Fast intra bunch train charge feedback for FELs based on photo injector laser pulse modulation

Bunch charge variations in Free Electron Lasers such as the Free Electron Laser in Hamburg (FLASH) or the European X-Ray Free Electron Laser (E-XFEL) impacts the longitudinal phase space distribution of the electrons resulting in different bunch peak currents, pulse duration and pulse shapes. The electron bunches are generated by short ultraviolet laser pulses impinging onto a photocathode inside a radio frequency (RF) accelerating cavity. At FLASH, bursts of bunches up to 800 pulses with an intra train repetition rate of 1 MHz are used and even higher repetition rates for the E-XFEL (up to 4.5 Mhz) are planned. Charge variations along these bunch-trains can be caused by variations of the laser pulse energies, instabilities of the accelerating fields in the RF cavity and time dependent effects in the photoemission process. To improve the intra bunch-train charge flatness and to compensate train-to-train fluctuations a dedicated digital control system, based on the Micro Telecommunication Computing Architecture (MicroTCA.4) standard, was designed, implemented and successfully tested at the FLASH. The system consists of a bunch charge detection module which analyzes data from toroid system and provides the input signal for the controller which drives a fast UV-Pockels Cell installed in the optical path of the photo-cathode laser. The Pockels cell alters the laser polarization and thus the transmission through a polarizer. The modulation of UV laser pulse energy with an iterative learning feed-forward minimizing the repetitive errors from bunch-train to bunch-train and a fast feedback algorithm implemented in a Field Programmable Gate Array (FPGA) allows for fast tuning of bunch charge inside the bunch-train. In this paper a detailed description of the system and first measurement results are presented.