3-FILTER COLORIMETRY IN DIII-D TO MEASURE IN-SITU EROSION AND DEPOSITION OF PLASMA-FACING SURFACES

Three-filter colorimetry has been introduced in the DIII-D tokamak to measure in situ the growth and the erosion of transparent wall coating s (a-C/B:H) in the divertor region. Colorimetry uses the interference of light on a transparent layer to measure the film thickness. The col orimetry setup at DIII-D consists of a light source, a black and white camera, and a filter wheel equipped with three broadband color filter s. A white light source illuminates the area of interest. A picture wi th each filter is recorded and digitized. Thus the interference color of a semitransparent layer is measured and the film thickness is compu ted using the formalism of thin film optics. Since color cameras are m ore sensitive to neutron damage, a set of three filters was chosen whi ch best matches the responsivity of a radiation-hardened B/W camera. I n fact, one infrared filter and two visible filters are used. The thre e 2D camera images obtained from rotating the three filters into the l ine of sight of the camera are used to produce a 2D image of the chang e in film thickness due to the high flux of particles and power incide nt on the film during a diverted discharge. In a first measurement, th e technique was used to measure the growth of a boron him in situ in t he DIII-D tokamak. (C) 1995 American Institute of Physics.