Faraday-Effect Polarimeter-Interferometer System for current density measurement on EAST*

An eleven-channel far-infrared laser-based \textbf{PO}larimeter-\textbf{INT}erferometer (\textbf{POINT}) system utilizing the three-wave technique has been implemented for current density and electron density profile measurements in the EAST tokamak. Both polarimetric and interferometric measurement are obtained in a long pulse ($\sim$ 52s) discharge. The electron line-integrated density resolution of POINT is less than 5 $\times$ 10$^{16}$ m$^{-2}$ ($\sim$ 2$^{\circ}$), and the Faraday rotation angle rms phase noise is \textless 0.1$^{\circ}$. With the high temporal ($\sim$ 1 $\mu $sec) and phase resolution(\textless 0.1$^{\circ}$), density perturbations associated with the sawteeth cycle and tearing mode activities have been observed. It is evident that tearing modes are well correlated to dynamics of equilibrium current profile (or q-profile). Faraday rotation angle shows clear variation with low hybrid current drive while line-integrated density remains little changed, implying the current drive in the core. A Digital Phase Detector with 250 kHz bandwidth provides real-time Faraday rotation angle and density phase shift output, which will be integrated into current profile control system in a long pulse discharge in future.