Seeded FELs Demonstrated in China

The first successful lasing of a free electron laser (FEL) with an echo-enabled harmonic-generation (EEHG) scheme holds promise for the production of a coherent laser at short wavelengths down to the x-ray regime.

FELs provide tunable, high-intensity, ultrashort, coherent radiation for a variety of applications in biology, chemistry, physics and materials science. In the x-ray wavelength range, most of the FELs operate in the self-amplified spontaneous emission (SASE) mode, which has excellent transverse coherence but limited temporal coherence and relatively large statistical fluctuations because they start from electron beam shot noise. Now, with the advent of x-ray FELs, a new era of x-ray science has arrived.

Seeded FEL vs. SASE FEL. (Top) SASE FEL with poor temporal coherence; (middle) HGHG FEL, full temporal coherence with limited harmonic number (N~10) for a single stage; (bottom) EEHG FEL, full temporal coherence with the potentially very high harmonic number in a single stage. (M: modulator; DS: dispersive section; R: radiator) (Images: ©2009 Shanghai Institute of Applied Physics, Chinese Academy of Sciences)

Scientists at Shanghai Institute of Applied Physics (SINAP), Chinese Academy of Sciences, have demonstrated a free-electron laser with ultrahigh brightness and excellent transverse coherence at x-ray wavelengths. Their experimental demonstrations of the EEHG mechanism were performed at the Shanghai Deep-Ultraviolet FEL (SDUV-FEL) facility and at the Next Linear Collider Test Accelerator (NLCTA) at SLAC National Accelerator Laboratory. No amplification was observed during the experiments.

To meet the desired temporal coherence in applications such as soft x-ray resonant inelastic scattering and spectroscopic studies of correlated electron materials, various high-gain seeded FEL schemes, external seeding or self-seeding were developed to produce stable and fully coherent laser pulses from the deep-UV down to the x-ray regime. Among these schemes is the EEHG, which holds promise for fully coherent short wavelength FELs with a single stage of seeded FEL setup.

(Left) Cover of the latest issue of Nature Photonics, which shows the phase spaces of modulated electron beams. FEL radiation from EEHG FEL (upper right), and gain curves of EEHG and HGHG FEL (bottom right).

The team showed the first lasing of a free electron laser with the EEHG scheme using the SDUV-FEL facility, combining a 135.4-MeV electron accelerator and an amplifier with a series of undulator magnets. The lasing was achieved at the third harmonic of the seed, with a gain of ~100,000 over the spontaneous radiation. The measurements showed a typical exponential growth, excellent spectral characteristics and good intensity stability.

The SDUV-FEL facility is composed of a linear electron accelerator with beam energy up to 200 MeV and a multipurpose FEL amplifier. The FEL amplifier with EEHG setup, including a pair of modulator and a 9-m-long “radiator,” designed to study the novel principles of seeded FELs, can do several kinds of FEL experiments and make thorough comparison studies.

The results agree with the team’s theoretical predictions: The premicrobunched electron beam with EEHG technique is amplified with a gain larger than 1000; the central wavelength of the high-gain harmonic-generation (HGHG) radiation is shorter than the third harmonic of the seed laser because of the negative energy chirp in the electron beam; the central wavelength of the EEHG radiation is different from HGHG because of a different wavelength dependence on the energy chirp; and the bandwidths of HGHG and EEHG spectrum are different as the result of a different bandwidth dependence on the nonlinear energy chirp.

The scientists now plan to improve the method to achieve full coherence. They hope to seed the free-electron laser interaction using a conventional source that has good temporal coherence.

The study appeared in Nature Photonics.

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