Synchrotron Radiation as a Modulated Source for Fluorescence Lifetime Measurements and for Time-Resolved Spectroscopy

The fundamental properties of synchrotron radiation as a spectroscopic source are enhanced by the additional quality of time modulation of the source intensity over a broad range of modulation frequencies. This time modulation is a result of the dynamics of accelerators and storage rings. It depends mainly on a balance between longitudinal focusing and damping due to the magnetic lattice and the radio frequency drive and to excitation due to the emission of synchrotron radiation. The spectral profile, state of polarization, and angular distribution of emitted radiation as a function of frequency can be calculated explicitly for any accelerator or storage ring source in terms of its known parameters. The time modulation of the emitted intensity is proportional to the electron beam current modulation seen traversing a single azimuth in the accelerator. The circulating beam in the storage ring is preserved by continuously restoring energy lost to synchrotron radiation with radio frequency (rf) power, usually in the range of from 10 to 500 MHz. In a working storage ring, although the bunch length is set primarily by the frequency of the accelerating field, the bunch shape is also modified by the magnitude of the circulating current and by the internal geometry of the vacuum chamber that contains the electron beam. The complexity of the interactions between the bunch and the walls of the vacuum chamber are such that the bunch shape is difficult to predict exactly for a new storage ring and is best determined empirically as a function of the various operating parameters of the storage ring.

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