Deuterium trapping and surface modification of polycrystalline tungsten exposed to a high-flux plasma at high fluences

Abstract

Deuterium (D) retention and surface modifications of hot-rolled polycrystalline tungsten (W) exposed to a low-energy (40 eV/D), high-flux (2-5×10 23 D/m 2 s) D plasma at temperatures of 380 K and 1140 K to fluences up to 1.2×10 28 D/m 2 have been examined by using nuclear reaction analysis, thermal desorption spectroscopy, and scanning electron microscopy. The samples exposed at 380 K exhibited various types of surface modifications: dome-shaped blister-like structures, stepped flat-topped protrusions, and various types of nanostructures. It was observed that a large fraction of the surface was covered with blisters and protrusions, but their average size and the number density showed almost no fluence dependence. The D depth distributions and total D inventories also barely changed with increasing fluence at 380 K. A substantial amount of D was retained in the subsurface region, which thickness correlated with the depth where the cavities of blisters and protrusions were located. It is therefore suggested that defects appearing during creation of blisters and protrusions govern the D trapping in the investigated fluence range. In addition, a large number of small cracks was observed on the exposed surfaces, which can serve as fast D release channels towards the surface, resulting in a reduction of the effective D influx into the W bulk. On the samples exposed at 1140 K no blisters and protrusions were found.However, wave-like and faceted terrace-like structures were formed instead. The concentrations of trapped D were very low (<10 5 at. fr.) after the exposure at 1140 K.

Physics of ultimate detachment of a tokamak divertor plasma

“…One of the most complete sets of the fluid equations for classical transport of a multicomponent plasma relevant for the conditions in the edge of the fusion devices can be found in Zhdanov (2002). These 3-D equations, derivation of which is based on the Grad approach (Grad 1949), can potentially describe both the edge plasma turbulence and transport of both the background plasma and impurities.…”

“…Zweben et al (2007) and the references therein). Moreover, studies of double-null magnetic configurations demonstrate much stronger plasma transport on the outer side of the torus in comparison to the inner one (Counsell et al 2002;LaBombard et al 2004).…”

“…Takamura et al 2006;Kajita et al 2009;Zibrov et al 2017) resulting in formation of the blisters (figure 15, dust, 'fuzz', pinholes, protrusions, etc. (see e.g.…”

“…The most complete set of fluid equations for fully ionized, multi-component, magnetized plasma can be found in Zhdanov (2002). It was derived from the kinetic equations for the plasma components by using the 21-moment Grad approximation.…”

“…Umansky et al 1998;LaBombard et al 2000;Greenwald 2002;Rudakov et al 2005;Wiesen et al 2011. Usually it is related to the increased contribution of intermittent blobby plasma transport (see Zweben et al 2007;Krasheninnikov et al 2008;D'Ippolito et al 2011, and the references therein). However, recent experimental data (Sun et al 2015) suggest that even in H-mode, ∆ SOL increases during evolution to detachment.…”