Slippage effect on energy modulation in seeded free-electron lasers with frequency chirped seed laser pulses

Abstract

Free-electron lasers (FELs) seeded with external lasers hold great promise for generating high power radiation with nearly transform-limited bandwidth in the soft x-ray region. However, it has been pointed out that the initial seed laser phase error will be amplified by the frequency up-conversion process, which may degrade the quality of the output radiation produced by a harmonic generation scheme. In this paper, theoretical and simulation studies on frequency chirp amplification in seeded FEL schemes with slippage effect taken into account are presented. It is found that the seed laser imperfection experienced by the electron beam can be significantly smoothed by the slippage effect in the modulator when the slippage length is comparable to the seed laser pulse length. This smoothing effect allows one to preserve the excellent temporal coherence of seeded FELs in the presence of large frequency chirp in the seed laser. Our studies show that the tolerance on frequency chirp in the seed laser for generating nearly transformlimited soft x-ray pulses in seeded FELs is much looser than previously thought and fully coherent radiation at nanometer wavelength may be reached with current technologies.

Demonstration of harmonic interaction in an undulator up to the 15th order

“…Alternatively, for a given radiation wavelength, harmonic lasing may be driven by electron beams with lower energy to reduce the size and cost of FEL facilities. The harmonic interaction has the further benefit that the slippage between beam and radiation is increased by the harmonic number, which provides an in situ method for communicating phase information over larger portions of the electron beam in order to enhance the temporal coherence of the output light [13,14]. For example, in a purified self-amplified spontaneous emission (pSASE) FEL, a few undulator sections placed in the middle stage of exponential growth (called the slippageboosted sections) with harmonics tuned to the FEL fundamental are predicted to reduce the FEL bandwidth [13].…”

Generation of intense and coherent sub-femtosecond X-ray pulses in electron storage rings

“…The bunching factor b k h quantifies the longitudinal, periodic density modulation of the electrons with respect to the radiation wavelength λ r ; = ∑ , where z j is the longitudinal position of j-th particle, and n k is the number of particles in the k-th slice. The slippage effect on energy modulation in the second wiggler is carefully evaluated and the single period wiggler is adopted for this scheme 35 . With the second wiggler with one period, a laser pulse power of 196 μJ at λ 2 = 800 nm and the duration of 5.0 fs FWHM is required for generating the energy modulation of 8.6σ E .…”

Soft x-ray seeding studies for the SLAC Linac Coherent Light Source II

“…These effects are all included in the simulations presented here, and the mitigating solutions discussed are implemented to achieve the best FEL performance (i) Laser spectral phase: In the simplest models, eikonal phase structure in the second laser gets multiplied by the large harmonic factor m [32,33,40,41]. However, this detrimental effect is suppressed by the combination of harmonic compression and slippage in the modulator [42,43]. Slippage smooths out laser phase errors if it is comparable in duration to the final <30 fs bunching envelope, which is 35 undulator periods with 260 nm lasers [44].…”