What Are Linear Accelerators?

Linear accelerators, commonly abbreviated as linacs, are devices that accelerate charged particles along a straight path using alternating radiofrequency electromagnetic fields. Unlike circular accelerators, which bend the beam repeatedly around a closed orbit, a linac passes the particles through a series of accelerating structures only once, imparting energy at each stage. The result is a beam of electrons, protons, ions, or other charged particles with precisely controlled energy, intensity, and emittance.

Linacs occupy a central place in both fundamental particle physics research and applied technology. In high-energy physics, they serve as injectors feeding particles into synchrotrons or as standalone sources for collider experiments. In medicine, electron linacs are the primary machines used to produce the megavoltage X-rays that treat tumors in radiation therapy. The underlying physics draws on electrodynamics, microwave engineering, and accelerator physics, with design practice governed by international standards and documented extensively in the CERN Accelerator School proceedings on linear accelerators.

RF Acceleration and Drift-Tube Design

Acceleration in a linac relies on the synchronism between the particle's velocity and the phase of an oscillating electric field. In a drift-tube linac (DTL), originally developed by Luis Alvarez at Berkeley in 1945, particles travel through a series of hollow metal tubes inside a resonant cavity. The tubes shield the particles from the decelerating phase of the RF cycle while they are inside, so that when the particles emerge into the gaps between tubes, the field polarity has reversed and the field is accelerating. The tube lengths increase along the structure to match the growing particle velocity.

At higher particle energies and higher RF frequencies, standing-wave or traveling-wave cavities replace drift tubes. Copper cavities operating in the 3 to 12 GHz range achieve accelerating gradients of 15 to 75 MeV per meter for electrons; superconducting niobium cavities operating near 1.3 GHz reach 10 to 30 MeV per meter but require cryogenic cooling to liquid helium temperatures. The Stanford Linear Accelerator Center (SLAC) linac, a 3-kilometer copper structure, was for decades the longest such machine and accelerated electrons and positrons to approximately 50 GeV.

Electron and Proton Linacs

Electron linacs and proton linacs differ significantly in structure because the two particle types reach relativistic velocities at very different energies. Electrons become relativistic at a few MeV, so the accelerating cavities in an electron linac can use a uniform cell geometry almost from the start of the structure. Protons require much heavier acceleration at low energies: a proton linac typically begins with a radiofrequency quadrupole (RFQ) that simultaneously focuses, bunches, and accelerates a low-energy beam, transitions to a drift-tube linac section, and then feeds higher-energy structures as the beam approaches relativistic velocities. Proton linacs at facilities such as CERN's Linac4 use this staged approach to prepare beams for injection into the Large Hadron Collider.

Beam Focusing and Collimation

Charged particle beams tend to diverge because of space-charge repulsion among the particles and because particles with slightly different momenta or trajectories accumulate transverse displacements along the linac. Focusing elements, typically magnetic quadrupole doublets or triplets placed periodically along the structure, confine the beam to a narrow channel by applying restoring forces in the transverse plane. Beam collimators, which are aperture-limiting devices placed at strategic points, remove halo particles that have migrated to large transverse amplitudes and would otherwise strike the linac walls or degrade beam quality at the target. Linear accelerator design principles address the coupled problems of focusing lattice design, emittance growth from space charge, and collimation strategy as a unified beam dynamics problem.

Applications

Linear accelerators have applications in a range of fields, including:

• Radiation oncology, where electron linacs produce megavoltage photon and electron beams for tumor treatment
• Particle physics, where proton and electron linacs inject beams into circular colliders or free-electron lasers
• Industrial radiography, where compact electron linacs produce high-energy X-rays to inspect welds and castings
• Synchrotron light sources, where electron linacs serve as injectors for storage rings producing X-ray beams
• Neutron spallation sources, where high-power proton linacs drive neutron production for materials research