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A linear accelerator is a device that accelerates matter to a high velocity by moving it down a linear path with electromagnetic fields. The term is most commonly used to refer to a linear particle accelerator, or linac, which accelerates atoms or subatomic particles. “Linear accelerator" can also refer to devices that use electromagnetism to propel larger objects, such as coilguns and railguns. Linear particle accelerators are commonly used in medicine, industry, and scientific experiments, and electromagnetic accelerators for larger objects may have future applications for purposes such as space travel and weapons.
A linear particle accelerator fires magnetically charged particles. These can be entire charged atoms, called ions, or subatomic particles such as protons and electrons. First, the particle to be accelerated is generated by an electromagnetic device such as a cathode or ion source and released into a pipe-shaped vacuum chamber lined with electrodes. The electrodes are then energized to create oscillating magnetic fields that impart energy to the particle and accelerate it down the tube towards the device's target. The precise arrangement of the electrodes within the tube, the power and frequency of the energy sent into the electrodes, and the size of the electrodes all vary according to the particles being accelerated and the purpose of the device.
A simple and very common example is the cathode ray tube, commonly used in televisions, monitors, and other display technologies. The cathode ray tube propels electrons down the tube until they strike a solid target at the tube's end made of luminescent materials called phosphors, which are usually metal sulfide compounds. This causes part of the energy of the electrons to be released as an emission of electromagnetic energy in the wavelengths that the human eye detects as visible light. X-ray machines used in medicine and biological research follow a similar principle, firing streams of electrons into copper, molybdenum, or tungsten to produce x-ray emissions that can be used for imaging or, with more powerful devices, radiotherapy.
Linear particle accelerators are also used in scientific research. Small devices are frequently used for imaging in biological and archeological research. Linear accelerators used for research vary greatly in size, and can reach truly colossal dimensions due to the extremely high energy levels needed to produce some of the phenomena studied in modern physics. The largest linear particle accelerator on Earth, located at the SLAC (Stanford Linear Accelerator Center) National Accelerator Laboratory in Menlo Park, California, is two miles long.
They are also used in some industrial processes. Some silicon chips used in modern electronics are manufactured in a process incorporating accelerators that propel entire charged atoms instead of subatomic particles, allowing very precise placement of atoms during production. Accelerators can also be used to implant ions in the surface of materials such as steel, altering the structure of the material to make it more resistant to cracks chemical corrosion.
The term “linear accelerator” is also sometimes used for devices that propel larger objects in a similar manner, using electromagnetism to accelerate a projectile along a straight path. These work by running electricity through either a metal coil wrapped around the barrel of the device, a design called a coilgun, mass driver, or Gauss gun, or through a pair of metal rails positioned parallel to each other, called a railgun. An object made of a ferromagnetic material, such as iron, can be accelerated down the barrel of the device with the magnetic fields produced by properly timed electric currents. Coilguns have been proposed as a possible way to launch cargoes from the Earth's surface into outer space, and both coilguns and railguns are being researched as possible weapons.