Breaking 50 Femtosecond Resolution Barrier in MeV Ultrafast Electron Diffraction with a Double Bend Achromat Compressor

We propose and demonstrate a novel scheme to produce ultrashort and ultrastable MeV electron beam. In this scheme, the electron beam produced in a photocathode radio frequency (rf) gun first expands under its own Coulomb force with which a positive energy chirp is imprinted in the beam longitudinal phase space. The beam is then sent through a double bend achromat with positive longitudinal dispersion where electrons at the bunch tail with lower energies follow shorter paths and thus catch up with the bunch head, leading to longitudinal bunch compression. We show that with optimized parameter sets, the whole beam path from the electron source to the compression point can be made isochronous such that the time of flight for the electron beam is immune to the fluctuations of rf amplitude. With a laser-driven THz deflector, the bunch length and arrival time jitter for a 20 fC beam after bunch compression are measured to be about 29 fs (FWHM) and 22 fs (FWHM), respectively. Such an ultrashort and ultrastable electron beam allows us to achieve 50 femtosecond (FWHM) resolution in MeV ultrafast electron diffraction where lattice oscillation at 2.6 THz corresponding to Bismuth $A_{1g}$ mode is clearly observed without correcting both the short-term timing jitter and long-term timing drift. Furthermore, oscillating weak diffuse scattering signal related to phonon coupling and decay is also clearly resolved thanks to the improved temporal resolution and increased electron flux. We expect that this technique will have a strong impact in emerging ultrashort electron beam based facilities and applications.

Figure 1

Schematic of DBA based bunch compression. The electron beam produced in a photocathode rf gun is compressed in DBA (region with dashed blue line). Two THz pulses are focused with off-axis parabolic (OAP) mirrors onto circular holes to measure the bunch length and timing jitter. Two solenoid lens ($S1$ and $S2$) are used to control the beam size and two phosphor screens ($P1$ and $P2$) are used to measure the beam transverse distribution. Electron beam longitudinal phase space evolution from the DBA entrance to the DBA exit, and finally to the sample are illustrated in (a),(b),(c), respectively.

Figure 2

(a) Measured streaking deflectogram as a function of time delay (in 33 fs steps) between the electron beam and THz pulse. (b) Relative beam arrival time measurements (offsets for display) when beam centroid energy is set at 2.8 MeV (red), 3.2 MeV (blue), and 3.0 MeV (black). In this measurement the beam charge is kept low enough such that compression effect is negligible.

Figure 3

Raw beam distribution measured at screen $P1$ when the beam energy chirp is low (a) and moderate (b) by controlling the transverse beam size. The bunch length after compression for the low and moderate energy chirp is about 100 fs FWHM (c) and 50 fs FWHM (d), respectively.

Figure 4

Full compression with DBA. (a) 100 consecutive measurements of the beam temporal profile with THz deflector off (the first 10 shots) and on (the rest 90 shots). Statistics of the beam arrival time (b) and electron beam FWHM pulse width (c) collected over 1000 consecutive shots. Long term stability of the bunch length and arrival time (d).

Figure 5

Electron diffraction pattern of Bismuth measured with integration over 100 pulses at normal incidence (a) and with the sample tilted by 26 degrees (b). Diffraction intensity change (c) for the highlighted areas (square, triangle, circle) in (b) and (d), and difference of the diffraction distribution (d) at 0.5 ps time delay between the laser pulse and electron bunch.