Measurement of n = 1 locked mode using locked mode coils in KSTAR

Abstract

The n = 1 static or quasi-static magnetohydrodynamic (MHD) mode such as locked mode (LM) is measured by using LM coils in KSTAR. The delicate signal processing scheme is established to measure the time-evolution of the amplitude and phase of n = 1 LM even low mode-signal sensitivity than non-mode poloidal field sources of LM coils. The effective area of each LM coils are calibrated by comparing the obtained signal and calculated signal responding to a current in the poloidal field (PF) coils without plasma. To pick up only the plasma response two kinds of analytic solutions - the lumped circuit model and the autoregressive with exogenous terms (ARX) model - are introduced to estimate the induced signals to the LM coils from the non-axisymmetric in-vessel coils (IVCs) including its eddy current effect. The developed mode diagnostic method is applied to KSTAR disrupted plasmas and shows consistency with other diagnostics.