CLASSE, Ithaca, New York
A need for femtosecond resolution beam arrival time measurements has arisen with the transition from many-picosecond-long bunches in ring-based accelerators to a few femtosecond-long bunches in high- gain free-electron lasers. Here we present an electro-optical detection scheme that uses the signal of a beam pick-up to modulate the intensity of a femtosecond laser pulse train. By detecting the energies of the laser pulses, the bunch arrival time can be deduced. We tested this scheme by distributing a laser pulse train to two locations in the FLASH linac, separated by 60 m, using length-stabilized optical fibers. By measuring the arrival times of the same electron bunches at these two locations, we determined an rms bunch arrival time resolution of 6 fs. This unprecedented monitor resolution allowed us to reduce the beam arrival time jitter from almost 200 fs down to 25 fs with an intra-bunch train feedback. Alternatively, the same detection scheme can be used for large dynamic range micrometer-resolution beam position measurements by using a stripline-type pickup mounted perpendicularly to the beam path, and then detecting the arrival time difference of both pick-up signals.
