Experiences With Tungsten Plasma Facing Components in ASDEX Upgrade and JET

Abstract

Abstract-ASDEX Upgrade (AUG) has been converted to all W plasma facing components (PFCs) in 2007 and JET has implemented the ITER like wall (ILW) project (2011) using the same PFC configuration as ITER during its active phase, namely Be in the main chamber and tungsten in the divertor. As a result of the all metal PFCs in both devices much less surface conditioning is needed to arrive at reproducible wall conditions. Specifically the Be PFCs of JET led to a very small low-Z content (reduction of C and O by at least a factor of 10), reducing the edge radiation in steady state operation as well as during disruptions. Both devices successfully employ massive gas injection to mitigate disruption forces and power loads to PFCs by radiating up to 100% of the available energy. Hydrogen retention is strongly reduced (AUG: factor 5, JET: factor 10) and the remaining retention is still dominated by co-deposition with residual C in AUG and intrinsic Be in JET. The very low edge and divertor radiation could be compensated by impurity seeding either by a single gas species (N2) (AUG and JET) or by combining N2 and Ar (AUG) injection for divertor and main chamber radiation, respectively. The W sputtering in the divertor increases when seeding small amounts of N2, but decreases for higher fluxes due to the plasma cooling provided by the nitrogen radiation. The tungsten content is controlled by the source as well as by its edge and central transport. It could be kept sufficiently small by using a minimum gas fuelling to reduce the W erosion and to diminish the W penetration. The control of the central W transport by central (wave) heating had been well established in AUG, however in both devices the W content is increased during ICRH operation most probably due to increased W sputtering caused by rectified sheaths. The H-Mode threshold is reduced by 20-30% in AUG and JET, but on average the confinement is lower in JET-ILW than with C PFCs. To date it is not yet clear, whether the reduced H-Mode confinement has to be attributed to the use of W PFCs, since such a clear trend as in JET was not found in AUG.

20 years of research on the Alcator C-Mod tokamak

“…All of these are ITER-typical and different from conditions behind most of the data in the confinement database. Recent results from AUG and JET find a drop in energy confinement under similar conditions, 44 suggesting that ITER98 may overestimate the results that will be obtained on ITER.…”

“…Operational issues with tungsten plasma facing components are now also under intensive study by the AUG and JET devices. 44 To keep the surface temperature of divertor plates within acceptable limits in a reactor, finite heat conduction dictates that no more than a few mm of material can intervene in front of the cooling channels. Thus, net erosion must be kept on the order of 1 mm over the lifetime of the first wall.…”

Mass spectrometry analysis of the impurity content in N2 seeded discharges in JET-ILW

“…W sputtering which determines the lifetime of the divertor components in the Be/W mix is dominated by intrinsic impurities such as Be, O and C, and extrinsic impurities like N 2 and Ar [2], though the C and Be content are orders of magnitude lower in quantity in comparison with the carbon wall [3]. As the JET-ILW does not include carbon-based PFCs, nitrogen is being seeded to replace carbon as the radiating species in the plasma edge and mitigate heat loads at the W target plates [4].…”

Impurity Seeding in ASDEX Upgrade Tokamak Modeled by COREDIV Code

“…Its value increases when the seeding gas flux is started. Improvement of the confinement due to impurity seeding is a known fact [10]. As in COREDIV the H 98 is a directly controlled, fixed www.cpp-journal.org input parameter at first we checked how does it influence the radiation fraction, f rad , tungsten concentration, c W , and electron temperature on the divertor plate, T plate e .…”