.com/public_html/rocket/heatrad.php#:~:text=Dusty%20Plasma%C2%A0radiator%3A

Example Image: https://twitter.com/toughsf/status/1154692082478526465

A dusty plasma radiator uses conductive plasma, in turn manipulated by magnetic fields, to manipulate dust particles. This could allow them to be cycled out of the ship and back within to disperse heat. There is some similarity to the idea of a liquid droplet radiator, except with dust and the conductive plasma gives greater control (and you are less likely to lose your radiative material during maneuvers). We mainly just need more understanding of how to precisely control the plasma and dust.

Assume we get over the control issue. We get a continuously flowing stream of dust going from the radiator, out into space, and back into the spacecraft.

Besides functioning as an effective radiator wouldn't these things also be fairly effective against laser attacks?

A laser trying to get through the dusty plasma would be subject to Rayleigh scattering, and because the dust is constantly being shuffled around the heat is actively dispersed instead of concentrated onto a single area. Not to mention the 'on the tin' purpose of a radiator is to radiate heat.

And depending on how much control we developed over magnetically manipulating the dusty plasma, more vulnerable areas could be covered in thicker layers/faster moving dusty plasma.

Does this make sense as a laser defense, or am I completely misundertanding things?

Answer 1

If you can see through it, so can the laser

@Daron's answer of "if it's dense enough and the laser's weak enough" is pretty accurate. I thought I'd need to do some math on the absorption of the dust particles and radiative area (the reason why dusty plasma is a good idea is because of their large radiative surface area to mass ratio, making them great heat exchangers), but the bold argument at the top is, in my opinion, pretty reasonable. If the microparticle plasma is only partially opaque, then that fraction of light that makes it through (e.g., sunlight reflecting from the vessel's hull, through the dust, to your eyes) is the same fraction of laser light that'll make it through (from just outside the magnetic confinement to the vessel's hull).

Actually, it'll be "slightly" worse than that. The dusty plasma will have an effective scattering length (what @Daron's answer describes), but as the laser bombards and the dust's temperature increases, the scattering length will increase until the plasma isn't scattering significantly at all (and the scattering length is already likely to be on the order of the dusty plasma's depth, anyway).

If you're being fired upon, it means you're probably within the effective range of a diffraction-limited laser, where spot size is likely on the order of a meter or less (a few cm spot size at 100,000 km distance is not unreasonable). Dusty plasma simply isn't going to do much against a 100 MW/m^2 pulse, as it's intended to absorb and radiate much, much less than that. Even if your dusty plasma shield was made super-dense and completely opaque, the principles of atmospheric hole burning can be used to quickly and effectively drill through it. Fire a powerful sub-millisecond pulse to ionize the first tens of meters of plasma shield, wait a few fractions of a second for the plasma to expand to near-vacuum, and then rinse and repeat until you've breached the shield and are now penetrating the hull. The first few milliseconds of beam power might have been wasted making it through the plasma shield, but the majority would have made contact.
(It's good to keep in mind that most of the laser's energy will be delivered in less than a second. There's not going to be enough time for the dust to "fill in" the hole before any competently powerful laser burns through and strikes the hull.)

You likely wouldn't want to surround your ship in an opaque cloud of magnetically-confined dust, anyway. Firstly, the upshot of dusty plasma radiators is their high radiative power to mass ratio, making them lightweight radiator options. Secondly, a cloud of the stuff dense enough to be opaque is going comparable in mass to a solid sheet of the same material laminating the hull.

Your best defense against lasers is likely to be some combination of ablative/NERA/composite/capacitive, etc. armor, spaced and angled. Especially angled, weathering the incident beam energy over a greater surface area.

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TL;DR: dusty plasma radiators are good at being low-mass radiators, but you might want to look elsewhere for passive laser defense.

Answer 2

Go with it

TLDR: If the cloud of plasma is big enough plasma and the radiators are good enough and the laser is weak enough then this might work. I do not know the correct equations to calculate it. I say don't bother calculating. Just say all the numbers are whatever numbers needed to make it work and get back to writing the story.

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The laser works by pumping enough heat into a small part of something to melt a hole through it. If your spaceship has a layer of plasma around it, then perhaps it will scatter some of the particles. The scattered particles start bouncing around in the plasma "atmosphere" like in this Shutterstock photo:

The big wavelengths zip straight through the atmosphere and heat the planet. The small wavelengths bounce around. Some hit the planet and some get absorbed by the atmosphere. Some zip back out into space. But here's the thing -- the guys that aren't scattered still heat the planet. You'd expect much less than half of the blue light to get reflected back.

Your layer of plasma works a similar way. It spreads out the laser beam rather than nullifying it. How much it spreads is a physics problem, and I don't know the equations. Ideally it spreads over the whole ship evenly.

In that case you are in luck. Because your ship is good at dissipating heat! If the the radiators are good enough then you cane dissipate the heat faster than the laser supplies it.

Perhaps if you had a really thick atmosphere and a really big spaceship it would keep you safe even without radiators.