Gap-tapered undulators for high-photon-energy synchrotron radiation production

Narrow-gap, short-period undulators are of interest to maximize the achievable photon energy at lower-energy storage rings. An important consideration is matching the e-beam beta function in the straight section to the vertical aperture at the insertion device so as to maximize vertical acceptance, beam lifetime, and injection efficiency. Various approaches have been considered such as in-vacuum undulators, undulators with flexible vacuum chambers, and superconducting undulators. In each of these the undulator gap is constant along the undulator length, in which case the optimum beta function is equal to one-half the length. We discuss an alternate approach in which the undulator gap is tapered to follow the transverse profile of the e-beam envelope. This allows the use of a relatively long undulator within a low-beta straight section. The undulator gap is physically and magnetically matched to the e-beam envelope throughout the straight section. The undulator period is varied to maintain constant photon resonance while everywhere maximizing the magnetic field strength. This approach optimizes the high-photon-energy synchrotron radiation generation. The achievable synchrotron radiation spectral properties and tunability of such a device are examined