Gas turbine impingement cooling structure and method of impingement cooling

An extended impingement cooling structure to cool outside an air supply plenum comprises an inner wall (102); an impingement sheet (104); a series of supports (106,106a) to maintain the inner wall (102) in spaced relation to the impingement sheet (104), and a baffle (112) supported between the inner wall (102) and the impingement sheet (104). The baffle (112) has a collector plenum area (150) that receives impingement cooling air from the air supply plenum (108) and a channel (154) in fluid communication with the collector plenum (150) and extending outside the air supply plenum with openings (156) to allow impingement cooling air to pass therethrough and having a series of lands (152) extending into the channel (154) wherein the lands (152) are located in proximity to impingement cooling air outlets (130) in the inner wall (102).