Calculating Rotational Transform Following Field Lines

author’s e-mail: todoroki@nifs.ac.jp Magnetic coordinates are commonly used in the study of plasma in toroidal magnetic confinement devices. To construct the magnetic coordinates, Boozer’s algorithm [1] following the magnetic field line is widely used. In such calculations the rotational transform is the most important quantity of the magnetic surface. In this paper the efficient and accurate method calculating the rotational transform is described. On the rational magnetic surface, magnetic lines of force are closed in n revolution around the long way and m revolution around the short way of the torus. The rotational transform is defined as