JET: progress in performance and understanding

In 1990 JET operated with a number of technical improvements which include beryllium antenna screens, a prototype lower hybrid current drive system, and modification of the NI system to enable the injection of ³He and ⁴He. Continued investigation of the hot-ion H-mode produced a value of n_D(O) tau _ET_i(O)=9*10²⁰ m^-3s keV, which is near conditions required for Q_DT=1, while a new peaked density profile H-mode was developed with only slightly lower performance. Progress towards steady state operation has been made by achieving ELMy H-modes under certain operating conditions, while maintaining good tau _E values. Experimental simulation of He ash transport indicates effective removal of alpha-particles from the plasma core for both L and H mode plasmas. Detailed analyses of particle and energy transport have helped establish a firmer link between particle and energy transport, and have suggested a connection between reduced energy transport and reversed shear. Numerical and analytic studies of divertor physics carried out for the pumped divertor phase of JET have helped clarify the key parameters governing impurity retention, and an intensive model validation effort has begun. Experimental simulation of alpha-particle effects with beta _fast up to 8% have shown that the slowing down processes are classical, and have given no evidence of deleterious collective effects.