Linear particle accelerator
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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. The design of a linac depends on the type of particle that is being accelerated: electron, proton or ion. They range in size from a cathode ray tube to the 3-km long Stanford Linear Accelerator Center in California.
Construction and operation
A linear particle accelerator consists of the following elements.
- The particle ion 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. Protons are generated in an ion source, which can have many different designes. If heavier particles are to be accelerated, (e.g. uranium ions, a specialized ion source is needed.
- 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. 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.
- 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. 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.
- 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 is lost due to corona discharge, with electrical energy dissipated 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 the 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 or to suddenly drop due to a spark. 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, 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 an even larger International Linear Collider may be built.
The first linear accelerator with the accelerating cavities made of superconductor was built by HEPL on the Stanford campus, mostly for low- and intermediate-energy nuclear physics experiments. It operated for several years, but not as well as originally hoped.
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 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. 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 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.
See also
- Accelerator physics
- Beam line
- Duoplasmatron
- Electromagnetism
- Particle accelerator
- Particle beam
- Particle physics
- List of particles
- Quadrupole magnet
- International Linear Collider
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