New designs of a defense system using a chemical oxygen iodine laser (COIL) are presented to realize a boost-phase interception of a ballistic missile. Although a space-based laser (SBL), in which the Hydrogen-Fluoride chemical laser is the primary candidate, can realize such a defense system, many SBLs are necessary to cover even a single missile site because they need to continuously go around the earth. This is an expensive system if the potential enemy is a small country. Meanwhile a high energy laser (HEL) carried by a high-altitude airship (HAA) can realize a geostationary defense system if the HEL is quite lightweight. A chemical oxygen-iodine laser (COIL) is suitable since it does not require a heavy electric power supply. But since the COIL should be as light as possible, it would be more advantageous if it can operate without a vacuum pump that requires a large electric-power supply and cooling water. Rate-equation based simulations have been performed to see if it can operate without a vacuum pump by filling a buffer gas at the pressure higher than the outside. The simulation results indicate that it can operate continuously at an altitude of 20 km where the atmospheric pressure becomes ~5,400 Pa (~40 Torr). Moreover, since atmospheric turbulence is greatly reduced at that altitude, adaptive optics is also not necessary for focusing the beam after a long propagation. A simple focusing mirror can focus the beam tightly enough to destroy the target of >100 km away.
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