Overview of the emissivity measurements performed in WEST: in situ and post-mortem observations

Abstract

This paper summarizes the emissivity measurements performed on the plasma facing units (PFU) of the WEST lower divertor during the first phase of WEST running with a mix of actively cooled ITER-like PFUs made of bulk tungsten and inertially cooled PFUs made of graphite with a coating of tungsten. In-situ assessment of the emissivity and laboratory measurements after removing the W-coated graphite and ITER-grade PFUs from the WEST device are shown. The measurements exhibit a complex pattern with strong emissivity variation as function of space and time mainly explained with the variation of magnetic equilibrium (strike point location) as well as the plasma performances during the experimental campaigns. The exposed ITER-grade PFU exhibits sharp spatial variation of the emissivity from 0.05 to 0.85 at a monoblock scale (12mm) at the transition of the erosion (strike point location) and deposition (next to the strike point location) areas on the high field side. In the low field side, the emissivity varies from 0.12, at the strike point location, to 0.2 few cm away in the low field side direction. This emissivity range after exposure is much higher than the emissivity variation of unexposed PFU with emissivity from 0.09 to 0.15. In-situ observation performed on the W-coated graphite PFU shows a rapid evolution, typically few pulses, of the emissivity in the inner and outer strike point location. The whole spatial distribution is discussed as well as his variation due to the plasma operation from the start-up of WEST to the removal of the tungsten (W) coated graphite components.

Surface temperature measurement from infrared synthetic diagnostic in preparation for ITER operations

“…Figure 1 shows an estimate of the total error on the measured temperature including the errors coming from the measurement chain (transmission, parasitic light) and coming from the observed thermal scene (emissivity, reflected flux) for the equatorial wide-angle viewing systems. We consider an uncertainty on the emissivity value ∆ε of 0.15 for a tungsten target around of 0.2 emissivity, which is in agreement with laboratory and tokamak measurements [2]. The total transmission of optical system is fixed to 10% with an error ∆τ of 50%, including the calibration uncertainties and the flux losses due to spatial resolution [3].…”

“…WEST experience has shown that the emissivity of metallic targets changes after plasma, mainly due to the erosiondeposition phenomenon that occurs during the reactor lifetime [2]. A challenge for getting more reliable IR measurements and safe operation is to be able to follow emissivity change during the experimental campaign.…”

Divertor heat load estimates on NSTX and DIII-D using new and open-source 2D inversion analysis code

“…The behavior of both emission and reflection can vary significantly with surface roughness, the machining process, and as the plasma operation progresses [17]. Researchers at WEST reported spatial gradients in the emissivity of exposed ITER-grade plasma-facing units that ranged from 0.05-0.85 [18]. However, this level of emissivity variation is not as severe in the case of devices like NSTX-U and DIII-D where the PFC armor is made of graphite.…”

Ex Situ LIBS Analysis of WEST Divertor Wall Tiles after C3 Campaign

“…The WEST tokamak has superconducting magnets and actively cooled metallic PFCs which operate with steady-state long pulses and high particle fluence; its aim is to test the W divertor technology in an integrated tokamak environment [22][23][24]. A thorough testing of the changes in the surface morphology and composition of PFCs has been carried out by various methods [25][26][27]. LIBS is one method planned for the testing of elemental composition of selected divertor samples and the fuel retention in them.…”