A slightly diverging beam of positive ions with a small opening angle a is admitted into a cylindrical capacitor at point A (Fig. 9.12). All ions in the beam have the same energy. Those ions whose velocity vector at point A is directed perpendicular to the segment OA move in a circle of radius r=OA, concentric with the capacitor plates. Prove that the ion beam will be focused at point B such that /AOB=n /√2. Determine the maximum beam width.
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