Approach to a window coating problem by in situ transmission monitoring and laser blow-off cleaning developed in the JT-60U Thomson scattering system

For solving a window coating problem, an in situ window transmission monitoring by inferring it precisely from a known attenuation of the deposited film has been developed in the JT-60U Thomson scattering system. Also the practicability of an in situ window cleaning based on a laser blow-off technique has been investigated extensively. The coated film composition could be guessed to be a resin such as polyacrylonitrile with a small amount of metals. The existence of chromatic upper limit has been found in the recovered transmission after the blow-off cleaning, which gives systematic errors only of less than 3% to an apparent measurement of both electron density and temperature at 10 keV or less. The attenuation itself is unchanged before and after the laser blow-off cleaning. A complementary use of both methods will provide the Thomson scattering measurement of high Te plasmas with durable reliability and sufficient precision in the existing tokamaks and the International Thermonuclear Experimental Reactor.