CALUTRONS

The other primitive method of enriching uranium is the calutron, a method abandoned by the Manhattan Project in 1944 in favor of diffusion. (The jet enricher was apparently not invented until the late 60s by West Germany and South Africa.) The prin-ciple of forcing the gas through a curve is the same, but it is accom-plished electromagnetically, not by a curved wall. The gas is ionized by an electric arc and goes through a velocity filter whose purpose will be explained in a moment. The now charged molecules (positive ions) will be acted on by a force perpendicular to both their velocity and the magnetic field, which is perpen-dicular to the page. The result is a curved path just as in the case of jet enrichment; the smaller mass hits the target with the inner curve. The targets are made of a heavy metal, such as lead, and the enriched uranium is then extracted chemically.

The reason why only one target is shown is that it is not really a figure of a calutron, but of a "mass spectrograph" which separates isotopes by mass for research purposes. The type shown was invented in 1932 by Bainbridge, and the wartime calutron was modeled after it. The first mass spectrograph was invented by Aston in 1919.

As every driver knows, the centrifugal force depends not only on mass, but also on velocity, and with a mass ratio barely differing from equality the separation would take place primarily by velocity unless precautions are taken to ensure exactly the same velocity for all ions entering the separation space through the slit S3.

This is accomplished by the velocity filter. [Its principle of operation, for technically minded readers, is based on the fact that both an electric and a magnetic field will act with a force on a moving charged particle, but only the magnetic force is dependent on its velocity. The velocity filter has two plates P with opposing voltages; their electric force acts on the charged particles entering through slit S2 and causes their path to curve (the negative plate attracting the positive ions), but this force does not depend on their velocity. On the other hand, the force of a magnetic field is directly proportional to the velocity (perpendicular to the field out of the page), and if it is so directed as to counteract the effect of the electric field, it will overcompensate for some velocities and not compensate enough for others; there will be only one value of velocity where the two forces will exactly cancel. For this velocity the particle will proceed in a straight line from slit 52 to slit S3. The ions with other velocities will follow curved paths and will end up in the walls of the velocity filter.]