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Old 09-06-2017, 10:35 AM   #1
Michael Thayne
 
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Default [Ultra-Tech] [Spaceships] Weapons that are hard or impossible to miniaturize

In Transhuman Space, particle beams have a minimum size that allows them to be installed on SDVs but not smaller ships. (SDVs are typically SM +10 or +11, though Spaceships 8 mentions particle beams can be installed on ships as small as SM +8.) Spaceships 7 has an optional rule to put similar limits on X-ray lasers, antiparticle beams, and grasers. Some questions about this:
  • How implausible are smaller versions of these weapons?
  • How plausible would it be to lift the limits 1 TL after a weapon is originally introduced?
  • SS7 gives the same size limits for all weapons covered by the optional rule. Is this realistic? I can see particle and antiparticle beams having the same minimum size (depending on how the antiparticles are generated), but would X-ray lasers and grasers have the same minimum size?
  • I've heard one possibility for x-ray laser weapons involves a "free electron laser", but IIUC these both have (a) an even larger minimum size than given in SS7 and (b) are tunable, so such a weapon could also function on other wavelengths for say planetary bombardment, a capability Spaceships lasers don't have. So how could an ~SM +10 x-ray laser with the capability in Spaceships work?
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Old 09-06-2017, 10:52 AM   #2
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Default Re: [Ultra-Tech] [Spaceships] Weapons that are hard or impossible to miniaturize

Quote:
Originally Posted by Michael Thayne View Post
[*]How implausible are smaller versions of these weapons?
Not very. The general problem is that you need a certain minimum length and/or radius for any given particle energy, and particle beam performance is heavily dependent on particle energy. This does plausibly vary with tech level, but there's a reason research particle accelerators have gotten bigger and bigger through the years. Free-electron lasers are based on electron accelerators and the wavelength that can be generated depends on the per-particle energy.
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[*]How plausible would it be to lift the limits 1 TL after a weapon is originally introduced?
As long as they're based on accelerator technology, higher TL would reduce but not eliminate the limit.
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[*]SS7 gives the same size limits for all weapons covered by the optional rule. Is this realistic?
No, except maybe by coincidence.
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Old 09-06-2017, 02:41 PM   #3
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Default Re: [Ultra-Tech] [Spaceships] Weapons that are hard or impossible to miniaturize

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Not very. Free-electron lasers are based on electron accelerators and the wavelength that can be generated depends on the per-particle energy.
Did you mean to say they're not very implausible or not very plausible?

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The general problem is that you need a certain minimum length and/or radius for any given particle energy, and particle beam performance is heavily dependent on particle energy. This does plausibly vary with tech level, but there's a reason research particle accelerators have gotten bigger and bigger through the years.
What's the relationship between minimum length and particle energy? As for radius, is that diameter of the circular accelerator?

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As long as they're based on accelerator technology, higher TL would reduce but not eliminate the limit.
What's plausible for how much they'd shrink with TL? Going for -1 SM per TL seems like it might work.

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No, except maybe by coincidence.
I assume grasers would be larger than X-ray lasers? How much bigger?
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Old 09-06-2017, 02:47 PM   #4
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Default Re: [Ultra-Tech] [Spaceships] Weapons that are hard or impossible to miniaturize

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Originally Posted by Michael Thayne View Post
Did you mean to say they're not very implausible or not very plausible?
Not very plausible.
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What's the relationship between minimum length and particle energy? As for radius, is that diameter of the circular accelerator?
Basically linear in both cases.
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What's plausible for how much they'd shrink with TL?
I'd probably go with -2 per TL.
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Old 09-06-2017, 05:01 PM   #5
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Default Re: [Ultra-Tech] [Spaceships] Weapons that are hard or impossible to miniaturize

Reading up on FELs, it looks like wavelength is inversely proportional to particle energy. Therefore would 1/10x wavelength require 10x the length? That would make FEL grasers limited to truly massive installations, even moreso than x-ray lasers, right?

Last edited by Michael Thayne; 09-06-2017 at 08:32 PM. Reason: said "10x" when I meant "1/10x"
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Old 09-06-2017, 05:07 PM   #6
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Default Re: [Ultra-Tech] [Spaceships] Weapons that are hard or impossible to miniaturize

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Originally Posted by Michael Thayne View Post
Reading up on FELs, it looks like wavelength is inversely proportional to particle energy. Therefore would 10x wavelength require 10x the length? That would make FEL grasers limited to truly massive installations, even moreso than x-ray lasers, right?
Presumably there's tech improvements if they're making them as weapons at all, but yes, you'd expect a FEL at gamma wavelengths to be larger than an X-ray installation.
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Old 09-06-2017, 02:18 PM   #7
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Default Re: [Ultra-Tech] [Spaceships] Weapons that are hard or impossible to miniaturize

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Originally Posted by Michael Thayne View Post
  • I've heard one possibility for x-ray laser weapons involves a "free electron laser", but IIUC these both have (a) an even larger minimum size than given in SS7 and (b) are tunable, so such a weapon could also function on other wavelengths for say planetary bombardment, a capability Spaceships lasers don't have. So how could an ~SM +10 x-ray laser with the capability in Spaceships work?
Only very recently has a trick to allow a free-electron laser to produce a clean x-ray beam been figured out, and I'm not sure how well it retains tunability.
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Old 09-06-2017, 09:34 PM   #8
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Default Re: [Ultra-Tech] [Spaceships] Weapons that are hard or impossible to miniaturize

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Originally Posted by Michael Thayne View Post
[*]How implausible are smaller versions of these weapons?
If you're willing to take a cut in efficiency, you could use laser wakefield accelerators to get a particle beam with lengths around 1 to 3 meters (SM -2 to +1).
https://en.wikipedia.org/wiki/Plasma_acceleration
We're still working out some of the kinks at TL 8 - we can get particle energies of a GeV in a few cm, but the emittance may not be tight enough to be usable as a spacecraft-to-spacecraft weapon. Also, the theoretical maximum amount of energy you can transfer from your laser pulse to a particle bunch is 50%, in practice current wakefield accelerators are more like 10% to 20%. At this point you need to ask yourself if you should just use that laser to fry your enemies instead.

If you use the particle beam from a wakefield accelerator to drive an x-ray free electron laser, you can get around the emittance problems - FELs self-organize their electron bunches to draw energy from the particles and transfer it to the x-ray field.

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Originally Posted by Michael Thayne View Post
[*]How plausible would it be to lift the limits 1 TL after a weapon is originally introduced?
Why not? TLs are supposed to represent advancement, and a lot of advancement is all sorts of stuff that is unexpected at lower TLs.

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Originally Posted by Michael Thayne View Post
[*]SS7 gives the same size limits for all weapons covered by the optional rule. Is this realistic? I can see particle and antiparticle beams having the same minimum size (depending on how the antiparticles are generated), but would X-ray lasers and grasers have the same minimum size?
Modern x-ray lasers are mostly generated by particle beams, so if you don't have the minimum size for the particle beam you don't get the laser.

We don't have gamma ray lasers, but you can use particle beams to get narrow gamma ray beams using laser Compton backscatter (in principle, you can also use positron annihilation in flight, but that's a tech path that we are not currently pursuing in favor of Compton backscatter).

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[*]I've heard one possibility for x-ray laser weapons involves a "free electron laser", but IIUC these both have (a) an even larger minimum size than given in SS7 and (b) are tunable, so such a weapon could also function on other wavelengths for say planetary bombardment, a capability Spaceships lasers don't have. So how could an ~SM +10 x-ray laser with the capability in Spaceships work?[/LIST]
How could it work? You accelerate up a bunch of electrons and send it through the gap between two Halbach arrays of magnets (a "wiggler").
https://en.wikipedia.org/wiki/Halbach_array
This forces the electrons to move in a sinusoidal pattern. The modes of the electromagnetic field whose force is constantly opposing the motion of the electrons will have work done on them and be amplified. With a suitably long wiggler or a resonant cavity, you can get a very narrow band of modes, like a laser beam. If the radiation field is intense enough and the wiggler is long enough, the electrons will self-organize into bunches to produce super-radiant coherent motion. Now you have a free electron laser.

For a tuneable FEL, you will need separate wigglers for different frequency bands. The wiggler for the x-rays will have a pitch way too short for when you need visible light, so you would divert the electron bunches to a visible light wiggler. (The visible light wiggler would probably also be inside a Fabry-Perot resonant cavity,
https://en.wikipedia.org/wiki/Fabry%...interferometer
allowing you to achieve your laser behavior threshold with a much shorter wiggler.)

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Old 09-07-2017, 09:47 AM   #9
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Default Re: [Ultra-Tech] [Spaceships] Weapons that are hard or impossible to miniaturize

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Originally Posted by lwcamp View Post
Modern x-ray lasers are mostly generated by particle beams, so if you don't have the minimum size for the particle beam you don't get the laser.

We don't have gamma ray lasers, but you can use particle beams to get narrow gamma ray beams using laser Compton backscatter (in principle, you can also use positron annihilation in flight, but that's a tech path that we are not currently pursuing in favor of Compton backscatter).
How likely is it that the size of particle accelerator you'd need for a particle beam weapon is significantly larger/smaller than the one you'd need for an x-ray laser weapon? Similarly, am I correct that the accelerator for a graser would likely need to be orders of magnitude larger than the one for an x-ray laser? Does Compton backscatter present its own issues with miniaturization? How does positron annihilation in flight lead to a coherent beam?

Quote:
How could it work? You accelerate up a bunch of electrons and send it through the gap between two Halbach arrays of magnets (a "wiggler").
https://en.wikipedia.org/wiki/Halbach_array
This forces the electrons to move in a sinusoidal pattern. The modes of the electromagnetic field whose force is constantly opposing the motion of the electrons will have work done on them and be amplified. With a suitably long wiggler or a resonant cavity, you can get a very narrow band of modes, like a laser beam. If the radiation field is intense enough and the wiggler is long enough, the electrons will self-organize into bunches to produce super-radiant coherent motion. Now you have a free electron laser.

For a tuneable FEL, you will need separate wigglers for different frequency bands. The wiggler for the x-rays will have a pitch way too short for when you need visible light, so you would divert the electron bunches to a visible light wiggler. (The visible light wiggler would probably also be inside a Fabry-Perot resonant cavity,
https://en.wikipedia.org/wiki/Fabry%...interferometer
allowing you to achieve your laser behavior threshold with a much shorter wiggler.)

Luke
So TLDR; if you want to justify a non-tunable x-ray laser you could assume the multiple wigglers was not worth the trouble?
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Old 09-07-2017, 08:07 PM   #10
lwcamp
 
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Default Re: [Ultra-Tech] [Spaceships] Weapons that are hard or impossible to miniaturize

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Originally Posted by Michael Thayne View Post
How likely is it that the size of particle accelerator you'd need for a particle beam weapon is significantly larger/smaller than the one you'd need for an x-ray laser weapon?
Judging by modern facilities, they'll be about the same size.

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Similarly, am I correct that the accelerator for a graser would likely need to be orders of magnitude larger than the one for an x-ray laser?
Roughly. Unfortunately, "x-ray" and "gamma-ray" are not all that well defined in terms of the photons themselves, but rather in how they were generated. X-rays are usually defined as photons created by core electron transitions in atomic systems; gamma rays are photons made by transitions between nuclear energy states. So the energy range of what is considered an x-ray can overlap with what is considered a gamma ray. Roughly, though, most x-rays that anyone has to deal with are between 10 and 100 keV, gamma rays tend to be between 100 and 2000 keV. So yeah, that's about an order of magnitude difference in energy and thus an order of magnitude difference in accelerator length.

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Does Compton backscatter present its own issues with miniaturization?
The Compton backscatter sources that people are working on now could easily fit in a van. They won't have enough energy to melt things, of course (mainly they're for detecting contraband nuclear material). The main issue is that the beam spread is likely to be too large for a long range weapon.

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How does positron annihilation in flight lead to a coherent beam?
You get a beam of positrons and a beam of equal energy electrons and collide them head-on. When you go work out the angular distribution of the resulting annihilation gamma rays, you find out that they are mostly directed along the beam axis. Make the electron and positron energies high enough and you can get fairly narrow beams. Again, like Compton laser backscatter, the beam spread may still be too large for practical offensive applications.

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So TLDR; if you want to justify a non-tunable x-ray laser you could assume the multiple wigglers was not worth the trouble?
Pretty much, yeah. But keep in mind that for less than an extra 10% mass and volume, you can get a visible light laser of the same power as well. Unless you have wakefield accelerators, which might be so compact that the wigglers are comparable in size to the accelerator - but in that case you already have a high energy laser in the near visible or visible band that you might as well use.

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