Error field measurement, correction and heat flux balancing on Wendelstein 7-X ^a

Abstract

Abstract:The measurement and correction of error fields in Wendelstein 7-X (W7-X) is critical to long pulse high beta operation, as small error fields may cause overloading of divertor plates. Accordingly, as part of a broad collaborative effort, the detection and correction of error fields on the W7-X experiment has been performed using the trim coil system in conjunction with the flux surface mapping diagnostic and high resolution infrared camera. In the early commissioning phase of the experiment, the trim coils were used to open an n/m=1/2 island chain in a specially designed magnetic configuration. The flux surfacing mapping diagnostic was then able to directly image the magnetic topology of the experiment, allowing the inference of a small ∼ 4 cm intrinsic island chain. The suspected main source of the error field, slight misalignment of the superconducting coils, is then confirmed through experimental modeling using the detailed measurements of the coil positions. Observations of the limiters temperatures in module 5 shows a clear dependence of the limiter heat flux pattern as the perturbing fields are rotated. Plasma experiments without applied correcting fields show a significant asymmetry in neutral pressure (centered in module 4) and light emission (visible, H-alpha, CII, and CIII). Such pressure asymmetry is associated with plasma-wall (limiter) interaction asymmetries between the modules. Application of trim coil fields with n=1 waveform correct the imbalance. Confirmation of the error fields allows the assessment of magnetic fields which resonate with the n/m=5/5 island chain.

Error fields in the Wendelstein 7-X stellarator

“…The differences arise from the large currents in the planar coils for the 'high-iota' configuration, as compared to the 'standard' and 'narrow mirror' configurations. Similar experiments performed with the limiter plasma suggested a slightly different phase for limiter heat flux balancing [32,33]. In this configuration the planar coils were energized with opposite sign and half the amplitude of the 'high-iota' configuration.…”

Measurements and correction of the 1/1 error field in Wendelstein 7-X

“…Error field components relevant for the divertor operation were not addressed, because of a minor effect on the limiter operation and due to restrictions for the operation of the magnet system at that time. A non-resonant modification of the heat flux to the limiters was studied in plasma experiments with a strong 1/1 field intentionally superimposed with trim coils [24,25].…”

Tuning of the rotational transform in Wendelstein 7-X

“…Figure13. Neutral compression as measured by neutral gas manometers at the outboard (AEE) and divertor ports (AEI).…”