DUV-FEL Project

Brookhaven National Laboratory has established an initiative in FEL science and technology which includes the Deep Ultra-Violet Free Electron Laser (DUV-FEL) experiment. The DUV-FEL will be used as a test-bed for experiments to utilize the UV radiation it produces, and serve as model for extending its principles to much shorter wavelengths (x-rays). It is a single pass device to avoid the wavelength limitations imposed by oscillator optics. It is configured as a sub-harmonically seeded High Gain Harmonic Generator (HGHG) to improve coherence and pulse control as contrasted with other schemes that start up from noise.

In this process UV light from a solid state laser (Titanium:Sapphire with conventional harmonic generation) is used to illuminate the cathode of an RF photo-injector. This generates an intense electron bunch that is accelerated by an electron linac. The electrons then pass through a short magnetic wiggler where they are coupled with light split off from the laser. The resulting energy modulation is then converted to spatial modulation (micro-bunching) in a dispersive section. A longer wiggler is then used to generate FEL radiation at harmonics of the initial seed laser. This process produces radiation with optical properties and stability that are controlled by the original seed laser (pulse length, bandwidth, coherence). This approach has been successfully demonstrated at BNL in the IR region, converting 10 micron wavelength seed radiation to 5 micron wavelength FEL radiation. In its initial configuration the DUV-FEL will allow operation at wavelengths down to 200 nm at pulse lengths below a picosecond. Planned enhancements can extend this performance to wavelengths the order of 50 nm and pulse lengths as short as 10 femtoseconds.

One way to view the DUV-FEL is essentially as a harmonic generation and amplification system for a solid-state laser. Whatever you can produce in the laser can in principle be carried through to the FEL output. This includes its stability as well as programmed pulse formats that can be used to make the FEL a chirped pulse amplification system, potentially yielding pulses shorter that 10 femtoseconds at wavelengths below 100 nm with energies up to a milli-Joule. These properties should prove valuable to experimental users of the DUV-FEL.

The other obvious feature of the facility is that pump-probe multicolor experiments should be readily possible with excellent timing jitter if the alternate colors can be derived from the facility laser. It should be noted that the tuning agility of the FEL depends on the tuning of the seed laser. The bandwidth of the Ti:Sapp is sufficient for small tuning ranges with relative ease, but broadly tunable operation will depend on enhancing its capabilities over the present system.

Schematic of DUV-FEL Configuration

Facility Layout

These simulations are representative cases of parameters that assume the mini undulator is used as the energy modulation wiggler and that the NISUS undulator is used as the amplifier. The amplifier parameters are fixed with a peak field of 0.31T (gap ~ 20 mm) providing a K of 1.14. All other parameters are as noted below.