What is a LINAC?

© 2026 Kieron Conway - All rights reserved.

By accelerating charged particles (electrons, protons, or other ions), the LINAC (linear accelerator), invented in the 1930s, became one of the key machines that advanced nuclear physics well into the second half of the twentieth century. One of the main uses of LINACs today is as the primary source of particle beams at high-energy laboratories around the world.

High-energy particle colliders are usually ring-based, such as the LHC (Large Hadron Collider). Ring-based accelerators are not efficient at accelerating particles from rest, but a LINAC is ideal as a primary source of high-speed ions, which can then be injected into a ring for further acceleration.

How Does a LINAC Work?

Unlike the Van de Graaff accelerator, which uses a high-voltage electrostatic charge to accelerate ions, a LINAC uses oscillating electric fields to accelerate ions in a straight line down the length of a fully evacuated tube or tank. These linear ion beams can be used to strike fixed targets for nuclear experiments, or they can serve as the primary beam source for synchrotrons such as the LHC complex at CERN.

CERN actually uses a chain of accelerators to increase beam energy step by step before final injection into the LHC itself, as described in Part 3 of A Journey into Modern Physics.

Types of LINAC Architecture

There are two main types of LINAC design:

1) Standing-Wave LINACs

In standing-wave LINACs, radio-frequency (RF) energy is pumped into an evacuated tank to generate standing waves, producing oscillating electric fields inside the tank. Tubes suspended inside the tank, known as drift-tubes, shield the particles during parts of the RF cycle. Ions receive acceleration as they cross the gap (called the RF gap) between adjacent drift-tubes.

2) Travelling-Wave LINACs

In travelling-wave LINACs, a series of cells act as waveguides, allowing the RF wave to propagate down the length of the evacuated accelerator structure. At the end of the accelerator, the wave energy is absorbed to prevent standing waves from forming. This design is particularly suitable for very long LINACs, as we shall see.

Precision Design

For a LINAC to function correctly, both the physical geometry of the accelerator and the precise frequency of the RF waves must be determined with great accuracy.

1) Standing-Wave LINACs

Charged particles are timed to pass through the gaps between drift tubes so that they receive an energy boost at each gap. While inside a drift tube, the ions coast at constant energy until they reach the next gap. With the correct design, alternating positive and negative charges are induced on the entrances and exits of the drift tubes, creating an oscillating potential difference between adjacent tubes.

2) Travelling-Wave LINACs

In travelling-wave LINACs, the RF wave moves continuously along the waveguides, constantly transferring energy to the particles. It is as if the particles are surfing on the crest of the wave!

Special Relativity Raises Its Head

As ions travel down the accelerator, they can quickly reach relativistic speeds. Consequently, design modifications are required to accommodate relativistic effects.

1) Standing-Wave LINACs

Further along the accelerator, the drift-tube lengths and the gaps between them are increased slightly, as indicated in the attached diagram, to ensure that particles remain in phase with the oscillating, electric field and continue to receive acceleration at each gap.

2) Travelling-Wave LINACs

In travelling-wave LINACs, discs are inserted into the waveguide cells to create reflections and interference patterns. These effectively reduce the phase velocity of the RF wave, allowing it to remain synchronised with the increasingly relativistic particles. If this was not done, the wave would quickly overtake the particles and could even slow them down.

Beam Focus and Energy

Magnetic fields are used to prevent the beam from diverging, keeping it tightly focused along the line of flight.

LINACs can produce beam energies ranging from the MeV scale up into the GeV range, depending on the length of the accelerator and the type of particle being accelerated. The particle type also strongly affects the beam energy. For example, a fully stripped xenon ion produces a much higher beam energy than a single proton, since xenon contains 54 protons and, including its neutrons, has far greater mass. It is rather like the difference between accelerating a tennis ball and a cannonball!

The beam energy delivered by a LINAC depends on the charge-to-mass ratio of the particle and the accelerating electric field. LINACs are therefore usually designed for specific particle types.

Uses of LINACs

Medical Applications

LINACs are widely used in medicine to produce electron beams that can be focused onto tumours, destroying cancerous tissue while minimising damage to surrounding healthy tissue. LINACs can also generate X-rays by accelerating electrons into metal targets. The sudden deceleration of the electrons produces bremsstrahlung (German for “braking”) radiation, which is used in cancer radiotherapy.

Industry

LINACs are used for sterilisation, imaging, and materials testing.

Research

LINACs are employed in low-energy nuclear physics and as injectors for large ring-based accelerators such as the LHC. Travelling-wave LINACs are particularly well suited for accelerating electrons and positrons.

CERN and SLAC Linear Accelerators

CERN uses a standing-wave LINAC to produce proton beams of 160 MeV for injection into the Proton-Synchrotron-Booster. These protons are then accelerated and passed successively through the Proton-Synchrotron and the Super-Proton-Synchrotron before final injection into the LHC.

A separate standing-wave LINAC is used as the primary source of heavy-ion beams, up to lead, delivering energies of about 4.2 MeV per nucleon. Fully stripped lead ions contain 82 protons.

The LHC complex can operate in several modes, including proton-proton collisions and lead-lead collisions, depending on which LINAC source is used.

SLAC (Stanford Linear Accelerator) is a remarkable 2-mile-long travelling-wave LINAC designed to accelerate electrons or positrons. It played a crucial role in confirming that protons and neutrons contain internal constituents: fractionally charged quarks. SLAC’s electron beam initially reached energies of 20 GeV for deep-inelastic scattering experiments. After upgrades, it can now accelerate electrons or positrons to energies of up to 50 GeV, although most experiments operate below this maximum.

Accelerating Cavities

The physics underlying standing-wave LINACs extends naturally to accelerating cavities used in many accelerator types, including the LHC. Each cavity has a beam entrance and exit, allowing particles to enter, gain energy, and rejoin the evacuated beam-line.

When RF energy, at the correct frequency, is fed into a fully evacuated cavity, an oscillating electric field is established. A standing wave forms such that, during one phase of the RF cycle, the cavity entrance becomes positive and the exit negative. During the next phase, the polarity reverses. If a positive ion enters the cavity when the entrance is positive and the exit negative, it receives an accelerating kick. Proper timing ensures that the particle always enters the cavity during the accelerating phase, gaining energy each time it passes through.

Part 3 of A Journey into Modern Physics explores the development of CERN’s accelerators from their beginnings in the 1950s to the present-day LHC.

--

Liked this article? Check out:

modern-physics-journey.com 

where you can read all about an exciting new science series: A Journey into Modern Physics, available from Amazon and Rakuten Kobo on-line shops.