Demonstration of Single-Crystal Self-Seeded Two-Color X-Ray Free-Electron Lasers

A scheme for generating two simultaneous hard-x-ray free-electron laser pulses with a controllable difference in photon energy is described and then demonstrated using the self-seeding setup at the Linac Coherent Light Source (LCLS). The scheme takes advantage of the existing LCLS equipment, which allows two independent rotations of the self-seeding diamond crystal. The two degrees of freedom are used to select two nearby crystal reflections, causing two wavelengths to be present in the forward transmitted seeding x-ray pulse. The free-electron laser system must support amplification at both desired wavelengths.

Figure 1

Single-crystal self-seeding scheme at LCLS. In the first undulator section a self-amplified spontaneous emission (SASE) photon pulse is generated. This photon pulse diffracts from a diamond crystal. The diffracted bandwidth is at least an order of magnitude smaller than the SASE bandwidth. The diffracted beam exits at some angle to the incident beam and is discarded. The undiffracted, transmitted pulse consists of a wide-bandwidth short pulse followed by a relatively weak narrow-bandwidth tail. The electron pulse is delayed by the magnetic chicane to overlap spatially with the narrow-bandwidth tail of the x-ray pulse in the second undulator section. The narrow-bandwidth tail, with long temporal coherence length, seeds FEL amplification in the second undulator section, generating a narrow-band intense x-ray output.

Figure 2

Photon energy values at which diffraction occurs in a diamond crystal, as a function of the pitch angle $\theta$ for fixed values of yaw angle $\psi$. Each curve corresponds to reflection from a different set of crystallographic planes. (a) Yaw angle $\psi =0°$. Stars represent demonstrated two-color working points. (b) Yaw angle $\psi =0.75°$. Different colors represent planes with different $h^2+k^2+l^2$ value. Dash-dotted lines are double lines (two sets of crystal planes excited) that are superposed for $\psi =0°$ and can be separated with a nonzero yaw angle. (c) Correcting for the small crystal orientation offset deduced from experimental measurements of the energies as a function of ($\theta$,$\psi$) values.

Figure 3

Average spectra from 111 consecutive shots for single-crystal two-color self-seeded beams with photon energy close to 8.43 keV. The pitch angle was fixed to $\theta =65.28°$. (Blue) yaw angle $\psi =1.47°$. (Green) $\psi =1.49°$. (Orange) $\psi =1.51°$. (Red) $\psi =1.53°$. The used reflection planes were ($13\overline{3}$) and ($31\overline{3}$). The average spectra are shown without postprocessing, including the SASE prepulse. The plateau between the two narrow colors is mainly due to the wide-bandwidth head part amplified in the second undulator section.

Figure 4

Single-crystal two-color self-seeding experimental results for 3500 consecutive shots. Photon energy was close to 8.43 keV and the reflections used were (202) and (022). (a) Average spectrum. (b) Correlation between the first and second color intensities in a 1 eV bandwidth around each color average peak energy. (c) Single-shot intensities in a 1 eV bandwidth for both colors vs electron beam energy; (blue) color 1; (red) color 2. (d) Histogram of the event rate as a function of the difference between the intensities of the two colors.

Figure 5

(a) Two-color self-seeded spectrum obtained by using the ($\overline{1}13$) and ($1\overline{1}3$) reflections, single shot (black) and average on 100 shots (red). The two color lines are separated by 43 eV ($\sim 0.6\%$), which is wider than the SASE bandwidth for the normal LCLS operation. The pulse energy for the shot was $120\mu \mathrm{J}$. (b) litrack simulated phase space obtained by manipulating the $X$ band linearizer. (c) Simulated two energy level electron beam energy profile.