In this instance forgo the particle part. I'm thinking more here like the way a railgun works. Or how some rollacosters can take you from 0 to 60kmph. Lets say we wanted to make a railgun that uses a large loop to continual increase velocity to a desired point, then release it into an adjacent chamber to let it launch vertically. How would gravity affect increasing the velocity? I'm thinking since we are using a vertical loop the effect of going up would be offset by when it is going down. Anybody have thoughts on a device like this?