Tungsten erosion in the outer divertor of JET

Abstract

Erosion of tungsten in the outer JET divertor was determined with a set of tungsten coated divertor tiles during the 2001-2004 discharge campaign. The tungsten marker was strongly eroded, with the largest erosion at the outer strike point position, where more than 75% of the initial W disappeared. Strong erosion is also observed at the outer baffle and horizontal apron of tile 8, where about half of the tungsten has been removed. These numbers are lower boundaries, because the W was locally completely eroded. The tungsten erosion is inhomogeneous on a microscopic level and depends on the micro-topography of the rough surface: large erosion with complete disappearance of the W layer is observed on plasma-facing areas of microscopic ridges, while a smaller erosion and sometimes even deposition of carbon is found on the far side of ridges and in pores.

Impact of gyro-motion and sheath acceleration on the flux distribution on rough surfaces

“…The erosion pattern visible in Fig 1 is very inhomogeneous: On the leading surface areas the W layer is complete eroded exposing the underlying C substrate, while on the shadowed surface areas the W layer is still intact. Such an inhomogeneous erosion pattern was already previously observed at the outer divertor of AUG [2], and was also observed in the outer JET divertor [3]. There is a slight tilt angle ∕ = 90 ∘ between the rows of leading surfaces and the magnetic field direction which already hints towards the influence of E×B forces on the particle trajectories responsible for eroding the W at these leading surfaces.…”

Tungsten erosion and redeposition in the all-tungsten divertor of ASDEX Upgrade

“…This indicates a very inhomogeneous erosion of the W marker, with a maximum erosion at plasma exposed areas which is more than 5 times larger than the mean erosion of the layer. Such an inhomogeneous erosion was already observed at tungsten marker layers in the outer divertor of JET [23], and in previous campaigns with marker layers in the outer divertor of ASDEX Upgrade [3]. This inhomogeneous erosion is difficult to understand, because the Larmor radius of both impurity and deuterium ions is larger than 100 µm: This is much larger than the surface roughness of a few µm, so that for the gyrating ions the surface should appear almost smooth.…”

“…8 for W marker stripes from the outer strike point tile of JET [23] and ASDEX Upgrade. The ASDEX Upgrade data points were obtained at different poloidal positions of tile 1.…”

Blistering and re-deposition on tungsten exposed to ASDEX Upgrade divertor plasma

“…The effect of net deposition or erosion depends sensitively on the flux ratios [14]. Roughness of the surface can lead to local net deposition even in erosion-dominated areas, e.g., close to the strike point in the outer divertor [15,16]. Recently a model was developed that calculates the element-resolved incident flux distribution for a pre-defined surface roughness in order to describe the observed erosiondeposition pattern [17].…”