Ultrafast time-resolved electron diffraction with megavolt electron beams

Abstract

An rf photocathode electron gun is used as an electron source for ultrafast time-resolved pump-probe electron diffraction. We observed single-shot diffraction patterns from a 160 nm Al foil using the 5.4 MeV electron beam from the Gun Test Facility at the Stanford Linear Accelerator. Excellent agreement with simulations suggests that single-shot diffraction experiments with a time resolution approaching 100 fs are possible. SLAC-PUB-12162 Submitted to Applied Physics Letters 2Our understanding about dynamical processes in chemistry, materials science and biology on the picosecond and sub-picosecond time scale stems almost exclusively from time-resolved spectroscopy. Structural changes, on atomic length scales, can only be inferred indirectly from the analysis of spectra. Both x-ray and electron diffraction share the goal of 'imaging' molecular structures with a time resolution that captures the motions as systems evolve, whether they be solids, liquids or gases. Lab scale experiments in both electron diffraction 1,2 and x-ray scattering 3 have produced impressive results. Recently, in anticipation of the construction of the Linac Coherent Light Source (LCLS) at the Stanford Linear Accelerator Center (SLAC), an experiment using the electron bunch from the SLAC Linac to produce spontaneous undulator radiation 4 has shown the possibilities for ultrafast x-ray scattering from condensed systems with 100 fs time resolution. 5 This has encouraged us to approach ultrafast electron diffraction (UED) using experimental techniques based on electron sources developed for particle accelerators, with the aim of obtaining single-shot diffraction patterns on a 100 fs time scale.Electron diffraction is complementary to x-ray scattering, but features much larger cross sections that allow the study of surface phenomena, the bulk structures of thin foils and membranes, as well as molecular structures of gas phase samples. 6 As with linac based x-ray sources there has been significant development of electron sources for UED based on the use of photocathodes. 7 Unfortunately, the space-charge interactions of the electrons within a pulse, and the initial kinetic energy distribution with which the electrons are generated, have made it difficult to obtain pulses much shorter than 1 ps 8,9,10 ,in 'conventional' UED experiments using ≈30 keV electron beams. To improve the time resolution one could use fewer electrons per pulse, but that requires longer data acquisition times to obtain the necessary signal-to-noise ratio. 11 Alternatively, it is possible to increase the electric field inside the electron gun, while reducing the flight distance between the gun and the target. 12 Both tend to reduce the time of flight of the electron pulse, thereby giving the electron pulse less time to spread. Even so, this 3 approach is limited because the maximum DC and pulsed electric fields are 12 MV/m and 25 MV/m, respectively. 13,14 In the present work we take a fresh approach to ultrafast time-resolved pump-probe diffraction by using MeV electron be...

Ultrashort electron pulses for diffraction, crystallography and microscopy: theoretical and experimental resolutions

“…Recent results suggest that single-shot electron diffraction patterns can be obtained using pulses containing 10 7 electrons, which were accelerated to 5.4 MeV. 33 It was suggested that this technique could reach sub-picosecond time resolution by utilizing the longitudinal pulse compression induced through time-dependent rf-acceleration. 34 Theoretically, longitudinal focusing at lower kinetic energies (r200 kV) can also be realized, either by acceleration through a static voltage gradient 32 or by rf-acceleration.…”

Demonstration of cathode emittance dominated high bunch charge beams in a DC gun-based photoinjector

“…Examples include: x-ray sources [1][2][3], electron-ion coolers [4], Ultra-fast Electron Diffraction (UED) experiments [5][6][7][8], and fixed-target nuclear physics experiments [9]. A key feature of many of these applications is the potential to produce beams where the initial beam quality, set by the source, dominates the final beam quality at the usage point.…”

An Injector Test Facility for the LCLS

“…However, an S-band photocathode gun operating at a few MeV can produce 100 fs pulses with up to 10 8 electrons. Such a source has already been used in a proof of principle high-energy electron diffraction experiment at SLAC [20]. A single shot diffraction pattern from a 160 nm thick Al foil is shown in figure 4 obtained with an approximately 500 fs rms long pulse containing 2 10 7 electrons.…”