Measurement of the Radial electric field in KSTAR using the Background Polychromator MSE (MSE-BP) system during Current drive experiments

It has been revealed that the Er X B_T shear-Poloidal flows at the plasma edge region can suppress turbulence by breaking up turbulent eddies [1]. It is generally acknowledged that this mechanism contributes significantly to enhanced plasma confinement. While radial electric field (𝐸𝑟) measurements have been extensively conducted in various tokamaks, no such attempt has been made in KSTAR to date [2-4]. In this work, we present the first attempt to measure the radial electric field in KSTAR using the MSE diagnostic. This work describes the approach to derive the 𝐸𝑟 by measuring and comparing the polarization angles of the full- and half-energy components of the neutral beam injected using the background polychromator-based MSE (MSE-BP). Since the measurement wavelengths of the two components depend on the beam energy, we calculated the wavelengths in advance, selected appropriate filters and then installed them in the MSE-BP system. The 𝐸𝑟 measurement was conducted concurrently with the current drive experiments to investigate the impact of non-inductive current drive on the 𝐸𝑟 structure. Preliminary results showed a clear difference in the 𝐸𝑟 distribution depending on the current drive location, and even for the same off-axis current drive, the distribution varied with the injection direction.