Experimental Demonstration to Control the Pulse Length of Coherent Undulator Radiation by Chirped Microbunching

In seeded free electron lasers (FELs), the temporal profile of FEL pulses usually reflects that of the seed pulse, and, thus, shorter FEL pulses are available with shorter seed pulses. In an extreme condition, however, this correlation is violated; the FEL pulse is stretched by the so-called slippage effect in undulators, when the seed pulse is ultimately short, e.g., few-cycles long. In a previous Letter, we have proposed a scheme to suppress the slippage effect and reduce the pulse length of FELs ultimately down to a single-cycle duration, which is based on “chirped microbunching,” or an electron density modulation with a varying modulation period. Toward realization of FELs based on the proposed scheme, experiments have been carried out to demonstrate its fundamental mechanism in the NewSUBARU synchrotron radiation facility, using an ultrashort seed pulse with the pulse length shorter than five cycles. Experimental results of spectral and cross-correlation measurements have been found to be in reasonable agreement with the theoretical predictions, which strongly suggests the successful demonstration of the proposed scheme.

Figure 1

Experimental setup to demonstrate the CM scheme.

Figure 2

Measured spectra of the seed pulse with (red line) and without (black line) the hollow fiber pulse compression scheme.

Figure 3

Experimental results of spectral measurement under the six conditions (i)–(vi) summarized in Table 1 and comparison with theoretical results: (a) measured and (b) computed spectra and (c),(d) FWHM bandwidth and pulse energy evaluated by Gaussian fitting of each spectrum.

Figure 4

Cross-correlation measurement for SFG signal between the seed pulse and coherent radiation, with (blue circles) and without (red squares) the CM scheme. The dashes lines show numerical predictions in the same conditions.