Experimental results with an optimized magnetic field configuration for JET breakdown

Experiments and modelling have been carried out to optimize the magnetic field null during breakdown at JET. Such optimization may prove to be essential for reliable plasma initiation at low voltages, e.g. in ITER where the value of the electric field available will be limited to 0.33 V m-1. A two-dimensional FEM electromagnetic model has been employed to predict the stray field configuration during JET breakdown. This model includes the active poloidal field circuits, a description of the passive structure and the JET magnetic circuit. In particular, the model includes the gap at the top of the iron circuit (but not at the bottom), which introduces a perturbing field, with radial and vertical components, not previously considered. A number of experiments were run using the optimized magnetic null configuration, allowing one to achieve a more robust breakdown at a low electric field. The model calculation to adjust the magnetic null position was validated using the recordings from a fast visible camera. The optimized position and dynamics of the plasma start lead to a smoother behaviour of the JET radial field control system, far from the amplifier limits. Finally, an important indication was obtained on the precision needed for the active current measurements during the low electric field breakdown relevant from the perspective of the ITER real-time acquisition system.