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Concept
Undulator
Summary
An undulator is an insertion device from high-energy physics and usually part of a larger
installation, a synchrotron storage ring, or it may be a component of a free electron laser. It consists of a periodic structure of dipole magnets. These can be permanent magnets or superconducting magnets. The static magnetic field alternates along the length of the undulator with a wavelength . Electrons traversing the periodic magnet structure are forced to undergo oscillations and thus to radiate energy. The radiation produced in an undulator is very intense and concentrated in narrow energy bands in the spectrum. It is also collimated on the orbit plane of the electrons. This radiation is guided through beamlines for experiments in various scientific areas.
The undulator strength parameter is:
where e is the electron charge, B is the magnetic field, is the spatial period of the undulator magnets, is the electron rest mass, and c is the speed of light.
This parameter characterizes the nature of the electron motion. For the oscillation amplitude of the motion is small and the radiation displays interference patterns which lead to narrow energy bands. If the oscillation amplitude is bigger and the radiation contributions from each field period sum up independently, leading to a broad energy spectrum. In this regime of fields the device is no longer called an undulator; it is called a wiggler.
The key difference between undulator and wiggler is coherence. In the case of an undulator, the emitted radiation is coherent with a wavelength determined by the period length and the beam energy, while in wiggler the electrons are not coherent.
The usual description of the undulator is relativistic but classical. This means that although a precise calculation is tedious, the undulator can be seen as a black box, where only functions inside the device affect how an input is converted to an output; an electron enters the box and an electromagnetic pulse exits through a small exit slit.
Official source
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