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Particle Accelerators: History, Types & Uses

Instructor: David Wood

David has taught Honors Physics, AP Physics, IB Physics and general science courses. He has a Masters in Education, and a Bachelors in Physics.

This lesson will introduce the history of particle accelerators, how they are used, as well as providing an overview of the types of particle accelerators in use today. A short quiz will follow.

What is a Particle Accelerator?

A particle accelerator is a piece of equipment that uses electric and/or magnetic fields to project subatomic particles at high speeds. Often those particles will then be collided with one another as part of physics experiments, enabling us to learn more about how the universe works. They're also sometimes used for particle therapy to treat cancer, or as light sources in the study of condensed matter physics.

History of Particle Accelerators

In 1930, Cockcroft and Walton built a 200,000 volt transformer and accelerated protons along a straight line to test for a phenomenon known as Gamow's tunneling. This was the first particle accelerator. Their attempt to observe the phenomenon failed, and they concluded that a higher energy accelerator would be needed. Thus began the quest for higher and higher energies that continues to this day.

Cockcroft and Walton, with another famous physicist: Earnest Rutherford (middle)
Cockcroft, Rutherford, Walton

Electrostatic Accelerators

The first particle accelerators were all electrostatic accelerators. These are accelerators that simply use an electric field to increase the speed of a charged particle. Opposites attract, so a negative particle will be attracted to a positively-charged plate, and a positive particle will be attracted to a negatively charged plate.

While this kind of particle accelerator is cheap and simple, they tend not to be used in modern particle physics experiments. This is because they are limited in the amount of energy they can give a particle. Once a certain energy is reached, the voltage being applied is so high that insulators experience a phenomenon known as electrical breakdown. Electrical breakdown is where materials that are normally insulators become conductors, and this stops a particle accelerator from operating correctly.

Oscillating Field

A more modern type of particle accelerator is called an oscillating field accelerator. These include linear accelerators, cyclotrons and synchrotrons which will be discussed later. Oscillating field accelerators work by applying smaller voltages that oscillate in direction (AC power). As a particle is attracted to and approaches a plate, the voltage is reversed. When the particle reaches the plate, it passes through a hole in it, and out the other side. As it goes through the hole the charge on the plate it switched so that the particle is then repelled away from the plate on the opposite side. This continues with a large number of plates, with oscillations that are faster and faster, until the particle reaches the intended speed.

Cyclotrons, Synchrotrons and Linear Accelerators

Oscillating field accelerators can be set up in multiple ways. They can be set up in a straight line, in which case they are called linear accelerators. Or they can be set up in a circular format, as is the case for cyclotrons and synchrotrons.

The benefit of circular particle accelerators is that they take up less space for a given amount of acceleration. Disadvantages include the fact that you need a huge magnet to keep the particles moving in a circle, and that charged particles moving in a circle produce synchrotron radiation, which fights against your attempts to give the particle more energy.

Diagram of a Cyclotron
Diagram of a Cyclotron

Cyclotrons were the first circular accelerators, and used a simple design made of two D-shaped plates and one large magnet. In a cyclotron, the particle gains speed and spirals outwards, until it reaches an opening and leaves the accelerator to arrive at the target. Cyclotrons are limited by relativity -- as you increase the energy of the particle to speeds approaching the speed of light, the mass of the particle begins to increase, making it harder and harder to accelerate it further. Since cyclotrons have a constant, unchanging magnetic field, there is no way to adjust for this.

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