Particle Beams vs Lasers

[Isolder74]

Yay! Plasma weapons are feasible! Woohoo! Although I thought they needed some sort of magnetic bottle crap.

Particle weapons are also feasable as well as lasers

All three have already been built and tested as part of the old STI program. The diferences in how each type of weapon causes its damage. A particle punches through a target using pure KE. A laserBurns through an target. A plasme weapom does both.

A magnetic bottle is a way to contain the Plamsa to make it easier to control. Its not nessisary to use one to fire the plasma in a beam.

The drawback right now is the size of the hardware required to generate the beams and magnetic fields. That is being miniturized and power generation problems are being dealt with. Truely Laser are the easiest of the three to miniturize.

[The Nomad]

So, a plasma weapon that would be relatively unaffected by magnetic fields then? Neat.

I'm not that sure. The Coulomb repulsion within the beam would probably be greatly dimished if not cancelled, but the charges are still there and would theoretically be affected by a magnetic field. Though if the magnetic or electrical field repelled a type of charge, it would attract the other .

But lasers tend to not vaporise things not in the line of the beam. A steam explosion as the point of beam contact would blow the person apaer at those power levels. The way a laser works would burn through a person before that would happen. The Laser does not normally ever heat anything outside of the beam.

True, though I expect that the hydrostatic shock generated by the supersonic explosive steam expansion in the line of laser beam propagation would actually compare favorably to that of a small firearm's bullet . But that's only my opinion.

[ClaysGhost]

(with lenses, gravity fields or whatever) it doesn't lose any of its energy. Thus lasers are flexible weapons.

Actually, lasers could lose energy if directed by gravitational fields. Such an approach can lead to a redshifting of the laser beam. Lenses can absorb some of the energy of the beam.

PBs, on the other hand, lose energy dramatically if redirected, and thus lend themselves more to mechanical means of training, such as turrets etc. PBs also require projectile tankage, which lasers do not.

You want to fit a powerful particle accelerator in a turret? I think that whilst laser beams may possibly be redirected into turrets from a central station (depending on a lot of things), particle weapons are almost certain to be fixed axis devices.

Both weapons have the problem of beam spread. Which system would spread less over a given distance? Does beam divergence affect weapon performance in the same way for both systems?

For the laser, the beam divergence can be easily estimated from

divergence (radians) = 1.2 * wavelength / aperture diameter

So a laser operating at a wavelength of 500nm (visible) through a 1 centimetre aperture would have a beam divergence of about 6x10^-5 radians = 0.003 degrees. The flux (power per unit area) of a laser beam is given by

flux (Watts per m^2) = 4L / (pi * divergence^2 * range^2)

L is the power of the beam in Watts. As you can see, the flux declines with the inverse square of the range.

For particle weapons, the particles interact and spread the beam, and aren't created in step in any case. Synchrotrons work by forcing the particles into bunches and so I might expect that synchrotrons have better divergence characteristics than beams from linacs, but I don't think either system can maintain particles in lock-step like a laser beam. The above formula for flux should work for both lasers and particle beams, but the divergence angle formula is probably inappropriate for particle beams.

[ClaysGhost]

Isnt an ion cannon/beam a particle beam?

And what could one do, exactly?

In the end it's just a charged particle beam with heavy particles (although you could describe a proton beam as a hydrogen ion beam). If it hit a target you would get the usual KE effects. Perhaps it would cause trouble for electrical systems, I don't know; satellites have had their electronics damaged by charged particles building up on their exterior surfaces before.

[ClaysGhost]

Hrmmm....aren't particle beams the same or similar to plasma beams?

Plasma is an assembly of free positive and negative charges, so no. In a charged particle beam, the charges involved are either positive or negative - no plasma there. In a neutral particle beam the charges are either not present at all or present but bound to one another - not free, hence no plasma.

So, a plasma weapon that would be relatively unaffected by magnetic fields then? Neat.

Not necessarily, no - see the magnetosphere, for instance. It shields us from a lot of solar plasma.

FELs are still lasers, but they use accelecrated electrons passing through a magnetic field to generate the laser beam. FELs should be able to produce very high energy beams that would be able to smash atoms like a particle beam fire netrons or elements (hydrogen, carbon, whatever - basically bigger elements = more damage and more input power). I believe a FEL was used in the late 90's to explode a Xenon atom. This means a FEL beam would probably destroy a mirror.

The point about the FEL is that it can generate X-rays now (and maybe in the distant future, gamma rays). At these wavelengths laser light does act a lot more like a bunch of particles than waves, as far as materials exposed to the beam are concerned. FELs aren't intrinsically more powerful than other types of laser, and they're a lot less efficient than some because they're being driven by a particle accelerator in the final analysis.

A magnetic bottle is a way to contain the Plamsa to make it easier to control. Its not nessisary to use one to fire the plasma in a beam

How hot is this plasma?

[18-Till-I-Die]

I have a question about plasma 'heat' too:

Is plasma part ofa star, and could it be artificially generated at such extreme heats if so?

[1337n1nj4]

Stars are plasma. Plasma is generated when gas is imparted with enough energy (heat) to ionize it.

[Symmetry]

What he said <nods in appreciation>.

A 'plasma beam weapon' might be a proton CPB and electron CPB mixed together for a net neutral charge. Kinda like a Quasi-NPB. I guess technically you could say it's a hydrogen plasma...

So, a plasma weapon that would be relatively unaffected by magnetic fields then? Neat.

Actually, a strong enough magnetic field will split the electrons off from the protons, and send them their seperate ways. It won't do this as easily as it would with a homogeneous beam due to things like turbulance and coulomb forces, but it will still be able to do it.

[Stark]

Okay, thanks guys, there's some excellent data here. I'm curious on a few points; could someone explain why bending a laser with some lam0r scifi 'gravity field' would tend to redshift the laser? Also, while I'm aware that PBs, when diverged too much to penetrate, can still mess with electronics etc, I was under the impression that the actual energy of the beam's impact was usually quite low at range (say tens of thousands of kms).

Kudos to ClaysGhost, the beam divergence equations have solved several problems for me

I'm asking because I was wondering which (in both realistic and scifi senses) weapons system would be superior for long-range engagement and point defense, and how the physical size and power requirements scale to power levels. Just a thought exersize, really, but EM and accelerator weapons appear to be the only sensible beam weapons. But one can't do everything with missiles

[SylasGaunt]

While we're on the laser subject I've seen it claimed that any laser with an intensity of greater than 10Kw/CM^2 is useless in atmosphere because beyond that intensity the air is opaque to the weapon. Seemed kinda wonky so I thought I'd ask.

[The Nomad]

I'm curious on a few points; could someone explain why bending a laser with some lam0r scifi 'gravity field' would tend to redshift the laser?

Photons are affected by gravity. An antigravity shield ( that pushes stuff away from the ship ) will make incident photons lose energy as they close, resulting in a frequency decrease ( Planck's equation is E=hf therefore if E decreases f decreases as h is Planck's constant ). Since frequency is linked to colour, colour will change ( here, it'll tend to redshift ). Fun stuff is that if the ship fires its own lasers through such a shield, it will increase the beam's power and make it blueshift !

I have a question about plasma 'heat' too:

Is plasma part ofa star, and could it be artificially generated at such extreme heats if so?

As Claysghost pointed out, plasma is a collection of positive and negative charges which must be free. Technically the electron gas in a metal could be ( though a bit academically ) described as a plasma, as well as the neon gas is some lighting devices. This definiton applies, in Sci-Fi, more specifically to the ionized gas that makes up stars.

That kind of plasma could be generated through the laser ignition of a deuterium pellet, for example. For a thermonuclear fusion reactor, say...


Question : in a neutron PB, how will neutron pressure affect the beam's properties, what's the critical density for measurable neutron pressure ?

[ClaysGhost]

Okay, thanks guys, there's some excellent data here. I'm curious on a few points; could someone explain why bending a laser with some lam0r scifi 'gravity field' would tend to redshift the laser?

If you do work on a photon, through a gravitational field or whatever else, you change its energy. The photon's wavelength is inversely proportional to its energy, so if you increase the photon energy you will decrease its wavelength. If the photon is approaching a mass it is gaining energy from the gravitational field of the mass, producing a blueshift. If the photon is heading away from a mass the opposite applies; the photon is redshifted.

If a photon passes by the Sun its wavelength is hardly affected because the blueshift it gains approaching the Sun is roughly matched by the redshift it experiences after passing the Sun (the field is weak, so the light bending angle is small). With a strong gravitational field designed to significantly bend a laser beam there's no reason to expect a zero change in photon wavelength, because the beam will most likely take a very different path out of the gravitational potential than it did heading in. The actual redshift or blueshift involved depends on the geometry and the field strength. The point is that photons can lose or gain energy to/from gravitational fields.

Also, while I'm aware that PBs, when diverged too much to penetrate, can still mess with electronics etc, I was under the impression that the actual energy of the beam's impact was usually quite low at range (say tens of thousands of kms).

Surely it depends on the details of the beam (luminosity, divergence angle, etc).

I'm asking because I was wondering which (in both realistic and scifi senses) weapons system would be superior for long-range engagement and point defense, and how the physical size and power requirements scale to power levels. Just a thought exersize, really, but EM and accelerator weapons appear to be the only sensible beam weapons. But one can't do everything with missiles

I did some rough calculations once based on what seemed to me reasonable assumptions about nuclear missiles and lasers. IR lasers were more "efficient" than nuclear missiles below about 100km for point defence, assuming a non-maneovring (but accurate) missile. This conclusion relied on a fusion source to power the laser. From this I decided that missiles will be more efficient uses of mass than beam weapons beyond what people might consider relatively short ranges. Also I didn't enforce realistic constraints on the laser's materials and output energy. In these calculations the only thing that would disadvantage a particle beam relative to the laser is a greater beam divergence, which at the moment I regard as likely. The particle beam might benefit from increased effectiveness against targets (which is not what I considered) but it would also be likely to consume more power and space than a laser, unless the laser is a FEL.

[ClaysGhost]

While we're on the laser subject I've seen it claimed that any laser with an intensity of greater than 10Kw/CM^2 is useless in atmosphere because beyond that intensity the air is opaque to the weapon. Seemed kinda wonky so I thought I'd ask.

The wavelength dependence of opacity is more important, I'd have thought.

Technically the electron gas in a metal could be ( though a bit academically ) described as a plasma, as well as the neon gas is some lighting devices. This definiton applies, in Sci-Fi, more specifically to the ionized gas that makes up stars.

Yes, in science fiction plasma usually appears as a lethal weapon, so hot plasma is required. But hot plasma is very difficult to force into well-behaved, long-ranged beams without surrounding magnets because a lot of the electrons and ions in the hot plasma will have large random velocities that will carry them out of the beam, spreading the cross-section.

Question : in a neutron PB, how will neutron pressure affect the beam's properties, what's the critical density for measurable neutron pressure ?

I don't think neutrons would start interacting with each other until the density became enormous, far beyond what would be required from a particle beam (look at the typical density of atomic nuclei). The divergence in a neutron beam would therefore depend only on the range of neutron momenta in the direction perpendicular to the beam's bulk velocity.

Neutron beams have other problems, I think. It's easy to see how you can generate and (most importantly accelerate) an electron or proton beam, or even a hydrogen atom beam. But for neutrons, how do you do it? They're neutral. You could generate a beam from a fission reactor, but how do you focus and accelerate a bunch of neutral particles? It's not as easy as charge stripping for hydrogen ions, because electrons won't associate with a free neutron. You need to add and remove protons, I suppose (or start with deuterium ions) and then remove the proton somehow. Better to use hydrogen ions.

[1337n1nj4]

Yes, in science fiction plasma usually appears as a lethal weapon, so hot plasma is required. But hot plasma is very difficult to force into well-behaved, long-ranged beams without surrounding magnets because a lot of the electrons and ions in the hot plasma will have large random velocities that will carry them out of the beam, spreading the cross-section.

This raises an interesting question. Would there be anyway to "direct" the trajectories of the particles in the plasma, or at least a bulk of them, in the desired direction? Something like NMR immediately comes to mind, though I imagine it would be trickier than that. Or would there be any way to impart a big-ass electrical charge to it so it would remain somewhat coherent? The heat is going to come into play and limit how effective something like that could be, I'd imagine, but what kind of feasibility are we potentially looking at here?

[ClaysGhost]

This raises an interesting question. Would there be anyway to "direct" the trajectories of the particles in the plasma, or at least a bulk of them, in the desired direction? Something like NMR immediately comes to mind, though I imagine it would be trickier than that. Or would there be any way to impart a big-ass electrical charge to it so it would remain somewhat coherent? The heat is going to come into play and limit how effective something like that could be, I'd imagine, but what kind of feasibility are we potentially looking at here?

A coherent plasma beam is in effect approaching an ordinary particle beam.

Electromagnetic fields can direct plasma, but a plasma beam must emerge from the weapon at some point and travel through free space. Certainly the confining electromagnetic field can exist in free space, but it will get weaker with distance away from the weapon very quickly. It's difficult enough to confine hot plasma when you have many tonnes of superconducting electromagnets surrounding it. If the plasma were to be given an overall charge it would probably just accelerate any dispersion.

I usually estimate that dispersion woes will become serious when a particle's speed due to thermal energy becomes roughly equal to or greater than the speed with which the beam approaches a target, but that ignores the magnetic/electric fields in the system.

It's been pointed out in other threads by other people how much more thermal energy it takes to inconvenience a target compared to kinetic energy. Yet hot plasma as a weapon directly trades kinetic energy for thermal energy compared to the equivalent particle beam, as far as I can see. I just don't think there's much to be gained by using a hot plasma weapon over a particle beam weapon.

[1337n1nj4]

Gotcha.

Particle beams for all!

[Ender]

I can't get that lasers vs beam page to come up, could someone copy paste it to a PM for me?

[harbringer]

Someone said that due to the fact that at high wave lengths light behaves more like particles, this then explains why a gamma ray laser (for arguments sake bomb pumped or something) has great penetration??.

Sorry my scientific knowledge is limited and I always wondered.

[buzz_knox]

How dense would a substance have to be for a gamma laser to interact fully, and what form would that interaction take? I had a thought that a FEL capable of emitting in the gamma and X-ray ranges would be an effective multipurpose weapon if the gamma were capable of penetrating the the target and causing a detonation/explosion in a vital area (engine room, magazine) when it struck a dense object (engine, reactor shielding). The X-ray would be available for less dense targets.

[Stark]

Is there any way to redirect a laser at all, either 'internally' (from a central laser source) or 'externally' at the muzzle? A laser point defence system would be limited by size if the whole system had to be mounted on a rotating turret to physically point the entire array at the target.

[phongn]

Is there any way to redirect a laser at all, either 'internally' (from a central laser source) or 'externally' at the muzzle? A laser point defence system would be limited by size if the whole system had to be mounted on a rotating turret to physically point the entire array at the target.

Yes. The Airborne Laser has a turret to direct the laser beam, but the actual laser itself is generated from a fixed area in the back of the airplane.

[Connor MacLeod]

At least 200 MJ IIRC. Necronlord or Connor MacLeod did that some time ago...

Depends on body weight actually. The human body is around 50-60% water IIRC, and the heat of vaporization for water is around 2.4-2.5 MJ/KG Assuming a 70 KG man, we're probably tlaking around 35-45 kg of mass... so around 80-100 MJ at minimum.

That's not a precise calc, but a rough calculation, since that doesnt factor in the fact that not all of the huhamn body will vaporize (part of it at least turns to ash as well.) and it assumes 100% efficiency (I also am not sure if boiling point and specific heat of water matter there.)

[Winston Blake]

FELs should be able to produce very high energy beams that would be able to smash atoms like a particle beam fire netrons or elements (hydrogen, carbon, whatever - basically bigger elements = more damage and more input power).

I'm not sure but i think that lighter particles (ie electrons, protons) would be better 'ammunition' because KE = 1/2mv^2. I figure that for a given force on a particle, when it comes out the barrel, smaller mass (therefore high velocity) gives more energy than a large mass (therefore low velocity).

While we're on the laser subject I've seen it claimed that any laser with an intensity of greater than 10Kw/CM^2 is useless in atmosphere because beyond that intensity the air is opaque to the weapon. Seemed kinda wonky so I thought I'd ask.

Probably something to do with turning the air into plasma which then interacts with the beam.

If a photon passes by the Sun its wavelength is hardly affected because the blueshift it gains approaching the Sun is roughly matched by the redshift it experiences after passing the Sun (the field is weak, so the light bending angle is small). With a strong gravitational field designed to significantly bend a laser beam there's no reason to expect a zero change in photon wavelength, because the beam will most likely take a very different path out of the gravitational potential than it did heading in. The actual redshift or blueshift involved depends on the geometry and the field strength. The point is that photons can lose or gain energy to/from gravitational fields.

The way i see it, if a small mass approaches a much larger mass (eg asteroid vs a planet), and then swings around it, the exact kinetic energy it gains as it approaches is lost as it moves away. I don't understand how a laser beam can 'lose' energy after passing through a gravity-lens-thingy and coming out in a different direction. Surely it would blueshift as it neared the gravity source and redshift the same amount as it moved away, meaning no total loss of energy?

[Stark]

Yes. The Airborne Laser has a turret to direct the laser beam, but the actual laser itself is generated from a fixed area in the back of the airplane.

How does the turret redirect the beam? (Lenses, I assume) How much loss is associated with the use of a turret?

[Ender]

So would a neutral particle beam suffer from the inverse square law, or would the focusing prevent that?