Advances in radiated power control at DIII-D

Abstract

Feedback control of radiated power from the lower divertor P rad, div, L has been implemented in the DIII-D Plasma Control System (PCS). A realtime sensor for P rad, div, L has been constructed from 12 foil bolometer channels which agrees with standard post-shot analysis to within 20%. Results with the 12-channel sensor are compared to initial proof-of-concept tests with a single channel as a proxy for P rad, div, L , showing that the upgraded sensor is necessary to overcome limitations of the proxy channel strategy in DIII-D. Using N 2 seeding under feedback control, P rad, div, L has been increased by up to 150% above unseeded levels, and a radiated power fraction f rad of 80% has been demonstrated, although feedback controlled gas flow is steadier at f rad = 55%. Spatial coverage is broad enough to enable P rad control during the strike point sweeps which are commonly used to generate pseudo-2D divertor Thomson measurements in DIII-D divertor experiments. Use of this control reveals challenges that may affect next step devices, which will require actively controlled extrinsic impurity seeding in order to manage heat loads . When operating at high f rad , changes in pedestal T e (caused by ELMs in these experiments but could come from other disturbances) resulted in large perturbations to P rad which were destabilizing to the feedback controller .