PAL-XFEL laser heater commissioning

X-ray free electron lasers (XFELs) require electron beams with a peak current of several kA and a transverse emittance below 1mm mrad. The uncorrelated energy spread of the beam should be as small as 0.01% of the beam energy. A photoinjector generates an electron beam satisfying the requirements, however the uncorrelated energy spread of the beam is too small, about one keV. A beam with such a low uncorrelated energy spread may suffer from the longitudinal microbunching instability when the beam is accelerated and compressed through a linear accelerator. A laser heater system in the injector minimizes the microbunching instability growth by controlling the uncorrelated energy spread of the beam. In this paper, we introduce the PAL-XFEL laser heater system and the commissioning result. The laser heating effects depending on the undulator gap, infrared laser pulse energy and laser beam size are studied. Experimental studies on the laser heating depending on the relative spatial position between the infrared laser and electron beam are discussed in detail.