EXPRESSION FOR THE THERMAL H-MODE ENERGY CONFINEMENT TIME UNDER ELM FREE CONDITIONS IN DEUTERIUM

The design of future tokamaks, which are supposed to reach ignition wi th the H mode, requires a reliable scaling expression for the H mode e nergy confinement time. In the present work, an H mode scaling express ion for the thermal plasma energy confinement time has been developed by combining data from four existing divertor tokamaks, ASDEX, DIII-D, JET and PBX-M. The plasma conditions, which were as similar as possib le to ensure a coherent set of data, were ELM free deuterium discharge s heated by deuterium neutral beam injection. By combining four tokama ks, the dependence of the thermal energy confinement on the main plasm a parameters, including the three main geometrical variables, was dete rmined. Assuming a power law dependence, one obtains for the H mode in deuterium by linear regression: tau(th) = 0.066I(p) 1.11+/-0.04 P(L)- 0.51+/-0.03 R1.75+/-0.08 a-0.30+/-0.07 kappa0.18+/-0.09 B(T)0.15+/-0.0 7. The density dependence is found to be close to zero. This expressio n is compared with the existing scaling relations and the differences are analysed. Predictions for ITER are given, and discussed with parti cular emphasis on the uncertainties. Correcting the data to take the c losed divertor of ASDEX into account yields tau(th) = 0.048I(p) 1.10+/ -0.04 P(L)-0.51+/-0.03 R1.83+/-0.08 a-0.34+/-0.07 kappa0.61+/-0.09 B(T )0.14+/-0.07 essentially characterized by a smaller numerical factor a nd a larger kappa dependence. It appears, however, that the prediction s for ITER are only weakly changed when this expression is used. Final ly, it is shown that taking the improved H mode confinement into accou nt by means of a simple multiplier in front of an L mode confinement s caling expression is valid for ITER, but is not necessarily universall y valid.