Limiter observations during W7-X first plasmas

Abstract

During the first operational phase (referred to as OP1.1) of the new Wendelstein 7-X (W7-X) stellarator, five poloidal graphite limiters were mounted on the inboard side of the vacuum vessel, one in each of the five toroidal modules which form the W7-X vacuum vessel. Each limiter consisted of nine specially shaped graphite tiles, designed to conform to the last closed field line geometry in the bean-shaped section of the standard OP1.1 magnetic field configuration (Sunn Pedersen et al 2015 Nucl. Fusion 55 126001). We observed the limiters with multiple infrared and visible camera systems, as well as filtered photomultipliers. Power loads are calculated from infrared (IR) temperature measurements using THEODOR, and heating patterns (dual stripes) compare well with field line mapping and EMC3-EIRENE predictions. While the poloidal symmetry of the heat loads was excellent, the toroidal heating pattern showed up to a factor of 2× variation, with peak heat loads on Limiter 1. The total power intercepted by the limiters was up to ~60% of the input ECRH heating power. Calorimetry using bulk tile heating (measured via post-shot IR thermography) on Limiter 3 showed a difference between short high power discharges, and longer lower power ones, with regards to the fraction of energy deposited on the limiters. Finally, fast heating transients, with frequency >1 kHz were detected, and their visibility was enhanced by the presence of surface coatings which developed on the limiters by the end of the campaign.

Reconstruction of recycling flux from synthetic camera images, evaluated for the Wendelstein 7-X startup limiter

“…due to mismatching plasma conditions related to transport assumptions). Analysis of the heat load patterns has shown reasonable agreement with predictions by EMC3-EIRENE [28,29], which suggest that particle loads onto the limiter should exhibit a similar pattern. Of considerable concern are perturbations from impurities which can contaminate the experimental observations because of the red-shifted transmission curve of the filter.…”

“…We apply an idealized coil configuration in order to exploit the n = 5 toroidal symmetry of W7-X, and we neglect any contributions from error fields due to coil misalignments or equilibrium effects (because the heat loads fit the prediction in the ideal configuration quite well [28,29] even though small error fields are present [30,31])). Stellarator symmetry then allows to focus the simulations on a toroidal domain of ∆ϕ = 36 deg.…”

Magnetic configuration effects on the Wendelstein 7-X stellarator

“…For both configurations, the heating power was set to 1.0 MW for one second and lowered to about 0.6 MW for the remaining 5 s. Given the energy limit of 4 MJ, these settings allowed for a total pulse length of 6 s. Power losses by radiation rad and convective heat losses onto the limiters limiter add up to more than 80% of the total heating power ( Fig. 3(a)) [24]. The remaining 20% presumably leave the plasma by charge-exchange processes with neutrals that are released by the plasma-facing components, and may also stem from measurement errors (discussed in the methods section).…”

Confinement in Wendelstein 7-X limiter plasmas

“…Generally, the radiated power fraction increases with integrated discharge time since the last glow discharge, which is indicative of increasing wall pollution [25]. P lim -the total power load to the limiters, is estimated from two IR camera systems [26,27], taking into account power load asymmetries that have been derived from thermocouple observations. For stationary conditions at low ECRH power (P ECRH =0.6 MW) ~60% of the power is deposit on the limiters.…”