Electro -optic characterization of femtosecond electron bunches.

Abstract

Linear-accelerator-based x-ray sources will revolutionize ultrafast science due to their unprecedented brightness and short pulse duration. However, time-resolved studies at the resolution of the x-ray pulse duration are severely hampered by the inability to precisely synchronize an external laser to the x-ray source. At the Sub-Picosecond Pulse Source (SPPS) at the Stanford Linear Accelerator Center (SLAC) we solved this problem by measuring the arrival time of each high energy electron bunch. We use single-shot spatially resolved electro-optic sampling (EOS)to generate a temporal image of the bunch. An ultrafast laser pulse (135 fs) passes through an electro-optic crystal adjacent to the 28.5 GeV electron beam at SLAC. The re fractive index of the crystal is distorted by the strong electromagnetic fields of the ultra-relativistic electrons, and this transient birefringence is imprinted on the laser polarization. A polarizer decodes this signal, producing a time-dependent image of the electron bunch. The signal is a convolution of the laser probe pulse duration, the EO crystal properties, and the electron bunch length which is calculated to be 80<italic>fs</italic> FWHM at SPPS. Currently, the measurement at EOS is resolution limited, primarily by the EO crystal response to 300<italic>fs</italic> FWHM. The centroid of the signal, which gives the arrival time of the electron bunch, can be determined <italic>to</italic>30<italic>fs</italic> (10% of the EO signal FWHM). The results described in this thesis are the highest resolution direct measurement of electron bunch charge distribution at an accelerator facility to date. Knowledge of the bunch duration is critical because it determines the duration of the x-ray pulse delivered to an experiment. The bunch duration is also a useful accelerator tuning parameter. We present a direct comparison between the electron bunch arrival time and the x-ray pulse. The x-ray pulse arrival time is obtained through a destructive measurement. The comparison shows agreement to within 60<italic>fs</italic> rms. This small discrepancy is due in part to environmental fluctuations and additional error in determining the arrival time of the x-ray pulse. We demonstrate that EOS allows observation of atomic-scale ultrafast dynamics at the SPPS source.