A dual chamber gas discharge laser light source, comprising: a master oscillator having a laser chamber containing a lasing medium gas comprising a halogen; an amplifier having a laser chamber containing a lasing medium gas comprising a halogen; a gas refill system including a controller automatically executing a refill scheme, the refill scheme comprising in sequence the steps of: obtaining a target pressure and a target concentration of halogen for a selected one of the laser chambers; evacuating the selected laser chamber to a first point at which the gas pressure in the chamber reaches a predetermined low value, and measuring the temperature and pressure at the first point (202, 301); adding an amount of non-halogen containing gas to the selected laser chamber to reach a second point and measuring the temperature and pressure at the second point (203, 204, 302, 303); adding an amount of halogen containing gas to the selected laser chamber such that the gas in the selected laser chamber reaches the target concentration of halogen at a pressure greater than the target pressure (205, 304); and bleeding gas from the selected laser chamber until the target pressure is reached (206, 207, 305, 306). a master oscillator having a laser chamber containing a lasing medium gas comprising a halogen; an amplifier having a laser chamber containing a lasing medium gas comprising a halogen; a gas refill system including a controller automatically executing a refill scheme, the refill scheme comprising in sequence the steps of: obtaining a target pressure and a target concentration of halogen for a selected one of the laser chambers; evacuating the selected laser chamber to a first point at which the gas pressure in the chamber reaches a predetermined low value, and measuring the temperature and pressure at the first point (202, 301); adding an amount of non-halogen containing gas to the selected laser chamber to reach a second point and measuring the temperature and pressure at the second point (203, 204, 302, 303); adding an amount of halogen containing gas to the selected laser chamber such that the gas in the selected laser chamber reaches the target concentration of halogen at a pressure greater than the target pressure (205, 304); and bleeding gas from the selected laser chamber until the target pressure is reached (206, 207, 305, 306). obtaining a target pressure and a target concentration of halogen for a selected one of the laser chambers; evacuating the selected laser chamber to a first point at which the gas pressure in the chamber reaches a predetermined low value, and measuring the temperature and pressure at the first point (202, 301); adding an amount of non-halogen containing gas to the selected laser chamber to reach a second point and measuring the temperature and pressure at the second point (203, 204, 302, 303); adding an amount of halogen containing gas to the selected laser chamber such that the gas in the selected laser chamber reaches the target concentration of halogen at a pressure greater than the target pressure (205, 304); and bleeding gas from the selected laser chamber until the target pressure is reached (206, 207, 305, 306).