The three-dimensional Cauchy-condition surface method to reconstruct the last closed magnetic surface in non-axisymmetric fusion plasma

Abstract

The three-dimensional (3D) Cauchy-condition surface (CCS) method code is now under development to reconstruct the 3D magnetic field profile outside a non-axisymmetric fusion plasma using only magnetic sensor signals. The boundary integral equations in terms of 3D vector potential for magnetic field sensors, flux loops and points along the CCS are solved simultaneously. Test calculations have been carried out for the Large Helical Device (LHD). The magnetic field line tracing based on the reconstructed field indicates the plasma boundary (the outer surface of the stochastic region) precisely. The Poincare plot points obtained from a field line tracing are converted to contours of a ‘quasi’ magnetic surface function using the expansion of radial basis functions. Introducing the ‘inside/outside’ ratio related to the scatter in the Poincare plot, the contour where the ratio jumps is taken as a best estimate for the location of the last closed magnetic surface (LCMS). This reconstructed LCMS agrees well with the reference LCMS.