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.