Impurity behaviour in the ASDEX Upgrade divertor tokamak with large area tungsten walls

Abstract

At the central column of ASDEX Upgrade, an area of 5.5 m 2 of graphite tiles was replaced by tungsten-coated tiles representing about two-thirds of the total area of the central column. No negative influence on the plasma performance was found, except for internal transport barrier limiter discharges. The tungsten influx W stayed below the detection limit only during direct plasma wall contact or for reduced clearance in divertor discharges spectroscopic evidence for W could be found. From these observations a penetration factor of the order of 1% and effective sputtering yields of about 10 −3 could be derived, pointing to a strong contribution by light intrinsic impurities to the total W-sputtering. The tungsten concentrations ranged from below 10 −6 up to a few times 10 −5 . Generally, in discharges with increased density peaking, a tendency for increased central tungsten concentrations or even accumulation was observed. Central heating (mostly) by ECRH led to a strong reduction of the central impurity content, accompanied by a very benign reduction of the energy confinement. The observations suggest that the W-source strength plays only an inferior role for the central W-content compared to the transport, since in the discharges with increased W-concentration neither an increase in the W-influx nor a change in the edge parameters was observed. In contrast, there is strong experimental evidence, that the central impurity concentration can be controlled externally by central heating.

Recent H majority inverted radio frequency heating scheme experiments in JET-ILW

“…When JET was equipped with a C wall, D minority heating experiments were unsuccesful since C -equally sharing the deuteron's Z/A -eroded from the wall contaminated the main plasma and increased the D-like ion population concentration well beyond the limit tolerable for promising minority heating [1]. The C being replaced by Be in the ITER-like wall [13], this problem no longer poses and hence the scheme was readdressed when the D concentration was very low (X[D]~0.7%); X[Be] was estimated to be of order 0.5%. Reaching the D minority heating regime at B o =3.33T requires lowering the driver frequency to 25.3MHz, beyond the reach of the ILA.…”

Chapter 4: Power and particle control

“…This technique has been demonstrated in TEXTOR [370], Alcator C-Mod for discharges with ITBs [151] and studied in detail in ASDEX Upgrade with ICRH and ECRH [371]. The reduction of the central concentration of W can be explained by the changes caused by the central heating in the plasma particle transport [371,372].…”

Tungsten impurity transport experiments in Alcator C-Mod to address high priority research and development for ITERa)

“…Uncontrolled increases of the core W concentration (for quasi-stationary values of the edge W concentration) are frequently observed for H-mode discharges in tokamaks operating with W PFCs whose plasma heating is dominated by neutral beam injection (NBI) [Neu 2002]. The physics picture recently demonstrated to explain this phenomenology in H-mode plasmas is that W transport is dominated by anomalous transport for most of the plasma cross section inside the ETB except the very central region close to the magnetic axis [Pütterich 2013, Angioni 2014a.…”

“…An important factor in this behaviour is the fact that NBI heating unavoidably provides a source of neutrals in the central plasma region thus leading to a sizeable peaking of the ion density profile there ]. This relation between the ion density and temperature gradients in the central region of the plasma for NBI dominated plasmas can be modified by the application of central RF heating (ECRH and ICRH) thus avoiding an uncontrolled increase of the core W density [Neu 2002, Goniche 2014]. This picture is complicated by the role that these additional heating systems play in sustaining poloidal variations in high-Z impurity density, which is shown to have an important impact on the flux-surface averaged neoclassical radial impurity transport [Angioni 2014b].…”