That's just a modification to your "tube" idea.What about an ellipse instead of the circle?
Your original idea is plausible if you account for the losses during the change in direction, ie., the losses that another poster pointed out previously. You could do this by using a rocket engine to give the object an extra push after each change in direction. However, I think your idea of having all of this housed in a closed self contained system will not work. Do you know why I think that? Have you asked yourself if any of Newton's laws are being violated with this idea?