A linear particle accelerator (also called a linac) is an electrical device for the acceleration of subatomic particles. This sort of particle accelerator has many applications, from the generation of X-rays in a hospital environment, to an injector into a higher energy synchrotron at a dedicated experimental particle physics laboratory. X-radiation (composed of X-rays) is a form of Electromagnetic radiation. A synchrotron is a particular type of cyclic Particle accelerator in which the magnetic field (to turn the particles so they circulate and the electric field (to accelerate Particle physics is a branch of Physics that studies the elementary constituents of Matter and Radiation, and the interactions between them The design of a linac depends on the type of particle that is being accelerated: electron, proton or ion. The electron is a fundamental Subatomic particle that was identified and assigned the negative charge in 1897 by J The proton ( Greek πρῶτον / proton "first" is a Subatomic particle with an Electric charge of one positive An ion is an Atom or Molecule which has lost or gained one or more Valence electrons giving it a positive or negative electrical charge They range in size from a cathode ray tube to the 2-mile long Stanford Linear Accelerator Center in California. The cathode ray tube (CRT is a Vacuum tube containing an Electron gun (a source of electrons and a Fluorescent screen with internal or The Stanford Linear Accelerator Center ( SLAC) is a United States Department of Energy National Laboratory operated by Stanford University under California ( is a US state on the West Coast of the United States, along the Pacific Ocean.

Construction and operation

A linear particle accelerator consists of the following elements:

• The particle source. The design of the source depends on the particle that is being accelerated. Electrons are generated by a cold cathode, a hot cathode, a photocathode, or RF ion sources. The electron is a fundamental Subatomic particle that was identified and assigned the negative charge in 1897 by J A cold cathode is an element used within some Nixie tubes Gas discharge lamps Gas filled tubes and Vacuum tubes Cold cathodes do not Hot cathode is also a name for a Hot filament ionization gauge, a vacuum measuring device In a Photomultiplier or Phototube, a photocathode is a negatively charged Electrode coated with a Photosensitive compound An RF antenna ion source (or Radio frequency antenna ion source is an internal multi- Cusp design that can produce a Particle beam of about ~30 to Protons are generated in an ion source, which can have many different designs. The proton ( Greek πρῶτον / proton "first" is a Subatomic particle with an Electric charge of one positive An ion source is an electro-magnetic device that is used to create charged particles If heavier particles are to be accelerated, (e. g. uranium ions), a specialized ion source is needed. Uranium (jʊˈreɪniəm is a silvery-gray Metallic Chemical element in the An ion is an Atom or Molecule which has lost or gained one or more Valence electrons giving it a positive or negative electrical charge An ion source is an electro-magnetic device that is used to create charged particles
• A high voltage source for the initial injection of particles.
• A hollow pipe vacuum chamber. The length will vary with the application. If the device is used for the production of X-rays for inspection or therapy the pipe may be only 0. 5 to 1. 5 meters long. If the device is to be an injector for a synchrotron it may be about ten meters long. A synchrotron is a particular type of cyclic Particle accelerator in which the magnetic field (to turn the particles so they circulate and the electric field (to accelerate If the device is used as the primary accelerator for nuclear particle investigations, it may be several thousand meters long.
• Within the chamber, electrically isolated cylindrical electrodes whose length varies with the distance along the pipe. The length of each electrode is determined by the frequency and power of the driving power source and the nature of the particle to be accelerated, with shorter segments near the source and longer segments near the target. The mass of the particle has a large effect on the length of the cylindrical electrodes; for example an electron is considerably lighter than a proton and so will generally require a much larger section of cylindrical electrodes as it accelerates very quickly - think about a concrete ball and a tennis ball; it is easier to accelerate the tennis ball from rest (this comes about because of the kinetic energy ( being equal to the energy gained by the electron as it is accelerated through the potential difference, usually in the region of 5KV. )
• One or more sources of radio frequency energy, used to energize the cylindrical electrodes. A very high power accelerator will use one source for each electrode. The sources must operate at precise power, frequency and phase appropriate to the particle type to be accelerated to obtain maximum device power.
• An appropriate target. If electrons are accelerated to produce X-rays then a water cooled tungsten target is used. X-radiation (composed of X-rays) is a form of Electromagnetic radiation. Various target materials are used when protons or other nuclei are accelerated, depending upon the specific investigation. The proton ( Greek πρῶτον / proton "first" is a Subatomic particle with an Electric charge of one positive For particle-to-particle collision investigations the beam may be directed to a pair of storage rings, with the particles kept within the ring by magnetic fields. The beams may then be extracted from the storage rings to create head on particle collisions.

As the particle bunch passes through the tube it is unaffected (the tube acts as a Faraday cage), while the frequency of the driving signal and the spacing of the gaps between electrodes are designed so that the maximum voltage differential appears as the particle crosses the gap. A Faraday cage or Faraday shield is an enclosure formed by conducting material, or by a mesh of such material This accelerates the particle, imparting energy to it in the form of increased velocity. At speeds near the speed of light, the incremental velocity increase will be small, with the energy appearing as an increase in the mass of the particles. In portions of the accelerator where this occurs, the tubular electrode lengths will be almost constant.

• Additional magnetic or electrostatic lens elements may be included to ensure that the beam remains in the center of the pipe and its electrodes.
• Very long accelerators may maintain a precise alignment of their components through the use of servo systems guided by a laser beam.

Types of Accelerator

The Stanford superconducting linear accelerator, housed on campus below the Hansen Labs until 2007. This facility is separate from SLAC

The acceleration of the particles can be made with three general methods:

• Electrostatically: The particles are accelerated by the electric field between two different fixed potentials. The Stanford Linear Accelerator Center ( SLAC) is a United States Department of Energy National Laboratory operated by Stanford University under In Physics, the space surrounding an Electric charge or in the presence of a time-varying Magnetic field has a property called an electric field (that can Examples include the Van de Graaf, Pelletron and Tandem accelerators. A Van de Graaff generator is an electrostatic machine which uses a moving belt to accumulate very high electrostatically stable Voltages on a hollow metal globe Pelletron is a type of electrostatic Particle accelerator similar to a Van de Graaff generator.
• Induction: A pulsed voltage is applied around magnetic cores. Faraday's law of induction describes an important basic law of electromagnetism which is involved in the working of Transformers Inductors and many forms of The electric field produced by this voltage is used to accelerate the particles. In Physics, the space surrounding an Electric charge or in the presence of a time-varying Magnetic field has a property called an electric field (that can
• Radio Frequency (RF): The electric field component of radio waves accelerates particles inside a partially closed conducting cavity acting as a RF cavity resonator. Radio waves are electromagnetic waves occurring on the Radio frequency portion of the Electromagnetic spectrum. Radio waves are electromagnetic waves occurring on the Radio frequency portion of the Electromagnetic spectrum. A resonator is a device or system that exhibits Resonance or resonant behavior that is it naturally oscillates at some frequencies, called its resonance Examples include the travelling wave, Alvarez, and Wideroe cavity type accelerators.

Advantages

Linacs of appropriate design are capable of accelerating heavy ions to energies exceeding those available in ring-type accelerators, which are limited by the strength of the magnetic fields required to maintain the ions on a curved path. High power linacs are also being developed for production of electrons at relativistic speeds, required since fast electrons traveling in an arc will lose energy through synchrotron radiation; this limits the maximum power that can be imparted to electrons in a synchrotron of given size. This article concerns the physical phenomenon of synchrotron radiation

Linacs are also capable of prodigious output, producing a nearly continuous stream of particles, whereas a synchrotron will only periodically raise the particles to sufficient energy to merit a "shot" at the target. (The burst can be held or stored in the ring at energy to give the experimental electronics time to work, but the average output current is still limited. ) The high density of the output makes the linac particularly attractive for use in loading storage ring facilities with particles in preparation for particle to particle collisions. The high mass output also makes the device practical for the production of antimatter particles, which are generally difficult to obtain, being only a small fraction of a target's collision products. In Particle physics and Quantum chemistry, antimatter is the extension of the concept of the Antiparticle to Matter, where antimatter is composed These may then be stored and further used to study matter-antimatter annihilation.

As there are no primary bending magnets, this cost of an accelerator is reduced.

Medical grade linacs accelerate electrons using tuned-cavity waveguide in which the RF power creates a standing wave. A standing wave, also known as a stationary wave, is a Wave that remains in a constant position Some linacs have short, vertically mounted waveguides, while higher energy machines tend to have a horizontal, longer waveguide and a bending magnet to turn the beam vertically towards the patient. Medical linacs utilise monoenergetic electron beams between 4 and 25 MeV, giving an x-ray output with a spectrum of energies up to and including the electron energy when the electrons are directed at a high-density (such as tungsten) target. Tungsten (ˈtʌŋstən also known as wolfram (/ˈwʊlfrəm/ is a Chemical element that has the symbol W and Atomic number 74 The electrons or x-rays can be used to treat both benign and malignant disease. The reliability, flexibility and accuracy of the radiation beam produced has largely supplanted cobalt therapy as a treatment tool. Radiation therapy (or radiotherapy) is the medical use of Ionizing radiation as part of Cancer treatment to control Malignant In addition, the device can simply be powered off when not in use; there is no source requiring heavy shielding.

Disadvantages

• The device length limits the locations where one may be placed.
• A great number of driver devices and their associated power supplies are required, increasing the construction and maintenance expense of this portion.
• If the walls of the accelerating cavities are made of normally conducting material and the accelerating fields are large, the wall resistivity converts electric energy into heat quickly. On the other hand superconductors have various limits and are too expensive for very large accelerators. Therefore, high energy accelerators such as SLAC, still the longest in the world, (in its various generations) are run in short pulses, limiting the average current output and forcing the experimental detectors to handle data coming in short bursts. The Stanford Linear Accelerator Center ( SLAC) is a United States Department of Energy National Laboratory operated by Stanford University under

Wake fields

The electrons from the klystron build up the driving field. A klystron is a specialized linear-beam Vacuum tube (evacuated electron tube The driven particles also generate a field, called the wakefield. For strong wakefields high frequencies are used, which also allow higher field strengths. A small dielectrically loaded waveguide or coupled cavity waveguides are used instead of large waveguides with small drift tubes.

At the end all fields are absorbed by a dummy load or cavity losses. A dummy load is a device used to simulate an electrical load usually for testing purposes

See also

• Accelerator physics
• Beam line
• Compact Linear Collider
• Duoplasmatron
• Electromagnetism
• Particle accelerator
• Particle beam
• Particle physics
• Superconducting Radio Frequency
• List of particles
• Quadrupole magnet
• International Linear Collider
• Anatoli Bugorski

External links

• Elekta Corporate
• Varian Medical Systems
• Siemens Medical Solutions
• Novalis