Linear accelerator

An animated diagram showing how a linear accelerator works

A linear accelerator, often called a linac from the first syllables of its name in English (linear accelerator) is an electrical device for the acceleration of particles that have an electrical charge, such as electrons, positrons, protons, or ions. Acceleration occurs incrementally, as the particles traverse a sequence of alternating electric fields.

The theoretical concept of a linear accelerator using an oscillating field of radio frequencies was conceived in 1924 by the Swedish physicist Gustaf Ising. Influenced by this idea, the Norwegian engineer Rolf Widerøe built the first one, with which he was able to accelerate potassium ions to an energy of 50 000 eV . The advent of more powerful radio frequency generators, developed for radars during World War II, marked an important advance in the design of linear accelerators, by enabling the acceleration of lighter particles, such as protons and electrons. In 1946 Luis Álvarez designed an 875 m longitude accelerator located in a resonant cavity, capable of accelerating protons up to an energy of 800 MeV. The longest linear accelerator, at 3.2 km, is at the SLAC National Accelerator Laboratory, California.

Linear accelerators are used in particle physics and for the production of radiation to study the structure and properties of matter. They also have practical applications in the semiconductor industry and medicine.

Development of the linear accelerator

Construction principle of a linear accelerator according to Ising and Wideröe.

The first electrically charged particle accelerators were based on the application of a continuous voltage; the maximum energy reached by the particles in this type of accelerator is equal to the voltage multiplied by their charge. The main limitation of this design is that increasing the voltage to a few tens of megavolts causes an electrical discharge in the medium. For this reason alternatives to this mode of operation were sought. Gustav Ising suggested the use of an alternating voltage and Rolf Widerøe first developed such a concept in 1928.

This type of accelerator is made up of a variable number of cylindrical tubes. The alternating tubes are connected together in such a way that an oscillating potential difference is applied between the two sets of tubes. Due to this potential difference, the charged particles experience an acceleration in the space between the tubes; once they enter the tube, it acts like a Faraday box, isolating them from the oscillating electric field until they emerge at the other end, where they experience a new push. The time it takes for the particles to pass through the tube must be constant to keep in sync with the period of oscillation of the electric field. Since the speed of the particles increases as they travel along the accelerator, the tubes must be longer the further they are from the source. Early accelerators of this type worked well with heavy particles such as ions, but were unable to impart high energies to subatomic particles such as protons or electrons. Due to their low mass, these particles reach a speed close to that of light and an oscillating field is required at frequencies of the order of gigahertz. Klystrons, devices invented in 1937 and capable of generating these radio frequencies, began to be used for uses non-military at the end of World War II. At the same time, Luis Álvarez, along with other collaborators from the University of California, proposed to place the accelerator in a resonant cavity to confine the electromagnetic field and limit radiation losses. This design, with some modifications, is often used for accelerators. of protons.

In the 1960s a new design was introduced at Los Alamos National Laboratory known as the SCL (Side Coupling Linac) or 'Side Coupling Linac'. This type of accelerator is composed of multiple coupled resonant cavities. Cavities along the particle beam are called accelerator cavities; each adjacent pair of accelerator cavities, of opposite phases to each other, have a lateral coupling cavity that contributes to the stabilization of the electromagnetic field in the accelerators of greater length. These types of accelerators can use high-powered klystrons and are predominantly used for the acceleration of particles to speeds greater than half the speed of light. In the 1980s the use of superconducting materials in accelerator components was proposed.. This technology is predominantly used in large accelerators operating at high energies, such as free electron lasers and Energy Recovery linacs.

Components

Klistrón

A modern linear particle accelerator has the following elements:

• A source of particles: the source depends primarily on the type of accelerator. For electron accelerators, thermoionic cathodes can be used, in which electrons are separated from atoms by heating the material, cold cathodes or photocathods excited by a laser, resulting in a more concentrated and less divergent beam. Proton and ion sources are very diverse; these particles are usually extracted from a plasma, generated, for example from a discharge or microwave radiation applied to a gas.
• A high voltage source for the initial injection of particles. The injector can be of continuous or alternate voltage. In ion accelerators magnetic quadrupoles operated on radiofrequency are used to keep the beam focused on low energies.
• A hollow structure that houses the components of the accelerator and which should remain at a high vacuum level, between 10^-6 and 10^-9 Torto limit the deceleration of particles and energy losses. Its length depends on the applications and varies between 1 or 2 m and miles.
• Electrically isolated cylindrical electrodes. Its length depends on the distance in the tube, as well as the type of particle to accelerate and the power and frequency of the applied voltage. The shortest segments are close to the source and the longest, to the other end.
• alternate voltage sources, which will feed the electrodes. The use of klisters to amplify the electromagnetic signal is indispensable for high-power accelerators. Although vacuum tubes have been obsolete for most of the applications for which they initially developed, there is no alternative capable of generating the same power to wavelengths of the millimeter order.
• Additional magnetic and electrical lenses may be required to keep the beam focused in the center of the tube and the accelerating elements, especially in proton and ion accelerators.
• Very long accelerators can specify the alignment of their components by servos and a laser beam as a guide.

Advantages and disadvantages of the linear accelerator

Linear accelerators generate an intense beam of charged particles, at high energy and with a range of characteristics that make it an ideal instrument for multiple applications. It is possible to obtain small beams, collimated, with pulses concentrated in time or with low energy distribution. Among the advantages of this type of accelerators are the following:

• The beam crosses the accelerator once, which prevents destructive resonance effects.
• The beam travels in a straight line, so there is no loss of energy from synchrotron radiation.
• No complicated devices are required to inject and extract the beam.
• It can produce pulsed beams or operate on continuous wave.

The main disadvantage of the linear accelerator is that, in order to achieve high energies, it is necessary to increase the number of acceleration elements, with the consequent increase in construction costs. In contrast, in circular accelerators, the particles traverse the radio frequency cavity an indefinite number of times.

Applications of linear accelerators

Aerial view of Stanford's linear accelerator. This accelerator, used for years for particle physics experiments, is the electron injector for LCLS free electron laser.

There is a wide variety of linear accelerators, dedicated to different purposes. They are used as particle injectors in synchrotrons, both for particle physics studies and for producing synchrotron radiation for materials study and other practical applications. They can also be used for this purpose in electron lasers. High-energy linear accelerators such as the Stanford linear accelerator at SLAC National Accelerator Laboratory and the linear collider at DESY (German Electron Synchrotron) allow to obtain X-ray laser light.

Linear accelerators can be used to deliver therapy against cancer tumors (radiotherapy), for the characterization and study of biological and inorganic materials, and in industrial manufacturing processes, for example in microelectronics.

Linear accelerators have played an important role in particle physics research. Because of the high energy required for these studies, the accelerators in operation in the first decade of the XXI century are predominantly circular, like the Large Hadron Collider at CERN. However, the next generation of accelerators will use the linear design again: there are plans to build the 35 km long International Linear Collider (ILC). and the Compact Linear Collider (CLIC).