Abstract: An apparatus for laser irradiation including a housing wherein the housing defines at least a portion of a test volume; a first conduit configured to provide a first flow of gas into the test volume; a second conduit configured to remove the first flow of gas from the test volume; a third conduit configured to provide a second flow of gas; and a flow controller configured to provide a region of increased flow impedance between the first conduit and the third conduit so that the first flow of gas is directed towards the test volume and the second flow of gas is directed away from the first flow of gas so as to restrict gas external to the housing from coming into contact with the first flow of gas.