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 2-mile long Stanford Linear Accelerator Center in California.
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 moved. 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 designs. 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. 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. 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 are 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.
Types of Accelerators
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. Examples include the Van de Graaf, Pelletron and Tandem accelerators.
- Induction: A pulsed voltage is applied around magnetic cores. The electric field produced by this voltage is used to accelerate the particles.
- Radio Frequency (RF): The electric field component of radio waves accelerates particles inside a partially closed conducting cavity acting as a RF cavity resonator. Examples include the travelling wave, Alvarez, and Wideroe cavity type accelerators.
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 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. 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. 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. 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. In addition, the device can simply be powered off when not in use; there is no source requiring heavy shielding.
- 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 electrons from the klystron
build up the driving field.
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.