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-ba nd linear accelerator of the University of Tokyo. An original single b unch with a guise width [full width at half maximum (FWHM)] of less th an 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, r espectively. Then, the above energy modulation is transferred to a pat h length modulation by a magnetic optics system formed by a dipole and a quadrupole magnet assembly to achieve pulse compression, in other w ords, bunch compression. The energy modulation was optimally matched t o the magnetic optics to achieve the most effective compression by tun ing 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 puls e shape of electron bunches by one shot via Cherenkov radiation emitte d by the electrons in xenon or air. The specification of optical compo nents was also optimized to avoid pulse broadening due to optical disp ersion. Finally, the shortest and average pulse widths in FWHM are 0.7 and 0.9 ps in the best operating mode, respectively. The compressed b unches have an electric charge of 0.15 nC (9.4 X 10(8) electrons) in a verage. Prior to the experiment, numerical tracking analysis for elect rons in the pulse compressor was performed to investigate the matching between the energy modulation and the magnetic optics. Experimental a nd numerical results with respect to pulse widths were compared with e ach other and discussed. Space charge effects on longitudinal pulse le ngthening were also analyzed using relativistic electrodynamics. The s ubpicosecond electron single bunch is going to be utilized for explora tion of ultrafast and fundamental radiation physics and chemistry.