Characteristics of Radiating Collapse at the Density Limit in the Large Helical Device

Abstract

Steady state densities of up to 1.6 × 10²⁰ m^-3 have been sustained using gas puff fuelling and NBI heating up to 11 MW in the Large Helical Device (LHD). The density limit in LHD is observed to be greater than 1.6 times the Sudo limit. The density is ultimately limited by a radiating collapse which is attributed to the onset of a radiative thermal instability of the light impurities in the edge region of the plasma based on several recent observations in LHD. First of all the onset of the radiative thermal instability is tied to a certain edge temperature threshold. Secondly, the onset of the thermal instability occurs first in oxygen and then carbon as expected from their cooling rate temperature dependencies. Finally, radiation profiles show that as the temperature drops and the plasma collapses the radiating zone broadens and moves inward. In addition, comparison of impurity lines with the total radiated power behaviour suggests that carbon is the dominant radiator. Two dimensional tomographic inversions of Absolute eXtreme UltraViolet Diode (AXUVD) array data and comparison of modelling with images of radiation brightness from imaging bolometers indicate that the poloidal asymmetry which accompanies the radiating collapse is roughly toroidally symmetric.