An apparatus for reducing thermal blooming in an optical subsystem of a high energy laser (HEL) comprises a fluid pump that receives a first fluid containing diatomic oxygen at a first concentration and a combustion chamber that combusts the first fluid to develop a second fluid containing diatomic oxygen at a second concentration, wherein the second concentration is less than the first concentration, and wherein the second fluid is supplied to the optical subsystem of the HEL A method of operating an HEL having an optical subsystem is also disclosed.