states to relax their equilibrium to a value that is more appropriate for the lower temperature. However, the molecules in the lower states relax very quickly, while the upper state molecules take much longer to relax. Since a good quantity of molecules remain in the upper state, a population inversion is created, which often extends for quite a distance downstream. Continuous wave outputs as high as 100 kilowatts have been obtained from dynamic carbon dioxide lasers. Similar methods of supersonic expansion are used to adiabatically cool carbon monoxide lasers, which are then pumped either through chemical reaction, electrical, or radio frequency pumping. The adiabatic cooling replaces bulky and costly cryogenic cooling with liquid nitrogen, increasing the carbon monoxide laser's efficiency. Lasers of this type have been able to produce outputs as high as a gigawatt, with efficiencies as high as 60%. Other types Charge-displacement self-channeling can give rise to high energy concentration along a column created and maintained by the ponderomotive expulsion of electrons. The channel will also columnate shorter wavelength secondary radiation and ultimately extremely short wavelength lasing. Chemical reaction is used as a power source in chemical lasers. This allows for very high output powers difficult to reach by other means. Nuclear fission is used in exotic nuclear pumped lasers (NPL), directly employing the energy of the fast neutrons released in a nuclear reactor. The United States military tested an X-ray laser pumped by a nuclear weapon in the 1980s, but the results of the test were inconclusive and it has not been repeated. See also Laser construction References Pumping