The key issue to realize single-shot ultrafast electron diffraction (UED) is to develop intense short electron-pulse sources. With conventional UED instruments, an electron pulse is generated at a photocathode irradiated by a femtosecond laser pulse and accelerated by an additional external static electric field. The amount of electrons in the pulse is limited because the electron pulse expands during its flight by space-charge forces in the pulse. There are two ways considerable to avoid the space charge effect; those are reduction of electrons in the pulse and acceleration to relativistic energy by RF accelerators. However, for the former large amount of pulses are necessary to obtain a UED image, which is not available to observe irreversible phenomena, and for the latter the energy is too high for conventional transmission electron microscopy (TEM). Furthermore, for the mid-energy range of around 100keV to 1MeV, corresponding to the energies of conventional TEM, there is no satisfactory method for generating femtosecond electron pulses. In this paper, we have demonstrated femtosecond pulse compression of a laser-accelerated electron beam with energy of around 350keV.[1] The electron pulses are generated by irradiating a tightly focused terawatt femtosecond laser pulse on a solid target; then, the pulses are compressed by using an achromatic bending magnet system. These femtosecond electron pulses have a sufficient intensity to take a single-shot diffraction pattern.[1, 2]
展开▼
