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I´m building a hard-sifi setting and want to use Stellasers (abbreviation for stellar laser) to provide thrust and power to interstellar vessels. This part one is only concerned with the construction and physics of the laser and its abilities. Matters concerning the ship-side issues will be addressed in Part II.

The Basics

My Knowledge
I possess a basic understanding of the way lasers work. I encountered the concept of Stellasers only two times before. On the website http://www.laserstars.org/ which seems to be no longer available and in videos made by the futurist Youtuber Isaac Arthur [1], who mentions the concept regularly. Episodes, where the concept is described, are "Colonising the Sun", "Interstellar Highways", "Exodus Fleet" and many more.

Technology Level
Everything should happen with known physics. Should "optimistic estimates" be required to make the Stellaser work, so be it. I also want this to be a project for a sub-Kardashev-Type-2 civilization. That said, the simpler the proposal the better.

What should be delivered

  • a continuous beam delivering 5 Petawatt should be sufficient for my purposes (the value is somewhat arbitrary, if you want to change it by a few orders of magnitude feel free to do it)
  • the beam needn´t be strongly focused, as the target is several thousand $km^2$ big and should survive
  • none of the designs ask for one giant beam source, large arrays containing thousands of the chosen design them may be built

The Designs

Coronal Laser
The first idea is to place two mirrors as low as possible in the star's atmosphere to use it as my gain medium.

  • energy is delivered to the system via the light of the star in whose corona it orbits
  • tethers would connect the mirrors to prevent them from drifting apart
  • an outside laser would be used to target, stabilize and force population inversion
  • the laser would be placed in a polar orbit tangential to the star
  • protection from the element of fire would be delivered by maintenance, clever shading, supermaterials, magnetic fields (also useful for keeping height and stabilization), and active cooling (radiators are kept in the mirrors shadows)
  • a Fresnel Lens [2] could be used to behind the partially reflective mirror

Dance of Mirrors
This isn´t a laser but rather a partial Nicolle-Dyson-Beam. But it might get the job done. So it´s on the list.

  • several huge mirrors are placed in specific orbits around the star, allowing them to focus their light on the same point
  • they are stabilized and maneuvered using other mirrors, lasers or rockets

Power satellite

  • a satellite gathers energy with solar panels and uses it to power a laser
  • also placed in a polar orbit
  • stabilized and targeted with rockets

Question

  • Which is the best one concerning energy output, practicality, and efficiency? (all designs are based on a spherical disc with a radius of 50 km for reference)

[1] https://www.youtube.com/channel/UCZFipeZtQM5CKUjx6grh54g

[2] https://en.wikipedia.org/wiki/Fresnel_lens

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  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$
    – L.Dutch ♦
    Apr 13, 2019 at 7:21
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    $\begingroup$ @TheDyingOfLight. Have you read Ringworld by Larry Niven? Larry has the Ringworld star lase as both defence and propulsion. Greg Benford also puts out some useful thoughts in Sundiver. $\endgroup$
    – pHred
    Apr 15, 2019 at 2:33
  • $\begingroup$ @pHred Larry Niven is on my reading list. How exactly does the ring world laser work? $\endgroup$
    – TheDyingOfLight
    Apr 16, 2019 at 13:36
  • $\begingroup$ @AtTheDyingOfTheLight The star's corona is magnetically manipulated & then pumped to lase in the high UV. It is used as both meteor defence & in later books in the series propulsion. Many more than 5 petawatts. $\endgroup$
    – pHred
    Apr 17, 2019 at 5:06
  • $\begingroup$ @pHred Um David Brin wrote Sundiver what story are you thinking of? $\endgroup$
    – Ash
    Apr 23, 2019 at 12:56

3 Answers 3

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1e15 W isn't that much actually

If we take approximate proportions for power densities for existing CO2 lasers(simple design and all that) the construction you require for such a laser maybe can be something like 2-3km in diameter and with 1:10 proportion(about one existing in actual designs) 20-30 km in length.

with 0.4 efficiencies of converting solar to energy, and with 0.05 efficiency of pumping the energy, we may talk about 6800km diameter solar energy collector(if we assume enegy collection and not some nuclear reaction happening in the laser medium, which probably possible at those scales of laser device)

so we talk in the conditions about something like 1/4 of K1 (even if you use Sagan version of scale it definitely not a K2 level, and even not wanna be K2)

size matters, the power required matters while you choose options, as each approach has its problems to be solved, and you need weight them against the result you get - are they worth solving or not.

Coronal laser will require statites or some other means to hold the producing beam in the direction you would like to have it, etc.

partial Nicolle-Dyson-Beam will have a fundamental problem of focusing light, so as will require dynamic adjustment while used and while used to with your target. Focusing light problems like different wave length's, and stuff like that(at least direction of problems) Diffraction-limited system

At the scale, you mention 3rd option is probably a choice, as all the complexity all the engineering problems are concentrated in the device itself, no need to count other stuff in and all they are solved by adjusting stuff in the design itself

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  • $\begingroup$ "if we assume enegy collection and not some nuclear reaction happening in the laser medium, which probably possible at those scales of laser device" This sounds intriguing. Wouldn't that mess up the lasers effect? Inertial confinement fusion would be the way to do this I suppose? $\endgroup$
    – TheDyingOfLight
    Apr 23, 2019 at 13:37
  • $\begingroup$ @TheDyingOfLight though in a direction of Gaseous fission reactor, seems a potential(unsuccessful) thing on its own Nuclear pumped laser. The point is that with certain concentration the fission reaction will happen in the volume without much effort like it would be with fusion. The problem of fission in the case, it requires sort of 83t of reacting U235(or similar stuff) each second, a bit too much, hassle/problems of mining at the speed, so as sources of it. $\endgroup$
    – MolbOrg
    Apr 24, 2019 at 12:37
  • $\begingroup$ Fusion can be, in fact, it may be a fusion reactor, as with scale some problems go away(like rate of cooling plasma, frequency of instabilities, requirements for confinement), so as there can side benefits of having it (like powerful enough neutron source, to use to transmute elements(like breeding fusion fuel, or else)) So yes, considering the scale of potential collector, it makes total sense to consider confined options - when laser device and energy production are combined in one volume. $\endgroup$
    – MolbOrg
    Apr 24, 2019 at 12:37
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    $\begingroup$ Thanks for the explanation and the website. This helps me a lot with my setting. $\endgroup$
    – TheDyingOfLight
    Apr 24, 2019 at 13:38
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    $\begingroup$ @TheDyingOfLight glad to be helpful to an IA fan, with a chance to be even more helpful, both of those have discord communities, quite large and quite active, SFIA and ToughSF $\endgroup$
    – MolbOrg
    Apr 24, 2019 at 23:16
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All but one of the "Stellar Laser" designs that I'm familiar with aren't true Lasers they're giant particle beams; this is true of Larry Niven's Ringworld and the several weapons/engines that can be produced using K-II Dyson sphere/swarm designs. The only true Star Laser I can think of actually comes from Crawford Kilian's Empire of Time and uses coherent gravity lenses to create a phased light beam using solar radiation, which is the only way of getting the necessary scale that I can think of. It doesn't require a K-II civilisation but you couldn't do it with our current technology either.

The issue with actually using a star directly as a power source is the output; 5 petawatts is just too low a number. As such the best option is power satellites, you can gather just the energy you need, control the input/output to smooth out fluctuations in stellar output and pump out just the frequency you want, as much as you want, and when you want it.

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Just start mass producing power satellites with lasers and launch a billion or so (depending on the size of each) of them. You might want to consider a solar pumped laser with a large mirror to collect sunlight, with the development needed at this scale that will be more efficient than a photovoltaic solar array. The reaction force on each individual satellite will be quite small, so making judicious use of the solar wind can compensate for that, no need for rocket motors to keep the satellite in place.

Add in SpaceX's reusable Starship rocket, or possibly an even bigger next version of that, and we already have the technology now to build this. It will just cost a lot of money and probably take a few decades, but no reason why this couldn't be done if we were motivated enough.

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