Performance of a fixed-taper in-vacuum undulator at SPring-8.

With an in-vacuum undulator, the smallest gaps can be used to achieve high-brilliance radiation within a small spectral width around the harmonics of the fundamental. However, some experiments require a scan over a much wider range of energy within timescales which are impossible to reach via gap tuning. For standard undulators a flat spectrum is usually obtained by using a variable tapered gap. Unfortunately, the mechanical design of the in-vacuum undulator used at SPring-8 is hardly compatible with the extra degree of freedom necessary to adjust the taper mechanically. New magnetic designs are investigated to overcome this problem; their performances are compared with the performances of a fixed-taper in-vacuum undulator for a source of photons in the 5-15 keV range (energy of the fundamental) with an energy width of 1.5 keV.