Schematic diagram of uranium isotope separation in the calutron. The fact that beams of uranium ions could be defined well enough to yield small quantities of isotopes suitable for laboratory research by no means assured that electromagnetic separation could be worked on the industrial scale necessary to make a kilogram of U-235. Because the lighter ions respond slightly more readily to the magnetic field than the heavier, their trajectories bend in a-tighter arc. At the end of their semicircular travel, the ions of U-235 are relatively more plentiful on the inside than on the outside of the beam. But the maximum separation even in the ideal case is small, only one tenth of an inch for an arc with a diameter of 37 inches.
Actual beams are far from ideal.
Design of receiver for alpha calutron. They contrived electric arcs to ionize the uranium chloride feed. Lawrence did not doubt that other means, particularly reactor production of fissile plutonium, might ultimately be the most efficient way to a bomb. But in mid-1942 no reactor worked, and the calutron did. Extravagant effort went into designing powerful ion sources and aptly shaped, eventually parabolic collecting slots.