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Abstract. In the present paper the JET threshold database from 1990 to 1999 is analysed. Sources of data scatter, which introduce considerable uncertainty in the estimate of the power needed for the H-mode transition in ITER, are identified and eliminated. The influence of divertor geometry and plasma configuration on the power threshold is analysed in detail. In particular, the height of the X-point over the divertor plate (or septum) and the plasma-first wall distance are considered. The data show consistency throughout the ten years of JET operation, and confirm that the power threshold is minimised with decreasing X-point height, while it increases sharply if the plasma is pushed too near the outer wall. Increased threshold due to operation on the vertical target plates can be also explained in terms of X-point height. Although geometry influences the scatter in the database obtained with single divertors, it is found to be negligible if the whole database is considered. More important seems to be the trend towards stronger density dependence with increasing divertor closure. If this trend is taken into account, a significant reduction in the database scatter is obtained and data from different divertors become consistent.
IAEA 2001
