Molybdenum test limiter experiments in TEXTOR

Abstract

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 O 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*10¹³ 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