An electron source, e.g. for a free electron laser used for EUV lithography comprises: • a cathode (203) configured to be connected to a negative potential (100, 101); • a laser (110) configured to direct pulses of radiation onto the cathode so as to cause the cathode to emit bunches of electrons; • an RF booster (180) connected to an RF source and configured to accelerate the bunches of electrons; and • a timing corrector (303, 313, 400, 401) configured to correct the time of arrival of bunches of electrons at the RF booster relative to the RF voltage provided by the RF source. The timing corrector may comprise a correction electrode (303, 313) surrounding a path of the bunches of electrons from the cathode to the RF booster and a correction voltage source (400, 401) configured to apply a correction voltage to the correction electrode.