Copper vapor laser application for surface monitoring of divertor and first wall in ITER

AbstractThe availability of copper vapor laser system for surfaces surveying of International Thermonuclear Experimental Reactor (ITER) divertor and first wall during plasma discharge are justified. The construction concept of laser projector with image intensity amplification of the objects removed at a distance up to 20 m under intensive plasma background conditions is developed. The preliminary optical scheme for divertor and first wall surface monitoring from upper port is developed. Diffraction angle resolution at the object space is estimated (equal 1.18×10−5 rad).The preliminary estimation of laser viewing system power parameters is carried out. Laser system operation under intensive plasma background conditions (Tth/n=104 eV—typical thermonuclear temperature). Power relation signal/noise, calculated at the image intensity amplifier input is equal S/N=104, i.e. plasma background radiation can be neglected in the considered scheme. Hence, the proposed technique application for monitoring of ITER discharge chamber internal surfaces is possible during running cycles, and in pauses especially. Experimental tests of laser system without scanning mirror and plasma noise background are carried out. Objects were located at a distance about 20 m from laser and amplifier. Probing laser had the next operating parameters: average radiation power, 10 W; pulse repetition rate, 10 kHz; output beam diameter, 14 mm. Linear resolution at object's plane about 1 mm was obtained. Degradation of resolution in comparison with estimated value—0.24 mm is related with insufficient degree of the image contrast, as the laser amplifier tube had unnormalized noise level. Method determined rate of the protecting cover erosion of surveyed surfaces is proposed. Under given measurement method application the laser viewing system can record decreasing of protection cover depth about 0.5 mm, and available value of protection cover erosion rate is equal 0.5 mm/ms. Object's surface observe methods are considered. Laser beam noninterlaced raster scanning on surveyed surface is proposed. The laser beam moving velocity is determined by required accuracy of the image reproduction. Pointing direction definition method based on checkpoint grids is offered. Quadratic pyramidal deepening center may be a checkpoint grid node of pointing direction system. Accuracy of pointing direction positioning at the object equals 2 mm provided an even decrease of protection cover thickness near the given deepening.