Fast control of the plasma surface

Plasma and Fusion Research

Ishimaru

Matsumoto

et al. 2013

In the previous 3D Cauchy-condition surface (CCS) method analysis to reconstruct the magnetic field profile in the Large Helical Device (LHD), one assumed an impractically large number of magnetic sensors, i.e., 440 field sensors and 126 flux loops. In the singular value decomposition (SVD) process employed in the CCS method, a gap is found in the magnitude of the singular values. The most accurate field results can be obtained if all the singular values smaller than the gap threshold are eliminated, independent of the number of boundary elements on the CCS and the number of sensors as well. With the reduction in the number of boundary elements, the required numbers of field sensors and flux loops are significantly reduced to 110 and 25, respectively, without losing the solution accuracy. They can be further reduced to 58 and 13 respectively if considering the symmetry of the field profile in the LHD. This result suggests the possibility of actual application to the LHD.

Section: Introductionsupporting

confidence: 54%

Section: Introductionmentioning

confidence: 99%

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

Plasma and Fusion Research

Ishimaru

Matsumoto

et al. 2013

“…With the use of a Bayesian approach, they inferred the distribution of possible flux surface topologies. Instead of such filaments, Feneberg et al [6] assumed a "control surface" inside the plasma. The current density distribution on the surface is expressed as a sum of Fourier modes.…”

Section: Introductionmentioning

confidence: 99%

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

Plasma Phys. Control. Fusion

Okubo

Akazawa

et al. 2012

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.

“…The currents in these filaments are then computed in such a way that one obtains the best fit to the measured magnetic fluxes and fields. Instead of such filaments, Feneberg et al [4] assumed a "control surface" inside the plasma.…”

Section: Introductionmentioning

confidence: 99%

Three-dimensional Cauchy-condition surface method to identify the shape of the last closed magnetic surface in the Large Helical Device

Plasma Phys. Control. Fusion

Maeda

Ishimaru

et al. 2011

Kurihara's Cauchy condition surface (CCS) method, originally developed for axisymmetric tokamak plasma, has been expanded to reconstruct the 3-D magnetic field profile outside the nonaxisymmetric plasma in the Large Helical Device (LHD). The boundary integral equations (BIE) in terms of 3-D vector potential for magnetic field sensors, flux loops and points along the CCS are solved simultaneously. In the BIE for a flux loop, the portions related to the fundamental solution are integrated along the loop. The rotational symmetry of the plasma is incorporated into the formulation to reduce the number of unknowns. The reconstructed magnetic field caused only by the plasma current agree fairly well with the reference solution for the LHD, while a good agreement is observed when adding the coil current effect to the magnetic field. The magnetic field line tracing using the reconstructed field indicates the plasma boundary (the outer surface of the stochastic region) precisely and the last closed magnetic surface agrees fairly well with the reference one.