Triggering of internal transport barrier in JET

Internal transport barriers (ITBs) can be produced in JET by the application of strong additional heating during the current rise phase of the plasma discharge. Using up to 3 MW of lower hybrid power to tailor the q-profile prior to the main heating phase, a large variety of q-profiles ranging from low positive to strong negative central shear have been obtained during the current rise (0.4 MA s(-1)). With negative central magnetic shear s = (r/q) (dr/dq), the analysis of ITB triggering reveals a correlation between the formation of the ITB and q(min) reaching an integer value (q = 2 or q = 3). This observation is confirmed by the analysis of the Alfven cascades. The minimum power required to access regimes with ITBs is probably related to the transport and magnetohydrodynamic properties of integer magnetic surfaces. Laser ablation and shallow pellet injection have also been attempted in recent JET ITB triggering experiments.