LRL Accelerators, The 184-Inch Synchrocyclotron — Text and Context

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THE 184-INCH SYNCHROCYCLOTRON

LAWRENCE RADIATION LABORATORY

UNIVERSITY OF CALIFORNIA, BERKELEY, CALIFORNIA

THE 184-INCH SYNCHROCYCLOTRON 2

PRINCIPLE OF OPERATION OF A CONVENTIONAL CYCLOTRON 3

THE PRINCIPLE OF PHASE STABILITY 6

DESIGN AND CONSTRUCTION OF THE 184-INCH SYNCHROCYCLOTRON 8

Radiofrequency System 10

Internal Targets and Beam Extractor 12

CYCLOTRON EXPERIMENTS 15

THE 184-INCH SYNCHROCYCLOTRON

His success with the 60-inch cyclotron in 1939 led Dr. E. O. Lawrence to propose a much more powerful accelerator, one which could produce new types of nuclear rearrangements and even create particles. Grants totaling $1,225,000 permitted work 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.

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

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 the time the ions again reach the dee gap, the sign of the electric potential on the dees is reversed, so that now the ions are attracted toward the opposite dee.

As this process of alternating the electric potential is repeated, the ions gain speed and energy with each revolution. This causes them to spiral outward. Finally they strike a target inserted into their path or are extracted from the cyclotron for use as an external beam.

The time required for an ion to complete one loop remains constant as it spirals outward. This is because its velocity increases sufficiently to make up for the increased distance it travels during each turn. This means that the electric potential applied to the dees must alternate at a constant frequency, called the "resonant frequency."

The resonant frequency f is given by the relationship

He f = --------- , (1) 2[pi]mc

where H, e, [pi], c, and m are constants. H is the strength of the magnetic field of the cyclotron, e is the electric charge carried by the ion, [pi] equals 3.14, c is a conversion factor, and m is the mass of the ion. For example, the resonant frequency for protons accelerated in a 15,000-gauss magnetic field is 23.7 megacycles (Mc).[3] We call such a rapidly alternating potential a "radiofrequency voltage" and the electronic circuit for producing it a "radiofrequency oscillator."

The energy E of an ion emerging from the cyclotron is given by

H^2 R^2 e^2 E = ------- ---- , (2) 2 mc^2

where H, e, and m are as defined above, and R is the radius at which the beam is extracted. From this equation we see that for a given type of ion (where e and m are constant), the energy depends on the diameter and strength of the magnet, but not directly upon the voltage applied to the dees.