Limiter experiments with a MO test limiter have been carried out in TEXTOR under various conditions with ohmic and NBI heating. Maximum power loads reached about 20 MW/m2 resulting in surface temperatures up to the melting point. A maximum fraction of 8% of the total convective energy in the plasma was deposited onto the MO limiter. Molybdenum impurities are mainly produced by physical sputtering due to the impact of C and 0 impurities. Under ohmic heating conditions the MO impurity radiation increased with increasing plasma density, though the relative source strength of the MO release decreased. Approaching a critical density of about 3 X lOI3 cm-3 MO accumulated in the plasma centre leading to strong central radiation with hollow temperature profiles, which resulted in a minor disruption. Under NBI heated plasma conditions (CO-NBI, 1.3 MW) MO radiation decreased with increasing plasma density as did the relative MO source at the limiter; at high plasma densities the influence of the MO limiter on plasma impurities and plasma performance became negligible. No instability was observed under these conditions. The different behaviour between ohmic and NBI heating conditions is explained by the different relative importance of neoclassical and anomalous transport under ohmic and NBI heating, respectively. The observed instability in high density ohmic plasmas is caused by MO accumulation in the plasma centre and is attributed to neoclassical transport processes.

In order to maintain the thermo-mechanical durability of ITER it is proposed to castellate the interior surface of the first wall and divertor by splitting them into small-size cells [W. Daener et al., Fusion Eng. Des. 61&62 (2002) 61]. A concern is the accumulation of fuel in the gaps of the castellation. In TEXTOR, molybdenum limiters were exposed in the scrape-off layer (SOL) plasma to assess fuel accumulation. The first limiter was exposed under deposition-dominated conditions. Carbon deposits were formed both on top surfaces and in the gaps. About 0.12% of the impinging D-fluence was found in the gaps. Another castellated limiter was exposed under erosion-dominated conditions. Deposited layers were found only on the plasma shadowed areas of the gaps. A significant amount of molybdenum from the limiter was found intermixed in the deposit. The gaps contained $0.03% of the impinging D-fluence. Modeling was performed to simulate carbon transport into the gaps.

The aim of this letter is to demonstrate the feasibility and potential of the novel in situ dust diagnostic method—capture by aerogel targets. Aerogel, a highly porous material with a density of a few tens of kg m−3, allows capturing of dust particles present during the discharge without destroying them. The first exposures in the TEXTOR scrape-off layer plasma showed that such targets are able to capture both slow and fast particles with sizes in the range from submicrometre to ∼100 µm. The technique provides information on dust velocity and size distribution as well as dust flux estimates. The composition and texture of the captured dust can also be studied in detail to shed light on dust formation processes.

No abstract

No abstract