Simultaneous full- and half-energy Stark polarization measurements using a modified background polychromator motional Stark effect system.

Abstract

The radial electric field (Er) is important for turbulence suppression and transport barrier formation in magnetically confined plasmas. To establish the diagnostic capability required for its future determination, the Korea Superconducting Tokamak Advanced Research (KSTAR) Motional Stark Effect Background Polychromator was modified to measure the polarization angles of the full-energy π and half-energy σ Stark components simultaneously along a common line of sight (LOS). Interference filters and operating temperatures were selected using the calculated Stark spectra and the existing filter inventory. The modified configuration was implemented on seven lines of sight and evaluated in four KSTAR discharges at a toroidal magnetic field of 2.5 T with a continuously injected 90-keV neutral beam injection 1-B diagnostic beam. Usable simultaneous measurements were obtained from four lines of sight. The polarization angle difference, Δγ = γF - γH, exhibited distinguishable temporal and channel-dependent responses under internal transport barrier and off-axis neutral-beam-current-drive scenarios. The propagated statistical uncertainties of the reference-subtracted 1-s mean values ranged from ∼0.010° to 0.079°. At LOS 14, the peak changes in two off-axis discharges were 0.358° ± 0.028° and 0.365° ± 0.028°; their 0.007° difference was smaller than the combined statistical uncertainty of 0.039°. These results demonstrate the feasibility of simultaneously resolving full- and half-energy Stark polarization responses using the existing KSTAR diagnostic infrastructure, providing an experimental basis for future quantitative Er reconstruction through updated calibration and complete forward analysis development.