A method for controlling an interaction between a droplet target and a high power and high-repetition-rate laser beam at a laserfocus position of said laser beam, comprising the steps of a. providing a droplet generator for generating a train of droplets as a droplet target with a predetermined droplet frequency and velocity in a predetermined direction; b. providing a high power, high-repetition-rate laser for emitting a pulsed laser beam, which is focused in said laser focus position; c. aligning said droplet generator such that said train of droplets of said droplet target runs through said laser focus position; d. generating a train of droplets as a droplet target; and e. emitting a pulsed laser beam in synchronization with said train of droplets of said droplet target, such that said droplet target interacts with said pulsed laser beam at said laser focus position; whereby f. the actual position of individual droplets of said droplet target is measured in a plane crossing, preferably perpendicular to, the direction of said droplet target; g. the velocity of individual droplets of said droplet target is measured; wherein h. said actual position and said velocity of individual droplets of said droplet target is measured optically; and i. the droplets to be measured are illuminated on their way from the droplet generator to the laser focus position by at least two fixed laser beams crossing the droplet target in the form of laser lines (8,12) extending in said plane crossing, preferably perpendicular to the beam direction and the direction of said droplet target, and the position of said droplets of said droplet target within said laser lines (8, 12) is determined by position sensitive photo detection means, preferably linear photodiode arrays (16, 19), which are illuminated by said laser beams after having crossed said droplet target; and j. the droplet generator is realigned in a feedback mode, when the measured actual position of said individual droplets deviates from a predefined set position; and k. the timing of the pulses of the pulsed laser beam is changed in a feedback mode, when the measured velocity of said individual droplets leads to an arrival time of the respective droplets at said laser focus position, which is out of synchronization with the pulsed laser beam. a. providing a droplet generator for generating a train of droplets as a droplet target with a predetermined droplet frequency and velocity in a predetermined direction; b. providing a high power, high-repetition-rate laser for emitting a pulsed laser beam, which is focused in said laser focus position; c. aligning said droplet generator such that said train of droplets of said droplet target runs through said laser focus position; d. generating a train of droplets as a droplet target; and e. emitting a pulsed laser beam in synchronization with said train of droplets of said droplet target, such that said droplet target interacts with said pulsed laser beam at said laser focus position; whereby f. the actual position of individual droplets of said droplet target is measured in a plane crossing, preferably perpendicular to, the direction of said droplet target; g. the velocity of individual droplets of said droplet target is measured; wherein h. said actual position and said velocity of individual droplets of said droplet target is measured optically; and i. the droplets to be measured are illuminated on their way from the droplet generator to the laser focus position by at least two fixed laser beams crossing the droplet target in the form of laser lines (8,12) extending in said plane crossing, preferably perpendicular to the beam direction and the direction of said droplet target, and the position of said droplets of said droplet target within said laser lines (8, 12) is determined by position sensitive photo detection means, preferably linear photodiode arrays (16, 19), which are illuminated by said laser beams after having crossed said droplet target; and j. the droplet generator is realigned in a feedback mode, when the measured actual position of said individual droplets deviates from a predefined set position; and k. the timing of the pulses of the pulsed laser beam is changed in a feedback mode, when the measured velocity of said individual droplets leads to an arrival time of the respective droplets at said laser focus position, which is out of synchronization with the pulsed laser beam.