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 m onitoring by inferring it precisely from a known attenuation of the de posited film has been developed in the JT-60U Thomson scattering syste m. Also the practicability of an in situ window cleaning based on a la ser blow-off technique has been investigated extensively. The coated f ilm composition could be guessed to be a resin such as polyacrylonitri le with a small amount of metals. The existence of chromatic upper lim it has been found in the recovered transmission after the blow-off cle aning, which gives systematic errors only of less than 3% to an appare nt measurement of both electron density and temperature at 10 keV or l ess. The attenuation itself is unchanged before and after the laser bl ow-off cleaning. A complementary use of both methods will provide the Thomson scattering measurement of high T-e plasmas with durable reliab ility and sufficient precision In the existing tokamaks and the Intern ational Thermonuclear Experimental Reactor. (C) 1997 American Institut e of Physics.