A method and apparatus is disclosed which may comprise: a gas discharge laser system energy controller which may comprise: a laser system energy controller providing a first laser operating parameter control signal based on an error signal related to a value of the output energy of the laser system compared to a target value for output energy and an energy controller model of the value of the first laser operating parameter necessary to change the value of the laser system output energy to the target value; a first laser system operating parameter control signal modifier providing a modification to the first laser system operating parameter control signal based upon a controller signal modification model of the impact of a second laser system operating parameter on the value of the first laser system operating parameter necessary to change the value of the output energy to the target value.