Scaling of ion parameters at the H-transition in JET

Shaing et al. have proposed a theory of the H-mode based on bifurcated solutions for poloidal flow. In this theory the driving mechanism for poloidal flow is ion orbit loss. It is predicted that as the ion collisionality, vi* is reduced below some (machine dependent) threshold close to unity the poloidal flow speed will suddenly increase, establishing the H-mode. Experiments on DIII-D involving a neutral beam power scan agreed with the predictions of the theory. vi* depends on Ti-2, thus the critical collisionally appeared as a critical Ti in the experiment. We report here on a series of experiments on JET aimed at testing this critical vi* prediction. In these experiments vi* was varied by changing the density in a sequence of discharges with constant neutral heating power. (author) 3 refs., 3 figs