Undulator

Working of the undulator. 1: magnets, 2: electron beam, 3: synchrotron radiation

An undulator is an insertion device from high-energy physics and usually part of a larger installation, a synchrotron storage ring. It consists of a periodic structure of dipole magnets (see dipole magnet). The static magnetic field is alternating along the length of the undulator with a wavelength $\lambda _{u}$. Electrons traversing the periodic magnet structure are forced to undergo oscillations and radiate. 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 important dimensionless parameter

$K={\frac {eB\lambda _{u}}{2\pi mc}},$

where e is the particle charge, B the magnetic field, m the electron rest mass and c the speed of light, characterises the nature of the electron motion. For $K\ll 1$ the oscillation amplitude of the motion is small and the radiation displays interference patterns which lead to narrow energy bands. If $K\gg 1$ 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.

Undulators can provide several orders of magnitue higher flux than a simple bending magnet and as such are in high demand at synchrotron radiation facilities.

External links

D. T. Attwood's page at Berkeley: Soft X-Rays and Extreme Ultraviolet Radiation. His lecture and viewgraphs are available online.