Use of a twisted 3D Cauchy condition surface to reconstruct the last closed magnetic surface in a non-axisymmetric fusion plasma

The three-dimensional (3D) Cauchy condition surface (CCS) method code, ?CCS3D?, is now under development to reconstruct the 3D magnetic field profile outside a non-axisymmetric fusion plasma using only magnetic sensor signals. A new ?twisted CCS? is introduced, whose elliptic cross-section rotates with the variation in plasma geometry in the toroidal direction of a helical-type device. Independent of the toroidal angle, this CCS can be placed at a certain distance from the last closed magnetic surface (LCMS). With this new CCS, it is found through test calculations for the Large Helical Device that the numerical accuracy in the reconstructed field is improved. Furthermore, the magnetic field line tracing indicates the LCMS more precisely than with the use of the axisymmetric CCS. A new idea to determine the LCMS numerically is also proposed.