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Chapter 32:
Synchrotron Radiation Sources for EUVL Applications
It is well known that when charged particles are accelerated they emit electromagnetic radiation. For example, when electrons traveling at nearly the speed of light are forced to move along a curved path by the magnetic fields of a storage ring, they emit photons into a narrow cone. Depending on the electron energy, this so-called synchrotron radiation can be extremely intense over a broad wavelength range extending all the way from the microwave to the x-ray spectral region. Because this radiation also tends to be vertically collimated and polarized, it has been used as a powerful research and development tool in chemistry, physics, material science, biology and medicine, and, in the last decade or so, has played a key role in the development of EUVL. A drawing of a typical synchrotron radiation source built around a storage ring is shown in Fig. 32.1. The storage ring, a many-sided doughnut-shaped tube, enables a current of electrons to circulate at essentially the speed of light in a closed orbit for many hours, all the while emitting synchrotron radiation. The electron beam decays slowly because of electron momentum loss during collisions with residual gas molecules, electron-electron collisions, and photoemission. If the momentum loss is too high, the electron is lost from within the storage-ring acceptance. To reduce electron-molecule collisions, the inside of a storage ring is maintained at extremely low pressure (in the range 10−9 to 10−10 Torr). Changing the electron beam's size can change the electron-electron collision rate, but the photoemission process is inherent in the quantum nature of photoemission. A system of magnetic lenses, bending magnets (dipoles), focusing magnets (quadrupoles), and steering magnets (sextupoles), strategically located around the ring, guides and focuses the electron beam. This so-called lattice determines the basic features of the circulating beam, such as its emittance and its transverse dimensions, and also determines the number, length, and location of any straight sections in the ring available for insertion magnets: devices that provide radiation with enhanced flux, brightness, and spectral range. The energy lost by the circulating electrons to synchrotron radiation is replenished by longitudinal electrical kicks imparted to the beam as it transverses one or more radio frequency (rf) cavities incorporated in the ring. The rf power in these cavities chops the circulating electron beam into bunches and hence determines the duration of the synchrotron radiation pulses; its frequency determines the minimum bunch spacing. The storage ring is filled with electrons from a source called an injector, e.g., a linear accelerator fed from an electron gun, a microtron, or a small booster ring fed by a linac or a microtron. Some of the synchrotron radiation from the bending magnets and most of the radiation from the insertion magnets (wigglers and undulators) escapes the storage ring through tangential ports, called beamlines, that allow the radiation to pass to experimental end stations located at convenient distances from the ring.
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