Abstract
Current FEL development efforts aim at improving the control of coherence at high repetition rate while keeping the wavelength tunability. Seeding schemes, like HGHG and EEHG, allow for the generation of fully coherent FEL pulses, but the powerful external seed laser required limits the repetition rate that can be achieved. In turn, this impacts the average brightness and the amount of statistics that experiments can do. In order to solve this issue, here we take a unique approach and discuss the use of one or more optical cavities to seed the electron bunches accelerated in a superconducting linac to modulate their energy. Like standard seeding schemes, the cavity is followed by a dispersive section, which manipulates the longitudinal phase space of the electron bunches, inducing longitudinal density modulations with high harmonic content that undergo the FEL process in an amplifier placed downstream. We will discuss technical requirements for implementing these setups and their operation range based on numerical simulations.
Highlights
Free-electron lasers (FELs) have been making enormous improvements during the past decades, delivering high-brightness radiation to users all over the world at wavelengths from mm to hard x-rays, covering a wide range of experiments
We focus on an high-gain harmonic generation (HGHG)-based oscillator scheme as shown in Figure 1 and in the case of an oscillator-FEL starting from shot noise
In all cases we use the same set of simulation parameters, which is summarized in Table 1, and the modulator is resonant with 50 nm wavelength
Summary
Free-electron lasers (FELs) have been making enormous improvements during the past decades, delivering high-brightness radiation to users all over the world at wavelengths from mm to hard x-rays, covering a wide range of experiments. At wavelengths in the nanometer range and longer, alternatives to generate fully coherent radiation are based on external seeding In this case, a seed laser of typically several tens MW of power is used to prepare an initial signal for a final FEL amplifier, usually tuned at a harmonic of its wavelength, imprinting its coherence properties upon the output FEL pulse. An FEL oscillator is employed and acts as a feedback system which recirculates a seed pulse, and seeds the electron bunches at high repetition rate In this case, one may either use a low repetition rate seed laser, or start from shot noise.
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