Undulator
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An undulator is a device from high-energy physics and usually part of a larger installation, a synchrotron. It consists of a periodic dipole magnet structure. The static magnetic field is alternating along the length of the undulator with a wavelength <math>\lambda_u<math>. Electrons traversing the periodic magnet structure are forced to undergo oscillations and radiate. This process is very similar to Thomson scattering.
The important dimensionless parameter
<math>K=\frac{e B \lambda_u}{2 \pi m c},<math>
where e is the electron charge, B the magnetic field, m the electron rest mass and c the speed of light, characterises the nature of the electron motion. For <math>K\ll1<math> the motion of the electron can be considered linear and this is the undulator regime. If <math>K\gg1<math> the electron motion becomes highly non-linear which leads to radiation of harmonics. This is the wiggler regime.
External Links
D. T. Attwood's page at Berkeley: Soft X-Rays and Extreme Ultraviolet Radiation (http://www.coe.berkeley.edu/AST/sxreuv/). His lecture and viewgraphs are available online.de:Wiggler und Undulator