THE TESTING OF TRANSPORT MODELS AGAINST DATA

Many different models for energy transport in a tokamak have been test ed against subsets of the wealth of data accumulated from tokamak expe riments and claimed to describe adequately the data used in the tests. Yet no single transport model has emerged as an obvious candidate for accurate extrapolations from today's large tokamaks to a reactor. The principles involved in testing a model against data are examined. It is shown how: (i) collinearities in the data, (ii) confinement degrada tion with power and (iii) measurement errors in the data, can make tes ts insensitive to particular details of transport models. This lack of sensitivity is illustrated via tests of models against global confine ment data from several tokamaks (ITER L mode database) and via tests o f different models against local confinement data from one single toka mak (JET database). The latter tests demonstrate that the testing of t ransport models must include data from two or more tokamaks of differe nt size.