Lawrence and the Development of the Cyclotron

DOE-HDBK-1108-97

Radiological Training for Accelerator Facilities

Student's Guide

Large linear accelerators are of two types:

the drift tube and

the wave guide.

N Drift tube accelerator: In the drift-tube accelerator, the particle is accelerated through gaps

between sections of a hollow conducting cylinder. When the particle crosses a gap, it is

accelerated and the voltage oscillates so that when the particle reaches the next gap, it is

accelerated again. Since the frequency of the voltage is kept constant, sections of the tube

must be of different lengths as the particle accelerates.

N Wave guide accelerator: The wave guide accelerator, invented in 1947 by D.W. Fry, uses

long wavelength electromagnetic waves guided through a hollow conductor. Since electrons

of moderate (about 2 MeV) energy already travel with nearly the speed of light, they can keep

up with the wave and thus ride the force of the electric-vector part of the wave somewhat as a

surfer can ride an ocean wave.

Lawrence and the Development of the Cyclotron

The attempt to reach higher energies than were available with the devices of the early days led

Ernest O. Lawrence of the University of California to propose the first cyclotron. The idea was

simple. Instead to trying to accelerate the particles in one pass through, it was proposed that

many small accelerations be used so that their sum would be what was desired. The idea was to

use a magnetic field to make the charged ion move in a circular path.

Lawrence succeeded in substituting a circular race track for the straight-line track of the linear

accelerator. Each time the particle went around, it could be accelerated a little bit thus attaining

energies above that used in previous accelerators and thus with comparatively low voltages. In

describing the first cyclotron, Lawrence and Livingston wrote in 1932 that "experimental

difficulties go up rapidly with increasing voltage." This first device, barely 1 foot across,

produced 1,200,000-volt (1.2 MeV) protons.