Legendre polynomial expansion method for evaluating multipole moments of current density in toroidal devices

It is found that multipole moments of toroidal current are given by the coefficients of the Legendre polynomial expansion of the magnetic field on a meridian contour. Using this fact and the orthogonality of the Legendre polynomials, a method is proposed for evaluating the moments from magnetic field measurements through an open-contour. As an application, exact expressions of the first few order moments and the current center position are formulated. Results show that this method is applicable to any aspect ratio tokamaks without the limitation of small displacement of the current center. (C) 2000 American Institute of Physics. [S1070-664X(00)00305-0].