Seeded Free Electron Lasers
In a Free Electron Laser (FEL), similar to insertion devices in modern synchrotron radiation sources, the radiated wavelength λ is related to the energy of the electron beam and the magnetic properties of the undulator. Unlike conventional laser systems, the FEL can be tuned to any desired wavelength and - compared to synchrotron radiation sources - has the advantage of:
- High peak power, typically up to nine orders of magnitude higher;
- High peak brilliance, up to ten orders of magnitude higher;
- Full transverse and partially longitudinal coherence of the output radiation.
- One approach, achieving lasing in a single pass is so called self-amplified spontaneous emission (SASE). In this scheme an electron beam with high current density is passed through an undulator, stimulated to emit coherently, amplifying its own spontaneous radiation by several orders of magnitude. It was soon recognized that the performance of SASE-FELs had to be improved. To avoid the consequences originating from the statistical nature of starting up the lasing process, in particular the uncontrollable spiky nature of the output, different and more sophisticated magnetic and optical configurations have been proposed: regenerative amplification, two stage SASE-FEL, side band seeded FELs and pulse compression, to name a few.
An alternative approach is the High Gain Harmonic Generation (HGHG) scheme. In this concept, the FEL process is initiated by the external seed field. The seeding radiation and the electron beam pass through several HGHG stages, where the light frequency is upconverted to a higher harmonic. This process will be repeated until the desired wavelength of a few nanometers is achieved. By each repetition heating effects occur, the noise will be amplified and the quality of the FEL output radiation degrades. These unwanted effects could be avoided, if we seed directly at shorter wavelengths close to or in the final output range, so that the number of repetitions could be minimized.
To produce these short wavelengths High-order Harmonic Generation (HHG) in gases is a promising tool. If a gas is irradiated by a high intensity laser, some atoms are ionized. Upon recombination, the atoms coherently radiate at odd integer multiples of the fundamental laser frequency.
One can concept a seeded FEL which allows seeding each HGHG stage, so that we can use a seed field which is produced by an HHG-process and hence miss some of the HGHG stages out. For this, scientists at HZB are participating in the research on HHG-Seeding, i.e. the seeding project sFLASH at DESY.
Particularly at HZB we have developed the program Rhyno which allows simulating the HHG process.
