Experimental and numerical study of error fields in the CNT stellarator

TL;DR

This study combines experimental measurements and numerical modeling to identify and correct coil positioning errors in the CNT stellarator, leading to improved flux surface agreement.

Contribution

It introduces an iterative Newton-Raphson method to deduce coil displacements and tilts from flux surface discrepancies in a stellarator.

Findings

Improved agreement between measured and computed flux surfaces after correction

Validated the numerical method with simulated coil misplacements

Enhanced understanding of coil error impacts on stellarator magnetic fields

Abstract

Sources of error fields were indirectly inferred in a stellarator by reconciling computed and numerical flux surfaces. Sources considered so far include the displacements and tilts (but not the deformations, yet) of the four circular coils featured in the simple CNT stellarator. The flux surfaces were measured by means of an electron beam and phosphor rod, and were computed by means of a Biot-Savart field-line tracing code. If the ideal coil locations and orientations are used in the computation, agreement with measurements is poor. Discrepancies are ascribed to errors in the positioning and orientation of the in-vessel interlocked coils. To that end, an iterative numerical method was developed. A Newton-Raphson algorithm searches for the coils' displacements and tilts that minimize the discrepancy between the measured and computed flux surfaces. This method was verified by misplacing and tilting the coils in a numerical model of CNT, calculating the flux surfaces that they generated, and testing the algorithm's ability to deduce the coils' displacements and tilts. Subsequently, the numerical method was applied to the experimental data, arriving at a set of coil displacements whose resulting field errors exhibited significantly improved quantitative and qualitative agreement with experimental results.