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SJTU Researchers Proposed New Scheme for Ultrashort MeV Electron Beam

April 08, 2020      Author: Xiang Dao

A paper titled “Breaking 50 Femtosecond Resolution Barrier in MeV Ultrafast Electron Diffraction with a Double Bend Achromat Compressor” (F. Qi et al., Phys. Rev. Lett. 124, 134803 (2020)) was published at Physical Review Letters. It presented the latest findings of a team led by professor Xiang Dao and academician Zhang Jie from School of Physics and Astronomy, SJTU, in cooperation with professor Wan Weishi from Shanghai Tech University.

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Funded by “national special funding for major scientific research equipment development”, SJTU Ultrafast Diffraction and Microscopy System has been built and is accessible to ultrafast scientific users at home and abroad. This research was supported by National Major Scientific Research Instrument Development Project (Grant No. 11327902), Innovation Group Project (Grant No. 11721091) and Outstanding Youth Project (Grant No. 11925505) of National Natural Science Foundation of China as well as by Major Project of Shanghai Science and Technology Commission (Grant No. 18JC1410700) .The first author of this paper is doctoral student Qi Fengfeng.

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ABSTRACT 

We propose and demonstrate a novel scheme to produce ultrashort and ultrastable MeV electron beam. In this scheme, the electron beam produced in a photocathode radio frequency (rf) gun first expands under its own Coulomb force with which a positive energy chirp is imprinted in the beam longitudinal phase space. The beam is then sent through a double bend achromat with positive longitudinal dispersion where electrons at the bunch tail with lower energies follow shorter paths and thus catch up with the bunch head, leading to longitudinal bunch compression. We show that with optimized parameter sets, the whole beam path from the electron source to the compression point can be made isochronous such that the time of flight for the electron beam is immune to the fluctuations of rf amplitude. With a laser-driven THz deflector, the bunch length and arrival time jitter for a 20 fC beam after bunch compression are measured to be about 29 fs (FWHM) and 22 fs (FWHM), respectively. Such an ultrashort and ultrastable electron beam allows us to achieve 50 femtosecond (FWHM) resolution in MeV ultrafast electron diffraction where lattice oscillation at 2.6 THz corresponding to Bismuth A1g mode is clearly observed without correcting both the short-term timing jitter and long-term timing drift. Furthermore, oscillating weak diffuse scattering signal related to phonon coupling and decay is also clearly resolved thanks to the improved temporal resolution and increased electron flux. We expect that this technique will have a strong impact in emerging ultrashort electron beam based facilities and applications.

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Link: https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.124.134803\

Translated by Fu Jing