A question about neutron cannons

[ClaysGhost]

Neutrons do respond to magnetic fields, as has been pointed out, through their magnetic moment. I think that the magnetic moment is comparatively useless for beam acceleration and guidance, though. The force on a particle through its magnetic moment depends on the spatial rate-of-change of the magnetic field, rather than its "absolute" value. That is, if you had an absolutely constant field throughout all of the relevant space and you sent a neutron through it, the neutron would align its magnetic moment with the direction of the magnetic field. But it wouldn't be deflected because the field has zero change across space.

To deflect the neutron rather than just spinning it (as the constant field would) you need a non-uniform field, one that varied from place to place: for the moment force on a low-energy neutron (moving at only 1km/s !) to compete with the magnitude (if not the direction) of the magnetic force on a proton produced by a magnetic field of 1 Tesla, I estimate that you would require a magnetic field gradient of over 10^10 Tesla/metre. I think that's unrealistic. Especially so since the magnetic force is used in accelerators to focus and contain rather than accelerate (because the magnetic force always acts at right-angles to the particle's velocity) so a moment-based accelerator wouldn't even be able to do the collimation job plausibly. It's really in competition with electric fields for the accelerator job.

The directions of the two magnetic forces are also different, and my rough guess is that the magnetic field configuration required for the neutral particle case would be extremely inconvenient, especially for a very high flux of neutrons (you might need the magnetic field generator to be positioned in the centre of the beam and hence exposed to lots of neutrons). I think the beam divergence is likely to be very great as well; perhaps a neutron spray-can rather than a neutron cannon.

The inverse beta-decay is a nice idea, but you need to coerce a stable particle (the proton) with copious neutrinos and electrons. I believe it to be unlikely that you'd be able to transmute much of your beam very quickly in the length of a convenient weapon. You could rewrite the relevant physics to make the decay faster, but if you did that you might as well start rewriting at the root and ignore beta decay altogether; accelerate a "new" charged particle that can be coerced into decaying to a neutron in a convenient fashion. If you are set on neutron cannons then this or inverse beta decay are favourite, I think.

Another option, pointed out by previous posters, is gravity control; recommended if you've used it elsewhere. I have some doubts, because everything in a gravitational field drops at the same rate independent of mass (or masslessness); I figure that to accelerate a neutron to 10,000 km/s (not very fast) in 100 metres you'd need to drop it through a field of over 50 billion gees, which I think poses some structural challenges, not to mention some relativistic issues. But! If you can produce such a gradient (free of consequences) outside your ship somehow, you're sorted (and can use a much longer path length to boot).

For general massive particle-based weapons, it's clearly easiest to neutralise a charged particle beam at the weapon's emitter, as has been described by other posters. You will get a neutral particle beam that will prove easily as damaging and much more reliable and efficient than a neutron cannon. At weapon-level energies and time scales there will be little to choose between the neutron and a hydrogen atom. You will even get a glow at the weapon's exit port, gratis, as the electrons and protons combine and the electrons radiate their excess energy. Ok, it will be faint, but neutron cannon beams will be completely invisible to EM.

[Karza]

That nailed it down. Hydrogen beams, here we come. Thank you.

[McC]

So, how does a hydrogen beam affect something? Is it a purely kinetic weapon? What makes it superior (or inferior) to, say, a laser? A 'realistic' beam weapon has always been an interest of mine, so if anyone can elaborate further on the specific functionality of a hydrogen beam, and its usefulness as compared to a laser, I'd much appreciate it.

[Darth Wong]

So, how does a hydrogen beam affect something? Is it a purely kinetic weapon?

Basically. The atoms (assuming near-c velocity) carry tremendous KE.

What makes it superior (or inferior) to, say, a laser?

Presumably the scaling limits of lasers. Eventually as you keep scaling it up you start running into problems with mirrors etc., whereas one could theoretically build an enormously powerful particle accelerator. Also, a hydrogen beam would be insensitive to the reflectivity of the target, and although I'm not sure about the probabilities, if the electrons attach themselves to the atoms at high energy levels and drop into lower levels as the beam propagates, then the beam might even be somewhat visible due to line radiation. This would help with targeting (the ability to see and track your own beam is of considerable assistance in targeting because you can adjust with negative feedback).

[McC]

Ah, ok. So it's fundamentally no different than using any other atom as an ammunition supply for a particle beam? For instance, would a carbon particle beam be significantly different (aside from particulate mass and therefore input energy necessary to accelerate it) from a hydrogen beam?

Also, how quickly would it diffuse compare to, say, a laser?

[Winston Blake]

Ah, ok. So it's fundamentally no different than using any other atom as an ammunition supply for a particle beam? For instance, would a carbon particle beam be significantly different (aside from particulate mass and therefore input energy necessary to accelerate it) from a hydrogen beam?

AFAIK, nope, not significantly. You can also think of them like high-energy alpha particles that are neutral (in which case it's a helium beam).

I assume you haven't read this already; it's a good general description of particle beam weapons.

Also, how quickly would it diffuse compare to, say, a laser?

Well it'd depend on the laser, but the trick is with either neutral atom method is the neutralisation of the beam. Stripping off an extra electron or attaching one both would cause a slight spreading of the beam, which would tend to limit it's effectiveness at long range.

The stripping idea is mentioned in that article, but i've only heard of the attaching method now from Mike regarding fusion (which isn't exactly long range), so perhaps stripping results in less diffusion, so it's better for a space weapon (and probably wouldn't glow). Anyway, a space-war society would probably have more experience with lasers, so they'd diffuse less simply through expertise.

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It's nitpicky, but with the neutron idea, i'm pretty sure that's called electron capture and that inverse b- decay is when a neutrino hits a proton and turns it into a neutron while ejecting a positron.

BTW, regarding the PBW article, there's a few things that feel iffy to me. Like how he says neutrons and alpha particles would be a side-effect of an endoatmospheric PBW. Could that happen in air, and if it did, wouldn't spallation only work with protons?

[Darth Wong]

From a writing perspective, you need some sort of plot-device explanation for their use of particle beams over lasers. Lasers seem like the most realistic sci-fi weapon.

[Winston Blake]

Apparently the neutralisation method of attaching an electron to a positive ion only works for relatively low energies. So an NPBW in space would have to be negative-ion based.

Also for anyone else interested in the neutral-beam-injection plasma heating DW mentioned, here's a nice intro.

[Il Saggiatore]

Also, how quickly would it diffuse compare to, say, a laser?

Well, we can send a laser beam to the Moon and back (search for Lunar Laser Range Reflectors deployed during the Apollo Missions).
Also, there is a laser used for atmospheric experiments, LIDAR.

[Darth Wong]

The longer the range, the more effective lasers would be relative to neutral-beams. But perhaps neutral-beams would be more powerful (assuming your futuristic society can keep developing the tech but runs into snags with mirrors and lasers), so if you're writing them into sci-fi you might have the neutral-beam weapon be something you try to use for heavier punch at closer range.

[Stofsk]

The longer the range, the more effective lasers would be relative to neutral-beams. But perhaps neutral-beams would be more powerful (assuming your futuristic society can keep developing the tech but runs into snags with mirrors and lasers), so if you're writing them into sci-fi you might have the neutral-beam weapon be something you try to use for heavier punch at closer range.

Hmm... how good would lasers and particle beams be at, say... orbital bombardment? No difference, or would one have a clearer advantage over the other in terms of effectiveness?

[Darth Wong]

The longer the range, the more effective lasers would be relative to neutral-beams. But perhaps neutral-beams would be more powerful (assuming your futuristic society can keep developing the tech but runs into snags with mirrors and lasers), so if you're writing them into sci-fi you might have the neutral-beam weapon be something you try to use for heavier punch at closer range.

Hmm... how good would lasers and particle beams be at, say... orbital bombardment? No difference, or would one have a clearer advantage over the other in terms of effectiveness?

Unless they're ridiculously and unrealistically powerful (which is actually quite common in sci-fi), nukes are a better method.

[Stofsk]

Unless they're ridiculously and unrealistically powerful (which is actually quite common in sci-fi), nukes are a better method.

Wow. Is there any method of orbital bombardment which doesn't leave massive destruction? Say if you wanted precision strikes rather than nuking the site from orbit?

[Winston Blake]

The longer the range, the more effective lasers would be relative to neutral-beams. But perhaps neutral-beams would be more powerful (assuming your futuristic society can keep developing the tech but runs into snags with mirrors and lasers), so if you're writing them into sci-fi you might have the neutral-beam weapon be something you try to use for heavier punch at closer range.

Hmm... how good would lasers and particle beams be at, say... orbital bombardment? No difference, or would one have a clearer advantage over the other in terms of effectiveness?

Both would be crap in any realistic implementation (assuming orbital bombardment means blowing stuff up on the ground, ABM stuff is fine). But lasers do have an advantage in that their penetration/transmission through matter is wildly dependent on wavelength, whereas that of particle beams tends to be comparatively constant (this is an advantage when it actually hits a target).

So find the best frequency for transmitting through air and you can get the energy to the ground (only on a clear day), but chances are it'll be too diffused out to do much and if you put up the power too far you'll get heaps of air heating and ionisation no matter what your frequency.

Wow. Is there any method of orbital bombardment which doesn't leave massive destruction? Say if you wanted precision strikes rather than nuking the site from orbit?

I think all you can really do is use mini-nukes or low-yield KE rods, or even conventional-warhead bombs with retro-thrusters/airbrakes/etc on them so their KE doesn't effectively nuke the place. With sufficient energy/current, of course, beam weapons could be useful at this. But beware sentences that begin with "With sufficient".

OTOH, projectiles take time to get to the surface, and a writer might need instant bombardment. Although if you consider blinding a bunch of people to be 'bombardment' then lasers would be ok (epileptic alien species, watch out!)