Speaker
Description
Reverse tapering can suppress the FEL field while preserving electron microbunching for downstream radiators over an appropriate range of taper strength. In a monochromatically seeded FEL, one-dimensional small-signal theory suggests that the response to reverse tapering can be compared with the response to an accumulated resonance detuning. Here, we report on an experimental investigation of the relationship between undulator reverse tapering and resonance detuning in a seeded hard X-ray FEL, where the fixed seed frequency eliminates the influence of SASE photon-energy jitter. A modulator-radiator configuration was used in which U19 to U24 were scanned either by changing the resonance setting or by applying reverse tapering, while U25 was kept as the downstream radiator. The scans were performed with U25 active, to characterize the bunching-driven radiator signal, and with U25 off, to characterize residual upstream radiation. The U25-active spectra comparison shows a good agreement between reverse tapering and resonance detuning. The U25-off spectra show a stronger reduction of residual upstream radiation on the negative-detuning side than the U25-active radiator signal, indicating that the radiation field is suppressed more efficiently than the bunching-related signal, as expected for reverse-taper operation.
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