Experimental set-up & measurement

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Figure 6.12: Experimental set-up for the relative timing measurement between FEL and external laser. An external laser is optically locked to the accelerator reference clock signal and used to generate single-cycle THz pulses by optical rectification in LiNbO3. The generated THz pulses and the FEL pulses are then overlapped in a Ne gas jet, where the FEL pulse profile and the relative arrival time of these pulses with respect to the THz field are measured using streaking spectroscopy. Since the THz pulse is phase-locked to the external laser, the relative arrival time between the FEL photon pulse and the THz pulse is nearly equivalent to the relative arrival time between the FEL photon pulse and external laser.

Figure 6.13 shows the center of mass (COM) of each Ne 2p single-shot photoelectron spectrum versus the set delay between the FEL and THz pulse. On average, the kinetic energy of the streaked Ne 2p photoelectron peak shifts as the average delay between the THz streaking pulse and the FEL pulse is varied continuously over the full range of the THz pulse duration. Since the residual timing jitter is small, in this case the vector potential of the single cycle THz pulse is clearly traced out. The complete set of measurements consists of ~5000 single shots collected over 3 ps of relative delay.

As the jitter between the FEL pulse and the THz pulse is low in comparison to the THz pulse duration, the transformation map from streaked kinetic energy to time can be retrieved directly from the streaking measurements. This was not possible for the measurements presented in chapter 5, because there the timing jitter was large and the transformation map had to be constructed with the independently measured (EO-sampled) THz pulse shape. Constructing the map directly from the streaking measurements eliminates uncertainties that may arise by sampling a portion of the THz focus that does not interact with the FEL pulse in the gas target (see section 5.2.2).

To retrieve the map, the acquired single-shots were binned and averaged in time and interpolated. In Figure 6.13 the resulting transformation map is plotted in orange color.

The furthest upshifted and downshifted single-shot photoelectron spectra were found at 225 eV and 192 eV, respectively, corresponding to a THz peak electric field of ~85 kV/cm.

107 Figure 6.13: The center of mass (COM) of single-shot Ne 2p photoelectron peak is plotted as the set delay between the ionizing FEL photon pulse with 234 eV photon energy and the THz streaking pulse is changed gradually. Since the two sources are tightly synchronized, the shift in kinetic energy as a function of temporal overlap can be accurately mapped by averaging the shift over several single-shot measurements while the relative delay is scanned. Interpolating the averaged COM at each delay results in a continuous curve that represents the THz vector potential, plotted in orange. This curve is used to map the streaked photoelectron spectra, generated by an FEL pulse that arrives within the

~720 fs single-valued streaking ramp, from kinetic energy to time.

To evaluate the timing jitter, the COM of the 2p photoemission peaks of 600 consecutive FEL pulses arriving near the zero-crossing of the THz vector potential, i.e. where the measurement is most sensitive, were mapped from kinetic energy to time. The arrival time distribution of these 600 single-shot measurements is plotted in Figure 6.15 and has a width of (28 .± 2) fs r.m.s, where the error is determined from the Gaussian fit.

The single-shot arrival-time measurement precision, which is governed by the fluctuations of the mean FEL photon energy (0.3 eV r.m.s for the measurement shown), is determined to be 3 fs r.m.s. The quality of the synchronization system should be evaluated and put into context with the FEL pulse duration, which is simultaneously provided by the THz streaking measurement.

To retrieve the pulse duration each streaked Ne 2p photoelectron spectrum of the corresponding 600 single-shots was first fitted with a Gaussian curve, then deconvolved with the 2.7 eV FWHM energy resolution of the TOF spectrometer, and finally converted to time using the gradient of the THz streaking field. In Figure 6.14 the averaged streaking spectrogram and some single shots that were evaluated are plotted. The resulting FEL pulse duration distribution is shown in Figure 6.15b, yielding a mean value of (90 +6/-7) fs FWHM. The accuracy of the pulse duration measurements is evaluated from the statistical error in the single-shot photoelectron spectrum, as described in chapter 5.

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Figure 6.14: a) An averaged Ne 2p photoelectron streaking spectrogram is plotted. The shots arriving close to time zero can be analyzed to provide the temporal duration. b) In this panel photoelectron spectra of 2 FEL pulses arriving near-time zero are plotted. The FEL pulse duration is retrieved from the width of the streaked spectra. The curve in red color shows a field-free photoelectron spectrum.

Figure 6.15: Total residual timing jitter characterized by THz streaking. The arrival time of 600 consecutive FEL photon pulses recorded at the zero-crossing of the streaking THz vector potential (see Figure 6.13) is evaluated from the streaked Ne 2p photoemission peaks and plotted in panel a).

The distribution of the arrival times yields a timing jitter of (28±2) fs r.m.s, where the error is determined numerically from the Gaussian fit. In b) the distribution of the pulse duration of these 600 consecutive shots is shown. The average pulse duration is ~90 fs FWHM. The FWHM of the single-shot duration was determined with an average measurement precision of 7 fs.

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