Migration and deposition of¹³C in the full-tungsten ASDEX Upgrade tokamak

Abstract

The migration of carbon in low-density, low-confinement plasmas of ASDEX Upgrade was studied by injecting 13 C into the main chamber of the torus at the end of the 2007 experimental campaign. A selection of standard tungsten-coated lower-divertor and main-chamber tiles as well as a complete set of lower-divertor tiles with an uncoated poloidal marker stripe were removed from one poloidal cross section and analyzed using secondary ion mass spectrometry (SIMS). The poloidal deposition profiles of 13 C on both the tungsten-coated tiles and on the uncoated graphite areas of the marker tiles were measured and compared. For the W-coated lower-divertor tiles, 13 C was deposited mainly on the high-field side tiles, while barely detectable amounts of 13 C were observed on low-field side samples. In contrast, on the uncoated marker stripes the deposition was equally pronounced in the high-field and low-field side divertor. The marker-tile results are in agreement with those obtained from graphite tiles after the 2003 and 2005 13 C experiments in ASDEX Upgrade. In the case of W-coated tiles, the 13 C measurements were complemented by determining the total amount of deposited carbon ( 12 C) on the tiles, which also shows strong deposition at the inner parts of the lower divertor. The estimated deposition of 13 C on W at the divertor areas was less than 1.5% of the injected amount of 13 C atoms. The 13 C analyses of the main-chamber tiles and small silicon samples mounted in remote areas revealed significant deposition in the upper divertor, in upper parts of the heat shield, in the limiter region close to the injection valve, and below the roof baffle. Approximately 8% of the injected 13 C is estimated to have accumulated in these regions. Possible reasons for the different deposition patterns on W and on graphite in different regions of the torus are discussed.

Long-term erosion of plasma-facing materials with different surface roughness in ASDEX Upgrade

“…The analyses were carried out using a VG Ionex IX-70S double-focusing magnetic sector instrument with a 5-keV O 2 + primary ion beam and with a current of 500 nA [6]. In the measurements, the beam was raster-scanned across an area of 0.3×0.4 mm 2 .…”

DIVIMP simulations of 13C puffing experiments in ASDEX Upgrade L-mode plasma

“…Here we concentrate on an injection experiment performed at the the end of the 2007 ASDEX Upgrade [1] experimental campaign, where isotopically marked methane ( 13 CH 4 ) was injected into 11 identical lower single null L-mode discharges #22573 -#22585 [2]. The plasma parameters for the discharges were: toroidal magnetic field B t = -2.5 T, plasma current I p = 0.8 MA with the gradient B drift pointing towards the active divertor, line averaged density n e = 3.3 × 10 19 m −3 , and external heating power P aux = 0.75 MW achieved by electron cyclotron resonance heating.…”

“…As the valve is located approximately 1 m radially away from the plasma surface, the point source of the methane is spread over the port volume. Considering this large source with the large shear in the SOL, one would expect a fairly symmetric toroidal deposition [2]. The injection experiment was done at the end of the campaign in order to limit re-erosion.…”

“…A total of 2.7 × 10 22 13 C atoms were injected. The subsequent deposition was measured using secondary ion mass spectrometry (SIMS) at VTT from a full poloidal set of lower divertor tiles and from selected main chamber tiles from the same toroidal location [2].…”

Monte Carlo method and High Performance Computing for solving Fokker–Planck equation of minority plasma particles

“…In an experiment carried out in 2007, the post mortem analysis brought the puzzling result that over 90 % of the injected 13 C was missing when assuming toroidally symmetric deposition (Hakola et al 2010). No obvious experimental explanation for the disappearance of the carbon was found.…”