Temporal profile measurements of relativistic electron bunch based on wakefield generation

A complete characterization of the time-resolved longitudinal beam phase space is important to optimize the final performances of an accelerator, and in particular this is crucial for Free Electron Laser (FEL) facilities. In this paper we propose a novel method to characterize the profile of a relativistic electron bunch by passively streaking the beam using its self-interaction with the transverse wakefield excited by the bunch itself passing off-axis through a dielectric-lined or a corrugated waveguide. Results of a proof-of-principle experiment at the SwissFEL Injector Test Facility are discussed.

Figure 1

Schematic layout of the dielectric-lined waveguide (passive streaker) used for the experiment at SITF. $L$ is the leading particle that travels along the cylindrical waveguide at an offset $r_0$, $T$ is the trailing particle with an offset $r$.

Figure 2

(a) Initial bunch current distribution (blue) and reconstructed temporal profile (red). (b) Transverse displacement (blue) and resolution (green) at the screen as a function of the temporal coordinate.

Figure 3

(a–c) Measured transverse beam images at YAG:Ce screen for different offsets of the passive streaker. The head of the bunch is on the right and the tail on the left. (d) Measured average vertical beam centroid as a function of the vertical offset of the passive streaker against model expectations based on the dipole wake function (dashed) and the dipole and quadrupole wake functions (solid).

Figure 4

(a) Temporal profile measured with the TDS (dashed). Temporal profile reconstructed from the passively streaked beam (solid). (b) Measured and reconstructed transverse profiles at the screen. (c) Left axis: transverse displacement at the screen calculated with Eq. (2) with offset $\mathrm{\Delta }y=0.89\mathrm{mm}$ (blue). Right axis: temporal resolution without defocusing (dashed) and with defocusing (red) of the passive streaker against the longitudinal coordinate inside the bunch.