ABSTRACT
FIGURE 21.1 Schematic of dipole magnet indicating the electron beam trajectory/velocity (v), magnetic field (H) and resulting centripetal force (F) and X-radiation fan.
dipoles, wigglers and undulators, that produce synchrotron X-radiation with individual spectral characteristics tailored to the needs of the user (see Figure 21.2). The whole configuration of synchrotron magnets forms a closed loop (or 'ring') so that electrons circulate continuously, their energetic losses (principally from synchrotron radiation) being restored as they pass through high frequency radio cavities placed around the ring, and the electron losses (e.g. from collisions with stray gas molecules, in spite of the 10~10 Torr vacuum) being restored by injection cycles performed typically once or twice per day. In order to maintain synchronism with the RF cavities, the electrons have to travel around the ring in 'bunches'; this requirement coupled with the fact that the electrons are travelling very close to the speed of light (e.g. -0.9999 of the speed of light at 2 GeV energy) means that the synchrotron X-ray light is in actuality 'flashing', though at a rate of around 109 times per second this would appear to be continuous to most observers! The relativistic nature of the electrons (they have a relativistic mass typically of many thousands of times the rest mass) also confers other unique properties to the fan of radiation emanating from the magnet devices. The attributes that are most relevant to the subject under discussion here are: • the X-ray beam is intense. Intensity is
often measured in units of 'brilliance' which
Is 1 1°17 S1016
102 103
FIGURE 21.2 Typical synchrotron X-ray intensity spectra obtained from dipole (bending magnet), wiggler and undulator devices.
