| Due to the remarkably high temporal and spatial resolution,ultrafast electron diffraction(UED)technology has been widely used in the fields of solid-state physics,gas phase femtosecond chemistry and solid organic chemistry.Utilizing this kind of advanced measurement method,abundant accomplishment of so-called ‘direct observation' in ultrafast dynamics process,e.g.metal melting,transition state in chemical reactions and structure phase transition in organic chemistry etc,have been achieved.In recent years,UED technology with high-brightness electron bunch has been applied to explore the non-reversible ultrafast process in chemistry and biology extensively.However,owing to the strong Coulomb interaction between massive electrons(>105)in single high brightness electron pulse,the pulse duration of which would be broadened intensively during propagation,what's more,the temporal resolution of UED system will be destroyed,and above mentioned investigations for ultrafast dynamics could not be achieved successfully.Therefore,how to realize the compression of a high-brightness electron pulse to sub-picosecond even to femtosecond is a great challenge to UED technology.The key point of this dissertation is the realization of the electron pulse duration compression and accurate measurement based on radio frequency(RF)technology,which can suppress the space-charge-effect effectively.In addition,we also study and analyze the effect of sideband noise and phase jitter induced by the RF signal theoretically and experimentally.The detailed work is presented as follow:(1)A high brightness UED system based on RF compression has been built and the traditional DC accelerated electron gun has been optimized.The new electron gun owning double convex structures in photodiode and anode could eliminate the local field enhancement effect intensively,and improve the intensity of DC accelerate field from 7 MV/m to 12 MV/m(equals to the electrons energy increase from 40 ke V to 120 ke V).(2)The phase-lock between the femtosecond laser system and RF signal generation system has been achieved by the frequency locking device.The phase noise of RF signal generator,3.2 GHz and 6.4 GHz high power RF signal have been measured(~700fs)in the frequency range of 1 Hz to 10 MHz.The time synchronization of the electron pulse and the RF signal is also measured experimentally.(3)The principle of RF compression technology has been analyzed theoretically and the compression process has been simulated by software package GPT.We achieve the maximal compression of the pulse width of an electron bunch with the electron energy of 40 ke V and electron number of 1.0×105 from 14.98 ps to 0.60 ps experimentally.Additionally,the principle of electron pulse duration measurement based on RF streak cavity has been introduced and the analysis of the difference between single-pulse and multi-pulse measurement has been demonstrated.(4)We developed a series of experiments including the ultrafast dynamics of metal and semi-conductive materials.Firstly,lattice dynamics of Aluminum film induced by the femtosecond laser with 10 m J/cm2 excitation fluence is observed.Secondly,laser induced structural phase change of Ge2Sb2Te5 film is studied.Besides,the feasible measurement method for the structure phase transformation of Ti O2 from anatase to rutile is proposed and the electron diffraction simulation of this two types of Ti O2 have been finished. |
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