Dumb question about PAWS and Lasers

[TheDS]

In FF&S1 (and issues of Challenge) great lengths are gone to to explain how lasers are merely high-intensity flashlights, and need huge focal areas to get them down to reasonable spot-sizes. Hence, they used grav-focusing to fudge this.

But Particle Accelerators (and Meson Guns) don't have any mention of this at all. In fact, MGs don't even tell you what the bore of your gun is (but you can figure it out). Does this mean that subatomic particles, which must travel less than the speed of light, are more accurate than photons, which travel at exactly the speed of light? That seems to me to be contradictive.

The only reason I can think of that PAWs and MGs don't need to worry about dispersal so much is because photons behave like waves and SAPs (sub-atomic particles) might not.

Any ideas?

------------------
"This is a spaceship! Why did you throw that grenade here?"
"I was just trying to get the bad guys."
"Uh guys,..."
BOOOMM!

 

[Anthony]

Well, sort of. The wavelength of the particle beam is several orders of magnitude better than the best X-ray laser, so in that sense yes. However, the big problem with neutral particle beams is that given current tech there's no decent way to focus them much at all, so you have to assume that a mechanism similar to the grav focus used for lasers is used to focus neutral particle beams.

 

[TJP]

I remember reading from somewhere (might have been just fiction...) of a theoretical charged PAW design where the charge was neutralised at the end of the focussing barrel. So you focus it like a Charged PAW but it behaves like an Neutral PAW when the particle beam leaves the barrel. Dunno how "hard" this theory is, though.

 

[Anthony]

Well, yes, except for two problems:

1) charged particles don't focus that well either, because the beam repels itself.
2) neutralizing the beam adds energy (and random motion) to the beam, which should be enough to drop ranges well below Traveller ranges (I was having trouble justifying less than a microradian, which would give an effective range of 10 kilometers...)

 

[Uncle Bob]

<BLOCKQUOTE>quote:</font><HR>Originally posted by Anthony:
Well, yes, except for two problems:

1) charged particles don't focus that well either, because the beam repels itself.
)<HR></BLOCKQUOTE>
Ah, you neglect the "plasma pinch" effect on a stream of charged particles moving at relatavistic speed (The moving charge creates a concentric magnetic field that overwhelms the repulsion. The only thing I remember from Physics 440.)

So charged PBW have good cohesion, but ambiant electric and magnetic fields deflect the beam.
Neutral beams have poor focus, but shoot straight.
Neither lasts more than kilometer in air.

The trick about neutralizing the beam after focus and aiming was the party line for SDI. I kinda doubted it would work myself, but I won't argue until I see the hardware.

Lasers, I always assumed the gravitic lens was necessary because a material mirror would not focus the very short wavelenth beam. I don't care what kind of a lens you have, the formula is still range x wavelenght / aperature = beam diameter. For a range of 25,000 km (250 mm) an effective laser needs a wavelength on the order of 400 Angstroms, on the ragged edge between UV and X-rays.

 

[Anthony]

Mostly because said 'plasma pinch' effect is related to using a charged particle beam in an atmosphere (the beam creates a jacket about itself which maintains cohesion); this is why CPAWs have relatively reasonable atmospheric range (but atrocious vacuum range).

 

[Uncle Bob]

<BLOCKQUOTE>quote:</font><HR>Originally posted by Anthony:
Mostly because said 'plasma pinch' effect is related to using a charged particle beam in an atmosphere (the beam creates a jacket about itself which maintains cohesion); this is why CPAWs have relatively reasonable atmospheric range (but atrocious vacuum range).<HR></BLOCKQUOTE>

Sorry, would have got back sooner, but I misplaced my E-M theory text. I did find some references on the web, notably http://www.aa.washington.edu/AERP/Propulsion/gunpage1.htm.

The "Plasma pinch" is related to the better known magnetic pinch effect. A rapidly moving column of charge acts as an electric current. As a particle diverges from the mass, it is treated as a seperate, paralel current and is drawn back into the column by the interaction of their magnetic fields according to Ampere's Law.

Firing either type of beam in the atmosphere will result in collision scattering that overwhelms all other effects. IIRC a GeV electron beam dissipates after about two hundred meters, and to get that far it has to use multiple pulses. (With their higher momentum, a multipulse proton beam was expected to get about 700m.)

 

[Anthony]

Unfortunately, that works because a plasma is uncharged (it consists of charged particles, but the net charge is close to zero). A beam of charged particles, in the absence of particles of the other charge, doesn't produce a magnetic field, and the whole effect is dominated by electrostatic repulsion.

Atmospheric particle beam research focuses on creating a tube of plasma, which reduces atmospheric resistance, and keeps the beam focused. The effect is very similar to a conventional lightning bolt.

 

[Uncle Bob]

<BLOCKQUOTE>quote:</font><HR>Originally posted by Anthony:
Unfortunately, that works because a plasma is uncharged (it consists of charged particles, but the net charge is close to zero). A beam of charged particles, in the absence of particles of the other charge, doesn't produce a magnetic field, and the whole effect is dominated by electrostatic repulsion.
Silly me, I still believe in Maxwells equations. A magnetic field is created by a moving charge and by nothing else (except a monopole). A moving neutral plasma is incapable of creating a net magnetic field. If you read the references I cited you would see that the precise definition of "magnetic pinch" is the ability to overcome electrostatic repulsion in a moving charge.

Atmospheric particle beam research focuses on creating a tube of plasma, which reduces atmospheric resistance, and keeps the beam focused. The effect is very similar to a conventional lightning bolt.
As I said, the main problem with firing a PBW in atmosphere is collision scattering. Creating an evacuated tube with a laser or a precurser bolt is the obvious way of dealing with this scattering. The displaced air forms a superheated but neutral plasma which has no effect on the cohesion of a CPBW or NPBW, but looks pretty: the important thing is that it gets the hell out of the way.

Now, it has been twenty years since I got that physics degree, and even though all the sources I checked agree with me I might still be missing something. Can you cite a reference (preferably with equations) to back up your novel interpretation?
<HR></BLOCKQUOTE>

 

[Anthony]

A magnetic field is caused by a moving charge, yes, but moving relative to what? From the reference frame of the particle beam, the relative velocity of other particles is zero -- ergo, no magnetic field effects. If the particles in the beam were travelling at different speeds, there would be a magnetic field effect, though I doubt it would be relevant compared to electrostatic repulsion.

It's a bit hard to find reliable WWW references on any of this, but here's a SDI paper from 1984 which agrees with me. I'm not aware of any change in underlying physics since then.
http://www.airpower.maxwell.af.mil/airchronicles/aureview/1984/jul-aug/roberds.html

 

[Uncle Bob]

Yeah, PBW references are harder to find than reliable data on megawatt lasers. I usually end up with articles on scientific particle accelerators, and pray they keep to linear diffy-Q.

Unfortunately, relativity makes intuitive resoning treacherous. The magnetic field created by the moving charge has a reality independent of the charge itself. Take the classic magnetic pinch experiment. In this case the attraction is between two currents moving parralel and in the same direction and at "c".

from the Roberds' article you cited.
But as the beam propagates through the air, it would also strip electrons from the surrounding air molecules, creating a region of charged particles (ions) intermingling with the beam. The result of this phenomenon is to neutralize the overall charge of the beam, thereby reducing the undesired effect of mutual repulsion among the charged particles in the beam that is a cause of beam spreading.
I am not convinced: I think he is grossly oversimplifying and sounds a lot like a manager who has imperfectly understood what his engineers have told him. I notice he is a professor of "Engineering technology," the one "Engineering" discipline that lacks rigour or advanced mathematics.

Another force that tends to prevent beam spreading is a surrounding magnetic field, created by the current of the charged particle beam. This field wraps itself around the beam and produces a conduit that inhibits beam divergence.
This may have confused you. This is the magnetic pinch, has nothing to do with the previous sentence, and agrees with me
.
Unfortunately his caption then reads Unless there is some neutralization of the charge, the mutually repulsive force will allways be the stronger force and the beam will blow itself apart.
This is obviously untrue because if you neutralize the charge you kill the magnetic field just as fast as the repulsion. And, near c, you can reach an equilibrium point where the magnetic field pinches the charges together until the electrostatic repulsion exactly balances the magnetic squeeze. Eventually, the speed of the charge will drop and the magnetic field will weaken, but that is not the same thing.

[This message has been edited by Uncle Bob (edited 25 November 2001).]

 

[Anthony]

The 'magnetic pinch' experiment you describe is a current flowing in a wire. If our particle beam consisted of negatively charged particles moving through a cloud of positively charged particles, it might be relevant to particle beams (and, in fact, that's pretty much what a particle beam in atmosphere amounts to), but it doesn't apply to particles moving in free space.

Relativity is very simple here: a cloud of particles moving at constant velocity through free space should behave the same regardless of reference frame. If you set your reference frame to match the velocity of the particles, you get an unmoving cloud of charged particles, which obviously explodes due to electrostatic repulsion.

A current flowing in a wire is _not_ the same as an electron beam in vacuum. You can generate a magnetic field with two charged particles moving relative to one another, but unless the net charge is near zero, electrostatic effects will simply make your system explode.

 

[Uncle Bob]

<BLOCKQUOTE>quote:</font><HR>Originally posted by Anthony:
A current flowing in a wire is _not_ the same as an electron beam in vacuum. You can generate a magnetic field with two charged particles moving relative to one another, but unless the net charge is near zero, electrostatic effects will simply make your system explode.<HR></BLOCKQUOTE>

I am sorry, you are wrong. I argue from E-M theory and suggest physics sites with pertinent equations, you wave your hands and expect me to believe white is black by the compelling beauty of your clumsy gestures. I ought start posting equations, but your fundamental misconceptions would cause you to discount them, even if you understood them.
Life is to short for this sh*t.