rk to start on the 184-inch cyclotron in
August 1940.[1] It was designed to accelerate atomic particles to an
energy of 100 million electron volts (Mev), five times that possible
with the 60-inch machine.

[Illustration: Fig. 1. The electromagnet under construction during the
period 1940 to 1942.]

Before the new cyclotron could be finished World War II began.
Construction on the cyclotron was therefore halted. However, because of
interest in separating the isotopes of uranium by the electromagnetic
method, work on the giant magnet continued at an even faster pace. This
magnet would contain 3700 tons of steel in its yoke and pole pieces, and
300 tons of copper in its exciting coils (Fig. 1). By May 1942 the
magnet was completed. During that summer it was used in a pilot plant to
separate the first significant amounts of U^{235} ever obtained. The
184-inch magnet remained in use in a research and development program at
Berkeley until the end of the war, supplying information to Oak Ridge,
Tennessee, where a large separation plant had been erected.

Construction on the rest of the cyclotron was resumed in 1945. By that
time a new principle had been discovered which made it possible to
obtain ion beams of much higher energy than originally hoped for. Yet a
considerably lower accelerating voltage could be used. This important
discovery was made independently by Dr. V. Veksler in Russia and by
Dr. Edwin M. McMillan, present Director of the Lawrence Radiation
Laboratory. Before attempting to discuss this principle, we should first
review the operation of a conventional cyclotron.

PRINCIPLE OF OPERATION OF A CONVENTIONAL CYCLOTRON

[Illustration: Fig. 2. Basic parts of a cyclotron.]

The main parts of a cyclotron are represented in Fig. 2. Charged
particles (ions) are accelerated inside an evacuated tank. This is to
prevent the beam from colliding with air molecules and being scattered.
The vacuum tank is placed between the poles of an electromagnet, whose
field bends the ion beam into a circular orbit.

The operation begins when the ions are introduced into the region
between two accelerating electrodes, or "dees."[2] Because the ions
carry a positive electric charge, they are attracted toward that dee
which is electrically negative at the moment. Were it not for the
magnetic field, the ions would be accelerated in a straight line;
instead they are deflected into a circular path back toward the dee gap.
By th