T. Dürbeck

Deuterium inventory in the full-tungsten divertor of ASDEX Upgrade

The deuterium inventory in tungsten-coated divertor tiles used during the first full-tungsten plasma-facing wall phase of ASDEX Upgrade was measured by various methods of analysis. The D inventory in the inner divertor was still dominated by codeposition with residual carbon, whereas it was dominated by trapping in the thicker vacuum plasma sprayed tungsten layers at the outer divertor. The total inventory in the divertor area decreased by a factor of 5–10 compared with the period of carbon-dominated plasma-facing wall.

Deuterium retention in different tungsten grades

Deuterium retention in different tungsten (W) grades was investigated for various incident ion energies ranging from 3 to 200?eV per deuterium atom and fluences ranging from 1?1023?m?2 to 2?1025?m?2. Irradiation temperatures were 320 and 500?K. The retained amount was determined by ion beam analysis and thermal desorption spectroscopy. For irradiation at 320?K, deuterium retention in plasma-sprayed tungsten (PSW) is higher compared to polycrystalline W for all investigated energies. For irradiation at 500?K, the deuterium retention for polycrystalline W increases strongly with fluence in contrast to PSW. The highest deuterium inventory was found for ITER grade W irradiated with 200?eV per deuterium at 500?K.

Statistical analysis of blister bursts during temperature-programmed desorption of deuterium-implanted polycrystalline tungsten

During temperature-programmed desorption (TPD) of stress-relieved polycrystalline tungsten samples exposed to a deuterium plasma, short, intense bursts of D2 were observed on the low-temperature flank of the main desorption peak. These bursts are attributed to the rupturing of blisters filled with high-pressure D2 gas. A statistical analysis of the size distribution and temporal correlation of the bursts is presented. The influence of different measurement intervals and TPD heating rates on the observed bursts is simulated based on these statistics and compared to the experimental results. The contribution of bursts to the total D inventory in the sample is also estimated.

Interaction of Deuterium Plasma with Sputter-deposited Tungsten Nitride Films

Magnetron-sputtered tungsten nitride (WNx) films were used as a model system to study the behaviour of re-deposited WNx layers which could form in fusion devices with tungsten (W) wall during nitrogen seeding. The interaction of such WNx layers with deuterium (D) plasmas was investigated in dedicated laboratory experiments. D retention and N removal due to D plasma exposure (D flux: 9.9 × 1019 D m−2 s−1, ion energy 215 eV) at different temperatures were measured with ion beam analysis (IBA). Low-energy argon sputtering followed by IBA was applied to resolve the D distribution in the top-most surface of WNx with significantly improved depth resolution compared with the standard D depth profiling method by nuclear reaction analysis. Experimentally determined thicknesses for the penetration of D in WNx were compared with the penetration depth for D calculated in SDTrimSP simulations. Results show that D is only retained within the ion penetration range for samples exposed at 300 K. In contrast to the 300 K case, D diffuses beyond the implantation depth in a sample exposed at 600 K. However, the D penetration depth is much lower than in pure W at comparable conditions. The total amount of retained D in WNx at 600 K is by 50% lower than for implantation at 300 K with the same D fluence. Nitrogen is removed only within the D ion range.

Deuterium retention in bulk tungsten exposed to the outer divertor plasma of ASDEX Upgrade

Deuterium (D)-retention and -release behaviour from bulk solid tungsten (W) samples exposed to the outer divertor of ASDEX Upgrade were investigated. In the present plasma loading condition, no remarkable blister formation and no severe cracking/melting were observed on the W sample. D retained at the surface (up to ~7 μm) was measured by nuclear reaction analysis (NRA) and total D retention was determined by thermal desorption spectroscopy (TDS). By comparison of the NRA and TDS results, it was found that D was dominantly retained in the bulk (up to 90% of retained D was in the bulk beyond the NRA detection range). The amount of D retention in bulk solid W increased almost in proportional to the square root of the incident fluence, which suggests that D diffusion plays a key role in the D accumulation in the bulk.