Potential of femtosecond electron diffraction using near-relativistic electrons from a photocathode RF electron gun

The electron beam from a photocathode RF electron gun is usually used to drive a free electron laser (FEL) after acceleration and to generate coherent radiation from IR to X-rays. Building on our earlier proposal that such an electron beam itself can be used as scattering particles to produce a time-dependent diffraction signal, which contains structural information on a femtosecond resolution, here we further elucidate several important practical factors relevant to realizing electron diffraction on this time scale. A higher electrical field on the cathode of the RF gun can provide a larger number of electrons per bunch with a shorter temporal width compared to a typical DC gun, owing to a significant reduction in the space-charge effect. The near-relativistic speed of the electrons will further reduce the velocity mismatch and significantly improve the overall time resolution. However, the de Broglie wavelength of near-relativistic electrons at ∼2 MeV is one order of magnitude shorter than that of the electrons generated by a typical DC gun operated at 30 keV. Consequently the Bragg angle is one order smaller and is comparable to the beam's intrinsic divergence, significantly blurring the observed diffraction pattern. Our simulations show that it is possible to restore the ideal diffraction pattern from the observed, blurred diffraction pattern; therefore, femtosecond electron diffraction using a photocathode RF gun can be a useful and practical tool. In addition, the sensitive dependence of the diffraction pattern on the electron beam divergence can be utilized to measure the divergence with high accuracy.