1. An extreme ultraviolet (EUV) radiation source, comprising:a target droplet generator configured to generate target droplets;
a first laser source configured to generate pre-pulses that heat the target droplets to produce target plumes;
a second laser source configured to generate main pulses that heat the target plumes to produce plasma emitting EUV radiation;
a controller configured to adjust at least one parameter of the first and second laser sources comprising a delay between one of the pre-pulses and a corresponding one of the main pulses, and positions of the pre-pulses in a Y direction that is different from an X direction along which the target droplets travel;
an energy detector configured to monitor an energy of the EUV radiation and record the parameters of the first and second laser sources with which the energy of the EUV radiation is maximized;
a first laser beam generator and a second laser beam generator configured to generate first and second laser beams, respectively, that are directed onto a travel path of the target plumes, wherein the first and second laser beams are substantially parallel; and
a laser beam monitor configured to receive the first and second laser beams reflected by the target plumes to determine a real-time velocity of the target plumes in their traveling path.