Laser-driven bunch compression for ultrashort free-electron laser pulses

Generation of ultrashort X-ray pulses in a free-electron laser relies on high-density electron bunches with a precisely adjusted current and energy distribution. To this end, robust and flexible electron bunch manipulation techniques are required that allow a high degree of control over the phase space density of the bunch. This paper reports on the demonstration of ultrashort current spikes with femtosecond duration, created by compressing an electron bunch after a laser-induced energy modulation with linearly varying envelope. This scheme is implemented at the free-electron laser FLASH, where the energy modulation is created early in the linear accelerator before the bunch is accelerated to its final energy. Formation of the spikes is observed in measurements of the longitudinal phase space density. It is demonstrated that, in conjunction with conventional compression techniques, this laser-based scheme allows to create two spikes with variable temporal separation. Therefore, the demonstrated compression scheme shows great potential of enabling flexible ultrashort single- and double-pulse operation modes of free-electron lasers.