Harmonic-generation FEL may lead to x-rays

Scientists at Brookhaven National Laboratory (Upton, NY), and Argonne National Laboratory (Argonne, IL), have demonstrated a high-gain harmonic-generation (HGHG) free-electron laser (FEL) that may ultimately be used to provide an intense, highly coherent source of hard x-rays. The laser is capable of providing the intensity and spatial coherence of self-amplified spontaneous emission (SASE) but with the excellent temporal coherence necessary for generation of short wavelength emission, meaning the coherence time is much less than pulse duration.

There are several configurations of a FEL source. The most widespread involves the use of a high-quality optical cavity and is very effective in wavelength regimes for which appropriate mirrors are available. As in the case of lasers, an optical resonator can provide a high degree of spatial and temporal coherence. For developing a hard x-ray FEL, one simple strategy uses the SASE approach in a high-gain single-pass amplifier to circumvent the lack of high-quality mirrors at short wavelengths. However, the ability of the SASE to provide hard x-rays is limited by poor temporal coherence due to random noise buildup.

In the HGHG FEL, the electron beam undergoes a small energy modulation via interaction with a pulsed seed laser (see figure). The energy modulation is converted to a coherent spatial density modulation as the electron beam traverses a dispersion magnet (a three-dipole chicane). A second undulator, the radiator, is tuned to a higher harmonic of the seed frequency, w, which causes the microbunched electron beam to emit coherent radiation at the harmonic frequency, nw, followed by exponential amplification until saturation is achieved. The output radiation has a single phase determined by the seed laser.

The HGHG experiment, performed at the Accelerator Test Facility at the Brookhaven National Lab, involved seeding a FEL with a 200-ps, 10.6-µm carbon dioxide (CO2) laser with a peak power, Ppk, of.0.5 MW. Intense, saturated amplified output occurred at the second-harmonic wavelength of 5.3 µm, with a FWHM bandwidth of about 20 nm, which was Fourier transform limited. The HGHG pulse energy was measured to be about 107 times as large as the spontaneous radiation and about 106 times as large as the SASE signal, which is a source of background noise.

The HGHG approach offers an alternative and attractive FEL scheme that combines the benefits of the coherence properties of a laser with the short wavelength capabilities of an accelerator-based light source. Future designs for an x-ray HGHG FEL could utilize short-wavelength tabletop lasers as seeds for amplifying and pushing toward ever-shorter wavelengths. The next step will be a deep ultraviolet FEL facility capable of vacuum ultraviolet operation, which will use a higher energy linear accelerator.