Research On Key Technologies Of Sub-10 Femtosecond Mega-electron-volt Ultrafast Electron Diffraction

Mega-electron-volt ultrafast electron diffraction(MeV UED)is a technique which probes the ultrafast structural evolutions of non-equilibrium states.In a MeV UED system,the electron bunch is boosted to near the speed of light by a photocathode RF gun to reduce the effect of space charge force,and thus maintaining relatively small beam emittance and pulse duration.Compared to kilo-electron-volt ultrafast electron diffraction system which accelerates electrons by a DC gun and suffers more serious space charge effect,MeV UED has already improved the time resolution from sub-ps to about 100 fs.But for studying the structural dynamics of which the time scale is 10-100 fs,the time resolution of MeV UED system requires further improvements.This thesis focused on the key technologies that have potential to improve the time resolution of MeV UED to 10 fs regime.The main contents are summarized as follows:Based on representative parameters of the MeV UED setup,the author analyzed the best time resolution of the original system to be about 100 fs.For improving the time resolution,a C-band RF buncher cavity and a deflecting cavity are designed and fabricated for electron pulse compression and measurement.At full compression condition by the RF buncher,the mean pulse duration of the beam is measured to be about 6 fs(rms)while the energy fluctuation of the beam is measured to be about 4 times larger than that without RF bunching.Analysis shows that the fluctuation is introduced by the phase jitter in the RF buncher which will also lead to increase of the beam arrival time jitter at the sample.To make full use of the RF buncher,the beam arrival time needs to be measured on a shot-to-shot basis such that the data can be sorted for improved temporal resolution.To this end,the author has built intense Terahertz(THz)sources based on Lithium Niobate(LiNbO3).Three methods for measuring the time of flight of a relativistic electron bunch are realized.1.THz streaking with sub-wavelength narrow slit: electric filed of the intense THz wave is enhanced by a sub-wavelength narrow slit,which gives the maximum deflecting gradient of 5.1 ?rad/fs and the accuracy of flight time determination of 1.5 fs.By streaking the beam linearly,the dynamic range of this method is limited to about a quarter of the period of the THz.2.THz oscilloscope: circular streaking is achieved by injecting circular polarized THz into a metal coated dielectric tube,which give a dynamic range of 1.5 the period of the THz and flight of time accuracy of 3 fs.With the inner diameter being 1 mm,the dielectric tube provides much larger aperture than the sub-wavelength slit which leads to much less charge loss.Thus,the performance of THz streaking is greatly improved.3.Determination by energy jitter measurement:a non-invasive way of flight of time measurement is achieved by measuring the energy fluctuation after compression and the accuracy of this method is determined to be about24 fs by THz streaking.The experimental results from the RF bunching reveals the importance of developing methods to produce ultrashort relativistic electron bunch with less timing jitter such that time-stamping is not needed.We have developed three ways to realize that goal.1.THz wakefield bunching: using a dielectric tube coated with metal layer we have demonstrated the beam bunching with THz wakefield generated by a leading drive beam,which gives almost the same bunching performance as a RF buncher.Because of the lasers used to generate the drive beam and witness beam are from the same source and thus naturally synchronized,this method doesn't introduce timing jitter to the witness beam.2.THz slicing: an electron beam with duration of 158 fs(rms)is transversely streaked by a THz source and sliced by a downstream slit.The duration of the electron beam after slicing is measured to be 24 fs(rms).And the energy jitter is also found to be reduced,which will result in lower jitter of the sliced beam.3.Effective longitudinal bunching filed is found within the deflection mode excited in a dielectric tube by THz with some offset to the center.Beam compression is realized by off-set injection where the bunching field is synchronized to an external laser,and thus can generate electron beam with both short duration and small timing jitter.Experimental results show that the beam duration and timing jitter are reduced from 130 fs(rms)and 97 fs(fs)to 28 fs(rms)and 36 fs(rms)respectively.The author also introduced the basis of electron diffraction of crystals and detailed experimental procedure in a MeV UED experiment.Using single crystal gold film as a test sample,we measured its ultrafast structural dynamics after being excited by a fs laser pulse.We also demonstrated an experimental method to get higher time resolution at single shot mode.A relatively long electron bunch containing enough charge to generate a single shot diffraction pattern is produced and then sent to a RF buncher for beam compression.The pulse duration after compression is measured to be 13 fs(rms).We record each diffraction pattern together with the mean energy of its direct beam,with which we are able to refine the nominal delay for each single shot data.In an experiment we measured the intensity evolution of the specific Bragg spots which have strong correlation to the optical phonon in the single crystal Bismuth film.The time constant of this dynamics is determined to be about 210 fs,while this value is about 397 fs without timing correction.The resulted time resolution of the single shot mode MeV UED experiment is greatly improved.