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

Abstract

The three-dimensional (3-D) Cauchy condition surface (CCS) method code, 'CCS3D', is now under development to reconstruct the 3-D magnetic field profile outside a non-axisymmetric fusion plasma using only magnetic sensor signals. A new "twisted CCS" has been 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 has been improved. Further, 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 has also been proposed.

Analysis Method for the Low-Order Resistive Interchange Instability in LHD with Stochastic Magnetic Field Line Structure

“…Using the RBF expansion method [10], we fit the group of discrete points by tracing a magnetic field line on a poloidal cross section to a continuous curve. The curve is recognized as a quasi-magnetic surface.…”

“…In the conventional use of VMEC, the LCFS is adopted as the boundary. In contrast, this study uses the quasi-magnetic surface [10] and constructs Boozer-like coordinates in the stochastic region. The shape of the largest boundary should be extracted from the quasi-magnetic surfaces for the VMEC input.…”

“…Without the coordinates, we cannot describe the singlemode perturbation in the stochastic field line structure. In section 3, reference closed surfaces outside the LCFS obtained by using the Radial Basis Function (RBF) expansion method [10] are defined. Although these reference surfaces outside LCFS are not physical, we use the surfaces as the fixed-boundary in the VMEC code to obtain Boozer-like coordinates.…”

Improved Three-Dimensional CCS Method Analysis for the Reconstruction of the Peripheral Magnetic Field Structure in a Finite Beta Helical Plasma

“…[8]) that is applicable to 3D problems. Recently, the authors' research group developed the 3D Cauchy condition surface method [8,9] to reconstruct the 3D magnetic field profile outside the non-axisymmetric plasma in the LHD. This 3D CCS method is not a straight-forward extension of Kurihara's 2D CCS method.…”

“…The 10-fold rotational symmetry in the toroidal direction of the LHD was considered to reduce the number of unknowns [8][9][10]. However, the condition number [11] of the solution matrix still exceeds 10 15 in some cases.…”