Optimization of the bootstrap current in a large helical system with L = 2

Neoclassical bootstrap currents in the banana and plateau regimes are evaluated for a large helical system with L = 2 and with a major radius of 5 m. Various vacuum magnetic field configurations are studied with a view to optimizing the bootstrap current. In the banana regime, shifting of the magnetic axis and shaping of magnetic surfaces have a remarkable influence on the bootstrap current. It has been found that a small outward shift of the magnetic axis and vertically elongated magnetic surfaces are favourable for a reduction of the bootstrap current. In contrast, in the plateau regime the bootstrap current depends strongly on the toroidal pitch number of the helical winding coils, but shifting of the axis and shaping of magnetic surfaces are not effective. The magnitude of the bootstrap current is estimated for plasmas in a large helical system with parameters given by a one-dimensional radial transport code.