 This decision was made both for theoretical reasons as well as practical; because the force is larger on the inside edge of the torus, there is a large net force pressing inward on the entire reactor.The D-shape also had the advantage of reducing the net force, as well as making the supported inside edge flatter so it was easier to support.Code exploring the general layout noticed that a non-circular shape would slowly drift vertically, which led to the addition of an active feedback system to hold it in the center.