Emitting a particle beam within the atmosphere

Question

In particle accelerators, particles are typically accelerated within a vacuum (or as close to it as possible) in order to minimize interference and collisions with air molecules during the acceleration process. This usually isn't a problem as the target of the particles are also contained within a vacuum.

However, if a particle beam were weaponized and intended to be used within the atmosphere then there would need to be a method to maintain a vacuum within the acceleration tube during acceleration, but then allow the particles to exit the weapon. Preferably this method would also need to hold up against repeat firings.

Considering the particles would be moving at a significant fraction of the speed of light if not right up against it, presumably no shutter system would be fast enough to open right when the particles have completed their acceleration to allow them to exit the weapon. Some sort of thin film might possibly be used if you'd be willing to accept some loss of energy to the particles penetrating the film, although the film would become degraded with each firing.

Are there any more sophisticated methods to allowing the particles to accelerate but then also exit a particle accelerator?

Answer 1

Not only is a device that suits your needs possible, the idea has actually been around long enough that it is actually used in industry! (although it is frustratingly difficult to find decent pictures of)

May I present the Plasma Window. As discussed in that 1998 TechBriefs article, it's basically a small, stubby tube within which you create a dense, hot plasma. The key is to get it to stay in contact with the walls of the tube and fill the space between. The idea is that as long as you pump enough energy in, you can maintain a wall of hot gas that won't move around too easily.

These are actually used in modern niche aerospace industries where someone wants a part to be electron-beam welded but can't put the part under vacuum because it's too big or they can't afford to wait hours and hours to pump out the air or both. In fact, this Science Direct article actually states "Weld quality for the non-vacuum plasma window electron beam welding approached the quality of in-vacuum electron beam welding", not only does this device exist, it works really well.

So, yes, there absolutely exists a real device as to allow a particle beam to transition from high vacuum to regular atmosphere. Just don't make the opening too big seeing as, at a power density of 8kW/cm^2, it's got about double the heat flux of the surface of the sun, but I suppose if you're planning on frying your target with a particle beam anyway, does it really matter if they get a bit roasted from looking at the "lens cap" too?

Answer 2

While accelerating the beam in a vacuum tube and firing through a plasma window is simple enough, you also need to consider the beam's passage through the atmosphere to the target as well. If the beam scatters or "blooms" due to the interaction of the beam with the atmosphere, then you are not going to be able to deliver much energy on the target.

This was actually considered in the 1980's, as one of the possible spin offs of the Strategic Defense Initiative. A warship mounting a particle beam weapon would have a potent method of dealing with incoming anti ship missiles, since the particle beam would deposit both an immense amount of thermal energy on the target, but also a gret deal of radiation that would penetrate the structure and fry the electronics, detonate the fuel or warhead and do other interesting things...

In order for the beam to have enough range to actually affect the incoming missile, the beam projector was teamed up with a laser. The laser was used at low power to identify and range the target, then a high power beam was emitted. This would essentially clear a channel through the air as the energy of the beam superheated the atmosphere along the beam channel, causing it to expand out of the way and creating a local vacuum (or at least region of very low pressure). The particle beam was emitted with the powerful laser pulse, and since the beam would be moving somewhat slower than light, it would be travelling through the channel of lower pressure created by the beam.

While the idea is sound in theory, the Navy began to wonder about the Rube Goldburg nature of the system, and why spend so much money on two powerful beam weapons working in tandem when one could do the job? At any rate the laser and beam technologies of the 1980 era were not mature enough to create a viable weapon capable of working in an actual service environment.

So for a particle beam to work effectively in the atmosphere, some sort of channel must be created for the beam to follow.

Answer 3

Accelerate the gas out of the barrel first.

You have some method to touchlessly accelerate the particles that you are using as a beam. Presumably you have conferred charge to them and then electromagnetically accelerated them.

Do that to whatever is in your barrel first. Confer charge and electromagnetically accelerate out the gas. The force behind the accelerated gas will be much more than atmospheric pressure trying to push gas back in and you will have produced a soothing vacuum for your particles. They will learn about the rough world of cold gas once they leave.

You do not need to accelerate the gas as fast as your particle ammo, but you could. I like to think that the charged gas rapidly leaving the barrel will be hot, and so produce a gout of incandescent flame that precedes the stream of charged radon ions or whatever your main particles are. Because plain particles are invisible, and boring.