Van de Graaff Generator

DOE-HDBK-1108-97

Radiological Training for Accelerator Facilities

Student's Guide

These free protons are now introduced into another chamber where again there is an electrical

force on them. This force accelerates the protons, i.e., increases their velocity (speed).

The kinetic energy (the energy of motion) of a particle (at least for velocities much less than the

speed of light) is given by the equation:

KE = mv2

where

KE = the kinetic energy,

m = the mass of the particle, and

v = the velocity of the particle.

The acceleration of the proton by this electrical force also increases the energy of the proton.

The amount of acceleration is determined by the potential difference measured in volts (V) in this

electrical field. One electron volt (eV) is the energy gained by an electron accelerated through an

electric potential of 1 volt. Thus, if a proton is accelerated across a gap by means of a 10,000

volt, or 10 kilovolt (kV), potential difference, it is said to have gained 10,000 electron volts (10

keV) of energy after crossing the gap.

Ten keV was a potential easily achieved, but scientists wanted to accelerate particles to many

times this energy. One way to accomplish this was to apply 10 kV potential to each of many

gaps and have the particle traverse these gaps one after another, gaining 10 keV in energy for

each gap traversed. Alternatively, a voltage many times larger than 10 kV might be applied to a

single gap.

Van de Graaff Generator

One of the first machines to produce laboratory-accelerated particles was the Robert Van de

Graaff generator, an electrostatic accumulator that produces large potential differences

essentially by rubbing. In 1929, Van de Graaff built a pilot machine capable of generating

80,000 volts (80 keV). In 1931, a 1.5 million-volt (MeV) machine was constructed at Princeton