Ultrafast Electron Diffraction Based On DC-RF Technique

Ultrafast electron diffraction (UED) technique, which can directly observe the transient and microscopic dynamics on picosecond to sub-picosecond temporal and mili-angstrom spatial scale by using ultrafast electron pulses, provides a powerful tool for ultrafast dynamics study. In traditional UED system, the electron pulses are accelerated by a direct-current (DC) high voltage, and will be greatly broadened during propagation due to the space charge (SC) effect, which limits the temporal resolution of the experiment system. SC effect depends on the electron brightness, and thus ultrafast electron pulses can be obtained by decreasing the number of electrons. However, long accumulation time will be taken at low electron number to obtain the diffraction patterns with sufficient signal-to-noise ratio, which reduces the experiment efficiency and is adverse to the study of irreversible dynamics. Therefore, how to generate high-brightness and ultrafast electron pulses becomes the crucial issue and hot spot in UED system. In this dissertation, based on the DC accelerated UED system, radio-frequency (RF) compression technique is introduced to obtain high-brightness and ultrafast electron pulses. We focused our study on the investigation of the theoretical and experimental challenges in DC-RF UED system. The detailed works are presented as follow:1) The UED system based on DC-RF technique is built. The system consists primarily of ultrahigh vacuum system, femtosecond laser system, high-brightness ultrafast electron gun, phase-locked high power solid-state RF amplification system, sample transfer system, image acquisition and process system as well as timing and software control system. Several difficulties have been overcome including of high voltage arcing, phase-locked high power RF signal generation, time sequence control, and so on.2) The propagation dynamics of electron pulses is theoretically calculated and simulated. The dependence of the temporal and spatial resolution of the UED system on electron quality in both transverse and longitudinal spaces is illustrated. The influence of electron number, dc acceleration electric field, magnetic lens, and RF compression cavity on the electron beam-size and pulse-width is simulated and studied. The electron property on the sample position is analyzed and the high-brightness electron (>105) source with50keV energy and sub-300fs pulse-width generated by this technique is theoretically demonstrated. The power efficiency compression cavity is designed based on the analysis of electromagnetic field distribution and the quality factor of the traditional pillbox cavity worked under TM010mode. The corresponding electromagnetic field distributions under different phases are simulated with the CST MWS software.3) The DC-RF UED system is experimentally tested. The zero time and electron pulses are simultaneously characterized by the cross-correlation technique invoving electron pulses and laser-induced plasmas on the Ag tip. Results show that high-brightness ultrafast electron pulses can be generated by using the DC-RF technology. Electron diffraction patterns of an Ag film with high spatial resolution have been obtained based on our UED system.