Novel seeding techniques for generation of high repetition rate coherent nanometer FEL radiation

Abstract

High gain free electron lasers (FELs) generate radiation of unprecedented brightness and unique properties and have proven to be a useful tool for applications in a wide range of studies in physics, biology, medicine and chemistry. External seeding techniques have been experimentally demonstrated and aim to improve the intrinsically poor temporal coherence of a high-gain FEL starting from shot noise. Seeded schemes, like the high gain harmonic generation (HGHG), are based on frequency up-conversion and generate fully coherent radiation thanks to the external seed laser that initiates the process in the FEL. However, the dependence of the properties of the seeded radiation on those of the seed laser is at the same time a limiting factor. The repetition rate of the state-of-the-art seed lasers used in external seeding is incompatible with the repetition rates of modern high-gain FELs based on superconducting technology. In addition, seeded radiation is a harmonic of the seed laser wavelength, a feature that limits the output wavelength to above a few nanometers and restricts its tunability. To overcome these limitations it is necessary to search for new possibilities in FELs.

The scope of this cumulative thesis is to introduce novel ideas that allow us to achieve high repetition rate and fully coherent radiation at an extended and tunable wavelength range. The three proposals of this thesis aim to reduce the dependence of external seeding schemes on the seed laser source and at the same time, maintain the full coherence of seeded radiation. The first proposal is an optical klystron-based HGHG, which modifies the conventional HGHG beamline in a way that relaxes the stringent requirements on the seed laser power by several orders of magnitude. This way, the repetition rate of the seed laser source can be increased, or seed laser sources of shorter wavelengths can be used instead. The second proposal is an HGHG seeded oscillator-amplifier setup: an optical cavity captures a conventional low repetition rate seed laser pulse and stores it to seed electron bunches at a high repetition rate. The third proposal is an HGHG oscillator-amplifier that eliminates the dependence on external seed lasers. Instead of the external laser, the electrons generate the light pulse themselves, starting from shot noise, and the radiation is stored in the optical cavity to seed electron bunches at high repetition rates. In addition to the high repetition rate, this scheme allows shorter and tunable seeded radiation. This type of radiation has never been possible in the past and can greatly benefit already existing experiments and support new experiments and more discoveries by accelerating the ongoing science at FELs.