Improvement of polarimeter-interferometer system for current profile measurement on EAST

Abstract Current profile shape and temporal evolution contains important information needed to understand plasma stability and confinement physics in tokamaks. An 11-chord POlarimeter-INTeferometer (POINT) system has been installed on Experimental Advanced Superconducting Tokamak (EAST), which is able to provide real-time measurement of density and Faraday rotation with time response up to 1 μs. Accuracy of Faraday rotation measurement is largely limited by polarimetry phase noise and stray-light due to optical feedback inherent in the double-pass optical arrangement. In recent EAST experimental campaign, we have identified an additional feedback path between optical lens and tokamak window. Elimination of this feedback loop reduced Faraday rotation measurement errors from about 4° to 0.5°. Besides, a new detector with 20 MHz bandwidth and low-noise amplifier is tested during plasma discharges and found to have approximately 20% reduced phase noise. All these improvements on Faraday rotation angle measurement will bring more accuracy to plasma profile reconstruction.