Longitudinal phase space diagnostics with a nonmovable corrugated passive wakefield streaker

Time-resolved diagnostics at free-electron laser (FEL) facilities, in particular electron beam longitudinal phase space (LPS) and FEL power profile measurements, provide information highly valuable for users, machine development studies, and beam setup. We investigate the slice energy resolution of passive streaker setups, in particular the effect of an energy chirp on the measured slice energy spread. Downstream of the hard x-ray SASE2 beamline at the European XFEL, these measurements are enabled by a single-plate nonmovable passive wakefield streaker, essentially a rectangular corrugated plate placed inside a vacuum chamber. We show measurements with a time resolution down to a few femtoseconds and an energy resolution down to a few MeVs.

Figure 1

Layout of EuXFEL after the final compression stage (taken from [37], modified).

Figure 2

Top: layout of the diagnostic beamline for LPS measurements after the SASE2 undulator beamline. Bottom: geometry of the corrugated plate. The growing wakefield forces along the bunch are indicated by purple arrows.

Figure 3

Ideal beam simulations of LPS measurements based on electron streaking. (a) Initially generated beam with zero energy spread. (b),(c) Simulated transverse distribution at the screen for two cases: linear streaking (b), and wakefield streaking (c). We display the $y$ coordinate in the horizontal direction for consistency with the conventional display of LPS measurements.

Figure 4

Slice mean energy and energy spread analysis of the simulated LPS of Fig. 3. (a) Time resolution of the measurements. (b) Reconstructed slice mean energy $\mathrm{\Delta }{E}_m$ (colors) and true $\mathrm{\Delta }{E}_m$ (dashed black). (c) Reconstructed slice energy spread ${\sigma }_{Em}$ in solid, and slice energy resolution $r_E^{*}$ in dashed and matching colors. Colors in (b),(c) refer to the legend of plot (a). (d) The three components of $r_E^{*}$ as in Eq. (5) for the PS case.

Figure 5

LPS measurement including distance ($d$) calibration procedure. (a) Measured transverse projection of the streaked beam, again with the $y$ coordinate in the horizontal direction for consistency. (b) CRISP (blue) and reconstructed (orange) current profiles. (c) Measured (black) and simulated (colors) streaked projections at the screen assuming three different $d$. (d) Simulated transverse center of mass shift. The dashed horizontal line indicates the measured shift. The dashed vertical line indicates the calibrated $d$ of $694\mathrm{\mu }\mathrm{m}$. (e) Time calibration curves, using the current profiles from (b) as input (same colors). (f) Reconstructed LPS.

Figure 6

(a)–(c) Three LPS measurements acquired under the same beam conditions but varying beam trajectory inside the PS. The calibrated $d$ are listed in the titles. (d) Reconstructed current profiles from the PS (colors) and from CRISP (dashed).

Figure 7

Slice mean energy analysis for the LPS measured at different $d$ as shown in Fig. 6. (a) Raw slice mean energy $\mathrm{\Delta }{E}_m$ (solid). Estimated longitudinal wakefield contribution to $\mathrm{\Delta }{E}_m$. The legend indicates $d$ in units of $\mathrm{\mu }\mathrm{m}$. (b) Corrected slice mean energy $\mathrm{\Delta }{E}_c$. In both plots, the CRISP current profile is overlaid in arbitrary units (dashed black), and the relative energy is shifted to zero at $t=0$.

Figure 8

Slice energy spread and resolution analysis for the LPS measured at different $d$ as shown in Fig. 6. (a), (b) Calculated time and slice energy resolution. (c) Measured slice energy spread. In all plots, the CRISP current profile is overlaid in arbitrary units (dashed black).

Figure 9

LPS measurements at lasing-disabled (a) and lasing-enabled (b) undulator configurations are combined for an FEL power profile measurement, (c) shows the ${\sigma }_{Em}$ values for the two conditions, and (d) shows the FEL power profiles reconstructed from the increased energy spread when the FEL is enabled. The lightly colored lines show single-shot values, and the darkly colored lines the average value of 20 shots.

Figure 10

Comparison of the pwfm (solid) and ocelot codes (dashed) in tracking a particle distribution from the PS to the screen. (a) Simulated streaked transverse distribution at the screen. (b) Evaluation of the time-resolved transverse quadrupole effect in streaking direction. (c),(d) Slice energy loss and slice energy spread.