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An alternative solution for tungsten as a plasma facing material could be the use of W heavy alloys as they are produced commercially by several companies. They consist of up to 97% W and Ni/Fe (or Ni/Cu) admixtures, they are readily machinable and considerably cheaper than bulk tungsten. Their major drawbacks in view of the application in fusion experiments are the rather low melting temperature and their magnetic properties (in case of a Ni/Fe binder phase). In a first step W heavy alloys from two manufacturers were investigated concerning their thermal and magnetic properties and subjected to screening tests and cyclic loading in the high heat flux test facility GLADIS with up to 20 MW m −2 and surface temperatures of up to 2200 °C, showing no macroscopic failure. SEM investigations show a segregation of Ni and Fe at the top surface after the thermal overloading, but no signs of micro-cracking. The long-term behaviour of a W–Ni/Fe tile under plasma and electromagnetic load was investigated in ASDEX Upgrade using its divertor manipulator. The tile was exposed in discharges with record values of injected energy and power. Despite the observed surface modifications (Ni/Fe segregation) the W heavy alloys seem to provide a pragmatic and cost-effective alternative to bulk W tiles in the divertor of ASDEX Upgrade. [ABSTRACT FROM AUTHOR]

Abstract: Divertor plasma-facing components of future fusion reactors should be able to withstand heat fluxes of 10–20MW/m2 in stationary operation. Tungsten blocks with an inner cooling tube made of CuCr1Zr, so-called monoblocks, are potential candidates for such water-cooled components. To increase the strength and reliability of the interface between the W and the cooling tube of a Cu-based alloy (CuCr1Zr), a novel advanced W-fibre/Cu metal matrix composite (MMC) was developed for operation temperatures up to 550°C. Based on optimization results to enhance the adhesion between fibre and matrix, W fibres (W f) were chemically etched, coated by physical vapour deposition with a continuously graded W/CuPVD interlayer and then heated to 800°C. The Wf/Cu MMC was implemented by hot-isostatic pressing and brazing process in monoblock mock-ups reinforcing the interface between the plasma-facing material and the cooling channel. The suitability of the MMC as an efficient heat sink interface for water-cooled divertor components was tested in the high heat flux (HHF) facility GLADIS. Predictions from finite element simulations of the thermal behaviour of the component under loading conditions were confirmed by the HHF tests. The Wf/Cu MMC interlayer of the mock-ups survived cyclic heat loads above 10MW/m2 without any damage. One W block of each tested mock-up showed stable thermal behaviour at heat fluxes of up to 10.5MW/m2. [Copyright &y& Elsevier]

A massive tungsten divertor, Div-III, was installed in ASDEX Upgrade (AUG) in 2014. Div-III is an adiabatically loaded component and consists of massive tungsten tiles clamped into their supporting structures. Before installing the new component, extensive studies, including Finite Element Modeling (FEM) and high heat flux tests in the test facility GLADIS, were carried out. After the first experimental campaigns the tile inspection reveals most of the tiles were cracked. The difference between the high heat flux tests and the AUG behavior was attributed to mechanical loads due to disruptions and/or the thermal load profile and history. The actions to understand the cracks comprise tests with the divertor manipulator, DIM-II, and FEM analysis of different target design options. DIM-II was used to test 'split' tiles, i.e. the deep crack is avoided by cutting a wide tile into two small tiles. FEM calculations were done to investigate the behavior of castellated targets with reduced tensile stress on top of the target and a clamping with a more elastic material, titanium instead of stainless steel. In addition more ductile tungsten heavy alloy was qualified for use in AUG. Based on this, a new set-up of tungsten tiles was installed in 2016 with different divertor configurations. After about 1000 plasma shots a thorough inspection of the tiles reveals that only 2 configuration out of 4 get rid of deep cracks in the tungsten tiles. Outcome of this inspection are here discussed and the final divertor setup is described. [ABSTRACT FROM AUTHOR]

Abstract To clarify the suitability of solid tungsten as plasma facing material, a new divertor, DIV-III, entered in operation in ASDEX Upgrade (AUG) in 2014. Since the first experimental campaign, AUG divertor tiles faced crack issues. Trying to identify the reasoning, a new set of tiles was installed in 2016: many precautions were taken to reduce the stresses possibly causing the cracks. The AUG campaign 2017 started in March and the first results were collected during the short summer opening in 2017. This paper reports on the modification of the tile setup, based on Finite Element Modelling, and on the progress achieved in the first operation phase. [ABSTRACT FROM AUTHOR]

Deuterium retention in two reduced-activation ferritic/martensitic (RAFM) steels, F82H (8Cr-2W) and EUROFER (9Cr-1W), was measured by nuclear reaction analysis and thermal desorption spectrometry after exposure to low-energy deuterium plasmas. In the RAFM steels, the deuterium retention was 10 17 –10 19 D/m 2 and the major fraction of deuterium was retained in the near-surface layer. The surface concentrations of tungsten increased with the D ion fluence and energy due to the selective sputtering of iron and chromium. Scanning electron microscopy images clearly indicated that the surface morphology changed with increasing D fluence (i.e. the grain boundaries became more visible because of erosion by ion bombardment). At a low fluence of about 10 23 D/m 2 , deuterium retention was affected by surface modification in RAFMs. [ABSTRACT FROM AUTHOR]

Abstract: Metallic mirrors are foreseen in ITER diagnostic systems as optical elements directly viewing the plasma radiation. In the frame of an EFDA contract, metallic mirror samples have been exposed for long pulse plasma discharges in Tore Supra (TS) in order to investigate surface modifications caused by erosion and re-deposition processes. Three different materials have been selected: mono-crystalline molybdenum (mc-Mo), polycrystalline stainless steel (SS) and copper (Cu). The mc-Mo samples showed after TS exposure almost no surface roughness modifications and the lowest net-erosion. A slight reflectivity reduction, most pronounced in the near UV, is attributed to light absorption in a thin carbon deposit. Cu mirrors showed by far the highest surface roughness, erosion and diffusive reflectivity. Comparative laboratory glow discharge experiments with virgin reference samples and numerical simulations of erosion/deposition confirm the dominant contribution of conditioning procedures to erosion of mirrors exposed (without shutter protection) in Tore Supra. [Copyright &y& Elsevier]

ASDEX Upgrade is a midsize tokamak operated since 1991. A scheduled maintenance and enhancement shut down is done about once a year. After pump down and leak search a baking procedure is started to condition the vacuum vessel for plasma operation. During operation the vacuum vessel and all in-vessel components are water cooled. This cooling system is also used for baking with hot water at 150 °C. During one of these regular baking periods a hot steam leak appears in November 2017. The detection of the leak has triggered a controlled cool down scenario. Nevertheless, between the appearance of the leak and the identification and closing about 100 L water escaped during the controlled cool down from 150 °C to 65 °C into the torus. A fraction of the hot water was distributed as steam all over the torus and condensed at the coldest parts of the vessel, preferentially in remote ports. In addition, the steam interacts with boron containing layers. The resulting acid steam caused additional erosion. The paper presents details of damages, actions taken to remove them as well as measures taken to avoid a future steam leak. [ABSTRACT FROM AUTHOR]