Abstract
Author(s): Hemsing, E; Marcus, G; Fawley, WM; Schoenlein, RW; Coffee, R; Dakovski, G; Hastings, J; Huang, Z; Ratner, D; Raubenheimer, T; Penn, G | Abstract: We present the results from studies of soft X-ray seeding options for the LCLS-II X-ray free electron laser (FEL) at SLAC. The LCLS-II will use superconducting accelerator technology to produce X-ray pulses at up to 1 MHz repetition rate using 4 GeV electron beams. If properly seeded, these pulses will be nearly fully coherent, and highly stable in photon energy, bandwidth, and intensity, thus enabling unique experiments with intense high-resolution soft X-rays. Given the expected electron beam parameters from start to end simulations and predicted FEL performance, our studies reveal echo enabled harmonic generation (EEHG) and soft X-ray self-seeding (SXRSS) as promising and complementary seeding methods. We find that SXRSS has the advantage of simplicity and will deliver 5-35 times higher spectral brightness than EEHG in the 1-2 nm range, but lacks some of the potential for phase-stable multipulse and multicolor FEL operations enabled by external laser seeding with EEHG.
Highlights
Free electron laser (FEL) seeding offers the potential to produce bright, nearly Fourier-transform limited (FTL), and highly repeatable pulses compared to pulses produced by self-amplified spontaneous emission (SASE)
Results suggest that the best overall FEL performance and flexibility is provided by a combination of soft x-ray selfseeding (SXRSS) and echo enabled harmonic generation (EEHG)
Flat and linear phase space distributions with minimal energy tails lead to better performance, and resistive wall wakefields (RWW) effects limit the maximum FTL pulse to 25–30 fs unless the undulator vacuum chamber aperture is increased
Summary
Free electron laser (FEL) seeding offers the potential to produce bright, nearly Fourier-transform limited (FTL), and highly repeatable pulses compared to pulses produced by self-amplified spontaneous emission (SASE). Results suggest that the best overall FEL performance and flexibility is provided by a combination of soft x-ray selfseeding (SXRSS) and echo enabled harmonic generation (EEHG) This conclusion is driven by the need to address key SXR science targets and the concomitant requirements for FEL spectral brightness, spectral purity, bandwidth, stability, and tunability. Our studies on LCLS-II indicate that both EEHG and SXRSS can produce FEL pulses that are, at minimum, several times brighter than SASE and with narrower bandwidth spectra that are highly stable in central wavelength. Includes implementation of a suitable high-repetition rate, high power external UV laser system to manipulate the electron beam (e-beam), though seed laser operation at reduced rates is straightforward Both seeding schemes require excellent linear e-beam phase space properties for best operation, but in general may favor different parameters and bunch profiles for optimal performance
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