Railgun Plasma Plume: What Causes it & How Long Does it Last?

[breadloaf]

Hi,

I've seen a video of the US navy test firing a rail gun and in the video there is a plasma plume that follows the projectile. I am unsure of what exactly is causing the plasma plume... specifically:

• Is it the electrical energy passing through the rails that ignites the air and causes the plasma?
• Is it leftover magnetic charge on the projectile that ignites the air as it moves and causes the plasma plume?
• I'm guessing it is not merely due to the velocity of the projectile but is mach 7 enough to cause just the friction of the projectile against the atmosphere to cause a plasma plume?

And also, for how long would one expect there to be a plasma plume following the projectile?

 

[DaveC426913]

1] How do you know it's a plasma plume?
2] If you could link to a video, we could see what you are talking about.

 

[breadloaf]

http://www.youtube.com/watch?v=F6ioLh7boMc&feature=related

It was stated to be a plasma plume (wikipedia also has a screen cap from the same video, stating it is a plasma plume as well).

 

[Danger]

Damned thing won't play for me.
My first thought would be that it's remnants of the "shot cup". With non-magnetic projectiles, a railgun vapourizes a sample of something that is then accelerated and pushes the payload ahead of it. Perhaps the power used in this case is sufficient to turn the material into a plasma rather than just a gas? I don't know a lot about the subject, though.

edit: Sorry... I generalized. In some cases, an actual open-ended container is used for a non-reactive warhead, something like a saboted bullet in a firearm. What I was talking about is a bridge wire (which I think is copper) that the launching current passes through and detonates before magnetizing the rails.