Spheres vs Toriods

To all,

        I have been trying to figure out the magic behind the toriodal
shape. I was under the impression that the dominant factor in a high voltage
collector was the radius of curvature.  This would make a sphere or an
oblate spheriod the best choice.  
        Could someone enlighten me as to why the toriod is the shape of
choice?  Wouldn't the sphere be better?  Is the toriod such a success
primarily because of the lower cost of fabrication, or is there a technical
reason for it's popularity?

        On an unrelated issue, are there any coilers in Tucson or it's environs?