A high resolution IR/visible imaging system for the W7-X limiter

Abstract

A high-resolution imaging system, consisting of megapixel mid-IR and visible cameras along the same line of sight, has been prepared for the new W7-X stellarator, and was operated during Operational Period 1.1 (OP1.1) to view one of the five inboard graphite limiters. The radial line of sight, through a large diameter (184 mm clear aperture) uncoated sapphire window, couples a direct viewing 1344x784 pixel FLIR SC8303HD camera. A germanium beam-splitter sends visible light to a 1024x1024 pixel Allied Vision Technologies Prosilica GX1050 color camera. Both achieve submillimeter resolution on the 161 mm wide, inertially-cooled, segmented graphite tiles. The IR and visible cameras are controlled via optical fibers over full Camera Link and dual GigE Ethernet (2 Gbit/second data rates) interfaces, respectively. While they are mounted outside the cryostat at a distance of 3.2 meters from the limiter, they are close to a large magnetic trim coil, and require soft iron shielding. We have taken IR data at 125 Hz to 1.25 kHz frame rates, and seen surface temperature increases in excess of 350 °C, especially on leading edges or defect hot spots. The IR camera sees heat-load stripe patterns on the limiter, and has been used to infer limiter power fluxes (~1 to 4.5 MW/m 2 ), during the ECRH heating phase. IR images have also been used calorimetrically between shots to measure equilibrated bulk tile temperature, and hence tile energy inputs (in the range of 10 kJ/tile with 4 MW, 300 millisecond heating pulses). Small UFO's can be seen and tracked by the FLIR camera in some discharges. The calibrated visible color camera (100 Hz frame rate) has also been equipped with narrow band C-III and H-alpha filters, to compare with other diagnostics, and is used for absolute particle flux determination from the limiter surface. Sometimes, but not always, hot-spots in the IR are also seen to be bright in C-III light.

Overview of diagnostic performance and results for the first operation phase in Wendelstein 7-X (invited)

“…21 The limiter was seen with high temporal resolution (∼400 Hz in the IR and ∼100 Hz in the visible) and sub-mm spatial resolution on five of its nine tiles. The filtered visible camera provided absolutely calibrated measurements of carbon and hydrogen light, and therefore inputs to particle balance calculations.…”

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

“…The effect of a perturbing n = 1 EX/P5-5 trim coil field on the limiter temperatures was examined. In figure 1 a stereotypical limiter IR camera image is shown [6,7]. Three lines are drawn across the limiter from which temperature data is extracted.…”

“…The effect of error fields on limiter heat loads were also explored with the trim coils. A high resolution infrared camera [6,7] gave detailed imagery of the limiters during the pulse. Low limiter temperatures limited the range of diagnostic coverage during this first campaign.…”

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

“…W7-AS [4,5], W7-X [6,7], LHD [8,9], HSX [10,11]) and tokamaks (e.g. DIII-D [12,13], NSTX [14,15], TEXTOR-DED [16,17]), yet their interpretation in terms of local quantities can be challenging due to line of sight integration effects.…”

“…One particular application in fusion plasmas is the characterization of particle fluxes and erosion on limiter or divertor targets inferred from observations by a camera with an appropriate filter. At W7-X, such a diagnostic system has been dedicated to the analysis of the startup limiter during the first operation phase (OP1.1) from observation of H α emission (λ = 656.2 nm) [6,7]. These measurements are based on the "S/XB formalism" [18,2] Γ = S X B · Φ (1) which relates the measured photon flux Φ to the particle flux onto the limiter Γ through an appropriate choice of conversion factor S X B .…”