Error Field Predictability and Consequences for ITER

TL;DR

This paper reevaluates ITER coil tolerances using modern plasma coupling models, demonstrating that current tolerances are conservative and that correction schemes remain effective even with sub-optimal metrology, ensuring reliable error field management.

Contribution

It introduces an updated, statistically validated model for ITER coil error tolerances and compares model-based and empirical correction schemes under various measurement conditions.

Findings

ITER tolerances are conservative based on linear error field analysis.

Model-based correction schemes remain effective with poor metrology.

Scenario matching currents are crucial for accurate error correction.

Abstract

ITER coil tolerances are re-evaluated using the modern understanding of coupling to least-stable plasma modes and an updated center-line-traced model of ITER's coil windings. This reassessment finds the tolerances to be conservative through a statistical, linear study of $n=1$ error fields (EFs) due to tilted, shifted misplacements and nominal windings of central solenoid and poloidal field coils within tolerance. We also show that a model-based correction scheme remains effective even when metrology quality is sub-optimal, and compare this to projected empirical correction schemes. We begin with an analysis of the necessity of error field correction (EFC) for daily operation in ITER using scaling laws for the EF penetration threshold. We then consider the predictability of EF dominant mode overlap across early planned ITER scenarios and, as measuring EFs in high power scenarios can pose risks to the device, the potential for extrapolation to the ITER Baseline Scenario (IBS). We find that carefully designing a scenario matching currents proportionally to those of the IBS is far more important than plasma shape or profiles in accurately measuring an optimal correction current set.