Try using either sodium nitrite or arm and hammer so clean, those will stop the rusting for a while. When I built mine I got a bag of sodium nitrite but haven't used it yet.. You don't need much apparently. Just look online for plasma quench recipes and you'll find some. The only thing I haven't found is a insecticide to add. You'll get some little bugs in there if you leave it..
The trade off for not having metal smoke in the air is a dirty table but that's a small price to pay, just try running it dry for an hour.
What plasma machine are you running?