Time-Resolved Phase Measurement of a Self-Amplified Free-Electron Laser

We report on the first time-resolved phase measurement on self-amplified spontaneous emission (SASE) free-electron laser (FEL) pulses. We observed that the spikes in the output of such free-electron laser pulses have an intrinsic positive chirp. We also observed that the energy chirp in the electron bunch mapped directly into the FEL output. Under certain conditions, the two chirps cancel each other. The experimental result was compared with simulations and interpreted with SASE theory.

Figure 1

SASE output energy versus distance. Data points are experimental measurement and the lines are from GINGER simulations using the conditions listed in Table  with an electron bunch chirp (dashed line) and without the chirp (solid line). The measured gain length is 0.68 m. There is no normalization between the experimental and simulation data.

Figure 2

Example FROG traces and their retrievals showing instantaneous frequency (black), the phase (blue), and intensity (red) as a function of time. (a) A single-spike dominated case and (b) a multiple-spike case. Note that both raw traces show a symmetric pattern in the time direction with unbalanced intensity. The sharp spikes in the frequency plots are due to the phase discontinuity. The retrievals are shown in the correct time direction.

Figure 3

Magnitude of the chirp of the SASE spikes. The data points are experimental measurement using the FROG. The curves are fit using Eq. (4) with an electron chirp of $-24{\mathrm{m}}^{-1}$ (dashed line), $-28{\mathrm{m}}^{-1}$ (solid line), and $-33{\mathrm{m}}^{-1}$ (dotted line).

Figure 4

GINGER simulation using the conditions listed in Table , showing the intensity (bold lines) and phase (thin lines) as a function of time. (a) No chirp; (b) $-17{\mathrm{m}}^{-1}$ chirp, with a single dominant spike; and (c) with a $-17{\mathrm{m}}^{-1}$ chirp, showing multiple spikes. The phase during the spikes is almost flat in (b) and (c) due to the opposite sign of the SASE intrinsic chirp and the electron bunch chirp.