This presentation was made at the 2019 NAFEMS World Congress in Quebec Canada

Resource Abstract

Tungsten (W) has been chosen as the main candidate for plasma facing material in the next step plasma fusion devices. In general, solid W is used for shielding the plasma chamber interior against the high heat generated from the extremely hot plasma. The most loaded structural parts inside the plasma chamber are the so-called divertor plates composed of individual components.



The ASDEX (Axial Symmetric Divertor EXperiment) Upgrade or shortly AUG (ASDEX UpGrade), a plasma fusion experimental device has been gradually evolved from a graphite dominated experiment to W as plasma facing material starting in 1999. 2007 AUG was started as full W experiment. In 2010 a project was started to replace the W coated graphite plates in the lower outer divertor by solid W plates. The full-scale prototypes, with the dimension of 250x80x15 mm3, have been intensively tested in the high heat flux test facility GLADIS (Garching LArge DIvertor Sample). The applied GLADIS heat loading profiles are Gaussian with a central heat flux of 10 - 30 MW/m2, resulting in an integrated absorbed power of the W tile between 100 and 280 kW. Thus simulating the expected highest power and energy loads during adiabatically loading in AUG. The corresponding measured surface temperatures reached values between 1500 °C and 3300 °C.



During the cyclic loading in the GLADIS facility, no crack initiation at the W plates has been detected. However, after one campaign of AUG operation (about 1200 plasma shots) almost all W divertor plates exhibited cracks. The inspection of the plasma exposed plates has revealed 126 plates with deep cracks going through the target. Nearly all of the 128 plates did show shallow cracks in the high heat load region. Nevertheless, none of these divertor tile damages have caused an operational interruption of AUG. A comprehensive investigation of the damages has been performed to find out the origin of the crack initiations.



This paper is presenting a bundle of numerical simulations of the AUG solid W divertor plates based on the theoretical hypothesis for failure of brittle materials. Finally, the design optimisation considerations of divertor plates are discussed.