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

Abstract

In this paper, vacuum flux surface mapping is used to measure the 1/1 error field in Wendelstein 7-X. Two methods for discriminating the 1/1 perturbation from other co-resonant modes are applied: measurements of the helical axis shift in a magnetic configuration with the rotational transform barely above one on the axis; and measurements of the edge island structure with cancellation of intrinsic 5/5 islands by divertor control coils. The normalized radial 1/1 amplitude in Boozer coordinates is found to be about 0.8 • 10 −4 in the first case and about 0.5 • 10 −4 in the second one, with the discrepancy being attributable to different currents in the planar coils. The 1/1 error field can be reliably compensated by using error field trim coils with currents about 100 A. This compensation also helps to identify the 2/2 error field of about 0.6 • 10 −4. During plasma operation in divertor configurations, it is confirmed that the present 1/1 field leads to asymmetries in the power distribution and that the 1/1 compensation improves this symmetry.

Coherence imaging spectroscopy at Wendelstein 7-X for impurity flow measurements

“…The selected plasma discharges are in the standard magnetic configuration, with similar parameters (line integrated density of 7 • 10 19 m −2 , input power of 4 MW of ECRH, electron temperature in the core of ∼3 keV), without (experiment #20181010.17) and with (experiment #20181010.18) control coils, which are components designed to induce changes of the island structure. In the latter program, a positive current of 2 kA has been applied to all 10 control coils which results in an increase of the radial component of the magnetic field, increasing the island size while decreasing the connection lengths to the divertors [33,40]. The modification of the SOL geometry translates in an effect on the flows, without modifying the number of islands crossed by the CIS lines of sight.…”

Measurement and modeling of magnetic configurations to mimic overload scenarios in the W7-X stellarator

“…The good correspondence between the measured heat fluxes and the Poincare plots indicate that the W7-X magnetic field is reasonably close to the ideal (no error field) stellarator symmetric magnetic field topology [15,16]. This correspondence can be further explored by comparing the fluxes to a simple field line diffusion model as implemented in the DIV3D code [1], which was used to design the scraper element and is similar to methods used to design the other W7-X divertor components.…”

Tuning of the rotational transform in Wendelstein 7-X

“…Thus the influence of error fields on these measurements could be substantial. In particular the presence of both n/m = 1/1 and n/m = 2/2 magnetic error fields, and their effect on the island divertor have been well documented [15,7]. [20]) associated with outgassing of the divertor but did not appear to affect heat fluxes to the divertor.…”

“…field was also demonstrated to be correctible using less than 10% of the rated maximum current capacity of the trim coils [7,14,15]. However, during divertor operation an anomalous interaction between the plasma and carbon tiled wall elements suggested that the edge island chain was not optimally located.…”