Plasmas for Weapons and Hypersonic Aircraft

kalamazoo904 writes "This free article at Jane's Defence Weekly reports that Boeing's Phantom Works is seriously considering a plasma gun for fighter planes. The basic idea appears to be using the air plasma that builds up above Mach 1 on a plane's forward surfaces as the power source. I can't tell whether the actual plasma 'bullets' are formed from the air plasma, or are metal bullets melted and accelerated by the air plasma. The aspect of this story that I find scary is that the plasma gun is apparently already under classified development."

Comments

Explanation of what they're planning to do.

[Christopher Thomas]

What they're talking about doing is diverting some of the plasma from the shockwave of hot gas that forms around any hypersonic craft, and then using that to power a weapon.

Item number one: This is just an indirect way of drawing power from the craft's engines (as they're what push the craft along, and what give the craft the kinetic energy that it's shedding when it makes this shockwave). This is still useful, because the engines are handling an obscene amount of power.

Item number two: All they're (sanely) talking about is a laser here, folks. This has the same advantages and drawbacks of any other laser weapon: Almost zero time to reach the target, but it takes a lot more energy to do damage than (say) a railgun would, and you lose about 95%-99% of your wall-plug power as heat in the laser.

The laser will also reflect off of the plasma shell (lots of free electrons in there). I hope they're planning to fire it backwards.

Item number three: While you might be able to fire packets of plasma, this weapon would be useless in an atmosphere. Plasma is much more tenuous even than air. As soon as it hits the air, the packet disperses. What you'd actually get is something resembling a plasma torch, not a gun.

Even in space something like this is iffy. A toroid of plasma with the right combination of toroidal and poloidal currents running within it *might* be stable (in vacuum) for a short time. Most "packets" of plasma would expand due to gas pressure (they're a gas) or due to magnetic forces if you try to stabilize them with internal currents (you usually get compression in one direction at the expense of expansion in another).

Summary: An interesting thought-experiment, but not the "plasma gun" of video game fame.

Re:Explanation of what they're planning to do.

[JanneM]

Damn, and I really wanted at least one Sci-Fi staple to come true... Maybe they can get some of the same effect by shooting really bright lightbulbs with a railgun :-)

/Janne

Re:Explanation of what they're planning to do.

[Christopher Thomas]

Someone I once worked with (a physics graduate) suggested that work was going on in specific places to improve some successful experiments for keeping a body of plasma from diverging within a gaseous field, i.e. the atmosphere.

This would be really neat if it could be done, but everything I know about physics says "not a chance", for a few reasons:

• Your plasma is cooling off rapidly. It's touching the atmosphere, and Newton's Laws of Heating and Cooling hold. As it cools, its properties change, so any stabilization mechanism would have to be robust over a wide range of temperatures _and_ stable with temperatures changing (at different rates in different parts of the packet).
  
  
• You have one hell of a lot of turbulence where the plasma packet touches the air. This would be bad enough for a packet of cold matter (e.g. water or compressed air), but with the added turbulence due to both plasma and air reacting to temperature change... Not a chance.
  
  
• Your plasma packet will slow down so fast it's not funny. The 1/e velocity point is where the cross-sectional mass of the projectile is the same as the cross-sectional mass of the air it's plowed through (though you can get a fudge factor for an aerodynamic projectile). For a plasma packet, that'll be a few _inches_. Firing a plasma packet more than a few feet without having it shed all of its energy in its wake would take truly silly amounts of power.
  
  

In summary, whatever experiment is being talked about is almost certainly not the firing of a plasma packet through open air with no further intervention.

Things that I can think of that _might_ give rise to a report like this:

• Firing within a pipe or in another bounded environment. If the shape is right, and the object is travelling slower than the speed of sound in the medium, you can get a "soliton wave" forming, where wake disturbances reflected off of the walls interfere constructively and stabilize the packet. This won't happen in open air. Your packet will still cool off; you'll end up with a puff of compressed air coming out the end of the pipe.
  
  
• Dumping energy into the packet with a laser. If you're firing a laser along the path of the plasma packet's motion, you might get what _looks_ like a stable plasma packet moving through the air if you fire the packet faster than the speed of sound. What actually happens is that the packet smacks into the air and disperses, but creates a shockwave when it does so. Your laser heats the air behind the point of impact, which a) raises pressure, propelling the remainder of the gas packet forwards, and b) heats up the plasma. If the rate of energy input is equal to the rate of energy loss and if between confinement by the shockwave and input of matter through the shockwave mass gain equals mass loss, you might get a stable shockwave with a plasma packet at the top powered by the laser beam.
  
  I'd personally just fire the laser beam, or dispense with the laser altogether and fire a kinetic slug, but I can see this working with enough twiddling.
  

So, there are a couple of effects that might look like a "plasma gun" that actually aren't, which might give rise to the reports both from your co-worker and the article.

Re:Explanation of what they're planning to do.

[Prof_Dagoski]

They're talking about more than a laser here. One application is indeed about using the plasma as a power source. The other application is more interesting. That one uses magnetic fields to focus the plasma and direct it. I have the same degree of doubt about the application's feasiblity, but they're talking about directing the plasma itself. One point worth noting is that the article has a sentence about the an anti-ballistic misslile project working on a similar principle. It talking about dumping the energy into the missile's electronics. If this the second application is an anti-electronics weapons, I can see it working. The plasma bolts don't need to remain compact bullets. They just have to deliver enough energy to the electronics to fry them.

Then again... The mentioned a torus of plasma. I pulled down my trusty copy of Introduction to Electrodynamics and started doddling a bit using that as a reference. It's a toroid of plasma. Right? A doughut of charged particles, moving towards a target. What's the magnetic force on the torus? F=Q(vXB). If I remember right, shouldn't the magnetic force resulting from this situation confine the plasma to its original torus? What happens when it hits air, I dunno. However, consider the following arrangement. A duct to divert plasma. A magnetic field separates electrons from postive particles. Introduce them into a tube or gun barrel. Some +s pile up at the very end while others get magically formed into a plasma torus. Electrical replusion sends the plasma torus down the end of the tube, while a strong magnetic field running perpendicular to the tube serves to increase the focusing force, generating a very compact fast moving bolt of plasma. Now, things to remember: 1)I barely passed this class. 2)That was eight years ago. 3)It's late. I'll take another hit off my crack pipe and really go out on a limb here. What about using powerful radio waves to keep the plasma focused as it travels down range? Anyone with better major GPA than me care to comment?

Re:Explanation of what they're planning to do.

[Christopher Thomas]

They're talking about more than a laser here. One application is indeed about using the plasma as a power source The other application is more interesting. That one uses magnetic fields to focus the plasma and direct it. I have the same degree of doubt about the application's feasiblity, but they're talking about directing the plasma itself.

And I think that that last part was vapour, as per my original post.

It's a toroid of plasma. Right? A doughut of charged particles moving towards a target. What's the magnetic force on the torus? F=Q(vXB). If I remember right, shouldn't the magnetic force resulting from this situation confine the plasma to its original torus?

Firstly, your plasma is electrically neutral. It acts like a conducting neutral gas. Separating out substantial amounts of charge from the packet would a) take one heck of a lot of work, and b) cause the resulting charged packet to expand due to it repelling itself. It might be useful to use electrostatic forces to _fire_ the bolt, but when the packet leaves the gun, it's either neutral or it becomes neutral really fast by stealing charge from the atmosphere.

What would actually stabilize a plasma packet would be electric currents within the plasma. Unfortunately, this doesn't work very well.

If it has no internal currents, then magnetic forces will do nothing, as there's no magnetic field. It will then expand due to gas pressure (ignoring the effects of the atmosphere for a moment).

If it has internal currents, they'll either be toroidal (in the direction of the tube) or poloidal (winding around the tube, going through the hole).

Toroidal currents would compress the tube into a ring, fighting one expansion tendency, but would cause the ring to expand outwards (remember what happens to a closed loop carrying current).

Poloidal currents would crush the ring toroidally (the forces would "want" to shorten the circumference of the toroid), but this force would be uneven. It would be strongest near the hole in the toroid, and weakest near the edge. This would not lend itself to a balanced configuration. The poloidal currents would also make the tube of the toroid "want" to expand, again due to the same effect that pushes outwards on a closed current-carrying loop.

You can set up both toroidal and poloidal currents to try and balance these effects against each other, but this isn't very stable. It has been done in "spheromak" plasma confinement experiments, but these experiments a) are in vacuum, and so minimize outside disturbances, and b) are inside of a conducting tube, which causes "mirror currents" to be set up in the walls of the tube which stabilize the toroid.

In free air, this won't be stable. Not a chance.

What about using powerful radio waves to keep the plasma focused as it travels down range?

This is probably possible if it's travelling down an enclosed tube, or if you can otherwise focus radio beams on to it from many directions. From a single radio source in open air, I have strong doubts.

Interesting idea, though.

Classified?

[Bearpaw]

The aspect of this story that I find scary is that the plasma gun is apparently already under classified development.

That's the speculation of some unnamed "observers". Another possibility is that the high-ranking military official who'd been getting kick-backs for keeping it alive retired, so they just shut it down.

Even if it was classified, that's not necessarily something to be scared about. I gather that there's sometimes a certain amount of whim involved in what does and doesn't get classified.