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High-power and high-beam-quality lasers are used in various fields such as high energy physics, laser fusion, aerospace systems and material processing and are expected to have higher output. Power scalability is important for those applications. Solid-state lasers have such a characteristic. One of major problems in solid-state lasers is heat, which may cause damage to the medium due to thermal stress, deterioration of the beam quality, thermal lens effect, etc. Thus an effective cooling method is required. Solid-state lasers include active mirror lasers, slab lasers, thin disc lasers, rod lasers and the like, as is generally known. We have been developing laser systems using active mirror media such as TRAM (Total-Reflection Active Mirror) and ZiZa-AM (Zig-Zag Active-Mirror). In these media, disk like active materials are bonded to the outside of the non-doped material. The seed laser travels in the medium coaxially with the pumping laser while being totally reflected, and is amplified in the active material layer. Since high reflection coating required for a thin disk laser or the like is unnecessary in the case of these media, this is an advantage for removal of heat. We developed a liquid nitrogen jet impingement cooling system. This is an effective system that enables high-level heat removal performance while keeping the medium at an extremely low temperature and jets subcooled liquid nitrogen directly to the active material layer of the medium. In our previous studies, we have achieved development of a kW-class amplifier using a ZiZa-AM. This report presents the various laser characteristics in the case of multi amplifier chain to confirm the scalability of these amplifier systems.
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