SHEAR OPTIMIZATION EXPERIMENTS WITH CURRENT PROFILE CONTROL ON JET

A record performance on JET has been obtained with shear optimization scenarios. A neutron yield of 5.6 x 10(16) s(-1) in deuterium discharg es, and a global energy confinement improvement above the ITER-89 L-mo de scaling with H less than or equal to 2.5 in L-mode and H less than or equal to 3 in H-mode have been achieved. The tailoring of plasma cu rrent, density and heating power waveforms and current profile control with lower hybrid current drive and ICRF phasing have been essential. Internal energy, particle and momentum transport barriers develop spo ntaneously upon heating above a threshold power of about 15 MW with ne utral beams and ICRH into a low-density target plasma, with a wide cen tral region of slightly negative or flat magnetic shear with q > 1 eve rywhere. An additional H-mode transition can also raise the pressure i n the region between internal and edge transport barriers. The ion hea t conductivity falls to the neoclassical level in the improved core co nfinement region. Pressure profile control through power deposition fe edback control makes it possible to work close to the marginal stabili ty boundary for pressure-driven MHD modes. First experiments in deuter ium/tritium plasmas, with up to 75% tritium target concentration, have established internal transport barriers already with heating powers a t the lowest threshold of pure deuterium plasmas, resulting in a fusio n power output of P-fusion = 2 MW.