DISTRIBUTION OF CODEPOSITED METALS ON THE GRAPHITE DIVERTOR SURFACES OF DIII-D

Since the introduction of new graphite tiles on the divertor regions and the center post of DIII-D, beta backscatter measurements have been made of the metals codeposited on these surfaces at three different times. At the first vent (June 1988), the upper and lower portions of the machine appeared very similar, with the average amount of metal on a surface increasing with increasing radius and indications that local tile alignment had an effect on the amount of metal deposited. Measurements made during subsequent vents show that there has been much more metal deposited on the lower divertor than on the upper. Metal seems to be concentrated at radii both less than and greater than that expected for the strike zones. From measurements made during the latest vent (December 1989) there is clear indication that material has been eroded from a position of the outer strike zone, probably to be redeposited at a larger radius.