Collection strategy, inner morphology, and size distribution of dust particles in ASDEX Upgrade

Abstract

Abstract.The dust collection and analysis strategy in ASDEX Upgrade (AUG) is described. During five consecutive operation campaigns (2007)(2008)(2009)(2010)(2011), Si collectors were installed, which were supported by filtered vacuum sampling and collection with adhesive tapes in 2009. The outer and inner morphology (e.g. shape) and elemental composition of the collected particles were analysed by scanning electron microscopy. The majority of the ~50000 analysed particles on the Si collectors of campaign 2009 contain tungsten -the plasma-facing material in AUG -and show basically two different types of outer appearance: spheroids and irregularly shaped particles. By far most of the W-dominated spheroids consist of a solid W core, i.e., solidified W droplets. A part of these particles is coated with low Z material; a process which seems to happen presumably in the far scrape off layer plasma. In addition, some conglomerates of B, C, and W appear as spherical particles after their contact with plasma. By far most of the particles classified as B-, C-, and W-dominated irregularly shaped particles consist of the same conglomerate with varying fraction of embedded W in the B-C matrix and some porosity, which can exceed 50%. The fragile structures of many conglomerates confirm the absence of intensive plasma contact. Both the ablation and mobilization of conglomerate material and the production of W droplets are proposed to be triggered by arcing. The size distribution of each dust particle class is best described by a lognormal distribution allowing an extrapolation of the dust volume and surface area. The maximum in this distribution is observed above the resolution limit of 0.28 µm only for the W-dominated spheroids, at around 1 µm. The amount of W-containing dust is extrapolated to be less than 300 mg on the horizontal areas of AUG.

Transport of tungsten in the H-mode edge transport barrier of ITER

“…The points lie on a straight line as predicted by eq. (17). The slope is 14.6 corresponding to Z n W /Z i =37.7/2.58 and f W is 1 for f ne =2.92.…”

“…An analysis of this dynamical situation for ITER has been performed using transport calculations including impurity transport and will be reported elsewhere [16]. Dust [17] might be another source of plasma contamination by tungsten which is outside the scope of this study.…”

Dust remobilization in fusion plasmas under steady state conditions

“…n e = 10 14 cm −3 , T e = T i = 10 eV, z = 3, v i = √ 2v Ti and E w = 30 kV/cm. We also assume relatively large tungsten dust of 10 micron diameter (several times larger than the median size of the tungsten grains collected in ASDEX Upgrade [40]). Substituting the above parameters in Eqs.…”

Nanosecond laser pulses for mimicking thermal effects on nanostructured tungsten-based materials

“…Due to plasma-wall interactions, W PFCs can be eroded, leading to prompt W redepositing in the divertor region, or to a mixing of W with other plasma impurities and the formation of co-deposits above the first wall [2][3][4]. The co/re-deposited layers are usually characterized by different morphologies, structure, composition and thickness, depending on the type of first wall materials, the specific first wall position and the history of all the operating conditions [3,[5][6][7]. Both pristine PFMs and co/re-deposits are subjected to extreme thermal loads delivered by the edge plasma.…”