Generation of a subpicosecond relativistic electron single bunch at the S-band linear accelerator

A subpicosecond 37-MeV electron single bunch was generated at the S-band linear accelerator of the University of Tokyo. An original single bunch with a pulse width [full width at half maximum (FWHM)] of less than 10 ps was successfully compressed to a subpicosecond time domain by magnetic pulse compression. Here the energy profile of electrons in a bunch is modulated in the longitudinal direction by tuning the phase of a traveling microwave in an accelerating tube. The electrons in the earlier and later halves in the bunch have higher and lower energy, respectively. Then, the above energy modulation is transferred to a path length modulation by a magnetic optics system formed by a dipole and a quadrupole magnet assembly to achieve pulse compression, in other words, bunch compression. The energy modulation was optimally matched to the magnetic optics to achieve the most effective compression by tuning the rf power and the phase of the microwave. A femtosecond streak camera with a time resolution of 600 fs was utilized to measure a pulse shape of electron bunches by one shot via Cherenkov radiation emitted by the electrons in xenon or air. The specification of optical components was also optimized to avoid pulse broadening due to optical dispersion. Finally, the shortest and average pulse widths in FWHM are 0.7 and 0.9 ps in the best operating mode, respectively. The compressed bunches have an electric charge of 0.15 nC (9.4×10⁸ electrons) in average. Prior to the experiment, numerical tracking analysis for electrons in the pulse compressor was performed to investigate the matching between the energy modulation and the magnetic optics. Experimental and numerical results with respect to pulse widths were compared with each other and discussed. Space charge effects on longitudinal pulse lengthening were also analyzed using relativistic electrodynamics. The subpicosecond electron single bunch is going to be utilized for exploration of ultrafast and fundamental radiation physics and chemistry.