Purified self-amplified spontaneous emission free-electron lasers with slippage-boosted filtering

We propose a simple method to significantly enhance the temporal coherence and spectral brightness of a self-amplified spontaneous emission (SASE) free-electron laser (FEL). In this purified SASE (pSASE) FEL, a few undulator sections (called slippage-boosted sections) resonant at a subharmonic of the FEL radiation are used in the middle stage of the exponential growth regime to amplify the radiation while simultaneously reducing the FEL bandwidth. In this slippage-boosted section, the average longitudinal velocity of electrons is reduced, which effectively increases the FEL slippage length that allows the radiation fields initially far apart to create a phase relation, leading to $n$ times increase in FEL cooperation length, where $n$ is the ratio of the resonant wavelength of the slippage-boosted section to that of the original FEL radiation. The purified radiation, as a seed with improved temporal coherence, is further amplified to saturation in the undulator sections tuned to the FEL wavelength. Using the linac coherent light source II (LCLS-II) parameters as an example, we show that with the proposed configuration the temporal coherence and spectral brightness of a SASE FEL can be significantly enhanced. This scheme may be applied to many SASE FEL light sources to enhance the FEL performance.

Figure 1

Ratio of power gain length $L_{1\mathrm{D}}^{(n)}/L_{1\mathrm{D}}^{(1)}$ for various undulator $K$ values.

Figure 2

Schematic layout of a pSASE FEL.

Figure 3

Power gain length at ${\lambda }_0=0.6\mathrm{nm}$ as a function of beam energy spread with LCLS-II parameters for various lasing scenarios: fundamental lasing with $K=2$, 3rd harmonic lasing with $K=4$, 5th harmonic lasing with $K=5.29$, and 7th harmonic lasing with $K=6.32$.

Figure 4

FEL power (a) and spectrum (b) at the exit of U1 with nine undulator sections.

Figure 5

SASE radiation power and spectrum at the exit of the 1st undulator in U2 [(a) and (b)]; at the exit of the 2nd undulator in U2 [(c) and (d)]; at the exit of the 3rd undulator in U2 [(e) and (f)]; at the exit of the 5th undulator in U2 [(g) and (h)]. The spectrum brightness $P(\lambda )$ is normalized to the radiation peak spectral brightness at the exit of U1 [Fig. 4b].

Figure 6

Representative radiation power profiles and spectra for a standard SASE FEL [(a) and (b)] and a pSASE FEL [(c) and (d)]. The average power for both cases is about 7 GW and in the simulation the beam parameters, lattice functions, and initial shot noise are all the same. The spectral brightness is normalized to the peak spectral brightness at the exit of U1.

Figure 7

Spectrum of the FEL radiation produced in the standard SASE (a) and the pSASE mode (b). Thin lines refer to single shot realizations and the bold line refers to the average over ten realizations.