I'm thinking it is either the tension due to the ball, or the ball's centrifugal force. But the thing is, are these two forces the same in the conditions I've set (no gravity, no air resistance)?
After all, the centripetal force would be due to the tension on the ball from the rope. Since the centrifugal force is essentially the opposite of the centripetal force, then it would be equal to the tension on the rope from the ball, right?