Real time current profile control at JET

Abstract

A plasma current profile control system has been designed and tested at Joint European Torus (JET). The system uses the one turn loop voltage for lower hybrid current drive (LHCD) control, and the normalised second current moment of the plasma current distribution for off-axis current control. These provide relatively simple measurements and calculations suitable for real time operation. Models for the response of the loop voltage and second current moment to Lower Hybrid power input are derived and validated against JET data. Based on these models, the components of the second current moment and loop voltage control systems are designed to suit prescribed performance. Loop voltage has been controlled in 2.5 MA discharges at 66% reduction, providing the means for LHCD regulation. Second current moment control during the current rise phase has contributed to 60% improvement of the neutron yield performance of shear optimisation discharges at JET.