Applicability of a double-undulator configuration

The applicability of the double-undulator concept for an electron storage ring of 3-GeV class is evaluated based on the parameters of Taiwan Photon Source. In the soft X-ray case, the fundamental harmonic is mainly used, the interference effect is preserved at some level, which means that the brilliance from a double-undulator is expected to be much greater than that of a single undulator. In the hard X-ray case, harmonics number greater than five are generally used, the interference effect cannot, however, be preserved, which means that a double undulator configuration can be assumed to comprise two independent and uncorrelated sources. The total coherent flux obtained from a double-undulator configuration is found to be much less than twice that of a single undulator. The double-undulator concept is hence inapplicable in the hard X-ray region from the viewpoint of high coherent flux performance.