Linear particle accelerator

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A Linear particle accelerator is an electrical device for the acceleration of subatomic particles. It may be used to produce X-rays, by accelerating electrons into a target window, or for the production of high energy particles so that the results of particle collisions may be studied, usually with a bubble chamber or other device.

Contents

Construction and operation

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2mv_accelerator-MJC01.jpg
A 1960s single stage 2MeV linear Van de Graaff accelerator, here opened for maintenance

A linear particle accelerator consists of the following elements.

  • A particle source: If electrons are to be accelerated, an hot cathode, or if heavier particles are to be accelerated (e.g. protons) an appropriate source of ions.
  • 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 this may be only .5 to 1.5 meters long. If the device is to be an injector for a synchrotron this may be about ten meters long. If the device is used as the primary accelerator for nuclear particle investigations this 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.
  • One or more sources of radio frequency energy, used to energize the cylindrical electrodes. 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. Various target materials are used when protons or other nuclei are accelerated, depending upon the specific investigation. 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 is designed so that the maximum voltage differential appears as the particle crosses the gap. 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.

History

The first linear accelerators used only a single stage of acceleration, with a direct current potential providing the energy. This could be provided by a Van de Graaff generator or a voltage multiplier power supply. Such accelerators are severely limited in accelerating power since at high voltage energy will be lost due to corona discharge, with electrical energy dissapated into the surrounding atmosphere. Such devices are still used as ion injectors for other accelerating devices. The accelerating potential (in electron volts) is equal to the voltage potential (volts) between the ion source and the target. The maximum potential relative to ground potential is generally not limited by the generator(s) but rather by the tendency of voltage potential to leak away due to corona discharge. While various techniques may be applied to raise this maximum potential the structures required become impractically massive and/or expensive.

Early multiple–stage accelerators were limited by the lack of suitable electron tubes capable of operating at high frequency and high power while maintaining both precise frequency and phase control. Various other types of accelerators such as the Cyclotron and Synchrocyclotron were developed to overcome these limitations. With the development of the high power klystron tube it became practical to continue the development of the linear accelerator (LINAC), first for use as a high speed injector for the synchrotron and finally as a high power accelerator for research use, culminating in the two mile long Stanford Linear Accelerator (SLAC). In the future the even larger International Linear Collider may be built.

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.

LINACs are also capable of prodigious output, producing a nearly continuous stream of particles, whereas a syncrotron will only periodically raise the particles to sufficient energy to merit a "shot" at the target. 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. 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 a complex bending magnet arrangement and a 6-30 million electron-volt potential 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. 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 is required, increasing the construction and maintenance expense of this portion.de:Linearbeschleuniger

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