EFDA-JET-CP(07)03/29
High bN JET H-modes for Steady-State Application
An efficient steady-state tokamak fusion power plant requires stable operation at high plasma pressure without excessive recirculating power. Access to high bN with a q profile compatible with large bootstrap current is, therefore, highly desirable. Recent experiments at JET have investigated the access to high bN for plasmas with H-mode edge and q0 1. The 1.2MA/1.8T (q95 5) plasmas were formed by the initial inductive ramp-up phase, similar to experiments developed at DIII-D. LHCD or ICRH was added in some discharges and the initial current ramp rate varied to tailor the q profile shape as the current penetrated towards the plasma centre. The resulting target q profile at the start of the main NBI heating phase had low or reversed magnetic shear in the core and the minimum value of q (qmin) was adjusted using the start time of the NBI pulse. The time evolution of a typical pulse is shown in figure 1. The initial rise in b was provided by pre-programmed NBI power. Real-time control was triggered at bN = 1 and bN was then controlled by feedback on the NBI power. In these experiments bN 3 was sustained for up to ~18tE and bN 2.8 for up to ~35tE, which is of the order of the resistive time and was limited by the allowed NBI pulse length for this particular configuration.