The tungsten divertor experiment at ASDEX Upgrade

Abstract

Tungsten-coated tiles, manufactured by plasma spray on graphite, were mounted in the divertor of the ASDEX Upgrade tokamak and cover almost 90% of the surface facing the plasma in the strike zone. Over 600 plasma discharges have been performed to date, around 300 of which were auxiliary heated with heating powers up to 10 MW. The production of tungsten in the divertor was monitored by a W I line at 400.8 nm. In the plasma centre an array of spectral lines at 5 nm emitted by ionization states around W XXX was measured. From the intensity of these lines the W content was derived. Under normal discharge conditions W-concentrations around 10 −5 or even lower were found. The influence on the main plasma parameters was found to be negligible. The maximum concentrations observed decrease with increasing heating power. In several low power discharges accumulation of tungsten occurred and the temperature profile was flattened. The concentrations of the intrinsic impurities carbon and oxygen were comparable to the discharges with the graphite divertor. Furthermore, the density and the β limits remained unchanged and no negative influence on the energy confinement or on the Hmode threshold was found. Discharges with neon radiative cooling showed the same behaviour as in the graphite divertor case.

Electron Impact Excitation and Dielectronic Recombination of Highly Charged Tungsten Ions

“…Owing to its physical properties of high melting temperature, excellent thermal conductivity, low sputtering rate and low affinity for tritium [1,2], tungsten has been used as a plasma-facing material in several magnetic confinement fusion facilities [3][4][5][6] and is planned to be used in the divertor region having the highest heat load in the International Thermonuclear Experimental Reactor (ITER) tokamak [7]. However, inevitably, tungsten appears as an intrinsic impurity in fusion plasmas, which results in some severe problems due to its high radiation efficiency.…”

“…The computations of DR rate coefficients from the ADAS are based on the generalized collisional-radiative framework [53] and the semiempirical Burgess formulas [54]. Although many experiments on spectral emissions from tungsten have been carried out, especially at the ASDEX Upgrade tokamak [2][3][4]7,17,51], the first measurement on DR rate coefficients of tungsten ions was performed just recently for W 20+ using the technique of the electron-ion merged beam at the heavy ion storage ring [39]. A big discrepancy with a factor of four was found between the measured and calculated rate coefficients from the ADAS database, which was attributed to the neglect of DR-associated fine-structure excitations of W 20+ in the calculations [39].…”

Experiences With Tungsten Plasma Facing Components in ASDEX Upgrade and JET

“…In 1996, ASDEX Upgrade equipped the divertor strike point area with W coated tiles, demonstrating that the use of tungsten is feasible in a divertor tokamak [3]. At the same time it became clear that not only the divertor PFCs are a significant erosion source but also the main chamber carbon sources contribute significantly to the impurity content in the plasma and lead to C deposits in deposition dominated zones [5].…”

“…It could be shown early in the AUG W programme that central deposition of heating power is very beneficial in reducing the W-peaking [65], [66]. This heating can be provided either by ICRH or ECRH and even the deposition profile of beams using different acceleration voltage [3] or injection geometry plays a role. The amount of necessary local heating depends delicately on the local W concentration itself which was tested in similar accumulating discharges with different timing of an ECRH heating pulse [47].…”

Development of W coatings for fusion applications

“…A complete divertor with W-VPS coatings of 500 m was installed in ASDEX Upgrade during 1995/1996 [5]. After 800 discharges with power densities on the tile surface of 7-9 MW/m 2 cracks have been detected on almost all tiles (approx.…”