For high density operation of the large helical device and for a future large plasma machine such as ITER, a powerful 57 μm CH3OD laser pumped by a continuous wave 9R(8) CO2 laser has been developed. The 57 μm (5.2 THz) laser light has been successfully detected by using a GaAs Schottky barrier diode detector with a corner reflector. For optical windows of the plasma vessel and the far-infrared laser crystal quartz etalons have been designed under a concept of a two-wavelength etalon for 119 and 57 μm lights.
Powerful lasers in the far-infrared wavelength range (47.6 and 57.2 μm) have been developed to measure the plasma density in the Large Helical Device at National Institute for Fusion Science and future plasma devices such as the International Thermonuclear Experimental Reactor. The intensification of these lasers has been done by cooling the laser tube wall, adding He as the buffer gas, and using a chemical-vapor-deposited diamond output window. The output powers for the 57.2 and 47.6 μm lasers have been found to be 1.6 and 0.8 W, respectively.
SUMMARYA powerful short-wavelength far-infrared (FIR) laser from 40 µm to 100 µm in wavelength is required for the optical source of diagnostics of high-density and large-volume plasmas, and a production of γ-rays by inverse Compton scattering. In order to design the optical system, we have measured the optical constants (refractive index and absorption coefficient) of crystal quartz, CVD-diamond, and silicon for the short-wavelength FIR lasers.
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