Predictive modelling of JET optimized shear discharges

Transport analysis of high performance JET plasmas with optimized magnetic shear (OMS) has revealed many interesting features which cannot be explaine d by the existing JET empirical transport model (Erba, M., et al., Plasma P hys. Control. Fusion 39 (1997) 261). TRANSP analysis shows that transport c oefficients in OMS plasmas are often reduced in the plasma core (Cottrell, G.A., et al., in Controlled Fusion and Plasma Physics (Proc. 24th fur. Conf . Berchtesgaden, 1997), Vol. 21A, Part I, European Physical Society, Geneva (1997) 81) to the level of ion neoclassical transport. TRANSP analysis and predictive modelling with JETTO show that this region of improved confinem ent appears near the plasma centre and then expands outwards in a way which does not follow either the evolution of the region with the negative magne tic shear or the propagation of the region with a large shear in plasma rot ation. The best agreement with experiment has been achieved by using a tran sport model which combines the effect of a long wavelength decoupling due t o small magnetic shear with its suppression by strong rotational shear. Pre dictive modelling of some of the characteristic JET OMS plasmas gives quite good agreement between such a model and the experimental data.