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Bootstrap current

Self-supporting current, which arises in tokamak plasma when its pressure in the centre is higher than at the edges. The magnetic field in the tokamak needs to be helical in shape to hold the charged plasma particles well. The helicity is achieved by a combination of a toroidal magnetic field and an induced electric current. The particles then circulate through the toroidal plasma on a helical curve, alternately on the outer and inner surface of the torus, which is circular in its cross-section. Under certain circumstances, however, some particles begin to move not in a circle but back and forth, so that the curve of their motion resembles a banana shape. These are called trapped particles and the trajectory along which they move is called a banana orbit. When these trapped particles collide with passing particles, a bootstrap current is generated. This could, under specific conditions, replace the induced current and make the tokamak a steady-state device instead of a pulse device.