LINEAR ACCELERATOR BASICS:
The What, How, and Why of Linear Accelerators
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What is a Linear Accelerator?
Linear Accelerators (linacs) are linear devices used to accelerate atomic and sub-atomic particles to high velocities.
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How do they accelerate the particles?
Accelerators employ electric and magnetic fields to accelerate, focus and steer the particles. The electric and magnetic fields exert forces only on charged particles such as ions (atoms with too few or too many electrons), atomic nuclei (atoms with no electrons), protons, and/or electrons; they do not exert forces on neutral particles such as neutral atoms or neutrons. The “particles” in these linear accelerators must be “charged particles”.
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How do you measure the energy of the particles?
The unit of energy for these particles is the electron-volt (or eV), which is the energy imparted to an electron or proton when it is accelerated through a potential difference of one volt. Related energy units include kilo-electron-volts (keV), mega-electron-volts (MeV), giga-electron-volts (GeV), and tera-electron-volts (TeV).
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What is an RF Linear Accelerator?
For most applications involving particle energies of one MeV or higher, radio frequency linear accelerators (rf linacs) are employed. In these accelerators, the electric and magnetic fields oscillate at high frequencies, commonly know as “radio frequencies”, in the range of millions to billions of cycles per second. Rf linacs are one of the best ways to accelerate charged particles to MeV and GeV energies.
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What is an RF Linac Structure?
In rf linacs, very high electric and magnetic fields are produced by injecting rf energy from a powerful rf system, similar to a radar transmitter, into a confined region of space (cavity) bounded by conducting materials (usually copper) to keep the energy from radiating away (as in radio, television or radar transmissions). The particles to be accelerated are injected into the linac structure. As the direction of the electric field reverses millions of times per second, the linac structure must be designed to manage the distribution of the electric fields within the structure and the distribution of the particles within the beam so that the particles are exposed to the electric fields when they are in the accelerating direction and shielded from them when they are in the decelerating direction.
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What is a drift tube?
One of the earliest practical linac structures, the Drift Tube Linac structure, accomplishes the complex chore of acceleration with the aid of “drift tubes” distributed along the axis of the structure. The particles are exposed to the longitudinal electric fields when they are in the accelerating direction and are hidden from the electric fields by the drift tubes when the electric fields are in the opposite direction. As the velocities of the particles increase, the lengths of the drift tubes must also increase to keep pace with the particles. Incorporated into the design of all rf linacs is the ability to bunch and focus the particles into small packets suitable for efficient acceleration.
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Why do we accelerate particles?
Particles in these energy ranges find scientific, industrial, medical, and military applications through their ability to produce radioisotopes for medical applications; radiation effects for cancer therapy; thermal and energetic neutron beams for medical applications, material inspection, and explosive detection; special types of light and electromagnetic radiation for solid state physics applications; and energetic particle beams for elementary particle physics research.
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