Ten years of W programme in ASDEX Upgrade—challenges and conclusions

Abstract

Abstract. Since 1999 ASDEX Upgrade increased its tungsten plasma facing components and finally reached a full W coverage in 2007. Most of the initial goals of the investigations were successfully achieved. A highlight of the investigations were multiple start-ups and operation without any boronisation demonstrating that performance and confinement similar to boronised operation with carbon PFCs can be reached in high power, high density discharges. This also allowed the investigation of the hydrogen retention without disturbing effects from the low-Z coating. A strong reduction of hydrogen retention was found in gas balance measurements as well as in post mortem analyses. On the other hand, an almost complete suppression of low-Z divertor radiation was achieved after boronisation, providing valuable information on the control requirements of radiative cooling by artificially introduced impurities. Among the challenges remains the strong increase of the W source and W concentration resulting from ICRH. At the same time it helped to identify the underlying physics and may lead to solutions superior to the presently used ones.

Optimized tomography methods for plasma emissivity reconstruction at the ASDEX Upgrade tokamak

“…This condition together with the minimization of the regularization functional leads to a constrained quadratic optimization problem, easily solvable by an iterative root-solver. A unique solution satisfying the condition (14) exists if…”

“…AUG is a mid-sized tokamak with R = 1.65 m, a = 0.5 m, I p ≤ 1.4 MA, B t ≤ 3.1 T, and tungsten covered PFCs 14 . The metal wall and the high heating power provided by 8 neutral beam injection (NBI) sources delivering up to 20 MW, 6 MW of ion cyclotron heating, and 4 MW of electron cyclotron heating make this tokamak well suited for power exhaust studies relevant for future fusion reactors.…”

Characterization of the fluctuating detachment state in ASDEX Upgrade

“…To study divertor detachment, a series of ohmic and L-mode density ramp discharges has been performed at ASDEX Upgrade, AUG, whose plasma facing surfaces are since 2007 completely covered with tungsten [6]. All discharges were in lower single null divertor configuration with a lower triangularity of δ = 0.36, a plasma current of I p = 1 MA, a toroidal magnetic field of B t = 2.5 T and a safety factor of q 95 ≈ 4.…”

Characterization of dust collected after plasma operation of all-tungsten ASDEX Upgrade

“…The short discussion concerning possible dust production sites and relevant processes of dust formation is presented below. Since 2007 AUG operates as a full tungsten machine [7][8][9]. In 2009 campaign two kinds of tungsten coatings were present: (i) physical vapour deposited (PVD) coatings with thickness up to 4 µm in the main chamber, the inner strike line module and the upper divertor and (ii) a 10 µm CMSII tungsten coating (combined magnetron sputtering and ion implantation technique) on 3 µm Mo interlayer [10] in the outer divertor.…”